2 * Copyright (c) 1998, 2010, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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13 * accompanied this code).
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25 # include "incls/_precompiled.incl"
26 # include "incls/_synchronizer.cpp.incl"
28 #if defined(__GNUC__) && !defined(IA64)
29 // Need to inhibit inlining for older versions of GCC to avoid build-time failures
30 #define ATTR __attribute__((noinline))
35 // Native markword accessors for synchronization and hashCode().
37 // The "core" versions of monitor enter and exit reside in this file.
38 // The interpreter and compilers contain specialized transliterated
39 // variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
40 // for instance. If you make changes here, make sure to modify the
41 // interpreter, and both C1 and C2 fast-path inline locking code emission.
43 // TODO: merge the objectMonitor and synchronizer classes.
45 // -----------------------------------------------------------------------------
49 // Only bother with this argument setup if dtrace is available
50 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
52 HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,
53 jlong, uintptr_t, char*, int, long);
54 HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,
55 jlong, uintptr_t, char*, int);
56 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
57 jlong, uintptr_t, char*, int);
58 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
59 jlong, uintptr_t, char*, int);
60 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
61 jlong, uintptr_t, char*, int);
62 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
63 jlong, uintptr_t, char*, int);
64 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
65 jlong, uintptr_t, char*, int);
67 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
70 jlong jtid = SharedRuntime::get_java_tid(thread); \
71 symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
72 if (klassname != NULL) { \
73 bytes = (char*)klassname->bytes(); \
74 len = klassname->utf8_length(); \
77 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
79 if (DTraceMonitorProbes) { \
80 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
81 HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
82 (monitor), bytes, len, (millis)); \
86 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
88 if (DTraceMonitorProbes) { \
89 DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
90 HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
91 (uintptr_t)(monitor), bytes, len); \
95 #else // ndef DTRACE_ENABLED
97 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
98 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
100 #endif // ndef DTRACE_ENABLED
102 // ObjectWaiter serves as a "proxy" or surrogate thread.
103 // TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
104 // ParkEvent instead. Beware, however, that the JVMTI code
105 // knows about ObjectWaiters, so we'll have to reconcile that code.
106 // See next_waiter(), first_waiter(), etc.
108 class ObjectWaiter : public StackObj {
110 enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ;
111 enum Sorted { PREPEND, APPEND, SORTED } ;
112 ObjectWaiter * volatile _next;
113 ObjectWaiter * volatile _prev;
116 volatile int _notified ;
117 volatile TStates TState ;
118 Sorted _Sorted ; // List placement disposition
119 bool _active ; // Contention monitoring is enabled
121 ObjectWaiter(Thread* thread) {
127 _event = thread->_ParkEvent ;
129 assert (_event != NULL, "invariant") ;
132 void wait_reenter_begin(ObjectMonitor *mon) {
133 JavaThread *jt = (JavaThread *)this->_thread;
134 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
137 void wait_reenter_end(ObjectMonitor *mon) {
138 JavaThread *jt = (JavaThread *)this->_thread;
139 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
143 enum ManifestConstants {
144 ClearResponsibleAtSTW = 0,
145 MaximumRecheckInterval = 1000
150 #define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); }
152 #define FEVENT(nom) { static volatile int ctr = 0 ; int v = ++ctr ; if ((v & (v-1)) == 0) { ::printf (#nom " : %d \n", v); ::fflush(stdout); }}
155 #define TEVENT(nom) {;}
157 // Performance concern:
158 // OrderAccess::storestore() calls release() which STs 0 into the global volatile
159 // OrderAccess::Dummy variable. This store is unnecessary for correctness.
160 // Many threads STing into a common location causes considerable cache migration
161 // or "sloshing" on large SMP system. As such, I avoid using OrderAccess::storestore()
162 // until it's repaired. In some cases OrderAccess::fence() -- which incurs local
163 // latency on the executing processor -- is a better choice as it scales on SMP
164 // systems. See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
165 // discussion of coherency costs. Note that all our current reference platforms
166 // provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
168 // As a general policy we use "volatile" to control compiler-based reordering
169 // and explicit fences (barriers) to control for architectural reordering performed
170 // by the CPU(s) or platform.
172 static int MBFence (int x) { OrderAccess::fence(); return x; }
174 struct SharedGlobals {
175 // These are highly shared mostly-read variables.
176 // To avoid false-sharing they need to be the sole occupants of a $ line.
177 double padPrefix [8];
178 volatile int stwRandom ;
179 volatile int stwCycle ;
181 // Hot RW variables -- Sequester to avoid false-sharing
182 double padSuffix [16];
183 volatile int hcSequence ;
184 double padFinal [8] ;
187 static SharedGlobals GVars ;
188 static int MonitorScavengeThreshold = 1000000 ;
189 static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending
192 // The knob* variables are effectively final. Once set they should
193 // never be modified hence. Consider using __read_mostly with GCC.
195 static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
196 static int Knob_HandOff = 0 ;
197 static int Knob_Verbose = 0 ;
198 static int Knob_ReportSettings = 0 ;
200 static int Knob_SpinLimit = 5000 ; // derived by an external tool -
201 static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
202 static int Knob_SpinBackOff = 0 ; // spin-loop backoff
203 static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
204 static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
205 static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
206 static int Knob_SpinEarly = 1 ;
207 static int Knob_SuccEnabled = 1 ; // futile wake throttling
208 static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
209 static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
210 static int Knob_Bonus = 100 ; // spin success bonus
211 static int Knob_BonusB = 100 ; // spin success bonus
212 static int Knob_Penalty = 200 ; // spin failure penalty
213 static int Knob_Poverty = 1000 ;
214 static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
215 static int Knob_FixedSpin = 0 ;
216 static int Knob_OState = 3 ; // Spinner checks thread state of _owner
217 static int Knob_UsePause = 1 ;
218 static int Knob_ExitPolicy = 0 ;
219 static int Knob_PreSpin = 10 ; // 20-100 likely better
220 static int Knob_ResetEvent = 0 ;
221 static int BackOffMask = 0 ;
223 static int Knob_FastHSSEC = 0 ;
224 static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
225 static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
226 static volatile int InitDone = 0 ;
229 // hashCode() generation :
232 // * MD5Digest of {obj,stwRandom}
233 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
234 // * A DES- or AES-style SBox[] mechanism
235 // * One of the Phi-based schemes, such as:
236 // 2654435761 = 2^32 * Phi (golden ratio)
237 // HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
238 // * A variation of Marsaglia's shift-xor RNG scheme.
239 // * (obj ^ stwRandom) is appealing, but can result
240 // in undesirable regularity in the hashCode values of adjacent objects
241 // (objects allocated back-to-back, in particular). This could potentially
242 // result in hashtable collisions and reduced hashtable efficiency.
243 // There are simple ways to "diffuse" the middle address bits over the
244 // generated hashCode values:
247 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
250 // This form uses an unguarded global Park-Miller RNG,
251 // so it's possible for two threads to race and generate the same RNG.
252 // On MP system we'll have lots of RW access to a global, so the
253 // mechanism induces lots of coherency traffic.
254 value = os::random() ;
257 // This variation has the property of being stable (idempotent)
258 // between STW operations. This can be useful in some of the 1-0
259 // synchronization schemes.
260 intptr_t addrBits = intptr_t(obj) >> 3 ;
261 value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
264 value = 1 ; // for sensitivity testing
267 value = ++GVars.hcSequence ;
270 value = intptr_t(obj) ;
272 // Marsaglia's xor-shift scheme with thread-specific state
273 // This is probably the best overall implementation -- we'll
274 // likely make this the default in future releases.
275 unsigned t = Self->_hashStateX ;
277 Self->_hashStateX = Self->_hashStateY ;
278 Self->_hashStateY = Self->_hashStateZ ;
279 Self->_hashStateZ = Self->_hashStateW ;
280 unsigned v = Self->_hashStateW ;
281 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
282 Self->_hashStateW = v ;
286 value &= markOopDesc::hash_mask;
287 if (value == 0) value = 0xBAD ;
288 assert (value != markOopDesc::no_hash, "invariant") ;
289 TEVENT (hashCode: GENERATE) ;
293 void BasicLock::print_on(outputStream* st) const {
294 st->print("monitor");
297 void BasicLock::move_to(oop obj, BasicLock* dest) {
298 // Check to see if we need to inflate the lock. This is only needed
299 // if an object is locked using "this" lightweight monitor. In that
300 // case, the displaced_header() is unlocked, because the
301 // displaced_header() contains the header for the originally unlocked
302 // object. However the object could have already been inflated. But it
303 // does not matter, the inflation will just a no-op. For other cases,
304 // the displaced header will be either 0x0 or 0x3, which are location
305 // independent, therefore the BasicLock is free to move.
307 // During OSR we may need to relocate a BasicLock (which contains a
308 // displaced word) from a location in an interpreter frame to a
309 // new location in a compiled frame. "this" refers to the source
310 // basiclock in the interpreter frame. "dest" refers to the destination
311 // basiclock in the new compiled frame. We *always* inflate in move_to().
312 // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
313 // cause performance problems in code that makes heavy use of a small # of
314 // uncontended locks. (We'd inflate during OSR, and then sync performance
315 // would subsequently plummet because the thread would be forced thru the slow-path).
316 // This problem has been made largely moot on IA32 by inlining the inflated fast-path
317 // operations in Fast_Lock and Fast_Unlock in i486.ad.
319 // Note that there is a way to safely swing the object's markword from
320 // one stack location to another. This avoids inflation. Obviously,
321 // we need to ensure that both locations refer to the current thread's stack.
322 // There are some subtle concurrency issues, however, and since the benefit is
323 // is small (given the support for inflated fast-path locking in the fast_lock, etc)
324 // we'll leave that optimization for another time.
326 if (displaced_header()->is_neutral()) {
327 ObjectSynchronizer::inflate_helper(obj);
328 // WARNING: We can not put check here, because the inflation
329 // will not update the displaced header. Once BasicLock is inflated,
330 // no one should ever look at its content.
332 // Typically the displaced header will be 0 (recursive stack lock) or
333 // unused_mark. Naively we'd like to assert that the displaced mark
334 // value is either 0, neutral, or 3. But with the advent of the
335 // store-before-CAS avoidance in fast_lock/compiler_lock_object
336 // we can find any flavor mark in the displaced mark.
338 // [RGV] The next line appears to do nothing!
339 intptr_t dh = (intptr_t) displaced_header();
340 dest->set_displaced_header(displaced_header());
343 // -----------------------------------------------------------------------------
345 // standard constructor, allows locking failures
346 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
349 debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
353 TEVENT (ObjectLocker) ;
355 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
359 ObjectLocker::~ObjectLocker() {
361 ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
365 // -----------------------------------------------------------------------------
368 PerfCounter * ObjectSynchronizer::_sync_Inflations = NULL ;
369 PerfCounter * ObjectSynchronizer::_sync_Deflations = NULL ;
370 PerfCounter * ObjectSynchronizer::_sync_ContendedLockAttempts = NULL ;
371 PerfCounter * ObjectSynchronizer::_sync_FutileWakeups = NULL ;
372 PerfCounter * ObjectSynchronizer::_sync_Parks = NULL ;
373 PerfCounter * ObjectSynchronizer::_sync_EmptyNotifications = NULL ;
374 PerfCounter * ObjectSynchronizer::_sync_Notifications = NULL ;
375 PerfCounter * ObjectSynchronizer::_sync_PrivateA = NULL ;
376 PerfCounter * ObjectSynchronizer::_sync_PrivateB = NULL ;
377 PerfCounter * ObjectSynchronizer::_sync_SlowExit = NULL ;
378 PerfCounter * ObjectSynchronizer::_sync_SlowEnter = NULL ;
379 PerfCounter * ObjectSynchronizer::_sync_SlowNotify = NULL ;
380 PerfCounter * ObjectSynchronizer::_sync_SlowNotifyAll = NULL ;
381 PerfCounter * ObjectSynchronizer::_sync_FailedSpins = NULL ;
382 PerfCounter * ObjectSynchronizer::_sync_SuccessfulSpins = NULL ;
383 PerfCounter * ObjectSynchronizer::_sync_MonInCirculation = NULL ;
384 PerfCounter * ObjectSynchronizer::_sync_MonScavenged = NULL ;
385 PerfLongVariable * ObjectSynchronizer::_sync_MonExtant = NULL ;
387 // One-shot global initialization for the sync subsystem.
388 // We could also defer initialization and initialize on-demand
389 // the first time we call inflate(). Initialization would
390 // be protected - like so many things - by the MonitorCache_lock.
392 void ObjectSynchronizer::Initialize () {
393 static int InitializationCompleted = 0 ;
394 assert (InitializationCompleted == 0, "invariant") ;
395 InitializationCompleted = 1 ;
398 #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
399 #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
400 NEWPERFCOUNTER(_sync_Inflations) ;
401 NEWPERFCOUNTER(_sync_Deflations) ;
402 NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
403 NEWPERFCOUNTER(_sync_FutileWakeups) ;
404 NEWPERFCOUNTER(_sync_Parks) ;
405 NEWPERFCOUNTER(_sync_EmptyNotifications) ;
406 NEWPERFCOUNTER(_sync_Notifications) ;
407 NEWPERFCOUNTER(_sync_SlowEnter) ;
408 NEWPERFCOUNTER(_sync_SlowExit) ;
409 NEWPERFCOUNTER(_sync_SlowNotify) ;
410 NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
411 NEWPERFCOUNTER(_sync_FailedSpins) ;
412 NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
413 NEWPERFCOUNTER(_sync_PrivateA) ;
414 NEWPERFCOUNTER(_sync_PrivateB) ;
415 NEWPERFCOUNTER(_sync_MonInCirculation) ;
416 NEWPERFCOUNTER(_sync_MonScavenged) ;
417 NEWPERFVARIABLE(_sync_MonExtant) ;
418 #undef NEWPERFCOUNTER
422 // Compile-time asserts
423 // When possible, it's better to catch errors deterministically at
424 // compile-time than at runtime. The down-side to using compile-time
425 // asserts is that error message -- often something about negative array
426 // indices -- is opaque.
428 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); }
430 void ObjectMonitor::ctAsserts() {
431 CTASSERT(offset_of (ObjectMonitor, _header) == 0);
434 static int Adjust (volatile int * adr, int dx) {
436 for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
440 // Ad-hoc mutual exclusion primitives: SpinLock and Mux
442 // We employ SpinLocks _only for low-contention, fixed-length
443 // short-duration critical sections where we're concerned
444 // about native mutex_t or HotSpot Mutex:: latency.
445 // The mux construct provides a spin-then-block mutual exclusion
448 // Testing has shown that contention on the ListLock guarding gFreeList
449 // is common. If we implement ListLock as a simple SpinLock it's common
450 // for the JVM to devolve to yielding with little progress. This is true
451 // despite the fact that the critical sections protected by ListLock are
454 // TODO-FIXME: ListLock should be of type SpinLock.
455 // We should make this a 1st-class type, integrated into the lock
456 // hierarchy as leaf-locks. Critically, the SpinLock structure
457 // should have sufficient padding to avoid false-sharing and excessive
458 // cache-coherency traffic.
461 typedef volatile int SpinLockT ;
463 void Thread::SpinAcquire (volatile int * adr, const char * LockName) {
464 if (Atomic::cmpxchg (1, adr, 0) == 0) {
465 return ; // normal fast-path return
468 // Slow-path : We've encountered contention -- Spin/Yield/Block strategy.
469 TEVENT (SpinAcquire - ctx) ;
475 if ((ctr & 0xFFF) == 0 || !os::is_MP()) {
477 // Consider using a simple NakedSleep() instead.
478 // Then SpinAcquire could be called by non-JVM threads
479 Thread::current()->_ParkEvent->park(1) ;
488 if (Atomic::cmpxchg (1, adr, 0) == 0) return ;
492 void Thread::SpinRelease (volatile int * adr) {
493 assert (*adr != 0, "invariant") ;
494 OrderAccess::fence() ; // guarantee at least release consistency.
495 // Roach-motel semantics.
496 // It's safe if subsequent LDs and STs float "up" into the critical section,
497 // but prior LDs and STs within the critical section can't be allowed
498 // to reorder or float past the ST that releases the lock.
502 // muxAcquire and muxRelease:
504 // * muxAcquire and muxRelease support a single-word lock-word construct.
505 // The LSB of the word is set IFF the lock is held.
506 // The remainder of the word points to the head of a singly-linked list
507 // of threads blocked on the lock.
509 // * The current implementation of muxAcquire-muxRelease uses its own
510 // dedicated Thread._MuxEvent instance. If we're interested in
511 // minimizing the peak number of extant ParkEvent instances then
512 // we could eliminate _MuxEvent and "borrow" _ParkEvent as long
513 // as certain invariants were satisfied. Specifically, care would need
514 // to be taken with regards to consuming unpark() "permits".
515 // A safe rule of thumb is that a thread would never call muxAcquire()
516 // if it's enqueued (cxq, EntryList, WaitList, etc) and will subsequently
517 // park(). Otherwise the _ParkEvent park() operation in muxAcquire() could
518 // consume an unpark() permit intended for monitorenter, for instance.
519 // One way around this would be to widen the restricted-range semaphore
520 // implemented in park(). Another alternative would be to provide
521 // multiple instances of the PlatformEvent() for each thread. One
522 // instance would be dedicated to muxAcquire-muxRelease, for instance.
525 // -- Only as leaf locks
526 // -- for short-term locking only as muxAcquire does not perform
527 // thread state transitions.
530 // * We could implement muxAcquire and muxRelease with MCS or CLH locks
531 // but with parking or spin-then-park instead of pure spinning.
532 // * Use Taura-Oyama-Yonenzawa locks.
533 // * It's possible to construct a 1-0 lock if we encode the lockword as
534 // (List,LockByte). Acquire will CAS the full lockword while Release
535 // will STB 0 into the LockByte. The 1-0 scheme admits stranding, so
536 // acquiring threads use timers (ParkTimed) to detect and recover from
537 // the stranding window. Thread/Node structures must be aligned on 256-byte
538 // boundaries by using placement-new.
539 // * Augment MCS with advisory back-link fields maintained with CAS().
540 // Pictorially: LockWord -> T1 <-> T2 <-> T3 <-> ... <-> Tn <-> Owner.
541 // The validity of the backlinks must be ratified before we trust the value.
542 // If the backlinks are invalid the exiting thread must back-track through the
543 // the forward links, which are always trustworthy.
544 // * Add a successor indication. The LockWord is currently encoded as
545 // (List, LOCKBIT:1). We could also add a SUCCBIT or an explicit _succ variable
546 // to provide the usual futile-wakeup optimization.
547 // See RTStt for details.
548 // * Consider schedctl.sc_nopreempt to cover the critical section.
552 typedef volatile intptr_t MutexT ; // Mux Lock-word
553 enum MuxBits { LOCKBIT = 1 } ;
555 void Thread::muxAcquire (volatile intptr_t * Lock, const char * LockName) {
556 intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
558 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
562 TEVENT (muxAcquire - Contention) ;
563 ParkEvent * const Self = Thread::current()->_MuxEvent ;
564 assert ((intptr_t(Self) & LOCKBIT) == 0, "invariant") ;
566 int its = (os::is_MP() ? 100 : 0) + 1 ;
568 // Optional spin phase: spin-then-park strategy
571 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
577 Self->OnList = intptr_t(Lock) ;
578 // The following fence() isn't _strictly necessary as the subsequent
579 // CAS() both serializes execution and ratifies the fetched *Lock value.
580 OrderAccess::fence();
583 if ((w & LOCKBIT) == 0) {
584 if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
585 Self->OnList = 0 ; // hygiene - allows stronger asserts
588 continue ; // Interference -- *Lock changed -- Just retry
590 assert (w & LOCKBIT, "invariant") ;
591 Self->ListNext = (ParkEvent *) (w & ~LOCKBIT );
592 if (Atomic::cmpxchg_ptr (intptr_t(Self)|LOCKBIT, Lock, w) == w) break ;
595 while (Self->OnList != 0) {
601 void Thread::muxAcquireW (volatile intptr_t * Lock, ParkEvent * ev) {
602 intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
604 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
608 TEVENT (muxAcquire - Contention) ;
609 ParkEvent * ReleaseAfter = NULL ;
611 ev = ReleaseAfter = ParkEvent::Allocate (NULL) ;
613 assert ((intptr_t(ev) & LOCKBIT) == 0, "invariant") ;
615 guarantee (ev->OnList == 0, "invariant") ;
616 int its = (os::is_MP() ? 100 : 0) + 1 ;
618 // Optional spin phase: spin-then-park strategy
621 if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
622 if (ReleaseAfter != NULL) {
623 ParkEvent::Release (ReleaseAfter) ;
630 ev->OnList = intptr_t(Lock) ;
631 // The following fence() isn't _strictly necessary as the subsequent
632 // CAS() both serializes execution and ratifies the fetched *Lock value.
633 OrderAccess::fence();
636 if ((w & LOCKBIT) == 0) {
637 if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
639 // We call ::Release while holding the outer lock, thus
640 // artificially lengthening the critical section.
641 // Consider deferring the ::Release() until the subsequent unlock(),
642 // after we've dropped the outer lock.
643 if (ReleaseAfter != NULL) {
644 ParkEvent::Release (ReleaseAfter) ;
648 continue ; // Interference -- *Lock changed -- Just retry
650 assert (w & LOCKBIT, "invariant") ;
651 ev->ListNext = (ParkEvent *) (w & ~LOCKBIT );
652 if (Atomic::cmpxchg_ptr (intptr_t(ev)|LOCKBIT, Lock, w) == w) break ;
655 while (ev->OnList != 0) {
661 // Release() must extract a successor from the list and then wake that thread.
662 // It can "pop" the front of the list or use a detach-modify-reattach (DMR) scheme
663 // similar to that used by ParkEvent::Allocate() and ::Release(). DMR-based
665 // (A) CAS() or swap() null to *Lock, releasing the lock and detaching the list.
666 // (B) Extract a successor from the private list "in-hand"
667 // (C) attempt to CAS() the residual back into *Lock over null.
668 // If there were any newly arrived threads and the CAS() would fail.
669 // In that case Release() would detach the RATs, re-merge the list in-hand
670 // with the RATs and repeat as needed. Alternately, Release() might
671 // detach and extract a successor, but then pass the residual list to the wakee.
672 // The wakee would be responsible for reattaching and remerging before it
673 // competed for the lock.
675 // Both "pop" and DMR are immune from ABA corruption -- there can be
676 // multiple concurrent pushers, but only one popper or detacher.
677 // This implementation pops from the head of the list. This is unfair,
678 // but tends to provide excellent throughput as hot threads remain hot.
679 // (We wake recently run threads first).
681 void Thread::muxRelease (volatile intptr_t * Lock) {
683 const intptr_t w = Atomic::cmpxchg_ptr (0, Lock, LOCKBIT) ;
684 assert (w & LOCKBIT, "invariant") ;
685 if (w == LOCKBIT) return ;
686 ParkEvent * List = (ParkEvent *) (w & ~LOCKBIT) ;
687 assert (List != NULL, "invariant") ;
688 assert (List->OnList == intptr_t(Lock), "invariant") ;
689 ParkEvent * nxt = List->ListNext ;
691 // The following CAS() releases the lock and pops the head element.
692 if (Atomic::cmpxchg_ptr (intptr_t(nxt), Lock, w) != w) {
696 OrderAccess::fence() ;
702 // ObjectMonitor Lifecycle
703 // -----------------------
704 // Inflation unlinks monitors from the global gFreeList and
705 // associates them with objects. Deflation -- which occurs at
706 // STW-time -- disassociates idle monitors from objects. Such
707 // scavenged monitors are returned to the gFreeList.
709 // The global list is protected by ListLock. All the critical sections
710 // are short and operate in constant-time.
712 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
715 // -- unassigned and on the global free list
716 // -- unassigned and on a thread's private omFreeList
717 // -- assigned to an object. The object is inflated and the mark refers
718 // to the objectmonitor.
722 // * We currently protect the gFreeList with a simple lock.
723 // An alternate lock-free scheme would be to pop elements from the gFreeList
724 // with CAS. This would be safe from ABA corruption as long we only
725 // recycled previously appearing elements onto the list in deflate_idle_monitors()
726 // at STW-time. Completely new elements could always be pushed onto the gFreeList
727 // with CAS. Elements that appeared previously on the list could only
728 // be installed at STW-time.
730 // * For efficiency and to help reduce the store-before-CAS penalty
731 // the objectmonitors on gFreeList or local free lists should be ready to install
732 // with the exception of _header and _object. _object can be set after inflation.
733 // In particular, keep all objectMonitors on a thread's private list in ready-to-install
734 // state with m.Owner set properly.
736 // * We could all diffuse contention by using multiple global (FreeList, Lock)
737 // pairs -- threads could use trylock() and a cyclic-scan strategy to search for
738 // an unlocked free list.
740 // * Add lifecycle tags and assert()s.
742 // * Be more consistent about when we clear an objectmonitor's fields:
743 // A. After extracting the objectmonitor from a free list.
744 // B. After adding an objectmonitor to a free list.
747 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
748 ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ;
749 static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache
750 static volatile int MonitorFreeCount = 0 ; // # on gFreeList
751 static volatile int MonitorPopulation = 0 ; // # Extant -- in circulation
752 #define CHAINMARKER ((oop)-1)
754 // Constraining monitor pool growth via MonitorBound ...
756 // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
757 // the rate of scavenging is driven primarily by GC. As such, we can find
758 // an inordinate number of monitors in circulation.
759 // To avoid that scenario we can artificially induce a STW safepoint
760 // if the pool appears to be growing past some reasonable bound.
761 // Generally we favor time in space-time tradeoffs, but as there's no
762 // natural back-pressure on the # of extant monitors we need to impose some
763 // type of limit. Beware that if MonitorBound is set to too low a value
764 // we could just loop. In addition, if MonitorBound is set to a low value
765 // we'll incur more safepoints, which are harmful to performance.
766 // See also: GuaranteedSafepointInterval
768 // As noted elsewhere, the correct long-term solution is to deflate at
769 // monitorexit-time, in which case the number of inflated objects is bounded
770 // by the number of threads. That policy obviates the need for scavenging at
771 // STW safepoint time. As an aside, scavenging can be time-consuming when the
772 // # of extant monitors is large. Unfortunately there's a day-1 assumption baked
773 // into much HotSpot code that the object::monitor relationship, once established
774 // or observed, will remain stable except over potential safepoints.
776 // We can use either a blocking synchronous VM operation or an async VM operation.
777 // -- If we use a blocking VM operation :
778 // Calls to ScavengeCheck() should be inserted only into 'safe' locations in paths
779 // that lead to ::inflate() or ::omAlloc().
780 // Even though the safepoint will not directly induce GC, a GC might
781 // piggyback on the safepoint operation, so the caller should hold no naked oops.
782 // Furthermore, monitor::object relationships are NOT necessarily stable over this call
783 // unless the caller has made provisions to "pin" the object to the monitor, say
784 // by incrementing the monitor's _count field.
785 // -- If we use a non-blocking asynchronous VM operation :
786 // the constraints above don't apply. The safepoint will fire in the future
787 // at a more convenient time. On the other hand the latency between posting and
788 // running the safepoint introduces or admits "slop" or laxity during which the
789 // monitor population can climb further above the threshold. The monitor population,
790 // however, tends to converge asymptotically over time to a count that's slightly
791 // above the target value specified by MonitorBound. That is, we avoid unbounded
792 // growth, albeit with some imprecision.
794 // The current implementation uses asynchronous VM operations.
796 // Ideally we'd check if (MonitorPopulation > MonitorBound) in omAlloc()
797 // immediately before trying to grow the global list via allocation.
798 // If the predicate was true then we'd induce a synchronous safepoint, wait
799 // for the safepoint to complete, and then again to allocate from the global
800 // free list. This approach is much simpler and precise, admitting no "slop".
801 // Unfortunately we can't safely safepoint in the midst of omAlloc(), so
802 // instead we use asynchronous safepoints.
804 static void InduceScavenge (Thread * Self, const char * Whence) {
805 // Induce STW safepoint to trim monitors
806 // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
807 // More precisely, trigger an asynchronous STW safepoint as the number
808 // of active monitors passes the specified threshold.
809 // TODO: assert thread state is reasonable
811 if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
813 ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;
816 // Induce a 'null' safepoint to scavenge monitors
817 // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
818 // to the VMthread and have a lifespan longer than that of this activation record.
819 // The VMThread will delete the op when completed.
820 VMThread::execute (new VM_ForceAsyncSafepoint()) ;
823 ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;
829 ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
830 // A large MAXPRIVATE value reduces both list lock contention
831 // and list coherency traffic, but also tends to increase the
832 // number of objectMonitors in circulation as well as the STW
833 // scavenge costs. As usual, we lean toward time in space-time
835 const int MAXPRIVATE = 1024 ;
839 // 1: try to allocate from the thread's local omFreeList.
840 // Threads will attempt to allocate first from their local list, then
841 // from the global list, and only after those attempts fail will the thread
842 // attempt to instantiate new monitors. Thread-local free lists take
843 // heat off the ListLock and improve allocation latency, as well as reducing
844 // coherency traffic on the shared global list.
845 m = Self->omFreeList ;
847 Self->omFreeList = m->FreeNext ;
848 Self->omFreeCount -- ;
849 // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
850 guarantee (m->object() == NULL, "invariant") ;
851 if (MonitorInUseLists) {
852 m->FreeNext = Self->omInUseList;
853 Self->omInUseList = m;
854 Self->omInUseCount ++;
859 // 2: try to allocate from the global gFreeList
860 // CONSIDER: use muxTry() instead of muxAcquire().
861 // If the muxTry() fails then drop immediately into case 3.
862 // If we're using thread-local free lists then try
863 // to reprovision the caller's free list.
864 if (gFreeList != NULL) {
865 // Reprovision the thread's omFreeList.
866 // Use bulk transfers to reduce the allocation rate and heat
868 Thread::muxAcquire (&ListLock, "omAlloc") ;
869 for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
871 ObjectMonitor * take = gFreeList ;
872 gFreeList = take->FreeNext ;
873 guarantee (take->object() == NULL, "invariant") ;
874 guarantee (!take->is_busy(), "invariant") ;
876 omRelease (Self, take) ;
878 Thread::muxRelease (&ListLock) ;
879 Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
880 if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
881 TEVENT (omFirst - reprovision) ;
884 const int mx = MonitorBound ;
885 if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) {
886 // We can't safely induce a STW safepoint from omAlloc() as our thread
887 // state may not be appropriate for such activities and callers may hold
888 // naked oops, so instead we defer the action.
889 InduceScavenge (Self, "omAlloc") ;
894 // 3: allocate a block of new ObjectMonitors
895 // Both the local and global free lists are empty -- resort to malloc().
896 // In the current implementation objectMonitors are TSM - immortal.
897 assert (_BLOCKSIZE > 1, "invariant") ;
898 ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
900 // NOTE: (almost) no way to recover if allocation failed.
901 // We might be able to induce a STW safepoint and scavenge enough
902 // objectMonitors to permit progress.
904 vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ;
908 // initialize the linked list, each monitor points to its next
909 // forming the single linked free list, the very first monitor
910 // will points to next block, which forms the block list.
911 // The trick of using the 1st element in the block as gBlockList
912 // linkage should be reconsidered. A better implementation would
913 // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
915 for (int i = 1; i < _BLOCKSIZE ; i++) {
916 temp[i].FreeNext = &temp[i+1];
919 // terminate the last monitor as the end of list
920 temp[_BLOCKSIZE - 1].FreeNext = NULL ;
922 // Element [0] is reserved for global list linkage
923 temp[0].set_object(CHAINMARKER);
925 // Consider carving out this thread's current request from the
926 // block in hand. This avoids some lock traffic and redundant
929 // Acquire the ListLock to manipulate BlockList and FreeList.
930 // An Oyama-Taura-Yonezawa scheme might be more efficient.
931 Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
932 MonitorPopulation += _BLOCKSIZE-1;
933 MonitorFreeCount += _BLOCKSIZE-1;
935 // Add the new block to the list of extant blocks (gBlockList).
936 // The very first objectMonitor in a block is reserved and dedicated.
937 // It serves as blocklist "next" linkage.
938 temp[0].FreeNext = gBlockList;
941 // Add the new string of objectMonitors to the global free list
942 temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
943 gFreeList = temp + 1;
944 Thread::muxRelease (&ListLock) ;
945 TEVENT (Allocate block of monitors) ;
949 // Place "m" on the caller's private per-thread omFreeList.
950 // In practice there's no need to clamp or limit the number of
951 // monitors on a thread's omFreeList as the only time we'll call
952 // omRelease is to return a monitor to the free list after a CAS
953 // attempt failed. This doesn't allow unbounded #s of monitors to
954 // accumulate on a thread's free list.
956 // In the future the usage of omRelease() might change and monitors
957 // could migrate between free lists. In that case to avoid excessive
958 // accumulation we could limit omCount to (omProvision*2), otherwise return
959 // the objectMonitor to the global list. We should drain (return) in reasonable chunks.
960 // That is, *not* one-at-a-time.
963 void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m) {
964 guarantee (m->object() == NULL, "invariant") ;
965 m->FreeNext = Self->omFreeList ;
966 Self->omFreeList = m ;
967 Self->omFreeCount ++ ;
970 // Return the monitors of a moribund thread's local free list to
971 // the global free list. Typically a thread calls omFlush() when
972 // it's dying. We could also consider having the VM thread steal
973 // monitors from threads that have not run java code over a few
974 // consecutive STW safepoints. Relatedly, we might decay
975 // omFreeProvision at STW safepoints.
977 // We currently call omFlush() from the Thread:: dtor _after the thread
978 // has been excised from the thread list and is no longer a mutator.
979 // That means that omFlush() can run concurrently with a safepoint and
980 // the scavenge operator. Calling omFlush() from JavaThread::exit() might
981 // be a better choice as we could safely reason that that the JVM is
982 // not at a safepoint at the time of the call, and thus there could
983 // be not inopportune interleavings between omFlush() and the scavenge
986 void ObjectSynchronizer::omFlush (Thread * Self) {
987 ObjectMonitor * List = Self->omFreeList ; // Null-terminated SLL
988 Self->omFreeList = NULL ;
989 if (List == NULL) return ;
990 ObjectMonitor * Tail = NULL ;
993 for (s = List ; s != NULL ; s = s->FreeNext) {
996 guarantee (s->object() == NULL, "invariant") ;
997 guarantee (!s->is_busy(), "invariant") ;
998 s->set_owner (NULL) ; // redundant but good hygiene
999 TEVENT (omFlush - Move one) ;
1002 guarantee (Tail != NULL && List != NULL, "invariant") ;
1003 Thread::muxAcquire (&ListLock, "omFlush") ;
1004 Tail->FreeNext = gFreeList ;
1006 MonitorFreeCount += Tally;
1007 Thread::muxRelease (&ListLock) ;
1012 // Get the next block in the block list.
1013 static inline ObjectMonitor* next(ObjectMonitor* block) {
1014 assert(block->object() == CHAINMARKER, "must be a block header");
1015 block = block->FreeNext ;
1016 assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
1020 // Fast path code shared by multiple functions
1021 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1022 markOop mark = obj->mark();
1023 if (mark->has_monitor()) {
1024 assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1025 assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1026 return mark->monitor();
1028 return ObjectSynchronizer::inflate(Thread::current(), obj);
1031 // Note that we could encounter some performance loss through false-sharing as
1032 // multiple locks occupy the same $ line. Padding might be appropriate.
1034 #define NINFLATIONLOCKS 256
1035 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;
1037 static markOop ReadStableMark (oop obj) {
1038 markOop mark = obj->mark() ;
1039 if (!mark->is_being_inflated()) {
1040 return mark ; // normal fast-path return
1045 markOop mark = obj->mark() ;
1046 if (!mark->is_being_inflated()) {
1047 return mark ; // normal fast-path return
1050 // The object is being inflated by some other thread.
1051 // The caller of ReadStableMark() must wait for inflation to complete.
1053 // TODO: consider calling SafepointSynchronize::do_call_back() while
1054 // spinning to see if there's a safepoint pending. If so, immediately
1055 // yielding or blocking would be appropriate. Avoid spinning while
1056 // there is a safepoint pending.
1057 // TODO: add inflation contention performance counters.
1058 // TODO: restrict the aggregate number of spinners.
1061 if (its > 10000 || !os::is_MP()) {
1064 TEVENT (Inflate: INFLATING - yield) ;
1066 // Note that the following code attenuates the livelock problem but is not
1067 // a complete remedy. A more complete solution would require that the inflating
1068 // thread hold the associated inflation lock. The following code simply restricts
1069 // the number of spinners to at most one. We'll have N-2 threads blocked
1070 // on the inflationlock, 1 thread holding the inflation lock and using
1071 // a yield/park strategy, and 1 thread in the midst of inflation.
1072 // A more refined approach would be to change the encoding of INFLATING
1073 // to allow encapsulation of a native thread pointer. Threads waiting for
1074 // inflation to complete would use CAS to push themselves onto a singly linked
1075 // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
1076 // and calling park(). When inflation was complete the thread that accomplished inflation
1077 // would detach the list and set the markword to inflated with a single CAS and
1078 // then for each thread on the list, set the flag and unpark() the thread.
1079 // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
1080 // wakes at most one thread whereas we need to wake the entire list.
1081 int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
1082 int YieldThenBlock = 0 ;
1083 assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
1084 assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
1085 Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
1086 while (obj->mark() == markOopDesc::INFLATING()) {
1087 // Beware: NakedYield() is advisory and has almost no effect on some platforms
1088 // so we periodically call Self->_ParkEvent->park(1).
1089 // We use a mixed spin/yield/block mechanism.
1090 if ((YieldThenBlock++) >= 16) {
1091 Thread::current()->_ParkEvent->park(1) ;
1096 Thread::muxRelease (InflationLocks + ix ) ;
1097 TEVENT (Inflate: INFLATING - yield/park) ;
1100 SpinPause() ; // SMP-polite spinning
1105 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
1106 // Inflate mutates the heap ...
1107 // Relaxing assertion for bug 6320749.
1108 assert (Universe::verify_in_progress() ||
1109 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1112 const markOop mark = object->mark() ;
1113 assert (!mark->has_bias_pattern(), "invariant") ;
1115 // The mark can be in one of the following states:
1116 // * Inflated - just return
1117 // * Stack-locked - coerce it to inflated
1118 // * INFLATING - busy wait for conversion to complete
1119 // * Neutral - aggressively inflate the object.
1120 // * BIASED - Illegal. We should never see this
1123 if (mark->has_monitor()) {
1124 ObjectMonitor * inf = mark->monitor() ;
1125 assert (inf->header()->is_neutral(), "invariant");
1126 assert (inf->object() == object, "invariant") ;
1127 assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1131 // CASE: inflation in progress - inflating over a stack-lock.
1132 // Some other thread is converting from stack-locked to inflated.
1133 // Only that thread can complete inflation -- other threads must wait.
1134 // The INFLATING value is transient.
1135 // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1136 // We could always eliminate polling by parking the thread on some auxiliary list.
1137 if (mark == markOopDesc::INFLATING()) {
1138 TEVENT (Inflate: spin while INFLATING) ;
1139 ReadStableMark(object) ;
1143 // CASE: stack-locked
1144 // Could be stack-locked either by this thread or by some other thread.
1146 // Note that we allocate the objectmonitor speculatively, _before_ attempting
1147 // to install INFLATING into the mark word. We originally installed INFLATING,
1148 // allocated the objectmonitor, and then finally STed the address of the
1149 // objectmonitor into the mark. This was correct, but artificially lengthened
1150 // the interval in which INFLATED appeared in the mark, thus increasing
1151 // the odds of inflation contention.
1153 // We now use per-thread private objectmonitor free lists.
1154 // These list are reprovisioned from the global free list outside the
1155 // critical INFLATING...ST interval. A thread can transfer
1156 // multiple objectmonitors en-mass from the global free list to its local free list.
1157 // This reduces coherency traffic and lock contention on the global free list.
1158 // Using such local free lists, it doesn't matter if the omAlloc() call appears
1159 // before or after the CAS(INFLATING) operation.
1160 // See the comments in omAlloc().
1162 if (mark->has_locker()) {
1163 ObjectMonitor * m = omAlloc (Self) ;
1164 // Optimistically prepare the objectmonitor - anticipate successful CAS
1165 // We do this before the CAS in order to minimize the length of time
1166 // in which INFLATING appears in the mark.
1168 m->FreeNext = NULL ;
1169 m->_Responsible = NULL ;
1170 m->OwnerIsThread = 0 ;
1171 m->_recursions = 0 ;
1172 m->_SpinDuration = Knob_SpinLimit ; // Consider: maintain by type/class
1174 markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
1176 omRelease (Self, m) ;
1177 continue ; // Interference -- just retry
1180 // We've successfully installed INFLATING (0) into the mark-word.
1181 // This is the only case where 0 will appear in a mark-work.
1182 // Only the singular thread that successfully swings the mark-word
1183 // to 0 can perform (or more precisely, complete) inflation.
1185 // Why do we CAS a 0 into the mark-word instead of just CASing the
1186 // mark-word from the stack-locked value directly to the new inflated state?
1187 // Consider what happens when a thread unlocks a stack-locked object.
1188 // It attempts to use CAS to swing the displaced header value from the
1189 // on-stack basiclock back into the object header. Recall also that the
1190 // header value (hashcode, etc) can reside in (a) the object header, or
1191 // (b) a displaced header associated with the stack-lock, or (c) a displaced
1192 // header in an objectMonitor. The inflate() routine must copy the header
1193 // value from the basiclock on the owner's stack to the objectMonitor, all
1194 // the while preserving the hashCode stability invariants. If the owner
1195 // decides to release the lock while the value is 0, the unlock will fail
1196 // and control will eventually pass from slow_exit() to inflate. The owner
1197 // will then spin, waiting for the 0 value to disappear. Put another way,
1198 // the 0 causes the owner to stall if the owner happens to try to
1199 // drop the lock (restoring the header from the basiclock to the object)
1200 // while inflation is in-progress. This protocol avoids races that might
1201 // would otherwise permit hashCode values to change or "flicker" for an object.
1202 // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1203 // 0 serves as a "BUSY" inflate-in-progress indicator.
1206 // fetch the displaced mark from the owner's stack.
1207 // The owner can't die or unwind past the lock while our INFLATING
1208 // object is in the mark. Furthermore the owner can't complete
1209 // an unlock on the object, either.
1210 markOop dmw = mark->displaced_mark_helper() ;
1211 assert (dmw->is_neutral(), "invariant") ;
1213 // Setup monitor fields to proper values -- prepare the monitor
1214 m->set_header(dmw) ;
1216 // Optimization: if the mark->locker stack address is associated
1217 // with this thread we could simply set m->_owner = Self and
1218 // m->OwnerIsThread = 1. Note that a thread can inflate an object
1219 // that it has stack-locked -- as might happen in wait() -- directly
1220 // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
1221 m->set_owner(mark->locker());
1222 m->set_object(object);
1223 // TODO-FIXME: assert BasicLock->dhw != 0.
1225 // Must preserve store ordering. The monitor state must
1226 // be stable at the time of publishing the monitor address.
1227 guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
1228 object->release_set_mark(markOopDesc::encode(m));
1230 // Hopefully the performance counters are allocated on distinct cache lines
1231 // to avoid false sharing on MP systems ...
1232 if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
1233 TEVENT(Inflate: overwrite stacklock) ;
1234 if (TraceMonitorInflation) {
1235 if (object->is_instance()) {
1237 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1238 (intptr_t) object, (intptr_t) object->mark(),
1239 Klass::cast(object->klass())->external_name());
1246 // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1247 // If we know we're inflating for entry it's better to inflate by swinging a
1248 // pre-locked objectMonitor pointer into the object header. A successful
1249 // CAS inflates the object *and* confers ownership to the inflating thread.
1250 // In the current implementation we use a 2-step mechanism where we CAS()
1251 // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1252 // An inflateTry() method that we could call from fast_enter() and slow_enter()
1255 assert (mark->is_neutral(), "invariant");
1256 ObjectMonitor * m = omAlloc (Self) ;
1257 // prepare m for installation - set monitor to initial state
1259 m->set_header(mark);
1261 m->set_object(object);
1262 m->OwnerIsThread = 1 ;
1263 m->_recursions = 0 ;
1264 m->FreeNext = NULL ;
1265 m->_Responsible = NULL ;
1266 m->_SpinDuration = Knob_SpinLimit ; // consider: keep metastats by type/class
1268 if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1269 m->set_object (NULL) ;
1270 m->set_owner (NULL) ;
1271 m->OwnerIsThread = 0 ;
1273 omRelease (Self, m) ;
1276 // interference - the markword changed - just retry.
1277 // The state-transitions are one-way, so there's no chance of
1278 // live-lock -- "Inflated" is an absorbing state.
1281 // Hopefully the performance counters are allocated on distinct
1282 // cache lines to avoid false sharing on MP systems ...
1283 if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
1284 TEVENT(Inflate: overwrite neutral) ;
1285 if (TraceMonitorInflation) {
1286 if (object->is_instance()) {
1288 tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1289 (intptr_t) object, (intptr_t) object->mark(),
1290 Klass::cast(object->klass())->external_name());
1298 // This the fast monitor enter. The interpreter and compiler use
1299 // some assembly copies of this code. Make sure update those code
1300 // if the following function is changed. The implementation is
1301 // extremely sensitive to race condition. Be careful.
1303 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
1304 if (UseBiasedLocking) {
1305 if (!SafepointSynchronize::is_at_safepoint()) {
1306 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
1307 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
1311 assert(!attempt_rebias, "can not rebias toward VM thread");
1312 BiasedLocking::revoke_at_safepoint(obj);
1314 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1317 slow_enter (obj, lock, THREAD) ;
1320 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
1321 assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
1322 // if displaced header is null, the previous enter is recursive enter, no-op
1323 markOop dhw = lock->displaced_header();
1326 // Recursive stack-lock.
1327 // Diagnostics -- Could be: stack-locked, inflating, inflated.
1328 mark = object->mark() ;
1329 assert (!mark->is_neutral(), "invariant") ;
1330 if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
1331 assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
1333 if (mark->has_monitor()) {
1334 ObjectMonitor * m = mark->monitor() ;
1335 assert(((oop)(m->object()))->mark() == mark, "invariant") ;
1336 assert(m->is_entered(THREAD), "invariant") ;
1341 mark = object->mark() ;
1343 // If the object is stack-locked by the current thread, try to
1344 // swing the displaced header from the box back to the mark.
1345 if (mark == (markOop) lock) {
1346 assert (dhw->is_neutral(), "invariant") ;
1347 if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
1348 TEVENT (fast_exit: release stacklock) ;
1353 ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ;
1356 // This routine is used to handle interpreter/compiler slow case
1357 // We don't need to use fast path here, because it must have been
1358 // failed in the interpreter/compiler code.
1359 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
1360 markOop mark = obj->mark();
1361 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
1363 if (mark->is_neutral()) {
1364 // Anticipate successful CAS -- the ST of the displaced mark must
1365 // be visible <= the ST performed by the CAS.
1366 lock->set_displaced_header(mark);
1367 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
1368 TEVENT (slow_enter: release stacklock) ;
1371 // Fall through to inflate() ...
1373 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1374 assert(lock != mark->locker(), "must not re-lock the same lock");
1375 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
1376 lock->set_displaced_header(NULL);
1381 // The following optimization isn't particularly useful.
1382 if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
1383 lock->set_displaced_header (NULL) ;
1388 // The object header will never be displaced to this lock,
1389 // so it does not matter what the value is, except that it
1390 // must be non-zero to avoid looking like a re-entrant lock,
1391 // and must not look locked either.
1392 lock->set_displaced_header(markOopDesc::unused_mark());
1393 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
1396 // This routine is used to handle interpreter/compiler slow case
1397 // We don't need to use fast path here, because it must have
1398 // failed in the interpreter/compiler code. Simply use the heavy
1399 // weight monitor should be ok, unless someone find otherwise.
1400 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
1401 fast_exit (object, lock, THREAD) ;
1404 // NOTE: must use heavy weight monitor to handle jni monitor enter
1405 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter
1406 // the current locking is from JNI instead of Java code
1407 TEVENT (jni_enter) ;
1408 if (UseBiasedLocking) {
1409 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1410 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1412 THREAD->set_current_pending_monitor_is_from_java(false);
1413 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
1414 THREAD->set_current_pending_monitor_is_from_java(true);
1417 // NOTE: must use heavy weight monitor to handle jni monitor enter
1418 bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
1419 if (UseBiasedLocking) {
1420 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1421 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1424 ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());
1425 return monitor->try_enter(THREAD);
1429 // NOTE: must use heavy weight monitor to handle jni monitor exit
1430 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
1432 if (UseBiasedLocking) {
1433 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1435 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1437 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
1438 // If this thread has locked the object, exit the monitor. Note: can't use
1439 // monitor->check(CHECK); must exit even if an exception is pending.
1440 if (monitor->check(THREAD)) {
1441 monitor->exit(THREAD);
1445 // complete_exit()/reenter() are used to wait on a nested lock
1446 // i.e. to give up an outer lock completely and then re-enter
1447 // Used when holding nested locks - lock acquisition order: lock1 then lock2
1448 // 1) complete_exit lock1 - saving recursion count
1450 // 3) when notified on lock2, unlock lock2
1451 // 4) reenter lock1 with original recursion count
1453 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
1454 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
1455 TEVENT (complete_exit) ;
1456 if (UseBiasedLocking) {
1457 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1458 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1461 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1463 return monitor->complete_exit(THREAD);
1466 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
1467 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
1469 if (UseBiasedLocking) {
1470 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1471 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1474 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1476 monitor->reenter(recursion, THREAD);
1479 // This exists only as a workaround of dtrace bug 6254741
1480 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
1481 DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
1485 // NOTE: must use heavy weight monitor to handle wait()
1486 void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
1487 if (UseBiasedLocking) {
1488 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1489 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1492 TEVENT (wait - throw IAX) ;
1493 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
1495 ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
1496 DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
1497 monitor->wait(millis, true, THREAD);
1499 /* This dummy call is in place to get around dtrace bug 6254741. Once
1500 that's fixed we can uncomment the following line and remove the call */
1501 // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
1502 dtrace_waited_probe(monitor, obj, THREAD);
1505 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
1506 if (UseBiasedLocking) {
1507 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1508 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1511 TEVENT (wait - throw IAX) ;
1512 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
1514 ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
1517 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
1518 if (UseBiasedLocking) {
1519 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1520 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1523 markOop mark = obj->mark();
1524 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1527 ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
1530 // NOTE: see comment of notify()
1531 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
1532 if (UseBiasedLocking) {
1533 BiasedLocking::revoke_and_rebias(obj, false, THREAD);
1534 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1537 markOop mark = obj->mark();
1538 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
1541 ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
1544 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
1545 if (UseBiasedLocking) {
1546 // NOTE: many places throughout the JVM do not expect a safepoint
1547 // to be taken here, in particular most operations on perm gen
1548 // objects. However, we only ever bias Java instances and all of
1549 // the call sites of identity_hash that might revoke biases have
1550 // been checked to make sure they can handle a safepoint. The
1551 // added check of the bias pattern is to avoid useless calls to
1552 // thread-local storage.
1553 if (obj->mark()->has_bias_pattern()) {
1554 // Box and unbox the raw reference just in case we cause a STW safepoint.
1555 Handle hobj (Self, obj) ;
1556 // Relaxing assertion for bug 6320749.
1557 assert (Universe::verify_in_progress() ||
1558 !SafepointSynchronize::is_at_safepoint(),
1559 "biases should not be seen by VM thread here");
1560 BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
1562 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1566 // hashCode() is a heap mutator ...
1567 // Relaxing assertion for bug 6320749.
1568 assert (Universe::verify_in_progress() ||
1569 !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1570 assert (Universe::verify_in_progress() ||
1571 Self->is_Java_thread() , "invariant") ;
1572 assert (Universe::verify_in_progress() ||
1573 ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
1575 ObjectMonitor* monitor = NULL;
1578 markOop mark = ReadStableMark (obj);
1580 // object should remain ineligible for biased locking
1581 assert (!mark->has_bias_pattern(), "invariant") ;
1583 if (mark->is_neutral()) {
1584 hash = mark->hash(); // this is a normal header
1585 if (hash) { // if it has hash, just return it
1588 hash = get_next_hash(Self, obj); // allocate a new hash code
1589 temp = mark->copy_set_hash(hash); // merge the hash code into header
1590 // use (machine word version) atomic operation to install the hash
1591 test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
1595 // If atomic operation failed, we must inflate the header
1596 // into heavy weight monitor. We could add more code here
1597 // for fast path, but it does not worth the complexity.
1598 } else if (mark->has_monitor()) {
1599 monitor = mark->monitor();
1600 temp = monitor->header();
1601 assert (temp->is_neutral(), "invariant") ;
1602 hash = temp->hash();
1606 // Skip to the following code to reduce code size
1607 } else if (Self->is_lock_owned((address)mark->locker())) {
1608 temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
1609 assert (temp->is_neutral(), "invariant") ;
1610 hash = temp->hash(); // by current thread, check if the displaced
1611 if (hash) { // header contains hash code
1615 // The displaced header is strictly immutable.
1616 // It can NOT be changed in ANY cases. So we have
1617 // to inflate the header into heavyweight monitor
1618 // even the current thread owns the lock. The reason
1619 // is the BasicLock (stack slot) will be asynchronously
1620 // read by other threads during the inflate() function.
1621 // Any change to stack may not propagate to other threads
1625 // Inflate the monitor to set hash code
1626 monitor = ObjectSynchronizer::inflate(Self, obj);
1627 // Load displaced header and check it has hash code
1628 mark = monitor->header();
1629 assert (mark->is_neutral(), "invariant") ;
1630 hash = mark->hash();
1632 hash = get_next_hash(Self, obj);
1633 temp = mark->copy_set_hash(hash); // merge hash code into header
1634 assert (temp->is_neutral(), "invariant") ;
1635 test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
1637 // The only update to the header in the monitor (outside GC)
1638 // is install the hash code. If someone add new usage of
1639 // displaced header, please update this code
1640 hash = test->hash();
1641 assert (test->is_neutral(), "invariant") ;
1642 assert (hash != 0, "Trivial unexpected object/monitor header usage.");
1645 // We finally get the hash
1649 // Deprecated -- use FastHashCode() instead.
1651 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
1652 return FastHashCode (Thread::current(), obj()) ;
1655 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
1657 if (UseBiasedLocking) {
1658 BiasedLocking::revoke_and_rebias(h_obj, false, thread);
1659 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1662 assert(thread == JavaThread::current(), "Can only be called on current thread");
1665 markOop mark = ReadStableMark (obj) ;
1667 // Uncontended case, header points to stack
1668 if (mark->has_locker()) {
1669 return thread->is_lock_owned((address)mark->locker());
1671 // Contended case, header points to ObjectMonitor (tagged pointer)
1672 if (mark->has_monitor()) {
1673 ObjectMonitor* monitor = mark->monitor();
1674 return monitor->is_entered(thread) != 0 ;
1676 // Unlocked case, header in place
1677 assert(mark->is_neutral(), "sanity check");
1681 // Be aware of this method could revoke bias of the lock object.
1682 // This method querys the ownership of the lock handle specified by 'h_obj'.
1683 // If the current thread owns the lock, it returns owner_self. If no
1684 // thread owns the lock, it returns owner_none. Otherwise, it will return
1686 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
1687 (JavaThread *self, Handle h_obj) {
1688 // The caller must beware this method can revoke bias, and
1689 // revocation can result in a safepoint.
1690 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
1691 assert (self->thread_state() != _thread_blocked , "invariant") ;
1693 // Possible mark states: neutral, biased, stack-locked, inflated
1695 if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
1697 BiasedLocking::revoke_and_rebias(h_obj, false, self);
1698 assert(!h_obj->mark()->has_bias_pattern(),
1699 "biases should be revoked by now");
1702 assert(self == JavaThread::current(), "Can only be called on current thread");
1704 markOop mark = ReadStableMark (obj) ;
1706 // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
1707 if (mark->has_locker()) {
1708 return self->is_lock_owned((address)mark->locker()) ?
1709 owner_self : owner_other;
1712 // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
1713 // The Object:ObjectMonitor relationship is stable as long as we're
1714 // not at a safepoint.
1715 if (mark->has_monitor()) {
1716 void * owner = mark->monitor()->_owner ;
1717 if (owner == NULL) return owner_none ;
1718 return (owner == self ||
1719 self->is_lock_owned((address)owner)) ? owner_self : owner_other;
1723 assert(mark->is_neutral(), "sanity check");
1724 return owner_none ; // it's unlocked
1727 // FIXME: jvmti should call this
1728 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
1729 if (UseBiasedLocking) {
1730 if (SafepointSynchronize::is_at_safepoint()) {
1731 BiasedLocking::revoke_at_safepoint(h_obj);
1733 BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
1735 assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
1739 address owner = NULL;
1741 markOop mark = ReadStableMark (obj) ;
1743 // Uncontended case, header points to stack
1744 if (mark->has_locker()) {
1745 owner = (address) mark->locker();
1748 // Contended case, header points to ObjectMonitor (tagged pointer)
1749 if (mark->has_monitor()) {
1750 ObjectMonitor* monitor = mark->monitor();
1751 assert(monitor != NULL, "monitor should be non-null");
1752 owner = (address) monitor->owner();
1755 if (owner != NULL) {
1756 return Threads::owning_thread_from_monitor_owner(owner, doLock);
1759 // Unlocked case, header in place
1760 // Cannot have assertion since this object may have been
1761 // locked by another thread when reaching here.
1762 // assert(mark->is_neutral(), "sanity check");
1767 // Iterate through monitor cache and attempt to release thread's monitors
1768 // Gives up on a particular monitor if an exception occurs, but continues
1769 // the overall iteration, swallowing the exception.
1770 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1775 ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1776 void do_monitor(ObjectMonitor* mid) {
1777 if (mid->owner() == THREAD) {
1778 (void)mid->complete_exit(CHECK);
1783 // Release all inflated monitors owned by THREAD. Lightweight monitors are
1784 // ignored. This is meant to be called during JNI thread detach which assumes
1785 // all remaining monitors are heavyweight. All exceptions are swallowed.
1786 // Scanning the extant monitor list can be time consuming.
1787 // A simple optimization is to add a per-thread flag that indicates a thread
1788 // called jni_monitorenter() during its lifetime.
1790 // Instead of No_Savepoint_Verifier it might be cheaper to
1791 // use an idiom of the form:
1792 // auto int tmp = SafepointSynchronize::_safepoint_counter ;
1793 // <code that must not run at safepoint>
1794 // guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1795 // Since the tests are extremely cheap we could leave them enabled
1796 // for normal product builds.
1798 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1799 assert(THREAD == JavaThread::current(), "must be current Java thread");
1800 No_Safepoint_Verifier nsv ;
1801 ReleaseJavaMonitorsClosure rjmc(THREAD);
1802 Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
1803 ObjectSynchronizer::monitors_iterate(&rjmc);
1804 Thread::muxRelease(&ListLock);
1805 THREAD->clear_pending_exception();
1810 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
1811 ObjectMonitor* block = gBlockList;
1814 assert(block->object() == CHAINMARKER, "must be a block header");
1815 for (int i = _BLOCKSIZE - 1; i > 0; i--) {
1817 oop object = (oop) mid->object();
1818 if (object != NULL) {
1819 closure->do_monitor(mid);
1822 block = (ObjectMonitor*) block->FreeNext;
1826 void ObjectSynchronizer::oops_do(OopClosure* f) {
1827 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1828 for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
1829 assert(block->object() == CHAINMARKER, "must be a block header");
1830 for (int i = 1; i < _BLOCKSIZE; i++) {
1831 ObjectMonitor* mid = &block[i];
1832 if (mid->object() != NULL) {
1833 f->do_oop((oop*)mid->object_addr());
1839 // Deflate_idle_monitors() is called at all safepoints, immediately
1840 // after all mutators are stopped, but before any objects have moved.
1841 // It traverses the list of known monitors, deflating where possible.
1842 // The scavenged monitor are returned to the monitor free list.
1844 // Beware that we scavenge at *every* stop-the-world point.
1845 // Having a large number of monitors in-circulation negatively
1846 // impacts the performance of some applications (e.g., PointBase).
1847 // Broadly, we want to minimize the # of monitors in circulation.
1849 // We have added a flag, MonitorInUseLists, which creates a list
1850 // of active monitors for each thread. deflate_idle_monitors()
1851 // only scans the per-thread inuse lists. omAlloc() puts all
1852 // assigned monitors on the per-thread list. deflate_idle_monitors()
1853 // returns the non-busy monitors to the global free list.
1854 // An alternative could have used a single global inuse list. The
1855 // downside would have been the additional cost of acquiring the global list lock
1856 // for every omAlloc().
1858 // Perversely, the heap size -- and thus the STW safepoint rate --
1859 // typically drives the scavenge rate. Large heaps can mean infrequent GC,
1860 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1861 // This is an unfortunate aspect of this design.
1863 // Another refinement would be to refrain from calling deflate_idle_monitors()
1864 // except at stop-the-world points associated with garbage collections.
1866 // An even better solution would be to deflate on-the-fly, aggressively,
1867 // at monitorexit-time as is done in EVM's metalock or Relaxed Locks.
1869 // Deflate a single monitor if not in use
1870 // Return true if deflated, false if in use
1871 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1872 ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
1874 // Normal case ... The monitor is associated with obj.
1875 guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
1876 guarantee (mid == obj->mark()->monitor(), "invariant");
1877 guarantee (mid->header()->is_neutral(), "invariant");
1879 if (mid->is_busy()) {
1880 if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
1883 // Deflate the monitor if it is no longer being used
1884 // It's idle - scavenge and return to the global free list
1885 // plain old deflation ...
1886 TEVENT (deflate_idle_monitors - scavenge1) ;
1887 if (TraceMonitorInflation) {
1888 if (obj->is_instance()) {
1890 tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1891 (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
1895 // Restore the header back to obj
1896 obj->release_set_mark(mid->header());
1899 assert (mid->object() == NULL, "invariant") ;
1901 // Move the object to the working free list defined by FreeHead,FreeTail.
1902 if (*FreeHeadp == NULL) *FreeHeadp = mid;
1903 if (*FreeTailp != NULL) {
1904 ObjectMonitor * prevtail = *FreeTailp;
1905 prevtail->FreeNext = mid;
1913 void ObjectSynchronizer::deflate_idle_monitors() {
1914 assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1915 int nInuse = 0 ; // currently associated with objects
1916 int nInCirculation = 0 ; // extant
1917 int nScavenged = 0 ; // reclaimed
1918 bool deflated = false;
1920 ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors
1921 ObjectMonitor * FreeTail = NULL ;
1923 TEVENT (deflate_idle_monitors) ;
1924 // Prevent omFlush from changing mids in Thread dtor's during deflation
1925 // And in case the vm thread is acquiring a lock during a safepoint
1927 Thread::muxAcquire (&ListLock, "scavenge - return") ;
1929 if (MonitorInUseLists) {
1931 ObjectMonitor* next;
1932 ObjectMonitor* curmidinuse;
1933 for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1935 for (mid = cur->omInUseList; mid != NULL; ) {
1936 oop obj = (oop) mid->object();
1939 deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
1942 // extract from per-thread in-use-list
1943 if (mid == cur->omInUseList) {
1944 cur->omInUseList = mid->FreeNext;
1945 } else if (curmidinuse != NULL) {
1946 curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
1948 next = mid->FreeNext;
1949 mid->FreeNext = NULL; // This mid is current tail in the FreeHead list
1951 cur->omInUseCount--;
1955 mid = mid->FreeNext;
1960 } else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
1961 // Iterate over all extant monitors - Scavenge all idle monitors.
1962 assert(block->object() == CHAINMARKER, "must be a block header");
1963 nInCirculation += _BLOCKSIZE ;
1964 for (int i = 1 ; i < _BLOCKSIZE; i++) {
1965 ObjectMonitor* mid = &block[i];
1966 oop obj = (oop) mid->object();
1969 // The monitor is not associated with an object.
1970 // The monitor should either be a thread-specific private
1971 // free list or the global free list.
1972 // obj == NULL IMPLIES mid->is_busy() == 0
1973 guarantee (!mid->is_busy(), "invariant") ;
1976 deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
1979 mid->FreeNext = NULL ;
1987 MonitorFreeCount += nScavenged;
1989 // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree.
1992 ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",
1993 nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1994 MonitorPopulation, MonitorFreeCount) ;
1998 ForceMonitorScavenge = 0; // Reset
2000 // Move the scavenged monitors back to the global free list.
2001 if (FreeHead != NULL) {
2002 guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
2003 assert (FreeTail->FreeNext == NULL, "invariant") ;
2004 // constant-time list splice - prepend scavenged segment to gFreeList
2005 FreeTail->FreeNext = gFreeList ;
2006 gFreeList = FreeHead ;
2008 Thread::muxRelease (&ListLock) ;
2010 if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ;
2011 if (_sync_MonExtant != NULL) _sync_MonExtant ->set_value(nInCirculation);
2013 // TODO: Add objectMonitor leak detection.
2014 // Audit/inventory the objectMonitors -- make sure they're all accounted for.
2015 GVars.stwRandom = os::random() ;
2019 // A macro is used below because there may already be a pending
2020 // exception which should not abort the execution of the routines
2021 // which use this (which is why we don't put this into check_slow and
2022 // call it with a CHECK argument).
2024 #define CHECK_OWNER() \
2026 if (THREAD != _owner) { \
2027 if (THREAD->is_lock_owned((address) _owner)) { \
2028 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
2030 OwnerIsThread = 1 ; \
2032 TEVENT (Throw IMSX) ; \
2033 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
2038 // TODO-FIXME: eliminate ObjectWaiters. Replace this visitor/enumerator
2039 // interface with a simple FirstWaitingThread(), NextWaitingThread() interface.
2041 ObjectWaiter* ObjectMonitor::first_waiter() {
2045 ObjectWaiter* ObjectMonitor::next_waiter(ObjectWaiter* o) {
2049 Thread* ObjectMonitor::thread_of_waiter(ObjectWaiter* o) {
2053 // initialize the monitor, exception the semaphore, all other fields
2054 // are simple integers or pointers
2055 ObjectMonitor::ObjectMonitor() {
2064 _Responsible = NULL ;
2074 ObjectMonitor::~ObjectMonitor() {
2075 // TODO: Add asserts ...
2076 // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
2077 // _count == 0 _EntryList == NULL etc
2080 intptr_t ObjectMonitor::is_busy() const {
2081 // TODO-FIXME: merge _count and _waiters.
2082 // TODO-FIXME: assert _owner == null implies _recursions = 0
2083 // TODO-FIXME: assert _WaitSet != null implies _count > 0
2084 return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;
2087 void ObjectMonitor::Recycle () {
2088 // TODO: add stronger asserts ...
2089 // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
2090 // _count == 0 EntryList == NULL
2091 // _recursions == 0 _WaitSet == NULL
2092 // TODO: assert (is_busy()|_recursions) == 0
2103 // WaitSet management ...
2105 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2106 assert(node != NULL, "should not dequeue NULL node");
2107 assert(node->_prev == NULL, "node already in list");
2108 assert(node->_next == NULL, "node already in list");
2109 // put node at end of queue (circular doubly linked list)
2110 if (_WaitSet == NULL) {
2115 ObjectWaiter* head = _WaitSet ;
2116 ObjectWaiter* tail = head->_prev;
2117 assert(tail->_next == head, "invariant check");
2125 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2126 // dequeue the very first waiter
2127 ObjectWaiter* waiter = _WaitSet;
2129 DequeueSpecificWaiter(waiter);
2134 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2135 assert(node != NULL, "should not dequeue NULL node");
2136 assert(node->_prev != NULL, "node already removed from list");
2137 assert(node->_next != NULL, "node already removed from list");
2138 // when the waiter has woken up because of interrupt,
2139 // timeout or other spurious wake-up, dequeue the
2140 // waiter from waiting list
2141 ObjectWaiter* next = node->_next;
2143 assert(node->_prev == node, "invariant check");
2146 ObjectWaiter* prev = node->_prev;
2147 assert(prev->_next == node, "invariant check");
2148 assert(next->_prev == node, "invariant check");
2151 if (_WaitSet == node) {
2159 static char * kvGet (char * kvList, const char * Key) {
2160 if (kvList == NULL) return NULL ;
2161 size_t n = strlen (Key) ;
2163 for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
2164 if (strncmp (Search, Key, n) == 0) {
2165 if (Search[n] == '=') return Search + n + 1 ;
2166 if (Search[n] == 0) return (char *) "1" ;
2172 static int kvGetInt (char * kvList, const char * Key, int Default) {
2173 char * v = kvGet (kvList, Key) ;
2174 int rslt = v ? ::strtol (v, NULL, 0) : Default ;
2175 if (Knob_ReportSettings && v != NULL) {
2176 ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
2182 // By convention we unlink a contending thread from EntryList|cxq immediately
2183 // after the thread acquires the lock in ::enter(). Equally, we could defer
2184 // unlinking the thread until ::exit()-time.
2186 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
2188 assert (_owner == Self, "invariant") ;
2189 assert (SelfNode->_thread == Self, "invariant") ;
2191 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
2192 // Normal case: remove Self from the DLL EntryList .
2193 // This is a constant-time operation.
2194 ObjectWaiter * nxt = SelfNode->_next ;
2195 ObjectWaiter * prv = SelfNode->_prev ;
2196 if (nxt != NULL) nxt->_prev = prv ;
2197 if (prv != NULL) prv->_next = nxt ;
2198 if (SelfNode == _EntryList ) _EntryList = nxt ;
2199 assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
2200 assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
2201 TEVENT (Unlink from EntryList) ;
2203 guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
2204 // Inopportune interleaving -- Self is still on the cxq.
2205 // This usually means the enqueue of self raced an exiting thread.
2206 // Normally we'll find Self near the front of the cxq, so
2207 // dequeueing is typically fast. If needbe we can accelerate
2208 // this with some MCS/CHL-like bidirectional list hints and advisory
2209 // back-links so dequeueing from the interior will normally operate
2210 // in constant-time.
2211 // Dequeue Self from either the head (with CAS) or from the interior
2212 // with a linear-time scan and normal non-atomic memory operations.
2213 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
2214 // and then unlink Self from EntryList. We have to drain eventually,
2215 // so it might as well be now.
2217 ObjectWaiter * v = _cxq ;
2218 assert (v != NULL, "invariant") ;
2219 if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
2220 // The CAS above can fail from interference IFF a "RAT" arrived.
2221 // In that case Self must be in the interior and can no longer be
2222 // at the head of cxq.
2223 if (v == SelfNode) {
2224 assert (_cxq != v, "invariant") ;
2225 v = _cxq ; // CAS above failed - start scan at head of list
2228 ObjectWaiter * q = NULL ;
2229 for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
2231 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
2233 assert (v != SelfNode, "invariant") ;
2234 assert (p == SelfNode, "Node not found on cxq") ;
2235 assert (p != _cxq, "invariant") ;
2236 assert (q != NULL, "invariant") ;
2237 assert (q->_next == p, "invariant") ;
2238 q->_next = p->_next ;
2240 TEVENT (Unlink from cxq) ;
2243 // Diagnostic hygiene ...
2244 SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
2245 SelfNode->_next = (ObjectWaiter *) 0xBAD ;
2246 SelfNode->TState = ObjectWaiter::TS_RUN ;
2249 // Caveat: TryLock() is not necessarily serializing if it returns failure.
2250 // Callers must compensate as needed.
2252 int ObjectMonitor::TryLock (Thread * Self) {
2254 void * own = _owner ;
2255 if (own != NULL) return 0 ;
2256 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
2257 // Either guarantee _recursions == 0 or set _recursions = 0.
2258 assert (_recursions == 0, "invariant") ;
2259 assert (_owner == Self, "invariant") ;
2260 // CONSIDER: set or assert that OwnerIsThread == 1
2263 // The lock had been free momentarily, but we lost the race to the lock.
2264 // Interference -- the CAS failed.
2265 // We can either return -1 or retry.
2266 // Retry doesn't make as much sense because the lock was just acquired.
2267 if (true) return -1 ;
2271 // NotRunnable() -- informed spinning
2273 // Don't bother spinning if the owner is not eligible to drop the lock.
2274 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
2275 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2276 // The thread must be runnable in order to drop the lock in timely fashion.
2277 // If the _owner is not runnable then spinning will not likely be
2278 // successful (profitable).
2280 // Beware -- the thread referenced by _owner could have died
2281 // so a simply fetch from _owner->_thread_state might trap.
2282 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2283 // Because of the lifecycle issues the schedctl and _thread_state values
2284 // observed by NotRunnable() might be garbage. NotRunnable must
2285 // tolerate this and consider the observed _thread_state value
2288 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2289 // a thread pointer. We differentiate the two cases with OwnerIsThread.
2290 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2291 // with the LSB of _owner. Another option would be to probablistically probe
2292 // the putative _owner->TypeTag value.
2294 // Checking _thread_state isn't perfect. Even if the thread is
2295 // in_java it might be blocked on a page-fault or have been preempted
2296 // and sitting on a ready/dispatch queue. _thread state in conjunction
2297 // with schedctl.sc_state gives us a good picture of what the
2298 // thread is doing, however.
2300 // TODO: check schedctl.sc_state.
2301 // We'll need to use SafeFetch32() to read from the schedctl block.
2302 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
2304 // The return value from NotRunnable() is *advisory* -- the
2305 // result is based on sampling and is not necessarily coherent.
2306 // The caller must tolerate false-negative and false-positive errors.
2307 // Spinning, in general, is probabilistic anyway.
2310 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
2311 // Check either OwnerIsThread or ox->TypeTag == 2BAD.
2312 if (!OwnerIsThread) return 0 ;
2314 if (ox == NULL) return 0 ;
2316 // Avoid transitive spinning ...
2317 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2318 // Immediately after T1 acquires L it's possible that T2, also
2319 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2320 // This occurs transiently after T1 acquired L but before
2321 // T1 managed to clear T1.Stalled. T2 does not need to abort
2322 // its spin in this circumstance.
2323 intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
2325 if (BlockedOn == 1) return 1 ;
2326 if (BlockedOn != 0) {
2327 return BlockedOn != intptr_t(this) && _owner == ox ;
2330 assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
2331 int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
2332 // consider also: jst != _thread_in_Java -- but that's overspecific.
2333 return jst == _thread_blocked || jst == _thread_in_native ;
2337 // Adaptive spin-then-block - rational spinning
2339 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2340 // algorithm. On high order SMP systems it would be better to start with
2341 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
2342 // a contending thread could enqueue itself on the cxq and then spin locally
2343 // on a thread-specific variable such as its ParkEvent._Event flag.
2344 // That's left as an exercise for the reader. Note that global spinning is
2345 // not problematic on Niagara, as the L2$ serves the interconnect and has both
2346 // low latency and massive bandwidth.
2348 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2349 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2350 // (duration) or we can fix the count at approximately the duration of
2351 // a context switch and vary the frequency. Of course we could also
2352 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2353 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
2355 // This implementation varies the duration "D", where D varies with
2356 // the success rate of recent spin attempts. (D is capped at approximately
2357 // length of a round-trip context switch). The success rate for recent
2358 // spin attempts is a good predictor of the success rate of future spin
2359 // attempts. The mechanism adapts automatically to varying critical
2360 // section length (lock modality), system load and degree of parallelism.
2361 // D is maintained per-monitor in _SpinDuration and is initialized
2362 // optimistically. Spin frequency is fixed at 100%.
2364 // Note that _SpinDuration is volatile, but we update it without locks
2365 // or atomics. The code is designed so that _SpinDuration stays within
2366 // a reasonable range even in the presence of races. The arithmetic
2367 // operations on _SpinDuration are closed over the domain of legal values,
2368 // so at worst a race will install and older but still legal value.
2369 // At the very worst this introduces some apparent non-determinism.
2370 // We might spin when we shouldn't or vice-versa, but since the spin
2371 // count are relatively short, even in the worst case, the effect is harmless.
2373 // Care must be taken that a low "D" value does not become an
2374 // an absorbing state. Transient spinning failures -- when spinning
2375 // is overall profitable -- should not cause the system to converge
2376 // on low "D" values. We want spinning to be stable and predictable
2377 // and fairly responsive to change and at the same time we don't want
2378 // it to oscillate, become metastable, be "too" non-deterministic,
2379 // or converge on or enter undesirable stable absorbing states.
2381 // We implement a feedback-based control system -- using past behavior
2382 // to predict future behavior. We face two issues: (a) if the
2383 // input signal is random then the spin predictor won't provide optimal
2384 // results, and (b) if the signal frequency is too high then the control
2385 // system, which has some natural response lag, will "chase" the signal.
2386 // (b) can arise from multimodal lock hold times. Transient preemption
2387 // can also result in apparent bimodal lock hold times.
2388 // Although sub-optimal, neither condition is particularly harmful, as
2389 // in the worst-case we'll spin when we shouldn't or vice-versa.
2390 // The maximum spin duration is rather short so the failure modes aren't bad.
2391 // To be conservative, I've tuned the gain in system to bias toward
2392 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2393 // "rings" or oscillates between spinning and not spinning. This happens
2394 // when spinning is just on the cusp of profitability, however, so the
2395 // situation is not dire. The state is benign -- there's no need to add
2396 // hysteresis control to damp the transition rate between spinning and
2399 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
2401 // Spin-then-block strategies ...
2403 // Thoughts on ways to improve spinning :
2405 // * Periodically call {psr_}getloadavg() while spinning, and
2406 // permit unbounded spinning if the load average is <
2407 // the number of processors. Beware, however, that getloadavg()
2408 // is exceptionally fast on solaris (about 1/10 the cost of a full
2409 // spin cycle, but quite expensive on linux. Beware also, that
2410 // multiple JVMs could "ring" or oscillate in a feedback loop.
2411 // Sufficient damping would solve that problem.
2413 // * We currently use spin loops with iteration counters to approximate
2414 // spinning for some interval. Given the availability of high-precision
2415 // time sources such as gethrtime(), %TICK, %STICK, RDTSC, etc., we should
2416 // someday reimplement the spin loops to duration-based instead of iteration-based.
2418 // * Don't spin if there are more than N = (CPUs/2) threads
2419 // currently spinning on the monitor (or globally).
2420 // That is, limit the number of concurrent spinners.
2421 // We might also limit the # of spinners in the JVM, globally.
2423 // * If a spinning thread observes _owner change hands it should
2424 // abort the spin (and park immediately) or at least debit
2425 // the spin counter by a large "penalty".
2427 // * Classically, the spin count is either K*(CPUs-1) or is a
2428 // simple constant that approximates the length of a context switch.
2429 // We currently use a value -- computed by a special utility -- that
2430 // approximates round-trip context switch times.
2432 // * Normally schedctl_start()/_stop() is used to advise the kernel
2433 // to avoid preempting threads that are running in short, bounded
2434 // critical sections. We could use the schedctl hooks in an inverted
2435 // sense -- spinners would set the nopreempt flag, but poll the preempt
2436 // pending flag. If a spinner observed a pending preemption it'd immediately
2437 // abort the spin and park. As such, the schedctl service acts as
2438 // a preemption warning mechanism.
2440 // * In lieu of spinning, if the system is running below saturation
2441 // (that is, loadavg() << #cpus), we can instead suppress futile
2442 // wakeup throttling, or even wake more than one successor at exit-time.
2443 // The net effect is largely equivalent to spinning. In both cases,
2444 // contending threads go ONPROC and opportunistically attempt to acquire
2445 // the lock, decreasing lock handover latency at the expense of wasted
2446 // cycles and context switching.
2448 // * We might to spin less after we've parked as the thread will
2449 // have less $ and TLB affinity with the processor.
2450 // Likewise, we might spin less if we come ONPROC on a different
2451 // processor or after a long period (>> rechose_interval).
2453 // * A table-driven state machine similar to Solaris' dispadmin scheduling
2454 // tables might be a better design. Instead of encoding information in
2455 // _SpinDuration, _SpinFreq and _SpinClock we'd just use explicit,
2456 // discrete states. Success or failure during a spin would drive
2457 // state transitions, and each state node would contain a spin count.
2459 // * If the processor is operating in a mode intended to conserve power
2460 // (such as Intel's SpeedStep) or to reduce thermal output (thermal
2461 // step-down mode) then the Java synchronization subsystem should
2464 // * The minimum spin duration should be approximately the worst-case
2465 // store propagation latency on the platform. That is, the time
2466 // it takes a store on CPU A to become visible on CPU B, where A and
2469 // * We might want to factor a thread's priority in the spin policy.
2470 // Threads with a higher priority might spin for slightly longer.
2471 // Similarly, if we use back-off in the TATAS loop, lower priority
2472 // threads might back-off longer. We don't currently use a
2473 // thread's priority when placing it on the entry queue. We may
2474 // want to consider doing so in future releases.
2476 // * We might transiently drop a thread's scheduling priority while it spins.
2477 // SCHED_BATCH on linux and FX scheduling class at priority=0 on Solaris
2478 // would suffice. We could even consider letting the thread spin indefinitely at
2479 // a depressed or "idle" priority. This brings up fairness issues, however --
2480 // in a saturated system a thread would with a reduced priority could languish
2481 // for extended periods on the ready queue.
2483 // * While spinning try to use the otherwise wasted time to help the VM make
2486 // -- YieldTo() the owner, if the owner is OFFPROC but ready
2487 // Done our remaining quantum directly to the ready thread.
2488 // This helps "push" the lock owner through the critical section.
2489 // It also tends to improve affinity/locality as the lock
2490 // "migrates" less frequently between CPUs.
2491 // -- Walk our own stack in anticipation of blocking. Memoize the roots.
2492 // -- Perform strand checking for other thread. Unpark potential strandees.
2493 // -- Help GC: trace or mark -- this would need to be a bounded unit of work.
2494 // Unfortunately this will pollute our $ and TLBs. Recall that we
2495 // spin to avoid context switching -- context switching has an
2496 // immediate cost in latency, a disruptive cost to other strands on a CMT
2497 // processor, and an amortized cost because of the D$ and TLB cache
2498 // reload transient when the thread comes back ONPROC and repopulates
2500 // -- call getloadavg() to see if the system is saturated. It'd probably
2501 // make sense to call getloadavg() half way through the spin.
2502 // If the system isn't at full capacity the we'd simply reset
2503 // the spin counter to and extend the spin attempt.
2504 // -- Doug points out that we should use the same "helping" policy
2505 // in thread.yield().
2507 // * Try MONITOR-MWAIT on systems that support those instructions.
2509 // * The spin statistics that drive spin decisions & frequency are
2510 // maintained in the objectmonitor structure so if we deflate and reinflate
2511 // we lose spin state. In practice this is not usually a concern
2512 // as the default spin state after inflation is aggressive (optimistic)
2513 // and tends toward spinning. So in the worst case for a lock where
2514 // spinning is not profitable we may spin unnecessarily for a brief
2515 // period. But then again, if a lock is contended it'll tend not to deflate
2516 // in the first place.
2519 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
2520 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
2522 // Spinning: Fixed frequency (100%), vary duration
2524 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
2526 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2527 int ctr = Knob_FixedSpin ;
2529 while (--ctr >= 0) {
2530 if (TryLock (Self) > 0) return 1 ;
2536 for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
2537 if (TryLock(Self) > 0) {
2538 // Increase _SpinDuration ...
2539 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2540 // Raising _SpurDuration to the poverty line is key.
2541 int x = _SpinDuration ;
2542 if (x < Knob_SpinLimit) {
2543 if (x < Knob_Poverty) x = Knob_Poverty ;
2544 _SpinDuration = x + Knob_BonusB ;
2551 // Admission control - verify preconditions for spinning
2553 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2554 // becoming an absorbing state. Put another way, we spin briefly to
2555 // sample, just in case the system load, parallelism, contention, or lock
2556 // modality changed.
2558 // Consider the following alternative:
2559 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2560 // spin attempt. "Periodically" might mean after a tally of
2561 // the # of failed spin attempts (or iterations) reaches some threshold.
2562 // This takes us into the realm of 1-out-of-N spinning, where we
2563 // hold the duration constant but vary the frequency.
2565 ctr = _SpinDuration ;
2566 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
2567 if (ctr <= 0) return 0 ;
2569 if (Knob_SuccRestrict && _succ != NULL) return 0 ;
2570 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
2571 TEVENT (Spin abort - notrunnable [TOP]);
2575 int MaxSpin = Knob_MaxSpinners ;
2577 if (_Spinner > MaxSpin) {
2578 TEVENT (Spin abort -- too many spinners) ;
2581 // Slighty racy, but benign ...
2582 Adjust (&_Spinner, 1) ;
2585 // We're good to spin ... spin ingress.
2586 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2587 // when preparing to LD...CAS _owner, etc and the CAS is likely
2591 int caspty = Knob_CASPenalty ;
2592 int oxpty = Knob_OXPenalty ;
2593 int sss = Knob_SpinSetSucc ;
2594 if (sss && _succ == NULL ) _succ = Self ;
2595 Thread * prv = NULL ;
2597 // There are three ways to exit the following loop:
2598 // 1. A successful spin where this thread has acquired the lock.
2599 // 2. Spin failure with prejudice
2600 // 3. Spin failure without prejudice
2602 while (--ctr >= 0) {
2604 // Periodic polling -- Check for pending GC
2605 // Threads may spin while they're unsafe.
2606 // We don't want spinning threads to delay the JVM from reaching
2607 // a stop-the-world safepoint or to steal cycles from GC.
2608 // If we detect a pending safepoint we abort in order that
2609 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2610 // this thread, if safe, doesn't steal cycles from GC.
2611 // This is in keeping with the "no loitering in runtime" rule.
2612 // We periodically check to see if there's a safepoint pending.
2613 if ((ctr & 0xFF) == 0) {
2614 if (SafepointSynchronize::do_call_back()) {
2615 TEVENT (Spin: safepoint) ;
2616 goto Abort ; // abrupt spin egress
2618 if (Knob_UsePause & 1) SpinPause () ;
2620 int (*scb)(intptr_t,int) = SpinCallbackFunction ;
2621 if (hits > 50 && scb != NULL) {
2622 int abend = (*scb)(SpinCallbackArgument, 0) ;
2626 if (Knob_UsePause & 2) SpinPause() ;
2628 // Exponential back-off ... Stay off the bus to reduce coherency traffic.
2629 // This is useful on classic SMP systems, but is of less utility on
2630 // N1-style CMT platforms.
2632 // Trade-off: lock acquisition latency vs coherency bandwidth.
2633 // Lock hold times are typically short. A histogram
2634 // of successful spin attempts shows that we usually acquire
2635 // the lock early in the spin. That suggests we want to
2636 // sample _owner frequently in the early phase of the spin,
2637 // but then back-off and sample less frequently as the spin
2638 // progresses. The back-off makes a good citizen on SMP big
2639 // SMP systems. Oversampling _owner can consume excessive
2640 // coherency bandwidth. Relatedly, if we _oversample _owner we
2641 // can inadvertently interfere with the the ST m->owner=null.
2642 // executed by the lock owner.
2643 if (ctr & msk) continue ;
2645 if ((hits & 0xF) == 0) {
2646 // The 0xF, above, corresponds to the exponent.
2647 // Consider: (msk+1)|msk
2648 msk = ((msk << 2)|3) & BackOffMask ;
2651 // Probe _owner with TATAS
2652 // If this thread observes the monitor transition or flicker
2653 // from locked to unlocked to locked, then the odds that this
2654 // thread will acquire the lock in this spin attempt go down
2655 // considerably. The same argument applies if the CAS fails
2656 // or if we observe _owner change from one non-null value to
2657 // another non-null value. In such cases we might abort
2658 // the spin without prejudice or apply a "penalty" to the
2659 // spin count-down variable "ctr", reducing it by 100, say.
2661 Thread * ox = (Thread *) _owner ;
2663 ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
2665 // The CAS succeeded -- this thread acquired ownership
2666 // Take care of some bookkeeping to exit spin state.
2667 if (sss && _succ == Self) {
2670 if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
2672 // Increase _SpinDuration :
2673 // The spin was successful (profitable) so we tend toward
2674 // longer spin attempts in the future.
2675 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2676 // If we acquired the lock early in the spin cycle it
2677 // makes sense to increase _SpinDuration proportionally.
2678 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2679 int x = _SpinDuration ;
2680 if (x < Knob_SpinLimit) {
2681 if (x < Knob_Poverty) x = Knob_Poverty ;
2682 _SpinDuration = x + Knob_Bonus ;
2687 // The CAS failed ... we can take any of the following actions:
2688 // * penalize: ctr -= Knob_CASPenalty
2689 // * exit spin with prejudice -- goto Abort;
2690 // * exit spin without prejudice.
2691 // * Since CAS is high-latency, retry again immediately.
2693 TEVENT (Spin: cas failed) ;
2694 if (caspty == -2) break ;
2695 if (caspty == -1) goto Abort ;
2700 // Did lock ownership change hands ?
2701 if (ox != prv && prv != NULL ) {
2702 TEVENT (spin: Owner changed)
2703 if (oxpty == -2) break ;
2704 if (oxpty == -1) goto Abort ;
2709 // Abort the spin if the owner is not executing.
2710 // The owner must be executing in order to drop the lock.
2711 // Spinning while the owner is OFFPROC is idiocy.
2712 // Consider: ctr -= RunnablePenalty ;
2713 if (Knob_OState && NotRunnable (Self, ox)) {
2714 TEVENT (Spin abort - notrunnable);
2717 if (sss && _succ == NULL ) _succ = Self ;
2720 // Spin failed with prejudice -- reduce _SpinDuration.
2721 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2722 // AIMD is globally stable.
2723 TEVENT (Spin failure) ;
2725 int x = _SpinDuration ;
2727 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2728 // This is globally sample and tends to damp the response.
2736 if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
2737 if (sss && _succ == Self) {
2739 // Invariant: after setting succ=null a contending thread
2740 // must recheck-retry _owner before parking. This usually happens
2741 // in the normal usage of TrySpin(), but it's safest
2742 // to make TrySpin() as foolproof as possible.
2743 OrderAccess::fence() ;
2744 if (TryLock(Self) > 0) return 1 ;
2749 #define TrySpin TrySpin_VaryDuration
2751 static void DeferredInitialize () {
2752 if (InitDone > 0) return ;
2753 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2754 while (InitDone != 1) ;
2758 // One-shot global initialization ...
2759 // The initialization is idempotent, so we don't need locks.
2760 // In the future consider doing this via os::init_2().
2761 // SyncKnobs consist of <Key>=<Value> pairs in the style
2762 // of environment variables. Start by converting ':' to NUL.
2764 if (SyncKnobs == NULL) SyncKnobs = "" ;
2766 size_t sz = strlen (SyncKnobs) ;
2767 char * knobs = (char *) malloc (sz + 2) ;
2768 if (knobs == NULL) {
2769 vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
2770 guarantee (0, "invariant") ;
2772 strcpy (knobs, SyncKnobs) ;
2774 for (char * p = knobs ; *p ; p++) {
2775 if (*p == ':') *p = 0 ;
2778 #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
2779 SETKNOB(ReportSettings) ;
2781 SETKNOB(FixedSpin) ;
2782 SETKNOB(SpinLimit) ;
2784 SETKNOB(SpinBackOff);
2785 SETKNOB(CASPenalty) ;
2786 SETKNOB(OXPenalty) ;
2788 SETKNOB(SpinSetSucc) ;
2789 SETKNOB(SuccEnabled) ;
2790 SETKNOB(SuccRestrict) ;
2795 SETKNOB(SpinAfterFutile) ;
2797 SETKNOB(SpinEarly) ;
2799 SETKNOB(MaxSpinners) ;
2801 SETKNOB(ExitPolicy) ;
2803 SETKNOB(ResetEvent) ;
2804 SETKNOB(MoveNotifyee) ;
2805 SETKNOB(FastHSSEC) ;
2809 BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
2810 if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
2811 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
2813 Knob_SpinLimit = 0 ;
2816 Knob_FixedSpin = -1 ;
2819 if (Knob_LogSpins == 0) {
2820 ObjectSynchronizer::_sync_FailedSpins = NULL ;
2824 OrderAccess::fence() ;
2828 // Theory of operations -- Monitors lists, thread residency, etc:
2830 // * A thread acquires ownership of a monitor by successfully
2831 // CAS()ing the _owner field from null to non-null.
2833 // * Invariant: A thread appears on at most one monitor list --
2834 // cxq, EntryList or WaitSet -- at any one time.
2836 // * Contending threads "push" themselves onto the cxq with CAS
2837 // and then spin/park.
2839 // * After a contending thread eventually acquires the lock it must
2840 // dequeue itself from either the EntryList or the cxq.
2842 // * The exiting thread identifies and unparks an "heir presumptive"
2843 // tentative successor thread on the EntryList. Critically, the
2844 // exiting thread doesn't unlink the successor thread from the EntryList.
2845 // After having been unparked, the wakee will recontend for ownership of
2846 // the monitor. The successor (wakee) will either acquire the lock or
2849 // Succession is provided for by a policy of competitive handoff.
2850 // The exiting thread does _not_ grant or pass ownership to the
2851 // successor thread. (This is also referred to as "handoff" succession").
2852 // Instead the exiting thread releases ownership and possibly wakes
2853 // a successor, so the successor can (re)compete for ownership of the lock.
2854 // If the EntryList is empty but the cxq is populated the exiting
2855 // thread will drain the cxq into the EntryList. It does so by
2856 // by detaching the cxq (installing null with CAS) and folding
2857 // the threads from the cxq into the EntryList. The EntryList is
2858 // doubly linked, while the cxq is singly linked because of the
2859 // CAS-based "push" used to enqueue recently arrived threads (RATs).
2861 // * Concurrency invariants:
2863 // -- only the monitor owner may access or mutate the EntryList.
2864 // The mutex property of the monitor itself protects the EntryList
2865 // from concurrent interference.
2866 // -- Only the monitor owner may detach the cxq.
2868 // * The monitor entry list operations avoid locks, but strictly speaking
2869 // they're not lock-free. Enter is lock-free, exit is not.
2870 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
2872 // * The cxq can have multiple concurrent "pushers" but only one concurrent
2873 // detaching thread. This mechanism is immune from the ABA corruption.
2874 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
2876 // * Taken together, the cxq and the EntryList constitute or form a
2877 // single logical queue of threads stalled trying to acquire the lock.
2878 // We use two distinct lists to improve the odds of a constant-time
2879 // dequeue operation after acquisition (in the ::enter() epilog) and
2880 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
2881 // A key desideratum is to minimize queue & monitor metadata manipulation
2882 // that occurs while holding the monitor lock -- that is, we want to
2883 // minimize monitor lock holds times. Note that even a small amount of
2884 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
2885 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
2886 // locks and monitor metadata.
2888 // Cxq points to the the set of Recently Arrived Threads attempting entry.
2889 // Because we push threads onto _cxq with CAS, the RATs must take the form of
2890 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
2891 // the unlocking thread notices that EntryList is null but _cxq is != null.
2893 // The EntryList is ordered by the prevailing queue discipline and
2894 // can be organized in any convenient fashion, such as a doubly-linked list or
2895 // a circular doubly-linked list. Critically, we want insert and delete operations
2896 // to operate in constant-time. If we need a priority queue then something akin
2897 // to Solaris' sleepq would work nicely. Viz.,
2898 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
2899 // Queue discipline is enforced at ::exit() time, when the unlocking thread
2900 // drains the cxq into the EntryList, and orders or reorders the threads on the
2901 // EntryList accordingly.
2903 // Barring "lock barging", this mechanism provides fair cyclic ordering,
2904 // somewhat similar to an elevator-scan.
2906 // * The monitor synchronization subsystem avoids the use of native
2907 // synchronization primitives except for the narrow platform-specific
2908 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
2909 // the semantics of park-unpark. Put another way, this monitor implementation
2910 // depends only on atomic operations and park-unpark. The monitor subsystem
2911 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
2912 // underlying OS manages the READY<->RUN transitions.
2914 // * Waiting threads reside on the WaitSet list -- wait() puts
2915 // the caller onto the WaitSet.
2917 // * notify() or notifyAll() simply transfers threads from the WaitSet to
2918 // either the EntryList or cxq. Subsequent exit() operations will
2919 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
2920 // it's likely the notifyee would simply impale itself on the lock held
2923 // * An interesting alternative is to encode cxq as (List,LockByte) where
2924 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
2925 // variable, like _recursions, in the scheme. The threads or Events that form
2926 // the list would have to be aligned in 256-byte addresses. A thread would
2927 // try to acquire the lock or enqueue itself with CAS, but exiting threads
2928 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
2929 // Note that is is *not* word-tearing, but it does presume that full-word
2930 // CAS operations are coherent with intermix with STB operations. That's true
2931 // on most common processors.
2933 // * See also http://blogs.sun.com/dave
2936 void ATTR ObjectMonitor::EnterI (TRAPS) {
2937 Thread * Self = THREAD ;
2938 assert (Self->is_Java_thread(), "invariant") ;
2939 assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
2941 // Try the lock - TATAS
2942 if (TryLock (Self) > 0) {
2943 assert (_succ != Self , "invariant") ;
2944 assert (_owner == Self , "invariant") ;
2945 assert (_Responsible != Self , "invariant") ;
2949 DeferredInitialize () ;
2951 // We try one round of spinning *before* enqueueing Self.
2953 // If the _owner is ready but OFFPROC we could use a YieldTo()
2954 // operation to donate the remainder of this thread's quantum
2955 // to the owner. This has subtle but beneficial affinity
2958 if (TrySpin (Self) > 0) {
2959 assert (_owner == Self , "invariant") ;
2960 assert (_succ != Self , "invariant") ;
2961 assert (_Responsible != Self , "invariant") ;
2965 // The Spin failed -- Enqueue and park the thread ...
2966 assert (_succ != Self , "invariant") ;
2967 assert (_owner != Self , "invariant") ;
2968 assert (_Responsible != Self , "invariant") ;
2970 // Enqueue "Self" on ObjectMonitor's _cxq.
2972 // Node acts as a proxy for Self.
2973 // As an aside, if were to ever rewrite the synchronization code mostly
2974 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
2975 // Java objects. This would avoid awkward lifecycle and liveness issues,
2976 // as well as eliminate a subset of ABA issues.
2977 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
2980 ObjectWaiter node(Self) ;
2981 Self->_ParkEvent->reset() ;
2982 node._prev = (ObjectWaiter *) 0xBAD ;
2983 node.TState = ObjectWaiter::TS_CXQ ;
2985 // Push "Self" onto the front of the _cxq.
2986 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
2987 // Note that spinning tends to reduce the rate at which threads
2988 // enqueue and dequeue on EntryList|cxq.
2989 ObjectWaiter * nxt ;
2991 node._next = nxt = _cxq ;
2992 if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
2994 // Interference - the CAS failed because _cxq changed. Just retry.
2995 // As an optional optimization we retry the lock.
2996 if (TryLock (Self) > 0) {
2997 assert (_succ != Self , "invariant") ;
2998 assert (_owner == Self , "invariant") ;
2999 assert (_Responsible != Self , "invariant") ;
3004 // Check for cxq|EntryList edge transition to non-null. This indicates
3005 // the onset of contention. While contention persists exiting threads
3006 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
3007 // operations revert to the faster 1-0 mode. This enter operation may interleave
3008 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
3009 // arrange for one of the contending thread to use a timed park() operations
3010 // to detect and recover from the race. (Stranding is form of progress failure
3011 // where the monitor is unlocked but all the contending threads remain parked).
3012 // That is, at least one of the contended threads will periodically poll _owner.
3013 // One of the contending threads will become the designated "Responsible" thread.
3014 // The Responsible thread uses a timed park instead of a normal indefinite park
3015 // operation -- it periodically wakes and checks for and recovers from potential
3016 // strandings admitted by 1-0 exit operations. We need at most one Responsible
3017 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
3018 // be responsible for a monitor.
3020 // Currently, one of the contended threads takes on the added role of "Responsible".
3021 // A viable alternative would be to use a dedicated "stranding checker" thread
3022 // that periodically iterated over all the threads (or active monitors) and unparked
3023 // successors where there was risk of stranding. This would help eliminate the
3024 // timer scalability issues we see on some platforms as we'd only have one thread
3025 // -- the checker -- parked on a timer.
3027 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
3028 // Try to assume the role of responsible thread for the monitor.
3029 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
3030 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
3033 // The lock have been released while this thread was occupied queueing
3034 // itself onto _cxq. To close the race and avoid "stranding" and
3035 // progress-liveness failure we must resample-retry _owner before parking.
3036 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
3037 // In this case the ST-MEMBAR is accomplished with CAS().
3039 // TODO: Defer all thread state transitions until park-time.
3040 // Since state transitions are heavy and inefficient we'd like
3041 // to defer the state transitions until absolutely necessary,
3042 // and in doing so avoid some transitions ...
3044 TEVENT (Inflated enter - Contention) ;
3046 int RecheckInterval = 1 ;
3050 if (TryLock (Self) > 0) break ;
3051 assert (_owner != Self, "invariant") ;
3053 if ((SyncFlags & 2) && _Responsible == NULL) {
3054 Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
3058 if (_Responsible == Self || (SyncFlags & 1)) {
3059 TEVENT (Inflated enter - park TIMED) ;
3060 Self->_ParkEvent->park ((jlong) RecheckInterval) ;
3061 // Increase the RecheckInterval, but clamp the value.
3062 RecheckInterval *= 8 ;
3063 if (RecheckInterval > 1000) RecheckInterval = 1000 ;
3065 TEVENT (Inflated enter - park UNTIMED) ;
3066 Self->_ParkEvent->park() ;
3069 if (TryLock(Self) > 0) break ;
3071 // The lock is still contested.
3072 // Keep a tally of the # of futile wakeups.
3073 // Note that the counter is not protected by a lock or updated by atomics.
3074 // That is by design - we trade "lossy" counters which are exposed to
3075 // races during updates for a lower probe effect.
3076 TEVENT (Inflated enter - Futile wakeup) ;
3077 if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
3078 ObjectSynchronizer::_sync_FutileWakeups->inc() ;
3082 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
3083 // We can defer clearing _succ until after the spin completes
3084 // TrySpin() must tolerate being called with _succ == Self.
3085 // Try yet another round of adaptive spinning.
3086 if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
3088 // We can find that we were unpark()ed and redesignated _succ while
3089 // we were spinning. That's harmless. If we iterate and call park(),
3090 // park() will consume the event and return immediately and we'll
3091 // just spin again. This pattern can repeat, leaving _succ to simply
3092 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
3093 // Alternately, we can sample fired() here, and if set, forgo spinning
3094 // in the next iteration.
3096 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
3097 Self->_ParkEvent->reset() ;
3098 OrderAccess::fence() ;
3100 if (_succ == Self) _succ = NULL ;
3102 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
3103 OrderAccess::fence() ;
3107 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
3108 // Normally we'll find Self on the EntryList .
3109 // From the perspective of the lock owner (this thread), the
3110 // EntryList is stable and cxq is prepend-only.
3111 // The head of cxq is volatile but the interior is stable.
3112 // In addition, Self.TState is stable.
3114 assert (_owner == Self , "invariant") ;
3115 assert (object() != NULL , "invariant") ;
3116 // I'd like to write:
3117 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3118 // but as we're at a safepoint that's not safe.
3120 UnlinkAfterAcquire (Self, &node) ;
3121 if (_succ == Self) _succ = NULL ;
3123 assert (_succ != Self, "invariant") ;
3124 if (_Responsible == Self) {
3125 _Responsible = NULL ;
3126 // Dekker pivot-point.
3127 // Consider OrderAccess::storeload() here
3129 // We may leave threads on cxq|EntryList without a designated
3130 // "Responsible" thread. This is benign. When this thread subsequently
3131 // exits the monitor it can "see" such preexisting "old" threads --
3132 // threads that arrived on the cxq|EntryList before the fence, above --
3133 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
3134 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
3135 // non-null and elect a new "Responsible" timer thread.
3137 // This thread executes:
3138 // ST Responsible=null; MEMBAR (in enter epilog - here)
3139 // LD cxq|EntryList (in subsequent exit)
3141 // Entering threads in the slow/contended path execute:
3142 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
3143 // The (ST cxq; MEMBAR) is accomplished with CAS().
3145 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
3146 // exit operation from floating above the ST Responsible=null.
3148 // In *practice* however, EnterI() is always followed by some atomic
3149 // operation such as the decrement of _count in ::enter(). Those atomics
3150 // obviate the need for the explicit MEMBAR, above.
3153 // We've acquired ownership with CAS().
3154 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
3155 // But since the CAS() this thread may have also stored into _succ,
3156 // EntryList, cxq or Responsible. These meta-data updates must be
3157 // visible __before this thread subsequently drops the lock.
3158 // Consider what could occur if we didn't enforce this constraint --
3159 // STs to monitor meta-data and user-data could reorder with (become
3160 // visible after) the ST in exit that drops ownership of the lock.
3161 // Some other thread could then acquire the lock, but observe inconsistent
3162 // or old monitor meta-data and heap data. That violates the JMM.
3163 // To that end, the 1-0 exit() operation must have at least STST|LDST
3164 // "release" barrier semantics. Specifically, there must be at least a
3165 // STST|LDST barrier in exit() before the ST of null into _owner that drops
3166 // the lock. The barrier ensures that changes to monitor meta-data and data
3167 // protected by the lock will be visible before we release the lock, and
3168 // therefore before some other thread (CPU) has a chance to acquire the lock.
3169 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
3171 // Critically, any prior STs to _succ or EntryList must be visible before
3172 // the ST of null into _owner in the *subsequent* (following) corresponding
3173 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
3174 // execute a serializing instruction.
3176 if (SyncFlags & 8) {
3177 OrderAccess::fence() ;
3182 // ExitSuspendEquivalent:
3183 // A faster alternate to handle_special_suspend_equivalent_condition()
3185 // handle_special_suspend_equivalent_condition() unconditionally
3186 // acquires the SR_lock. On some platforms uncontended MutexLocker()
3187 // operations have high latency. Note that in ::enter() we call HSSEC
3188 // while holding the monitor, so we effectively lengthen the critical sections.
3190 // There are a number of possible solutions:
3192 // A. To ameliorate the problem we might also defer state transitions
3193 // to as late as possible -- just prior to parking.
3194 // Given that, we'd call HSSEC after having returned from park(),
3195 // but before attempting to acquire the monitor. This is only a
3196 // partial solution. It avoids calling HSSEC while holding the
3197 // monitor (good), but it still increases successor reacquisition latency --
3198 // the interval between unparking a successor and the time the successor
3199 // resumes and retries the lock. See ReenterI(), which defers state transitions.
3200 // If we use this technique we can also avoid EnterI()-exit() loop
3201 // in ::enter() where we iteratively drop the lock and then attempt
3202 // to reacquire it after suspending.
3204 // B. In the future we might fold all the suspend bits into a
3205 // composite per-thread suspend flag and then update it with CAS().
3206 // Alternately, a Dekker-like mechanism with multiple variables
3208 // ST Self->_suspend_equivalent = false
3210 // LD Self_>_suspend_flags
3214 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
3215 int Mode = Knob_FastHSSEC ;
3216 if (Mode && !jSelf->is_external_suspend()) {
3217 assert (jSelf->is_suspend_equivalent(), "invariant") ;
3218 jSelf->clear_suspend_equivalent() ;
3219 if (2 == Mode) OrderAccess::storeload() ;
3220 if (!jSelf->is_external_suspend()) return false ;
3221 // We raced a suspension -- fall thru into the slow path
3222 TEVENT (ExitSuspendEquivalent - raced) ;
3223 jSelf->set_suspend_equivalent() ;
3225 return jSelf->handle_special_suspend_equivalent_condition() ;
3229 // ReenterI() is a specialized inline form of the latter half of the
3230 // contended slow-path from EnterI(). We use ReenterI() only for
3231 // monitor reentry in wait().
3233 // In the future we should reconcile EnterI() and ReenterI(), adding
3234 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
3235 // loop accordingly.
3237 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
3238 assert (Self != NULL , "invariant") ;
3239 assert (SelfNode != NULL , "invariant") ;
3240 assert (SelfNode->_thread == Self , "invariant") ;
3241 assert (_waiters > 0 , "invariant") ;
3242 assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
3243 assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
3244 JavaThread * jt = (JavaThread *) Self ;
3248 ObjectWaiter::TStates v = SelfNode->TState ;
3249 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
3250 assert (_owner != Self, "invariant") ;
3252 if (TryLock (Self) > 0) break ;
3253 if (TrySpin (Self) > 0) break ;
3255 TEVENT (Wait Reentry - parking) ;
3257 // State transition wrappers around park() ...
3258 // ReenterI() wisely defers state transitions until
3259 // it's clear we must park the thread.
3261 OSThreadContendState osts(Self->osthread());
3262 ThreadBlockInVM tbivm(jt);
3264 // cleared by handle_special_suspend_equivalent_condition()
3265 // or java_suspend_self()
3266 jt->set_suspend_equivalent();
3267 if (SyncFlags & 1) {
3268 Self->_ParkEvent->park ((jlong)1000) ;
3270 Self->_ParkEvent->park () ;
3273 // were we externally suspended while we were waiting?
3275 if (!ExitSuspendEquivalent (jt)) break ;
3276 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
3277 jt->java_suspend_self();
3278 jt->set_suspend_equivalent();
3282 // Try again, but just so we distinguish between futile wakeups and
3283 // successful wakeups. The following test isn't algorithmically
3284 // necessary, but it helps us maintain sensible statistics.
3285 if (TryLock(Self) > 0) break ;
3287 // The lock is still contested.
3288 // Keep a tally of the # of futile wakeups.
3289 // Note that the counter is not protected by a lock or updated by atomics.
3290 // That is by design - we trade "lossy" counters which are exposed to
3291 // races during updates for a lower probe effect.
3292 TEVENT (Wait Reentry - futile wakeup) ;
3295 // Assuming this is not a spurious wakeup we'll normally
3296 // find that _succ == Self.
3297 if (_succ == Self) _succ = NULL ;
3299 // Invariant: after clearing _succ a contending thread
3300 // *must* retry _owner before parking.
3301 OrderAccess::fence() ;
3303 if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
3304 ObjectSynchronizer::_sync_FutileWakeups->inc() ;
3308 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
3309 // Normally we'll find Self on the EntryList.
3310 // Unlinking from the EntryList is constant-time and atomic-free.
3311 // From the perspective of the lock owner (this thread), the
3312 // EntryList is stable and cxq is prepend-only.
3313 // The head of cxq is volatile but the interior is stable.
3314 // In addition, Self.TState is stable.
3316 assert (_owner == Self, "invariant") ;
3317 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3318 UnlinkAfterAcquire (Self, SelfNode) ;
3319 if (_succ == Self) _succ = NULL ;
3320 assert (_succ != Self, "invariant") ;
3321 SelfNode->TState = ObjectWaiter::TS_RUN ;
3322 OrderAccess::fence() ; // see comments at the end of EnterI()
3325 bool ObjectMonitor::try_enter(Thread* THREAD) {
3326 if (THREAD != _owner) {
3327 if (THREAD->is_lock_owned ((address)_owner)) {
3328 assert(_recursions == 0, "internal state error");
3334 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3344 void ATTR ObjectMonitor::enter(TRAPS) {
3345 // The following code is ordered to check the most common cases first
3346 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
3347 Thread * const Self = THREAD ;
3350 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
3352 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
3353 assert (_recursions == 0 , "invariant") ;
3354 assert (_owner == Self, "invariant") ;
3355 // CONSIDER: set or assert OwnerIsThread == 1
3360 // TODO-FIXME: check for integer overflow! BUGID 6557169.
3365 if (Self->is_lock_owned ((address)cur)) {
3366 assert (_recursions == 0, "internal state error");
3368 // Commute owner from a thread-specific on-stack BasicLockObject address to
3369 // a full-fledged "Thread *".
3375 // We've encountered genuine contention.
3376 assert (Self->_Stalled == 0, "invariant") ;
3377 Self->_Stalled = intptr_t(this) ;
3379 // Try one round of spinning *before* enqueueing Self
3380 // and before going through the awkward and expensive state
3381 // transitions. The following spin is strictly optional ...
3382 // Note that if we acquire the monitor from an initial spin
3383 // we forgo posting JVMTI events and firing DTRACE probes.
3384 if (Knob_SpinEarly && TrySpin (Self) > 0) {
3385 assert (_owner == Self , "invariant") ;
3386 assert (_recursions == 0 , "invariant") ;
3387 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3388 Self->_Stalled = 0 ;
3392 assert (_owner != Self , "invariant") ;
3393 assert (_succ != Self , "invariant") ;
3394 assert (Self->is_Java_thread() , "invariant") ;
3395 JavaThread * jt = (JavaThread *) Self ;
3396 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
3397 assert (jt->thread_state() != _thread_blocked , "invariant") ;
3398 assert (this->object() != NULL , "invariant") ;
3399 assert (_count >= 0, "invariant") ;
3401 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
3402 // Ensure the object-monitor relationship remains stable while there's contention.
3403 Atomic::inc_ptr(&_count);
3405 { // Change java thread status to indicate blocked on monitor enter.
3406 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
3408 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
3409 if (JvmtiExport::should_post_monitor_contended_enter()) {
3410 JvmtiExport::post_monitor_contended_enter(jt, this);
3413 OSThreadContendState osts(Self->osthread());
3414 ThreadBlockInVM tbivm(jt);
3416 Self->set_current_pending_monitor(this);
3418 // TODO-FIXME: change the following for(;;) loop to straight-line code.
3420 jt->set_suspend_equivalent();
3421 // cleared by handle_special_suspend_equivalent_condition()
3422 // or java_suspend_self()
3426 if (!ExitSuspendEquivalent(jt)) break ;
3429 // We have acquired the contended monitor, but while we were
3430 // waiting another thread suspended us. We don't want to enter
3431 // the monitor while suspended because that would surprise the
3432 // thread that suspended us.
3438 jt->java_suspend_self();
3440 Self->set_current_pending_monitor(NULL);
3443 Atomic::dec_ptr(&_count);
3444 assert (_count >= 0, "invariant") ;
3445 Self->_Stalled = 0 ;
3447 // Must either set _recursions = 0 or ASSERT _recursions == 0.
3448 assert (_recursions == 0 , "invariant") ;
3449 assert (_owner == Self , "invariant") ;
3450 assert (_succ != Self , "invariant") ;
3451 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
3453 // The thread -- now the owner -- is back in vm mode.
3454 // Report the glorious news via TI,DTrace and jvmstat.
3455 // The probe effect is non-trivial. All the reportage occurs
3456 // while we hold the monitor, increasing the length of the critical
3457 // section. Amdahl's parallel speedup law comes vividly into play.
3459 // Another option might be to aggregate the events (thread local or
3460 // per-monitor aggregation) and defer reporting until a more opportune
3461 // time -- such as next time some thread encounters contention but has
3462 // yet to acquire the lock. While spinning that thread could
3463 // spinning we could increment JVMStat counters, etc.
3465 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
3466 if (JvmtiExport::should_post_monitor_contended_entered()) {
3467 JvmtiExport::post_monitor_contended_entered(jt, this);
3469 if (ObjectSynchronizer::_sync_ContendedLockAttempts != NULL) {
3470 ObjectSynchronizer::_sync_ContendedLockAttempts->inc() ;
3474 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
3475 assert (_owner == Self, "invariant") ;
3478 // 1. ST _succ = wakee
3479 // 2. membar #loadstore|#storestore;
3480 // 2. ST _owner = NULL
3483 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
3484 ParkEvent * Trigger = Wakee->_event ;
3486 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
3487 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
3488 // out-of-scope (non-extant).
3492 OrderAccess::release_store_ptr (&_owner, NULL) ;
3493 OrderAccess::fence() ; // ST _owner vs LD in unpark()
3496 // If there's a safepoint pending the best policy would be to
3497 // get _this thread to a safepoint and only wake the successor
3498 // after the safepoint completed. monitorexit uses a "leaf"
3499 // state transition, however, so this thread can't become
3500 // safe at this point in time. (Its stack isn't walkable).
3501 // The next best thing is to defer waking the successor by
3502 // adding to a list of thread to be unparked after at the
3503 // end of the forthcoming STW).
3504 if (SafepointSynchronize::do_call_back()) {
3505 TEVENT (unpark before SAFEPOINT) ;
3508 // Possible optimizations ...
3510 // * Consider: set Wakee->UnparkTime = timeNow()
3511 // When the thread wakes up it'll compute (timeNow() - Self->UnparkTime()).
3512 // By measuring recent ONPROC latency we can approximate the
3513 // system load. In turn, we can feed that information back
3514 // into the spinning & succession policies.
3515 // (ONPROC latency correlates strongly with load).
3518 // If the wakee is cold then transiently setting it's affinity
3519 // to the current CPU is a good idea.
3520 // See http://j2se.east/~dice/PERSIST/050624-PullAffinity.txt
3521 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
3524 // Maintain stats and report events to JVMTI
3525 if (ObjectSynchronizer::_sync_Parks != NULL) {
3526 ObjectSynchronizer::_sync_Parks->inc() ;
3533 // Note that the collector can't reclaim the objectMonitor or deflate
3534 // the object out from underneath the thread calling ::exit() as the
3535 // thread calling ::exit() never transitions to a stable state.
3536 // This inhibits GC, which in turn inhibits asynchronous (and
3537 // inopportune) reclamation of "this".
3539 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
3540 // There's one exception to the claim above, however. EnterI() can call
3541 // exit() to drop a lock if the acquirer has been externally suspended.
3542 // In that case exit() is called with _thread_state as _thread_blocked,
3543 // but the monitor's _count field is > 0, which inhibits reclamation.
3547 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
3548 // the fast-path operators have been optimized so the common ::exit()
3549 // operation is 1-0. See i486.ad fast_unlock(), for instance.
3550 // The code emitted by fast_unlock() elides the usual MEMBAR. This
3551 // greatly improves latency -- MEMBAR and CAS having considerable local
3552 // latency on modern processors -- but at the cost of "stranding". Absent the
3553 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
3554 // ::enter() path, resulting in the entering thread being stranding
3555 // and a progress-liveness failure. Stranding is extremely rare.
3556 // We use timers (timed park operations) & periodic polling to detect
3557 // and recover from stranding. Potentially stranded threads periodically
3558 // wake up and poll the lock. See the usage of the _Responsible variable.
3560 // The CAS() in enter provides for safety and exclusion, while the CAS or
3561 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
3562 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
3563 // We detect and recover from stranding with timers.
3565 // If a thread transiently strands it'll park until (a) another
3566 // thread acquires the lock and then drops the lock, at which time the
3567 // exiting thread will notice and unpark the stranded thread, or, (b)
3568 // the timer expires. If the lock is high traffic then the stranding latency
3569 // will be low due to (a). If the lock is low traffic then the odds of
3570 // stranding are lower, although the worst-case stranding latency
3571 // is longer. Critically, we don't want to put excessive load in the
3572 // platform's timer subsystem. We want to minimize both the timer injection
3573 // rate (timers created/sec) as well as the number of timers active at
3574 // any one time. (more precisely, we want to minimize timer-seconds, which is
3575 // the integral of the # of active timers at any instant over time).
3576 // Both impinge on OS scalability. Given that, at most one thread parked on
3577 // a monitor will use a timer.
3579 void ATTR ObjectMonitor::exit(TRAPS) {
3580 Thread * Self = THREAD ;
3581 if (THREAD != _owner) {
3582 if (THREAD->is_lock_owned((address) _owner)) {
3583 // Transmute _owner from a BasicLock pointer to a Thread address.
3584 // We don't need to hold _mutex for this transition.
3585 // Non-null to Non-null is safe as long as all readers can
3586 // tolerate either flavor.
3587 assert (_recursions == 0, "invariant") ;
3592 // NOTE: we need to handle unbalanced monitor enter/exit
3593 // in native code by throwing an exception.
3594 // TODO: Throw an IllegalMonitorStateException ?
3595 TEVENT (Exit - Throw IMSX) ;
3596 assert(false, "Non-balanced monitor enter/exit!");
3598 THROW(vmSymbols::java_lang_IllegalMonitorStateException());
3604 if (_recursions != 0) {
3605 _recursions--; // this is simple recursive enter
3606 TEVENT (Inflated exit - recursive) ;
3610 // Invariant: after setting Responsible=null an thread must execute
3611 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
3612 if ((SyncFlags & 4) == 0) {
3613 _Responsible = NULL ;
3617 assert (THREAD == _owner, "invariant") ;
3619 // Fast-path monitor exit:
3621 // Observe the Dekker/Lamport duality:
3622 // A thread in ::exit() executes:
3623 // ST Owner=null; MEMBAR; LD EntryList|cxq.
3624 // A thread in the contended ::enter() path executes the complementary:
3625 // ST EntryList|cxq = nonnull; MEMBAR; LD Owner.
3627 // Note that there's a benign race in the exit path. We can drop the
3628 // lock, another thread can reacquire the lock immediately, and we can
3629 // then wake a thread unnecessarily (yet another flavor of futile wakeup).
3630 // This is benign, and we've structured the code so the windows are short
3631 // and the frequency of such futile wakeups is low.
3633 // We could eliminate the race by encoding both the "LOCKED" state and
3634 // the queue head in a single word. Exit would then use either CAS to
3635 // clear the LOCKED bit/byte. This precludes the desirable 1-0 optimization,
3638 // Possible fast-path ::exit() optimization:
3639 // The current fast-path exit implementation fetches both cxq and EntryList.
3640 // See also i486.ad fast_unlock(). Testing has shown that two LDs
3641 // isn't measurably slower than a single LD on any platforms.
3642 // Still, we could reduce the 2 LDs to one or zero by one of the following:
3644 // - Use _count instead of cxq|EntryList
3645 // We intend to eliminate _count, however, when we switch
3646 // to on-the-fly deflation in ::exit() as is used in
3647 // Metalocks and RelaxedLocks.
3649 // - Establish the invariant that cxq == null implies EntryList == null.
3650 // set cxq == EMPTY (1) to encode the state where cxq is empty
3651 // by EntryList != null. EMPTY is a distinguished value.
3652 // The fast-path exit() would fetch cxq but not EntryList.
3654 // - Encode succ as follows:
3655 // succ = t : Thread t is the successor -- t is ready or is spinning.
3656 // Exiting thread does not need to wake a successor.
3657 // succ = 0 : No successor required -> (EntryList|cxq) == null
3658 // Exiting thread does not need to wake a successor
3659 // succ = 1 : Successor required -> (EntryList|cxq) != null and
3660 // logically succ == null.
3661 // Exiting thread must wake a successor.
3663 // The 1-1 fast-exit path would appear as :
3664 // _owner = null ; membar ;
3665 // if (_succ == 1 && CAS (&_owner, null, Self) == null) goto SlowPath
3666 // goto FastPathDone ;
3668 // and the 1-0 fast-exit path would appear as:
3669 // if (_succ == 1) goto SlowPath
3671 // goto FastPathDone
3673 // - Encode the LSB of _owner as 1 to indicate that exit()
3674 // must use the slow-path and make a successor ready.
3675 // (_owner & 1) == 0 IFF succ != null || (EntryList|cxq) == null
3676 // (_owner & 1) == 0 IFF succ == null && (EntryList|cxq) != null (obviously)
3677 // The 1-0 fast exit path would read:
3678 // if (_owner != Self) goto SlowPath
3680 // goto FastPathDone
3682 if (Knob_ExitPolicy == 0) {
3683 // release semantics: prior loads and stores from within the critical section
3684 // must not float (reorder) past the following store that drops the lock.
3685 // On SPARC that requires MEMBAR #loadstore|#storestore.
3686 // But of course in TSO #loadstore|#storestore is not required.
3687 // I'd like to write one of the following:
3688 // A. OrderAccess::release() ; _owner = NULL
3689 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
3690 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
3691 // store into a _dummy variable. That store is not needed, but can result
3692 // in massive wasteful coherency traffic on classic SMP systems.
3693 // Instead, I use release_store(), which is implemented as just a simple
3694 // ST on x64, x86 and SPARC.
3695 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
3696 OrderAccess::storeload() ; // See if we need to wake a successor
3697 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
3698 TEVENT (Inflated exit - simple egress) ;
3701 TEVENT (Inflated exit - complex egress) ;
3703 // Normally the exiting thread is responsible for ensuring succession,
3704 // but if other successors are ready or other entering threads are spinning
3705 // then this thread can simply store NULL into _owner and exit without
3706 // waking a successor. The existence of spinners or ready successors
3707 // guarantees proper succession (liveness). Responsibility passes to the
3708 // ready or running successors. The exiting thread delegates the duty.
3709 // More precisely, if a successor already exists this thread is absolved
3710 // of the responsibility of waking (unparking) one.
3712 // The _succ variable is critical to reducing futile wakeup frequency.
3713 // _succ identifies the "heir presumptive" thread that has been made
3714 // ready (unparked) but that has not yet run. We need only one such
3715 // successor thread to guarantee progress.
3716 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
3717 // section 3.3 "Futile Wakeup Throttling" for details.
3719 // Note that spinners in Enter() also set _succ non-null.
3720 // In the current implementation spinners opportunistically set
3721 // _succ so that exiting threads might avoid waking a successor.
3722 // Another less appealing alternative would be for the exiting thread
3723 // to drop the lock and then spin briefly to see if a spinner managed
3724 // to acquire the lock. If so, the exiting thread could exit
3725 // immediately without waking a successor, otherwise the exiting
3726 // thread would need to dequeue and wake a successor.
3727 // (Note that we'd need to make the post-drop spin short, but no
3728 // shorter than the worst-case round-trip cache-line migration time.
3729 // The dropped lock needs to become visible to the spinner, and then
3730 // the acquisition of the lock by the spinner must become visible to
3731 // the exiting thread).
3734 // It appears that an heir-presumptive (successor) must be made ready.
3735 // Only the current lock owner can manipulate the EntryList or
3736 // drain _cxq, so we need to reacquire the lock. If we fail
3737 // to reacquire the lock the responsibility for ensuring succession
3738 // falls to the new owner.
3740 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3743 TEVENT (Exit - Reacquired) ;
3745 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
3746 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
3747 OrderAccess::storeload() ;
3748 // Ratify the previously observed values.
3749 if (_cxq == NULL || _succ != NULL) {
3750 TEVENT (Inflated exit - simple egress) ;
3754 // inopportune interleaving -- the exiting thread (this thread)
3755 // in the fast-exit path raced an entering thread in the slow-enter
3757 // We have two choices:
3758 // A. Try to reacquire the lock.
3759 // If the CAS() fails return immediately, otherwise
3760 // we either restart/rerun the exit operation, or simply
3761 // fall-through into the code below which wakes a successor.
3762 // B. If the elements forming the EntryList|cxq are TSM
3763 // we could simply unpark() the lead thread and return
3764 // without having set _succ.
3765 if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
3766 TEVENT (Inflated exit - reacquired succeeded) ;
3769 TEVENT (Inflated exit - reacquired failed) ;
3771 TEVENT (Inflated exit - complex egress) ;
3775 guarantee (_owner == THREAD, "invariant") ;
3777 // Select an appropriate successor ("heir presumptive") from the EntryList
3778 // and make it ready. Generally we just wake the head of EntryList .
3779 // There's no algorithmic constraint that we use the head - it's just
3780 // a policy decision. Note that the thread at head of the EntryList
3781 // remains at the head until it acquires the lock. This means we'll
3782 // repeatedly wake the same thread until it manages to grab the lock.
3783 // This is generally a good policy - if we're seeing lots of futile wakeups
3784 // at least we're waking/rewaking a thread that's like to be hot or warm
3785 // (have residual D$ and TLB affinity).
3787 // "Wakeup locality" optimization:
3788 // http://j2se.east/~dice/PERSIST/040825-WakeLocality.txt
3789 // In the future we'll try to bias the selection mechanism
3790 // to preferentially pick a thread that recently ran on
3791 // a processor element that shares cache with the CPU on which
3792 // the exiting thread is running. We need access to Solaris'
3793 // schedctl.sc_cpu to make that work.
3795 ObjectWaiter * w = NULL ;
3796 int QMode = Knob_QMode ;
3798 if (QMode == 2 && _cxq != NULL) {
3799 // QMode == 2 : cxq has precedence over EntryList.
3800 // Try to directly wake a successor from the cxq.
3801 // If successful, the successor will need to unlink itself from cxq.
3803 assert (w != NULL, "invariant") ;
3804 assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3805 ExitEpilog (Self, w) ;
3809 if (QMode == 3 && _cxq != NULL) {
3810 // Aggressively drain cxq into EntryList at the first opportunity.
3811 // This policy ensure that recently-run threads live at the head of EntryList.
3812 // Drain _cxq into EntryList - bulk transfer.
3813 // First, detach _cxq.
3814 // The following loop is tantamount to: w = swap (&cxq, NULL)
3817 assert (w != NULL, "Invariant") ;
3818 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3822 assert (w != NULL , "invariant") ;
3824 ObjectWaiter * q = NULL ;
3826 for (p = w ; p != NULL ; p = p->_next) {
3827 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3828 p->TState = ObjectWaiter::TS_ENTER ;
3833 // Append the RATs to the EntryList
3834 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
3835 ObjectWaiter * Tail ;
3836 for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
3844 // Fall thru into code that tries to wake a successor from EntryList
3847 if (QMode == 4 && _cxq != NULL) {
3848 // Aggressively drain cxq into EntryList at the first opportunity.
3849 // This policy ensure that recently-run threads live at the head of EntryList.
3851 // Drain _cxq into EntryList - bulk transfer.
3852 // First, detach _cxq.
3853 // The following loop is tantamount to: w = swap (&cxq, NULL)
3856 assert (w != NULL, "Invariant") ;
3857 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3861 assert (w != NULL , "invariant") ;
3863 ObjectWaiter * q = NULL ;
3865 for (p = w ; p != NULL ; p = p->_next) {
3866 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3867 p->TState = ObjectWaiter::TS_ENTER ;
3872 // Prepend the RATs to the EntryList
3873 if (_EntryList != NULL) {
3874 q->_next = _EntryList ;
3875 _EntryList->_prev = q ;
3879 // Fall thru into code that tries to wake a successor from EntryList
3884 // I'd like to write: guarantee (w->_thread != Self).
3885 // But in practice an exiting thread may find itself on the EntryList.
3886 // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
3887 // then calls exit(). Exit release the lock by setting O._owner to NULL.
3888 // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
3889 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
3890 // release the lock "O". T2 resumes immediately after the ST of null into
3891 // _owner, above. T2 notices that the EntryList is populated, so it
3892 // reacquires the lock and then finds itself on the EntryList.
3893 // Given all that, we have to tolerate the circumstance where "w" is
3894 // associated with Self.
3895 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
3896 ExitEpilog (Self, w) ;
3900 // If we find that both _cxq and EntryList are null then just
3901 // re-run the exit protocol from the top.
3903 if (w == NULL) continue ;
3905 // Drain _cxq into EntryList - bulk transfer.
3906 // First, detach _cxq.
3907 // The following loop is tantamount to: w = swap (&cxq, NULL)
3909 assert (w != NULL, "Invariant") ;
3910 ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
3914 TEVENT (Inflated exit - drain cxq into EntryList) ;
3916 assert (w != NULL , "invariant") ;
3917 assert (_EntryList == NULL , "invariant") ;
3919 // Convert the LIFO SLL anchored by _cxq into a DLL.
3920 // The list reorganization step operates in O(LENGTH(w)) time.
3921 // It's critical that this step operate quickly as
3922 // "Self" still holds the outer-lock, restricting parallelism
3923 // and effectively lengthening the critical section.
3924 // Invariant: s chases t chases u.
3925 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
3926 // we have faster access to the tail.
3929 // QMode == 1 : drain cxq to EntryList, reversing order
3930 // We also reverse the order of the list.
3931 ObjectWaiter * s = NULL ;
3932 ObjectWaiter * t = w ;
3933 ObjectWaiter * u = NULL ;
3935 guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
3936 t->TState = ObjectWaiter::TS_ENTER ;
3944 assert (s != NULL, "invariant") ;
3946 // QMode == 0 or QMode == 2
3948 ObjectWaiter * q = NULL ;
3950 for (p = w ; p != NULL ; p = p->_next) {
3951 guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
3952 p->TState = ObjectWaiter::TS_ENTER ;
3958 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
3959 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
3961 // See if we can abdicate to a spinner instead of waking a thread.
3962 // A primary goal of the implementation is to reduce the
3963 // context-switch rate.
3964 if (_succ != NULL) continue;
3968 guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
3969 ExitEpilog (Self, w) ;
3974 // complete_exit exits a lock returning recursion count
3975 // complete_exit/reenter operate as a wait without waiting
3976 // complete_exit requires an inflated monitor
3977 // The _owner field is not always the Thread addr even with an
3978 // inflated monitor, e.g. the monitor can be inflated by a non-owning
3979 // thread due to contention.
3980 intptr_t ObjectMonitor::complete_exit(TRAPS) {
3981 Thread * const Self = THREAD;
3982 assert(Self->is_Java_thread(), "Must be Java thread!");
3983 JavaThread *jt = (JavaThread *)THREAD;
3985 DeferredInitialize();
3987 if (THREAD != _owner) {
3988 if (THREAD->is_lock_owned ((address)_owner)) {
3989 assert(_recursions == 0, "internal state error");
3990 _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
3996 guarantee(Self == _owner, "complete_exit not owner");
3997 intptr_t save = _recursions; // record the old recursion count
3998 _recursions = 0; // set the recursion level to be 0
3999 exit (Self) ; // exit the monitor
4000 guarantee (_owner != Self, "invariant");
4004 // reenter() enters a lock and sets recursion count
4005 // complete_exit/reenter operate as a wait without waiting
4006 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
4007 Thread * const Self = THREAD;
4008 assert(Self->is_Java_thread(), "Must be Java thread!");
4009 JavaThread *jt = (JavaThread *)THREAD;
4011 guarantee(_owner != Self, "reenter already owner");
4012 enter (THREAD); // enter the monitor
4013 guarantee (_recursions == 0, "reenter recursion");
4014 _recursions = recursions;
4018 // Note: a subset of changes to ObjectMonitor::wait()
4019 // will need to be replicated in complete_exit above
4020 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
4021 Thread * const Self = THREAD ;
4022 assert(Self->is_Java_thread(), "Must be Java thread!");
4023 JavaThread *jt = (JavaThread *)THREAD;
4025 DeferredInitialize () ;
4027 // Throw IMSX or IEX.
4030 // check for a pending interrupt
4031 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
4032 // post monitor waited event. Note that this is past-tense, we are done waiting.
4033 if (JvmtiExport::should_post_monitor_waited()) {
4034 // Note: 'false' parameter is passed here because the
4035 // wait was not timed out due to thread interrupt.
4036 JvmtiExport::post_monitor_waited(jt, this, false);
4038 TEVENT (Wait - Throw IEX) ;
4039 THROW(vmSymbols::java_lang_InterruptedException());
4044 assert (Self->_Stalled == 0, "invariant") ;
4045 Self->_Stalled = intptr_t(this) ;
4046 jt->set_current_waiting_monitor(this);
4048 // create a node to be put into the queue
4049 // Critically, after we reset() the event but prior to park(), we must check
4050 // for a pending interrupt.
4051 ObjectWaiter node(Self);
4052 node.TState = ObjectWaiter::TS_WAIT ;
4053 Self->_ParkEvent->reset() ;
4054 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
4056 // Enter the waiting queue, which is a circular doubly linked list in this case
4057 // but it could be a priority queue or any data structure.
4058 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
4059 // by the the owner of the monitor *except* in the case where park()
4060 // returns because of a timeout of interrupt. Contention is exceptionally rare
4061 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
4063 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
4065 Thread::SpinRelease (&_WaitSetLock) ;
4067 if ((SyncFlags & 4) == 0) {
4068 _Responsible = NULL ;
4070 intptr_t save = _recursions; // record the old recursion count
4071 _waiters++; // increment the number of waiters
4072 _recursions = 0; // set the recursion level to be 1
4073 exit (Self) ; // exit the monitor
4074 guarantee (_owner != Self, "invariant") ;
4076 // As soon as the ObjectMonitor's ownership is dropped in the exit()
4077 // call above, another thread can enter() the ObjectMonitor, do the
4078 // notify(), and exit() the ObjectMonitor. If the other thread's
4079 // exit() call chooses this thread as the successor and the unpark()
4080 // call happens to occur while this thread is posting a
4081 // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
4082 // handler using RawMonitors and consuming the unpark().
4084 // To avoid the problem, we re-post the event. This does no harm
4085 // even if the original unpark() was not consumed because we are the
4086 // chosen successor for this monitor.
4087 if (node._notified != 0 && _succ == Self) {
4088 node._event->unpark();
4091 // The thread is on the WaitSet list - now park() it.
4092 // On MP systems it's conceivable that a brief spin before we park
4093 // could be profitable.
4095 // TODO-FIXME: change the following logic to a loop of the form
4096 // while (!timeout && !interrupted && _notified == 0) park()
4099 int WasNotified = 0 ;
4100 { // State transition wrappers
4101 OSThread* osthread = Self->osthread();
4102 OSThreadWaitState osts(osthread, true);
4104 ThreadBlockInVM tbivm(jt);
4105 // Thread is in thread_blocked state and oop access is unsafe.
4106 jt->set_suspend_equivalent();
4108 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
4109 // Intentionally empty
4111 if (node._notified == 0) {
4113 Self->_ParkEvent->park () ;
4115 ret = Self->_ParkEvent->park (millis) ;
4119 // were we externally suspended while we were waiting?
4120 if (ExitSuspendEquivalent (jt)) {
4121 // TODO-FIXME: add -- if succ == Self then succ = null.
4122 jt->java_suspend_self();
4125 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
4128 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
4129 // from the WaitSet to the EntryList.
4130 // See if we need to remove Node from the WaitSet.
4131 // We use double-checked locking to avoid grabbing _WaitSetLock
4132 // if the thread is not on the wait queue.
4134 // Note that we don't need a fence before the fetch of TState.
4135 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
4136 // written by the is thread. (perhaps the fetch might even be satisfied
4137 // by a look-aside into the processor's own store buffer, although given
4138 // the length of the code path between the prior ST and this load that's
4139 // highly unlikely). If the following LD fetches a stale TS_WAIT value
4140 // then we'll acquire the lock and then re-fetch a fresh TState value.
4141 // That is, we fail toward safety.
4143 if (node.TState == ObjectWaiter::TS_WAIT) {
4144 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
4145 if (node.TState == ObjectWaiter::TS_WAIT) {
4146 DequeueSpecificWaiter (&node) ; // unlink from WaitSet
4147 assert(node._notified == 0, "invariant");
4148 node.TState = ObjectWaiter::TS_RUN ;
4150 Thread::SpinRelease (&_WaitSetLock) ;
4153 // The thread is now either on off-list (TS_RUN),
4154 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
4155 // The Node's TState variable is stable from the perspective of this thread.
4156 // No other threads will asynchronously modify TState.
4157 guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
4158 OrderAccess::loadload() ;
4159 if (_succ == Self) _succ = NULL ;
4160 WasNotified = node._notified ;
4162 // Reentry phase -- reacquire the monitor.
4163 // re-enter contended monitor after object.wait().
4164 // retain OBJECT_WAIT state until re-enter successfully completes
4165 // Thread state is thread_in_vm and oop access is again safe,
4166 // although the raw address of the object may have changed.
4167 // (Don't cache naked oops over safepoints, of course).
4169 // post monitor waited event. Note that this is past-tense, we are done waiting.
4170 if (JvmtiExport::should_post_monitor_waited()) {
4171 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
4173 OrderAccess::fence() ;
4175 assert (Self->_Stalled != 0, "invariant") ;
4176 Self->_Stalled = 0 ;
4178 assert (_owner != Self, "invariant") ;
4179 ObjectWaiter::TStates v = node.TState ;
4180 if (v == ObjectWaiter::TS_RUN) {
4183 guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
4184 ReenterI (Self, &node) ;
4185 node.wait_reenter_end(this);
4188 // Self has reacquired the lock.
4189 // Lifecycle - the node representing Self must not appear on any queues.
4190 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
4191 // want residual elements associated with this thread left on any lists.
4192 guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
4193 assert (_owner == Self, "invariant") ;
4194 assert (_succ != Self , "invariant") ;
4195 } // OSThreadWaitState()
4197 jt->set_current_waiting_monitor(NULL);
4199 guarantee (_recursions == 0, "invariant") ;
4200 _recursions = save; // restore the old recursion count
4201 _waiters--; // decrement the number of waiters
4203 // Verify a few postconditions
4204 assert (_owner == Self , "invariant") ;
4205 assert (_succ != Self , "invariant") ;
4206 assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
4208 if (SyncFlags & 32) {
4209 OrderAccess::fence() ;
4212 // check if the notification happened
4214 // no, it could be timeout or Thread.interrupt() or both
4215 // check for interrupt event, otherwise it is timeout
4216 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
4217 TEVENT (Wait - throw IEX from epilog) ;
4218 THROW(vmSymbols::java_lang_InterruptedException());
4222 // NOTE: Spurious wake up will be consider as timeout.
4223 // Monitor notify has precedence over thread interrupt.
4228 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
4229 // then instead of transferring a thread from the WaitSet to the EntryList
4230 // we might just dequeue a thread from the WaitSet and directly unpark() it.
4232 void ObjectMonitor::notify(TRAPS) {
4234 if (_WaitSet == NULL) {
4235 TEVENT (Empty-Notify) ;
4238 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
4240 int Policy = Knob_MoveNotifyee ;
4242 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
4243 ObjectWaiter * iterator = DequeueWaiter() ;
4244 if (iterator != NULL) {
4245 TEVENT (Notify1 - Transfer) ;
4246 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
4247 guarantee (iterator->_notified == 0, "invariant") ;
4248 // Disposition - what might we do with iterator ?
4249 // a. add it directly to the EntryList - either tail or head.
4250 // b. push it onto the front of the _cxq.
4251 // For now we use (a).
4253 iterator->TState = ObjectWaiter::TS_ENTER ;
4255 iterator->_notified = 1 ;
4257 ObjectWaiter * List = _EntryList ;
4259 assert (List->_prev == NULL, "invariant") ;
4260 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4261 assert (List != iterator, "invariant") ;
4264 if (Policy == 0) { // prepend to EntryList
4266 iterator->_next = iterator->_prev = NULL ;
4267 _EntryList = iterator ;
4269 List->_prev = iterator ;
4270 iterator->_next = List ;
4271 iterator->_prev = NULL ;
4272 _EntryList = iterator ;
4275 if (Policy == 1) { // append to EntryList
4277 iterator->_next = iterator->_prev = NULL ;
4278 _EntryList = iterator ;
4280 // CONSIDER: finding the tail currently requires a linear-time walk of
4281 // the EntryList. We can make tail access constant-time by converting to
4282 // a CDLL instead of using our current DLL.
4283 ObjectWaiter * Tail ;
4284 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
4285 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
4286 Tail->_next = iterator ;
4287 iterator->_prev = Tail ;
4288 iterator->_next = NULL ;
4291 if (Policy == 2) { // prepend to cxq
4294 iterator->_next = iterator->_prev = NULL ;
4295 _EntryList = iterator ;
4297 iterator->TState = ObjectWaiter::TS_CXQ ;
4299 ObjectWaiter * Front = _cxq ;
4300 iterator->_next = Front ;
4301 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
4307 if (Policy == 3) { // append to cxq
4308 iterator->TState = ObjectWaiter::TS_CXQ ;
4310 ObjectWaiter * Tail ;
4313 iterator->_next = NULL ;
4314 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
4318 while (Tail->_next != NULL) Tail = Tail->_next ;
4319 Tail->_next = iterator ;
4320 iterator->_prev = Tail ;
4321 iterator->_next = NULL ;
4326 ParkEvent * ev = iterator->_event ;
4327 iterator->TState = ObjectWaiter::TS_RUN ;
4328 OrderAccess::fence() ;
4333 iterator->wait_reenter_begin(this);
4336 // _WaitSetLock protects the wait queue, not the EntryList. We could
4337 // move the add-to-EntryList operation, above, outside the critical section
4338 // protected by _WaitSetLock. In practice that's not useful. With the
4339 // exception of wait() timeouts and interrupts the monitor owner
4340 // is the only thread that grabs _WaitSetLock. There's almost no contention
4341 // on _WaitSetLock so it's not profitable to reduce the length of the
4342 // critical section.
4345 Thread::SpinRelease (&_WaitSetLock) ;
4347 if (iterator != NULL && ObjectSynchronizer::_sync_Notifications != NULL) {
4348 ObjectSynchronizer::_sync_Notifications->inc() ;
4353 void ObjectMonitor::notifyAll(TRAPS) {
4355 ObjectWaiter* iterator;
4356 if (_WaitSet == NULL) {
4357 TEVENT (Empty-NotifyAll) ;
4360 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
4362 int Policy = Knob_MoveNotifyee ;
4364 Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
4367 iterator = DequeueWaiter () ;
4368 if (iterator == NULL) break ;
4369 TEVENT (NotifyAll - Transfer1) ;
4372 // Disposition - what might we do with iterator ?
4373 // a. add it directly to the EntryList - either tail or head.
4374 // b. push it onto the front of the _cxq.
4375 // For now we use (a).
4377 // TODO-FIXME: currently notifyAll() transfers the waiters one-at-a-time from the waitset
4378 // to the EntryList. This could be done more efficiently with a single bulk transfer,
4379 // but in practice it's not time-critical. Beware too, that in prepend-mode we invert the
4380 // order of the waiters. Lets say that the waitset is "ABCD" and the EntryList is "XYZ".
4381 // After a notifyAll() in prepend mode the waitset will be empty and the EntryList will
4384 guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
4385 guarantee (iterator->_notified == 0, "invariant") ;
4386 iterator->_notified = 1 ;
4388 iterator->TState = ObjectWaiter::TS_ENTER ;
4391 ObjectWaiter * List = _EntryList ;
4393 assert (List->_prev == NULL, "invariant") ;
4394 assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4395 assert (List != iterator, "invariant") ;
4398 if (Policy == 0) { // prepend to EntryList
4400 iterator->_next = iterator->_prev = NULL ;
4401 _EntryList = iterator ;
4403 List->_prev = iterator ;
4404 iterator->_next = List ;
4405 iterator->_prev = NULL ;
4406 _EntryList = iterator ;
4409 if (Policy == 1) { // append to EntryList
4411 iterator->_next = iterator->_prev = NULL ;
4412 _EntryList = iterator ;
4414 // CONSIDER: finding the tail currently requires a linear-time walk of
4415 // the EntryList. We can make tail access constant-time by converting to
4416 // a CDLL instead of using our current DLL.
4417 ObjectWaiter * Tail ;
4418 for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
4419 assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
4420 Tail->_next = iterator ;
4421 iterator->_prev = Tail ;
4422 iterator->_next = NULL ;
4425 if (Policy == 2) { // prepend to cxq
4427 iterator->TState = ObjectWaiter::TS_CXQ ;
4429 ObjectWaiter * Front = _cxq ;
4430 iterator->_next = Front ;
4431 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
4436 if (Policy == 3) { // append to cxq
4437 iterator->TState = ObjectWaiter::TS_CXQ ;
4439 ObjectWaiter * Tail ;
4442 iterator->_next = NULL ;
4443 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
4447 while (Tail->_next != NULL) Tail = Tail->_next ;
4448 Tail->_next = iterator ;
4449 iterator->_prev = Tail ;
4450 iterator->_next = NULL ;
4455 ParkEvent * ev = iterator->_event ;
4456 iterator->TState = ObjectWaiter::TS_RUN ;
4457 OrderAccess::fence() ;
4462 iterator->wait_reenter_begin(this);
4465 // _WaitSetLock protects the wait queue, not the EntryList. We could
4466 // move the add-to-EntryList operation, above, outside the critical section
4467 // protected by _WaitSetLock. In practice that's not useful. With the
4468 // exception of wait() timeouts and interrupts the monitor owner
4469 // is the only thread that grabs _WaitSetLock. There's almost no contention
4470 // on _WaitSetLock so it's not profitable to reduce the length of the
4471 // critical section.
4474 Thread::SpinRelease (&_WaitSetLock) ;
4476 if (Tally != 0 && ObjectSynchronizer::_sync_Notifications != NULL) {
4477 ObjectSynchronizer::_sync_Notifications->inc(Tally) ;
4481 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
4482 // TODO-FIXME: remove check_slow() -- it's likely dead.
4484 void ObjectMonitor::check_slow(TRAPS) {
4485 TEVENT (check_slow - throw IMSX) ;
4486 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
4487 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
4491 // -------------------------------------------------------------------------
4492 // The raw monitor subsystem is entirely distinct from normal
4493 // java-synchronization or jni-synchronization. raw monitors are not
4494 // associated with objects. They can be implemented in any manner
4495 // that makes sense. The original implementors decided to piggy-back
4496 // the raw-monitor implementation on the existing Java objectMonitor mechanism.
4497 // This flaw needs to fixed. We should reimplement raw monitors as sui-generis.
4498 // Specifically, we should not implement raw monitors via java monitors.
4499 // Time permitting, we should disentangle and deconvolve the two implementations
4500 // and move the resulting raw monitor implementation over to the JVMTI directories.
4501 // Ideally, the raw monitor implementation would be built on top of
4502 // park-unpark and nothing else.
4504 // raw monitors are used mainly by JVMTI
4505 // The raw monitor implementation borrows the ObjectMonitor structure,
4506 // but the operators are degenerate and extremely simple.
4508 // Mixed use of a single objectMonitor instance -- as both a raw monitor
4509 // and a normal java monitor -- is not permissible.
4511 // Note that we use the single RawMonitor_lock to protect queue operations for
4512 // _all_ raw monitors. This is a scalability impediment, but since raw monitor usage
4513 // is deprecated and rare, this is not of concern. The RawMonitor_lock can not
4514 // be held indefinitely. The critical sections must be short and bounded.
4516 // -------------------------------------------------------------------------
4518 int ObjectMonitor::SimpleEnter (Thread * Self) {
4520 if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
4524 ObjectWaiter Node (Self) ;
4525 Self->_ParkEvent->reset() ; // strictly optional
4526 Node.TState = ObjectWaiter::TS_ENTER ;
4528 RawMonitor_lock->lock_without_safepoint_check() ;
4529 Node._next = _EntryList ;
4530 _EntryList = &Node ;
4531 OrderAccess::fence() ;
4532 if (_owner == NULL && Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
4533 _EntryList = Node._next ;
4534 RawMonitor_lock->unlock() ;
4537 RawMonitor_lock->unlock() ;
4538 while (Node.TState == ObjectWaiter::TS_ENTER) {
4539 Self->_ParkEvent->park() ;
4544 int ObjectMonitor::SimpleExit (Thread * Self) {
4545 guarantee (_owner == Self, "invariant") ;
4546 OrderAccess::release_store_ptr (&_owner, NULL) ;
4547 OrderAccess::fence() ;
4548 if (_EntryList == NULL) return OS_OK ;
4551 RawMonitor_lock->lock_without_safepoint_check() ;
4554 _EntryList = w->_next ;
4556 RawMonitor_lock->unlock() ;
4558 guarantee (w ->TState == ObjectWaiter::TS_ENTER, "invariant") ;
4559 ParkEvent * ev = w->_event ;
4560 w->TState = ObjectWaiter::TS_RUN ;
4561 OrderAccess::fence() ;
4567 int ObjectMonitor::SimpleWait (Thread * Self, jlong millis) {
4568 guarantee (_owner == Self , "invariant") ;
4569 guarantee (_recursions == 0, "invariant") ;
4571 ObjectWaiter Node (Self) ;
4572 Node._notified = 0 ;
4573 Node.TState = ObjectWaiter::TS_WAIT ;
4575 RawMonitor_lock->lock_without_safepoint_check() ;
4576 Node._next = _WaitSet ;
4578 RawMonitor_lock->unlock() ;
4581 guarantee (_owner != Self, "invariant") ;
4585 Self->_ParkEvent->park();
4587 ret = Self->_ParkEvent->park(millis);
4590 // If thread still resides on the waitset then unlink it.
4591 // Double-checked locking -- the usage is safe in this context
4592 // as we TState is volatile and the lock-unlock operators are
4593 // serializing (barrier-equivalent).
4595 if (Node.TState == ObjectWaiter::TS_WAIT) {
4596 RawMonitor_lock->lock_without_safepoint_check() ;
4597 if (Node.TState == ObjectWaiter::TS_WAIT) {
4598 // Simple O(n) unlink, but performance isn't critical here.
4600 ObjectWaiter * q = NULL ;
4601 for (p = _WaitSet ; p != &Node; p = p->_next) {
4604 guarantee (p == &Node, "invariant") ;
4606 guarantee (p == _WaitSet, "invariant") ;
4607 _WaitSet = p->_next ;
4609 guarantee (p == q->_next, "invariant") ;
4610 q->_next = p->_next ;
4612 Node.TState = ObjectWaiter::TS_RUN ;
4614 RawMonitor_lock->unlock() ;
4617 guarantee (Node.TState == ObjectWaiter::TS_RUN, "invariant") ;
4618 SimpleEnter (Self) ;
4620 guarantee (_owner == Self, "invariant") ;
4621 guarantee (_recursions == 0, "invariant") ;
4625 int ObjectMonitor::SimpleNotify (Thread * Self, bool All) {
4626 guarantee (_owner == Self, "invariant") ;
4627 if (_WaitSet == NULL) return OS_OK ;
4629 // We have two options:
4630 // A. Transfer the threads from the WaitSet to the EntryList
4631 // B. Remove the thread from the WaitSet and unpark() it.
4633 // We use (B), which is crude and results in lots of futile
4634 // context switching. In particular (B) induces lots of contention.
4636 ParkEvent * ev = NULL ; // consider using a small auto array ...
4637 RawMonitor_lock->lock_without_safepoint_check() ;
4639 ObjectWaiter * w = _WaitSet ;
4640 if (w == NULL) break ;
4641 _WaitSet = w->_next ;
4642 if (ev != NULL) { ev->unpark(); ev = NULL; }
4644 OrderAccess::loadstore() ;
4645 w->TState = ObjectWaiter::TS_RUN ;
4646 OrderAccess::storeload();
4649 RawMonitor_lock->unlock() ;
4650 if (ev != NULL) ev->unpark();
4654 // Any JavaThread will enter here with state _thread_blocked
4655 int ObjectMonitor::raw_enter(TRAPS) {
4656 TEVENT (raw_enter) ;
4659 // don't enter raw monitor if thread is being externally suspended, it will
4660 // surprise the suspender if a "suspended" thread can still enter monitor
4661 JavaThread * jt = (JavaThread *)THREAD;
4662 if (THREAD->is_Java_thread()) {
4663 jt->SR_lock()->lock_without_safepoint_check();
4664 while (jt->is_external_suspend()) {
4665 jt->SR_lock()->unlock();
4666 jt->java_suspend_self();
4667 jt->SR_lock()->lock_without_safepoint_check();
4669 // guarded by SR_lock to avoid racing with new external suspend requests.
4670 Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
4671 jt->SR_lock()->unlock();
4673 Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
4676 if (Contended == THREAD) {
4681 if (Contended == NULL) {
4682 guarantee (_owner == THREAD, "invariant") ;
4683 guarantee (_recursions == 0, "invariant") ;
4687 THREAD->set_current_pending_monitor(this);
4689 if (!THREAD->is_Java_thread()) {
4690 // No other non-Java threads besides VM thread would acquire
4692 assert(THREAD->is_VM_thread(), "must be VM thread");
4693 SimpleEnter (THREAD) ;
4695 guarantee (jt->thread_state() == _thread_blocked, "invariant") ;
4697 jt->set_suspend_equivalent();
4698 // cleared by handle_special_suspend_equivalent_condition() or
4699 // java_suspend_self()
4700 SimpleEnter (THREAD) ;
4702 // were we externally suspended while we were waiting?
4703 if (!jt->handle_special_suspend_equivalent_condition()) break ;
4705 // This thread was externally suspended
4707 // This logic isn't needed for JVMTI raw monitors,
4708 // but doesn't hurt just in case the suspend rules change. This
4709 // logic is needed for the ObjectMonitor.wait() reentry phase.
4710 // We have reentered the contended monitor, but while we were
4711 // waiting another thread suspended us. We don't want to reenter
4712 // the monitor while suspended because that would surprise the
4713 // thread that suspended us.
4716 SimpleExit (THREAD) ;
4718 jt->java_suspend_self();
4721 assert(_owner == THREAD, "Fatal error with monitor owner!");
4722 assert(_recursions == 0, "Fatal error with monitor recursions!");
4725 THREAD->set_current_pending_monitor(NULL);
4726 guarantee (_recursions == 0, "invariant") ;
4730 // Used mainly for JVMTI raw monitor implementation
4731 // Also used for ObjectMonitor::wait().
4732 int ObjectMonitor::raw_exit(TRAPS) {
4734 if (THREAD != _owner) {
4735 return OM_ILLEGAL_MONITOR_STATE;
4737 if (_recursions > 0) {
4742 void * List = _EntryList ;
4743 SimpleExit (THREAD) ;
4748 // Used for JVMTI raw monitor implementation.
4749 // All JavaThreads will enter here with state _thread_blocked
4751 int ObjectMonitor::raw_wait(jlong millis, bool interruptible, TRAPS) {
4753 if (THREAD != _owner) {
4754 return OM_ILLEGAL_MONITOR_STATE;
4757 // To avoid spurious wakeups we reset the parkevent -- This is strictly optional.
4758 // The caller must be able to tolerate spurious returns from raw_wait().
4759 THREAD->_ParkEvent->reset() ;
4760 OrderAccess::fence() ;
4762 // check interrupt event
4763 if (interruptible && Thread::is_interrupted(THREAD, true)) {
4764 return OM_INTERRUPTED;
4767 intptr_t save = _recursions ;
4770 if (THREAD->is_Java_thread()) {
4771 guarantee (((JavaThread *) THREAD)->thread_state() == _thread_blocked, "invariant") ;
4772 ((JavaThread *)THREAD)->set_suspend_equivalent();
4774 int rv = SimpleWait (THREAD, millis) ;
4775 _recursions = save ;
4778 guarantee (THREAD == _owner, "invariant") ;
4779 if (THREAD->is_Java_thread()) {
4780 JavaThread * jSelf = (JavaThread *) THREAD ;
4782 if (!jSelf->handle_special_suspend_equivalent_condition()) break ;
4783 SimpleExit (THREAD) ;
4784 jSelf->java_suspend_self();
4785 SimpleEnter (THREAD) ;
4786 jSelf->set_suspend_equivalent() ;
4789 guarantee (THREAD == _owner, "invariant") ;
4791 if (interruptible && Thread::is_interrupted(THREAD, true)) {
4792 return OM_INTERRUPTED;
4797 int ObjectMonitor::raw_notify(TRAPS) {
4798 TEVENT (raw_notify) ;
4799 if (THREAD != _owner) {
4800 return OM_ILLEGAL_MONITOR_STATE;
4802 SimpleNotify (THREAD, false) ;
4806 int ObjectMonitor::raw_notifyAll(TRAPS) {
4807 TEVENT (raw_notifyAll) ;
4808 if (THREAD != _owner) {
4809 return OM_ILLEGAL_MONITOR_STATE;
4811 SimpleNotify (THREAD, true) ;
4816 void ObjectMonitor::verify() {
4819 void ObjectMonitor::print() {
4823 //------------------------------------------------------------------------------
4828 void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
4829 bool is_method, bool is_locking) {
4830 // Don't know what to do here
4833 // Verify all monitors in the monitor cache, the verification is weak.
4834 void ObjectSynchronizer::verify() {
4835 ObjectMonitor* block = gBlockList;
4838 assert(block->object() == CHAINMARKER, "must be a block header");
4839 for (int i = 1; i < _BLOCKSIZE; i++) {
4841 oop object = (oop) mid->object();
4842 if (object != NULL) {
4846 block = (ObjectMonitor*) block->FreeNext;
4850 // Check if monitor belongs to the monitor cache
4851 // The list is grow-only so it's *relatively* safe to traverse
4852 // the list of extant blocks without taking a lock.
4854 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
4855 ObjectMonitor* block = gBlockList;
4858 assert(block->object() == CHAINMARKER, "must be a block header");
4859 if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
4860 address mon = (address) monitor;
4861 address blk = (address) block;
4862 size_t diff = mon - blk;
4863 assert((diff % sizeof(ObjectMonitor)) == 0, "check");
4866 block = (ObjectMonitor*) block->FreeNext;