changeset 1894:42f65821fa4e

Merge
author coleenp
date Mon, 06 Dec 2010 15:37:00 -0500
parents 9bc798875b2a d9310331a29c
children 684faacebf20
files
diffstat 10 files changed, 1060 insertions(+), 403 deletions(-) [+]
line wrap: on
line diff
--- a/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Mon Dec 06 15:37:00 2010 -0500
@@ -1051,6 +1051,7 @@
   void work(int worker_i) {
     assert(Thread::current()->is_ConcurrentGC_thread(),
            "this should only be done by a conc GC thread");
+    ResourceMark rm;
 
     double start_vtime = os::elapsedVTime();
 
@@ -1888,6 +1889,9 @@
   G1CollectedHeap* g1h   = G1CollectedHeap::heap();
   ReferenceProcessor* rp = g1h->ref_processor();
 
+  // See the comment in G1CollectedHeap::ref_processing_init()
+  // about how reference processing currently works in G1.
+
   // Process weak references.
   rp->setup_policy(clear_all_soft_refs);
   assert(_markStack.isEmpty(), "mark stack should be empty");
@@ -2918,7 +2922,11 @@
   CMOopClosure(G1CollectedHeap* g1h,
                ConcurrentMark* cm,
                CMTask* task)
-    : _g1h(g1h), _cm(cm), _task(task) { }
+    : _g1h(g1h), _cm(cm), _task(task)
+  {
+    _ref_processor = g1h->ref_processor();
+    assert(_ref_processor != NULL, "should not be NULL");
+  }
 };
 
 void CMTask::setup_for_region(HeapRegion* hr) {
--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Mon Dec 06 15:37:00 2010 -0500
@@ -58,10 +58,11 @@
 // INVARIANTS/NOTES
 //
 // All allocation activity covered by the G1CollectedHeap interface is
-//   serialized by acquiring the HeapLock.  This happens in
-//   mem_allocate_work, which all such allocation functions call.
-//   (Note that this does not apply to TLAB allocation, which is not part
-//   of this interface: it is done by clients of this interface.)
+// serialized by acquiring the HeapLock.  This happens in mem_allocate
+// and allocate_new_tlab, which are the "entry" points to the
+// allocation code from the rest of the JVM.  (Note that this does not
+// apply to TLAB allocation, which is not part of this interface: it
+// is done by clients of this interface.)
 
 // Local to this file.
 
@@ -536,18 +537,20 @@
 // If could fit into free regions w/o expansion, try.
 // Otherwise, if can expand, do so.
 // Otherwise, if using ex regions might help, try with ex given back.
-HeapWord* G1CollectedHeap::humongousObjAllocate(size_t word_size) {
+HeapWord* G1CollectedHeap::humongous_obj_allocate(size_t word_size) {
+  assert_heap_locked_or_at_safepoint();
   assert(regions_accounted_for(), "Region leakage!");
 
-  // We can't allocate H regions while cleanupComplete is running, since
-  // some of the regions we find to be empty might not yet be added to the
-  // unclean list.  (If we're already at a safepoint, this call is
-  // unnecessary, not to mention wrong.)
-  if (!SafepointSynchronize::is_at_safepoint())
+  // We can't allocate humongous regions while cleanupComplete is
+  // running, since some of the regions we find to be empty might not
+  // yet be added to the unclean list. If we're already at a
+  // safepoint, this call is unnecessary, not to mention wrong.
+  if (!SafepointSynchronize::is_at_safepoint()) {
     wait_for_cleanup_complete();
+  }
 
   size_t num_regions =
-    round_to(word_size, HeapRegion::GrainWords) / HeapRegion::GrainWords;
+         round_to(word_size, HeapRegion::GrainWords) / HeapRegion::GrainWords;
 
   // Special case if < one region???
 
@@ -598,153 +601,474 @@
   return res;
 }
 
+void
+G1CollectedHeap::retire_cur_alloc_region(HeapRegion* cur_alloc_region) {
+  // The cleanup operation might update _summary_bytes_used
+  // concurrently with this method. So, right now, if we don't wait
+  // for it to complete, updates to _summary_bytes_used might get
+  // lost. This will be resolved in the near future when the operation
+  // of the free region list is revamped as part of CR 6977804.
+  wait_for_cleanup_complete();
+
+  retire_cur_alloc_region_common(cur_alloc_region);
+  assert(_cur_alloc_region == NULL, "post-condition");
+}
+
+// See the comment in the .hpp file about the locking protocol and
+// assumptions of this method (and other related ones).
 HeapWord*
-G1CollectedHeap::attempt_allocation_slow(size_t word_size,
-                                         bool permit_collection_pause) {
-  HeapWord* res = NULL;
-  HeapRegion* allocated_young_region = NULL;
-
-  assert( SafepointSynchronize::is_at_safepoint() ||
-          Heap_lock->owned_by_self(), "pre condition of the call" );
-
-  if (isHumongous(word_size)) {
-    // Allocation of a humongous object can, in a sense, complete a
-    // partial region, if the previous alloc was also humongous, and
-    // caused the test below to succeed.
-    if (permit_collection_pause)
-      do_collection_pause_if_appropriate(word_size);
-    res = humongousObjAllocate(word_size);
-    assert(_cur_alloc_region == NULL
-           || !_cur_alloc_region->isHumongous(),
-           "Prevent a regression of this bug.");
-
-  } else {
-    // We may have concurrent cleanup working at the time. Wait for it
-    // to complete. In the future we would probably want to make the
-    // concurrent cleanup truly concurrent by decoupling it from the
-    // allocation.
-    if (!SafepointSynchronize::is_at_safepoint())
+G1CollectedHeap::replace_cur_alloc_region_and_allocate(size_t word_size,
+                                                       bool at_safepoint,
+                                                       bool do_dirtying) {
+  assert_heap_locked_or_at_safepoint();
+  assert(_cur_alloc_region == NULL,
+         "replace_cur_alloc_region_and_allocate() should only be called "
+         "after retiring the previous current alloc region");
+  assert(SafepointSynchronize::is_at_safepoint() == at_safepoint,
+         "at_safepoint and is_at_safepoint() should be a tautology");
+
+  if (!g1_policy()->is_young_list_full()) {
+    if (!at_safepoint) {
+      // The cleanup operation might update _summary_bytes_used
+      // concurrently with this method. So, right now, if we don't
+      // wait for it to complete, updates to _summary_bytes_used might
+      // get lost. This will be resolved in the near future when the
+      // operation of the free region list is revamped as part of
+      // CR 6977804. If we're already at a safepoint, this call is
+      // unnecessary, not to mention wrong.
       wait_for_cleanup_complete();
-    // If we do a collection pause, this will be reset to a non-NULL
-    // value.  If we don't, nulling here ensures that we allocate a new
-    // region below.
-    if (_cur_alloc_region != NULL) {
-      // We're finished with the _cur_alloc_region.
-      // As we're builing (at least the young portion) of the collection
-      // set incrementally we'll add the current allocation region to
-      // the collection set here.
-      if (_cur_alloc_region->is_young()) {
-        g1_policy()->add_region_to_incremental_cset_lhs(_cur_alloc_region);
+    }
+
+    HeapRegion* new_cur_alloc_region = newAllocRegion(word_size,
+                                                      false /* zero_filled */);
+    if (new_cur_alloc_region != NULL) {
+      assert(new_cur_alloc_region->is_empty(),
+             "the newly-allocated region should be empty, "
+             "as right now we only allocate new regions out of the free list");
+      g1_policy()->update_region_num(true /* next_is_young */);
+      _summary_bytes_used -= new_cur_alloc_region->used();
+      set_region_short_lived_locked(new_cur_alloc_region);
+
+      assert(!new_cur_alloc_region->isHumongous(),
+             "Catch a regression of this bug.");
+
+      // We need to ensure that the stores to _cur_alloc_region and,
+      // subsequently, to top do not float above the setting of the
+      // young type.
+      OrderAccess::storestore();
+
+      // Now allocate out of the new current alloc region. We could
+      // have re-used allocate_from_cur_alloc_region() but its
+      // operation is slightly different to what we need here. First,
+      // allocate_from_cur_alloc_region() is only called outside a
+      // safepoint and will always unlock the Heap_lock if it returns
+      // a non-NULL result. Second, it assumes that the current alloc
+      // region is what's already assigned in _cur_alloc_region. What
+      // we want here is to actually do the allocation first before we
+      // assign the new region to _cur_alloc_region. This ordering is
+      // not currently important, but it will be essential when we
+      // change the code to support CAS allocation in the future (see
+      // CR 6994297).
+      //
+      // This allocate method does BOT updates and we don't need them in
+      // the young generation. This will be fixed in the near future by
+      // CR 6994297.
+      HeapWord* result = new_cur_alloc_region->allocate(word_size);
+      assert(result != NULL, "we just allocate out of an empty region "
+             "so allocation should have been successful");
+      assert(is_in(result), "result should be in the heap");
+
+      _cur_alloc_region = new_cur_alloc_region;
+
+      if (!at_safepoint) {
+        Heap_lock->unlock();
       }
-      _summary_bytes_used += _cur_alloc_region->used();
-      _cur_alloc_region = NULL;
+
+      // do the dirtying, if necessary, after we release the Heap_lock
+      if (do_dirtying) {
+        dirty_young_block(result, word_size);
+      }
+      return result;
     }
-    assert(_cur_alloc_region == NULL, "Invariant.");
-    // Completion of a heap region is perhaps a good point at which to do
-    // a collection pause.
-    if (permit_collection_pause)
-      do_collection_pause_if_appropriate(word_size);
-    // Make sure we have an allocation region available.
-    if (_cur_alloc_region == NULL) {
-      if (!SafepointSynchronize::is_at_safepoint())
-        wait_for_cleanup_complete();
-      bool next_is_young = should_set_young_locked();
-      // If the next region is not young, make sure it's zero-filled.
-      _cur_alloc_region = newAllocRegion(word_size, !next_is_young);
-      if (_cur_alloc_region != NULL) {
-        _summary_bytes_used -= _cur_alloc_region->used();
-        if (next_is_young) {
-          set_region_short_lived_locked(_cur_alloc_region);
-          allocated_young_region = _cur_alloc_region;
-        }
+  }
+
+  assert(_cur_alloc_region == NULL, "we failed to allocate a new current "
+         "alloc region, it should still be NULL");
+  assert_heap_locked_or_at_safepoint();
+  return NULL;
+}
+
+// See the comment in the .hpp file about the locking protocol and
+// assumptions of this method (and other related ones).
+HeapWord*
+G1CollectedHeap::attempt_allocation_slow(size_t word_size) {
+  assert_heap_locked_and_not_at_safepoint();
+  assert(!isHumongous(word_size), "attempt_allocation_slow() should not be "
+         "used for humongous allocations");
+
+  // We will loop while succeeded is false, which means that we tried
+  // to do a collection, but the VM op did not succeed. So, when we
+  // exit the loop, either one of the allocation attempts was
+  // successful, or we succeeded in doing the VM op but which was
+  // unable to allocate after the collection.
+  for (int try_count = 1; /* we'll return or break */; try_count += 1) {
+    bool succeeded = true;
+
+    {
+      // We may have concurrent cleanup working at the time. Wait for
+      // it to complete. In the future we would probably want to make
+      // the concurrent cleanup truly concurrent by decoupling it from
+      // the allocation. This will happen in the near future as part
+      // of CR 6977804 which will revamp the operation of the free
+      // region list. The fact that wait_for_cleanup_complete() will
+      // do a wait() means that we'll give up the Heap_lock. So, it's
+      // possible that when we exit wait_for_cleanup_complete() we
+      // might be able to allocate successfully (since somebody else
+      // might have done a collection meanwhile). So, we'll attempt to
+      // allocate again, just in case. When we make cleanup truly
+      // concurrent with allocation, we should remove this allocation
+      // attempt as it's redundant (we only reach here after an
+      // allocation attempt has been unsuccessful).
+      wait_for_cleanup_complete();
+      HeapWord* result = attempt_allocation(word_size);
+      if (result != NULL) {
+        assert_heap_not_locked();
+        return result;
       }
     }
+
+    if (GC_locker::is_active_and_needs_gc()) {
+      // We are locked out of GC because of the GC locker. Right now,
+      // we'll just stall until the GC locker-induced GC
+      // completes. This will be fixed in the near future by extending
+      // the eden while waiting for the GC locker to schedule the GC
+      // (see CR 6994056).
+
+      // If this thread is not in a jni critical section, we stall
+      // the requestor until the critical section has cleared and
+      // GC allowed. When the critical section clears, a GC is
+      // initiated by the last thread exiting the critical section; so
+      // we retry the allocation sequence from the beginning of the loop,
+      // rather than causing more, now probably unnecessary, GC attempts.
+      JavaThread* jthr = JavaThread::current();
+      assert(jthr != NULL, "sanity");
+      if (!jthr->in_critical()) {
+        MutexUnlocker mul(Heap_lock);
+        GC_locker::stall_until_clear();
+
+        // We'll then fall off the end of the ("if GC locker active")
+        // if-statement and retry the allocation further down in the
+        // loop.
+      } else {
+        if (CheckJNICalls) {
+          fatal("Possible deadlock due to allocating while"
+                " in jni critical section");
+        }
+        return NULL;
+      }
+    } else {
+      // We are not locked out. So, let's try to do a GC. The VM op
+      // will retry the allocation before it completes.
+
+      // Read the GC count while holding the Heap_lock
+      unsigned int gc_count_before = SharedHeap::heap()->total_collections();
+
+      Heap_lock->unlock();
+
+      HeapWord* result =
+        do_collection_pause(word_size, gc_count_before, &succeeded);
+      assert_heap_not_locked();
+      if (result != NULL) {
+        assert(succeeded, "the VM op should have succeeded");
+
+        // Allocations that take place on VM operations do not do any
+        // card dirtying and we have to do it here.
+        dirty_young_block(result, word_size);
+        return result;
+      }
+
+      Heap_lock->lock();
+    }
+
+    assert_heap_locked();
+
+    // We can reach here when we were unsuccessful in doing a GC,
+    // because another thread beat us to it, or because we were locked
+    // out of GC due to the GC locker. In either case a new alloc
+    // region might be available so we will retry the allocation.
+    HeapWord* result = attempt_allocation(word_size);
+    if (result != NULL) {
+      assert_heap_not_locked();
+      return result;
+    }
+
+    // So far our attempts to allocate failed. The only time we'll go
+    // around the loop and try again is if we tried to do a GC and the
+    // VM op that we tried to schedule was not successful because
+    // another thread beat us to it. If that happened it's possible
+    // that by the time we grabbed the Heap_lock again and tried to
+    // allocate other threads filled up the young generation, which
+    // means that the allocation attempt after the GC also failed. So,
+    // it's worth trying to schedule another GC pause.
+    if (succeeded) {
+      break;
+    }
+
+    // Give a warning if we seem to be looping forever.
+    if ((QueuedAllocationWarningCount > 0) &&
+        (try_count % QueuedAllocationWarningCount == 0)) {
+      warning("G1CollectedHeap::attempt_allocation_slow() "
+              "retries %d times", try_count);
+    }
+  }
+
+  assert_heap_locked();
+  return NULL;
+}
+
+// See the comment in the .hpp file about the locking protocol and
+// assumptions of this method (and other related ones).
+HeapWord*
+G1CollectedHeap::attempt_allocation_humongous(size_t word_size,
+                                              bool at_safepoint) {
+  // This is the method that will allocate a humongous object. All
+  // allocation paths that attempt to allocate a humongous object
+  // should eventually reach here. Currently, the only paths are from
+  // mem_allocate() and attempt_allocation_at_safepoint().
+  assert_heap_locked_or_at_safepoint();
+  assert(isHumongous(word_size), "attempt_allocation_humongous() "
+         "should only be used for humongous allocations");
+  assert(SafepointSynchronize::is_at_safepoint() == at_safepoint,
+         "at_safepoint and is_at_safepoint() should be a tautology");
+
+  HeapWord* result = NULL;
+
+  // We will loop while succeeded is false, which means that we tried
+  // to do a collection, but the VM op did not succeed. So, when we
+  // exit the loop, either one of the allocation attempts was
+  // successful, or we succeeded in doing the VM op but which was
+  // unable to allocate after the collection.
+  for (int try_count = 1; /* we'll return or break */; try_count += 1) {
+    bool succeeded = true;
+
+    // Given that humongous objects are not allocated in young
+    // regions, we'll first try to do the allocation without doing a
+    // collection hoping that there's enough space in the heap.
+    result = humongous_obj_allocate(word_size);
     assert(_cur_alloc_region == NULL || !_cur_alloc_region->isHumongous(),
-           "Prevent a regression of this bug.");
-
-    // Now retry the allocation.
-    if (_cur_alloc_region != NULL) {
-      if (allocated_young_region != NULL) {
-        // We need to ensure that the store to top does not
-        // float above the setting of the young type.
-        OrderAccess::storestore();
+           "catch a regression of this bug.");
+    if (result != NULL) {
+      if (!at_safepoint) {
+        // If we're not at a safepoint, unlock the Heap_lock.
+        Heap_lock->unlock();
       }
-      res = _cur_alloc_region->allocate(word_size);
+      return result;
     }
-  }
-
-  // NOTE: fails frequently in PRT
-  assert(regions_accounted_for(), "Region leakage!");
-
-  if (res != NULL) {
-    if (!SafepointSynchronize::is_at_safepoint()) {
-      assert( permit_collection_pause, "invariant" );
-      assert( Heap_lock->owned_by_self(), "invariant" );
+
+    // If we failed to allocate the humongous object, we should try to
+    // do a collection pause (if we're allowed) in case it reclaims
+    // enough space for the allocation to succeed after the pause.
+    if (!at_safepoint) {
+      // Read the GC count while holding the Heap_lock
+      unsigned int gc_count_before = SharedHeap::heap()->total_collections();
+
+      // If we're allowed to do a collection we're not at a
+      // safepoint, so it is safe to unlock the Heap_lock.
       Heap_lock->unlock();
+
+      result = do_collection_pause(word_size, gc_count_before, &succeeded);
+      assert_heap_not_locked();
+      if (result != NULL) {
+        assert(succeeded, "the VM op should have succeeded");
+        return result;
+      }
+
+      // If we get here, the VM operation either did not succeed
+      // (i.e., another thread beat us to it) or it succeeded but
+      // failed to allocate the object.
+
+      // If we're allowed to do a collection we're not at a
+      // safepoint, so it is safe to lock the Heap_lock.
+      Heap_lock->lock();
     }
 
-    if (allocated_young_region != NULL) {
-      HeapRegion* hr = allocated_young_region;
-      HeapWord* bottom = hr->bottom();
-      HeapWord* end = hr->end();
-      MemRegion mr(bottom, end);
-      ((CardTableModRefBS*)_g1h->barrier_set())->dirty(mr);
+    assert(result == NULL, "otherwise we should have exited the loop earlier");
+
+    // So far our attempts to allocate failed. The only time we'll go
+    // around the loop and try again is if we tried to do a GC and the
+    // VM op that we tried to schedule was not successful because
+    // another thread beat us to it. That way it's possible that some
+    // space was freed up by the thread that successfully scheduled a
+    // GC. So it's worth trying to allocate again.
+    if (succeeded) {
+      break;
     }
-  }
-
-  assert( SafepointSynchronize::is_at_safepoint() ||
-          (res == NULL && Heap_lock->owned_by_self()) ||
-          (res != NULL && !Heap_lock->owned_by_self()),
-          "post condition of the call" );
-
-  return res;
+
+    // Give a warning if we seem to be looping forever.
+    if ((QueuedAllocationWarningCount > 0) &&
+        (try_count % QueuedAllocationWarningCount == 0)) {
+      warning("G1CollectedHeap::attempt_allocation_humongous "
+              "retries %d times", try_count);
+    }
+  }
+
+  assert_heap_locked_or_at_safepoint();
+  return NULL;
+}
+
+HeapWord* G1CollectedHeap::attempt_allocation_at_safepoint(size_t word_size,
+                                           bool expect_null_cur_alloc_region) {
+  assert_at_safepoint();
+  assert(_cur_alloc_region == NULL || !expect_null_cur_alloc_region,
+         err_msg("the current alloc region was unexpectedly found "
+                 "to be non-NULL, cur alloc region: "PTR_FORMAT" "
+                 "expect_null_cur_alloc_region: %d word_size: "SIZE_FORMAT,
+                 _cur_alloc_region, expect_null_cur_alloc_region, word_size));
+
+  if (!isHumongous(word_size)) {
+    if (!expect_null_cur_alloc_region) {
+      HeapRegion* cur_alloc_region = _cur_alloc_region;
+      if (cur_alloc_region != NULL) {
+        // This allocate method does BOT updates and we don't need them in
+        // the young generation. This will be fixed in the near future by
+        // CR 6994297.
+        HeapWord* result = cur_alloc_region->allocate(word_size);
+        if (result != NULL) {
+          assert(is_in(result), "result should be in the heap");
+
+          // We will not do any dirtying here. This is guaranteed to be
+          // called during a safepoint and the thread that scheduled the
+          // pause will do the dirtying if we return a non-NULL result.
+          return result;
+        }
+
+        retire_cur_alloc_region_common(cur_alloc_region);
+      }
+    }
+
+    assert(_cur_alloc_region == NULL,
+           "at this point we should have no cur alloc region");
+    return replace_cur_alloc_region_and_allocate(word_size,
+                                                 true, /* at_safepoint */
+                                                 false /* do_dirtying */);
+  } else {
+    return attempt_allocation_humongous(word_size,
+                                        true /* at_safepoint */);
+  }
+
+  ShouldNotReachHere();
+}
+
+HeapWord* G1CollectedHeap::allocate_new_tlab(size_t word_size) {
+  assert_heap_not_locked_and_not_at_safepoint();
+  assert(!isHumongous(word_size), "we do not allow TLABs of humongous size");
+
+  Heap_lock->lock();
+
+  // First attempt: try allocating out of the current alloc region or
+  // after replacing the current alloc region.
+  HeapWord* result = attempt_allocation(word_size);
+  if (result != NULL) {
+    assert_heap_not_locked();
+    return result;
+  }
+
+  assert_heap_locked();
+
+  // Second attempt: go into the even slower path where we might
+  // try to schedule a collection.
+  result = attempt_allocation_slow(word_size);
+  if (result != NULL) {
+    assert_heap_not_locked();
+    return result;
+  }
+
+  assert_heap_locked();
+  Heap_lock->unlock();
+  return NULL;
 }
 
 HeapWord*
 G1CollectedHeap::mem_allocate(size_t word_size,
                               bool   is_noref,
                               bool   is_tlab,
-                              bool* gc_overhead_limit_was_exceeded) {
-  debug_only(check_for_valid_allocation_state());
-  assert(no_gc_in_progress(), "Allocation during gc not allowed");
-  HeapWord* result = NULL;
+                              bool*  gc_overhead_limit_was_exceeded) {
+  assert_heap_not_locked_and_not_at_safepoint();
+  assert(!is_tlab, "mem_allocate() this should not be called directly "
+         "to allocate TLABs");
 
   // Loop until the allocation is satisified,
   // or unsatisfied after GC.
-  for (int try_count = 1; /* return or throw */; try_count += 1) {
-    int gc_count_before;
+  for (int try_count = 1; /* we'll return */; try_count += 1) {
+    unsigned int gc_count_before;
     {
       Heap_lock->lock();
-      result = attempt_allocation(word_size);
-      if (result != NULL) {
-        // attempt_allocation should have unlocked the heap lock
-        assert(is_in(result), "result not in heap");
-        return result;
+
+      if (!isHumongous(word_size)) {
+        // First attempt: try allocating out of the current alloc
+        // region or after replacing the current alloc region.
+        HeapWord* result = attempt_allocation(word_size);
+        if (result != NULL) {
+          assert_heap_not_locked();
+          return result;
+        }
+
+        assert_heap_locked();
+
+        // Second attempt: go into the even slower path where we might
+        // try to schedule a collection.
+        result = attempt_allocation_slow(word_size);
+        if (result != NULL) {
+          assert_heap_not_locked();
+          return result;
+        }
+      } else {
+        HeapWord* result = attempt_allocation_humongous(word_size,
+                                                     false /* at_safepoint */);
+        if (result != NULL) {
+          assert_heap_not_locked();
+          return result;
+        }
       }
+
+      assert_heap_locked();
       // Read the gc count while the heap lock is held.
       gc_count_before = SharedHeap::heap()->total_collections();
+      // We cannot be at a safepoint, so it is safe to unlock the Heap_lock
       Heap_lock->unlock();
     }
 
     // Create the garbage collection operation...
-    VM_G1CollectForAllocation op(word_size,
-                                 gc_count_before);
-
+    VM_G1CollectForAllocation op(gc_count_before, word_size);
     // ...and get the VM thread to execute it.
     VMThread::execute(&op);
-    if (op.prologue_succeeded()) {
-      result = op.result();
-      assert(result == NULL || is_in(result), "result not in heap");
+
+    assert_heap_not_locked();
+    if (op.prologue_succeeded() && op.pause_succeeded()) {
+      // If the operation was successful we'll return the result even
+      // if it is NULL. If the allocation attempt failed immediately
+      // after a Full GC, it's unlikely we'll be able to allocate now.
+      HeapWord* result = op.result();
+      if (result != NULL && !isHumongous(word_size)) {
+        // Allocations that take place on VM operations do not do any
+        // card dirtying and we have to do it here. We only have to do
+        // this for non-humongous allocations, though.
+        dirty_young_block(result, word_size);
+      }
       return result;
+    } else {
+      assert(op.result() == NULL,
+             "the result should be NULL if the VM op did not succeed");
     }
 
     // Give a warning if we seem to be looping forever.
     if ((QueuedAllocationWarningCount > 0) &&
         (try_count % QueuedAllocationWarningCount == 0)) {
-      warning("G1CollectedHeap::mem_allocate_work retries %d times",
-              try_count);
+      warning("G1CollectedHeap::mem_allocate retries %d times", try_count);
     }
   }
+
+  ShouldNotReachHere();
 }
 
 void G1CollectedHeap::abandon_cur_alloc_region() {
@@ -841,11 +1165,11 @@
   }
 };
 
-void G1CollectedHeap::do_collection(bool explicit_gc,
+bool G1CollectedHeap::do_collection(bool explicit_gc,
                                     bool clear_all_soft_refs,
                                     size_t word_size) {
   if (GC_locker::check_active_before_gc()) {
-    return; // GC is disabled (e.g. JNI GetXXXCritical operation)
+    return false;
   }
 
   ResourceMark rm;
@@ -929,6 +1253,9 @@
       g1_policy()->set_full_young_gcs(true);
     }
 
+    // See the comment in G1CollectedHeap::ref_processing_init() about
+    // how reference processing currently works in G1.
+
     // Temporarily make reference _discovery_ single threaded (non-MT).
     ReferenceProcessorMTMutator rp_disc_ser(ref_processor(), false);
 
@@ -1047,12 +1374,19 @@
   if (PrintHeapAtGC) {
     Universe::print_heap_after_gc();
   }
+
+  return true;
 }
 
 void G1CollectedHeap::do_full_collection(bool clear_all_soft_refs) {
-  do_collection(true,                /* explicit_gc */
-                clear_all_soft_refs,
-                0                    /* word_size */);
+  // do_collection() will return whether it succeeded in performing
+  // the GC. Currently, there is no facility on the
+  // do_full_collection() API to notify the caller than the collection
+  // did not succeed (e.g., because it was locked out by the GC
+  // locker). So, right now, we'll ignore the return value.
+  bool dummy = do_collection(true,                /* explicit_gc */
+                             clear_all_soft_refs,
+                             0                    /* word_size */);
 }
 
 // This code is mostly copied from TenuredGeneration.
@@ -1175,46 +1509,74 @@
 
 
 HeapWord*
-G1CollectedHeap::satisfy_failed_allocation(size_t word_size) {
-  HeapWord* result = NULL;
+G1CollectedHeap::satisfy_failed_allocation(size_t word_size,
+                                           bool* succeeded) {
+  assert(SafepointSynchronize::is_at_safepoint(),
+         "satisfy_failed_allocation() should only be called at a safepoint");
+  assert(Thread::current()->is_VM_thread(),
+         "satisfy_failed_allocation() should only be called by the VM thread");
+
+  *succeeded = true;
+  // Let's attempt the allocation first.
+  HeapWord* result = attempt_allocation_at_safepoint(word_size,
+                                     false /* expect_null_cur_alloc_region */);
+  if (result != NULL) {
+    assert(*succeeded, "sanity");
+    return result;
+  }
 
   // In a G1 heap, we're supposed to keep allocation from failing by
   // incremental pauses.  Therefore, at least for now, we'll favor
   // expansion over collection.  (This might change in the future if we can
   // do something smarter than full collection to satisfy a failed alloc.)
-
   result = expand_and_allocate(word_size);
   if (result != NULL) {
-    assert(is_in(result), "result not in heap");
+    assert(*succeeded, "sanity");
     return result;
   }
 
-  // OK, I guess we have to try collection.
-
-  do_collection(false, false, word_size);
-
-  result = attempt_allocation(word_size, /*permit_collection_pause*/false);
-
+  // Expansion didn't work, we'll try to do a Full GC.
+  bool gc_succeeded = do_collection(false, /* explicit_gc */
+                                    false, /* clear_all_soft_refs */
+                                    word_size);
+  if (!gc_succeeded) {
+    *succeeded = false;
+    return NULL;
+  }
+
+  // Retry the allocation
+  result = attempt_allocation_at_safepoint(word_size,
+                                      true /* expect_null_cur_alloc_region */);
   if (result != NULL) {
-    assert(is_in(result), "result not in heap");
+    assert(*succeeded, "sanity");
     return result;
   }
 
-  // Try collecting soft references.
-  do_collection(false, true, word_size);
-  result = attempt_allocation(word_size, /*permit_collection_pause*/false);
+  // Then, try a Full GC that will collect all soft references.
+  gc_succeeded = do_collection(false, /* explicit_gc */
+                               true,  /* clear_all_soft_refs */
+                               word_size);
+  if (!gc_succeeded) {
+    *succeeded = false;
+    return NULL;
+  }
+
+  // Retry the allocation once more
+  result = attempt_allocation_at_safepoint(word_size,
+                                      true /* expect_null_cur_alloc_region */);
   if (result != NULL) {
-    assert(is_in(result), "result not in heap");
+    assert(*succeeded, "sanity");
     return result;
   }
 
   assert(!collector_policy()->should_clear_all_soft_refs(),
-    "Flag should have been handled and cleared prior to this point");
+         "Flag should have been handled and cleared prior to this point");
 
   // What else?  We might try synchronous finalization later.  If the total
   // space available is large enough for the allocation, then a more
   // complete compaction phase than we've tried so far might be
   // appropriate.
+  assert(*succeeded, "sanity");
   return NULL;
 }
 
@@ -1224,14 +1586,20 @@
 // allocated block, or else "NULL".
 
 HeapWord* G1CollectedHeap::expand_and_allocate(size_t word_size) {
+  assert(SafepointSynchronize::is_at_safepoint(),
+         "expand_and_allocate() should only be called at a safepoint");
+  assert(Thread::current()->is_VM_thread(),
+         "expand_and_allocate() should only be called by the VM thread");
+
   size_t expand_bytes = word_size * HeapWordSize;
   if (expand_bytes < MinHeapDeltaBytes) {
     expand_bytes = MinHeapDeltaBytes;
   }
   expand(expand_bytes);
   assert(regions_accounted_for(), "Region leakage!");
-  HeapWord* result = attempt_allocation(word_size, false /* permit_collection_pause */);
-  return result;
+
+  return attempt_allocation_at_safepoint(word_size,
+                                     false /* expect_null_cur_alloc_region */);
 }
 
 size_t G1CollectedHeap::free_region_if_totally_empty(HeapRegion* hr) {
@@ -1649,6 +2017,24 @@
 }
 
 void G1CollectedHeap::ref_processing_init() {
+  // Reference processing in G1 currently works as follows:
+  //
+  // * There is only one reference processor instance that
+  //   'spans' the entire heap. It is created by the code
+  //   below.
+  // * Reference discovery is not enabled during an incremental
+  //   pause (see 6484982).
+  // * Discoverered refs are not enqueued nor are they processed
+  //   during an incremental pause (see 6484982).
+  // * Reference discovery is enabled at initial marking.
+  // * Reference discovery is disabled and the discovered
+  //   references processed etc during remarking.
+  // * Reference discovery is MT (see below).
+  // * Reference discovery requires a barrier (see below).
+  // * Reference processing is currently not MT (see 6608385).
+  // * A full GC enables (non-MT) reference discovery and
+  //   processes any discovered references.
+
   SharedHeap::ref_processing_init();
   MemRegion mr = reserved_region();
   _ref_processor = ReferenceProcessor::create_ref_processor(
@@ -1842,21 +2228,25 @@
   unsigned int full_gc_count_before;
   {
     MutexLocker ml(Heap_lock);
+
+    // Don't want to do a GC until cleanup is completed. This
+    // limitation will be removed in the near future when the
+    // operation of the free region list is revamped as part of
+    // CR 6977804.
+    wait_for_cleanup_complete();
+
     // Read the GC count while holding the Heap_lock
     gc_count_before = SharedHeap::heap()->total_collections();
     full_gc_count_before = SharedHeap::heap()->total_full_collections();
-
-    // Don't want to do a GC until cleanup is completed.
-    wait_for_cleanup_complete();
-
-    // We give up heap lock; VMThread::execute gets it back below
   }
 
   if (should_do_concurrent_full_gc(cause)) {
     // Schedule an initial-mark evacuation pause that will start a
-    // concurrent cycle.
+    // concurrent cycle. We're setting word_size to 0 which means that
+    // we are not requesting a post-GC allocation.
     VM_G1IncCollectionPause op(gc_count_before,
-                               true, /* should_initiate_conc_mark */
+                               0,     /* word_size */
+                               true,  /* should_initiate_conc_mark */
                                g1_policy()->max_pause_time_ms(),
                                cause);
     VMThread::execute(&op);
@@ -1864,8 +2254,10 @@
     if (cause == GCCause::_gc_locker
         DEBUG_ONLY(|| cause == GCCause::_scavenge_alot)) {
 
-      // Schedule a standard evacuation pause.
+      // Schedule a standard evacuation pause. We're setting word_size
+      // to 0 which means that we are not requesting a post-GC allocation.
       VM_G1IncCollectionPause op(gc_count_before,
+                                 0,     /* word_size */
                                  false, /* should_initiate_conc_mark */
                                  g1_policy()->max_pause_time_ms(),
                                  cause);
@@ -2221,14 +2613,6 @@
   }
 }
 
-HeapWord* G1CollectedHeap::allocate_new_tlab(size_t word_size) {
-  assert(!isHumongous(word_size),
-         err_msg("a TLAB should not be of humongous size, "
-                 "word_size = "SIZE_FORMAT, word_size));
-  bool dummy;
-  return G1CollectedHeap::mem_allocate(word_size, false, true, &dummy);
-}
-
 bool G1CollectedHeap::allocs_are_zero_filled() {
   return false;
 }
@@ -2633,27 +3017,26 @@
   // always_do_update_barrier = true;
 }
 
-void G1CollectedHeap::do_collection_pause() {
-  assert(Heap_lock->owned_by_self(), "we assume we'reholding the Heap_lock");
-
-  // Read the GC count while holding the Heap_lock
-  // we need to do this _before_ wait_for_cleanup_complete(), to
-  // ensure that we do not give up the heap lock and potentially
-  // pick up the wrong count
-  unsigned int gc_count_before = SharedHeap::heap()->total_collections();
-
-  // Don't want to do a GC pause while cleanup is being completed!
-  wait_for_cleanup_complete();
-
+HeapWord* G1CollectedHeap::do_collection_pause(size_t word_size,
+                                               unsigned int gc_count_before,
+                                               bool* succeeded) {
+  assert_heap_not_locked_and_not_at_safepoint();
   g1_policy()->record_stop_world_start();
-  {
-    MutexUnlocker mu(Heap_lock);  // give up heap lock, execute gets it back
-    VM_G1IncCollectionPause op(gc_count_before,
-                               false, /* should_initiate_conc_mark */
-                               g1_policy()->max_pause_time_ms(),
-                               GCCause::_g1_inc_collection_pause);
-    VMThread::execute(&op);
-  }
+  VM_G1IncCollectionPause op(gc_count_before,
+                             word_size,
+                             false, /* should_initiate_conc_mark */
+                             g1_policy()->max_pause_time_ms(),
+                             GCCause::_g1_inc_collection_pause);
+  VMThread::execute(&op);
+
+  HeapWord* result = op.result();
+  bool ret_succeeded = op.prologue_succeeded() && op.pause_succeeded();
+  assert(result == NULL || ret_succeeded,
+         "the result should be NULL if the VM did not succeed");
+  *succeeded = ret_succeeded;
+
+  assert_heap_not_locked();
+  return result;
 }
 
 void
@@ -2797,10 +3180,10 @@
 }
 #endif // TASKQUEUE_STATS
 
-void
+bool
 G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
   if (GC_locker::check_active_before_gc()) {
-    return; // GC is disabled (e.g. JNI GetXXXCritical operation)
+    return false;
   }
 
   if (PrintHeapAtGC) {
@@ -2871,6 +3254,9 @@
 
       COMPILER2_PRESENT(DerivedPointerTable::clear());
 
+      // Please see comment in G1CollectedHeap::ref_processing_init()
+      // to see how reference processing currently works in G1.
+      //
       // We want to turn off ref discovery, if necessary, and turn it back on
       // on again later if we do. XXX Dubious: why is discovery disabled?
       bool was_enabled = ref_processor()->discovery_enabled();
@@ -3068,6 +3454,8 @@
       (total_collections() % G1SummarizeRSetStatsPeriod == 0)) {
     g1_rem_set()->print_summary_info();
   }
+
+  return true;
 }
 
 size_t G1CollectedHeap::desired_plab_sz(GCAllocPurpose purpose)
@@ -3298,6 +3686,7 @@
   // untag the GC alloc regions and tear down the GC alloc region
   // list. It's desirable that no regions are tagged as GC alloc
   // outside GCs.
+
   forget_alloc_region_list();
 
   // The current alloc regions contain objs that have survived
@@ -3361,19 +3750,6 @@
 
 // *** Sequential G1 Evacuation
 
-HeapWord* G1CollectedHeap::allocate_during_gc(GCAllocPurpose purpose, size_t word_size) {
-  HeapRegion* alloc_region = _gc_alloc_regions[purpose];
-  // let the caller handle alloc failure
-  if (alloc_region == NULL) return NULL;
-  assert(isHumongous(word_size) || !alloc_region->isHumongous(),
-         "Either the object is humongous or the region isn't");
-  HeapWord* block = alloc_region->allocate(word_size);
-  if (block == NULL) {
-    block = allocate_during_gc_slow(purpose, alloc_region, false, word_size);
-  }
-  return block;
-}
-
 class G1IsAliveClosure: public BoolObjectClosure {
   G1CollectedHeap* _g1;
 public:
@@ -4316,6 +4692,10 @@
   }
   // Finish with the ref_processor roots.
   if (!_process_strong_tasks->is_task_claimed(G1H_PS_refProcessor_oops_do)) {
+    // We need to treat the discovered reference lists as roots and
+    // keep entries (which are added by the marking threads) on them
+    // live until they can be processed at the end of marking.
+    ref_processor()->weak_oops_do(scan_non_heap_roots);
     ref_processor()->oops_do(scan_non_heap_roots);
   }
   g1_policy()->record_collection_pause_end_G1_strong_roots();
@@ -4381,6 +4761,11 @@
   // on individual heap regions when we allocate from
   // them in parallel, so this seems like the correct place for this.
   retire_all_alloc_regions();
+
+  // Weak root processing.
+  // Note: when JSR 292 is enabled and code blobs can contain
+  // non-perm oops then we will need to process the code blobs
+  // here too.
   {
     G1IsAliveClosure is_alive(this);
     G1KeepAliveClosure keep_alive(this);
@@ -4625,12 +5010,6 @@
 #endif
 }
 
-void G1CollectedHeap::do_collection_pause_if_appropriate(size_t word_size) {
-  if (g1_policy()->should_do_collection_pause(word_size)) {
-    do_collection_pause();
-  }
-}
-
 void G1CollectedHeap::free_collection_set(HeapRegion* cs_head) {
   double young_time_ms     = 0.0;
   double non_young_time_ms = 0.0;
@@ -4789,6 +5168,7 @@
 }
 
 void G1CollectedHeap::wait_for_cleanup_complete() {
+  assert_not_at_safepoint();
   MutexLockerEx x(Cleanup_mon);
   wait_for_cleanup_complete_locked();
 }
@@ -5093,13 +5473,6 @@
   return n + m;
 }
 
-bool G1CollectedHeap::should_set_young_locked() {
-  assert(heap_lock_held_for_gc(),
-              "the heap lock should already be held by or for this thread");
-  return  (g1_policy()->in_young_gc_mode() &&
-           g1_policy()->should_add_next_region_to_young_list());
-}
-
 void G1CollectedHeap::set_region_short_lived_locked(HeapRegion* hr) {
   assert(heap_lock_held_for_gc(),
               "the heap lock should already be held by or for this thread");
--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp	Mon Dec 06 15:37:00 2010 -0500
@@ -290,6 +290,63 @@
   // started is maintained in _total_full_collections in CollectedHeap.
   volatile unsigned int _full_collections_completed;
 
+  // These are macros so that, if the assert fires, we get the correct
+  // line number, file, etc.
+
+#define heap_locking_asserts_err_msg(__extra_message)                         \
+  err_msg("%s : Heap_lock %slocked, %sat a safepoint",                        \
+          (__extra_message),                                                  \
+          (!Heap_lock->owned_by_self()) ? "NOT " : "",                        \
+          (!SafepointSynchronize::is_at_safepoint()) ? "NOT " : "")
+
+#define assert_heap_locked()                                                  \
+  do {                                                                        \
+    assert(Heap_lock->owned_by_self(),                                        \
+           heap_locking_asserts_err_msg("should be holding the Heap_lock"));  \
+  } while (0)
+
+#define assert_heap_locked_or_at_safepoint()                                  \
+  do {                                                                        \
+    assert(Heap_lock->owned_by_self() ||                                      \
+                                     SafepointSynchronize::is_at_safepoint(), \
+           heap_locking_asserts_err_msg("should be holding the Heap_lock or " \
+                                        "should be at a safepoint"));         \
+  } while (0)
+
+#define assert_heap_locked_and_not_at_safepoint()                             \
+  do {                                                                        \
+    assert(Heap_lock->owned_by_self() &&                                      \
+                                    !SafepointSynchronize::is_at_safepoint(), \
+          heap_locking_asserts_err_msg("should be holding the Heap_lock and " \
+                                       "should not be at a safepoint"));      \
+  } while (0)
+
+#define assert_heap_not_locked()                                              \
+  do {                                                                        \
+    assert(!Heap_lock->owned_by_self(),                                       \
+        heap_locking_asserts_err_msg("should not be holding the Heap_lock")); \
+  } while (0)
+
+#define assert_heap_not_locked_and_not_at_safepoint()                         \
+  do {                                                                        \
+    assert(!Heap_lock->owned_by_self() &&                                     \
+                                    !SafepointSynchronize::is_at_safepoint(), \
+      heap_locking_asserts_err_msg("should not be holding the Heap_lock and " \
+                                   "should not be at a safepoint"));          \
+  } while (0)
+
+#define assert_at_safepoint()                                                 \
+  do {                                                                        \
+    assert(SafepointSynchronize::is_at_safepoint(),                           \
+           heap_locking_asserts_err_msg("should be at a safepoint"));         \
+  } while (0)
+
+#define assert_not_at_safepoint()                                             \
+  do {                                                                        \
+    assert(!SafepointSynchronize::is_at_safepoint(),                          \
+           heap_locking_asserts_err_msg("should not be at a safepoint"));     \
+  } while (0)
+
 protected:
 
   // Returns "true" iff none of the gc alloc regions have any allocations
@@ -329,31 +386,162 @@
 
   // Attempt to allocate an object of the given (very large) "word_size".
   // Returns "NULL" on failure.
-  virtual HeapWord* humongousObjAllocate(size_t word_size);
+  virtual HeapWord* humongous_obj_allocate(size_t word_size);
 
-  // If possible, allocate a block of the given word_size, else return "NULL".
-  // Returning NULL will trigger GC or heap expansion.
-  // These two methods have rather awkward pre- and
-  // post-conditions. If they are called outside a safepoint, then
-  // they assume that the caller is holding the heap lock. Upon return
-  // they release the heap lock, if they are returning a non-NULL
-  // value. attempt_allocation_slow() also dirties the cards of a
-  // newly-allocated young region after it releases the heap
-  // lock. This change in interface was the neatest way to achieve
-  // this card dirtying without affecting mem_allocate(), which is a
-  // more frequently called method. We tried two or three different
-  // approaches, but they were even more hacky.
-  HeapWord* attempt_allocation(size_t word_size,
-                               bool permit_collection_pause = true);
+  // The following two methods, allocate_new_tlab() and
+  // mem_allocate(), are the two main entry points from the runtime
+  // into the G1's allocation routines. They have the following
+  // assumptions:
+  //
+  // * They should both be called outside safepoints.
+  //
+  // * They should both be called without holding the Heap_lock.
+  //
+  // * All allocation requests for new TLABs should go to
+  //   allocate_new_tlab().
+  //
+  // * All non-TLAB allocation requests should go to mem_allocate()
+  //   and mem_allocate() should never be called with is_tlab == true.
+  //
+  // * If the GC locker is active we currently stall until we can
+  //   allocate a new young region. This will be changed in the
+  //   near future (see CR 6994056).
+  //
+  // * If either call cannot satisfy the allocation request using the
+  //   current allocating region, they will try to get a new one. If
+  //   this fails, they will attempt to do an evacuation pause and
+  //   retry the allocation.
+  //
+  // * If all allocation attempts fail, even after trying to schedule
+  //   an evacuation pause, allocate_new_tlab() will return NULL,
+  //   whereas mem_allocate() will attempt a heap expansion and/or
+  //   schedule a Full GC.
+  //
+  // * We do not allow humongous-sized TLABs. So, allocate_new_tlab
+  //   should never be called with word_size being humongous. All
+  //   humongous allocation requests should go to mem_allocate() which
+  //   will satisfy them with a special path.
 
-  HeapWord* attempt_allocation_slow(size_t word_size,
-                                    bool permit_collection_pause = true);
+  virtual HeapWord* allocate_new_tlab(size_t word_size);
+
+  virtual HeapWord* mem_allocate(size_t word_size,
+                                 bool   is_noref,
+                                 bool   is_tlab, /* expected to be false */
+                                 bool*  gc_overhead_limit_was_exceeded);
+
+  // The following methods, allocate_from_cur_allocation_region(),
+  // attempt_allocation(), replace_cur_alloc_region_and_allocate(),
+  // attempt_allocation_slow(), and attempt_allocation_humongous()
+  // have very awkward pre- and post-conditions with respect to
+  // locking:
+  //
+  // If they are called outside a safepoint they assume the caller
+  // holds the Heap_lock when it calls them. However, on exit they
+  // will release the Heap_lock if they return a non-NULL result, but
+  // keep holding the Heap_lock if they return a NULL result. The
+  // reason for this is that we need to dirty the cards that span
+  // allocated blocks on young regions to avoid having to take the
+  // slow path of the write barrier (for performance reasons we don't
+  // update RSets for references whose source is a young region, so we
+  // don't need to look at dirty cards on young regions). But, doing
+  // this card dirtying while holding the Heap_lock can be a
+  // scalability bottleneck, especially given that some allocation
+  // requests might be of non-trivial size (and the larger the region
+  // size is, the fewer allocations requests will be considered
+  // humongous, as the humongous size limit is a fraction of the
+  // region size). So, when one of these calls succeeds in allocating
+  // a block it does the card dirtying after it releases the Heap_lock
+  // which is why it will return without holding it.
+  //
+  // The above assymetry is the reason why locking / unlocking is done
+  // explicitly (i.e., with Heap_lock->lock() and
+  // Heap_lock->unlocked()) instead of using MutexLocker and
+  // MutexUnlocker objects. The latter would ensure that the lock is
+  // unlocked / re-locked at every possible exit out of the basic
+  // block. However, we only want that action to happen in selected
+  // places.
+  //
+  // Further, if the above methods are called during a safepoint, then
+  // naturally there's no assumption about the Heap_lock being held or
+  // there's no attempt to unlock it. The parameter at_safepoint
+  // indicates whether the call is made during a safepoint or not (as
+  // an optimization, to avoid reading the global flag with
+  // SafepointSynchronize::is_at_safepoint()).
+  //
+  // The methods share these parameters:
+  //
+  // * word_size     : the size of the allocation request in words
+  // * at_safepoint  : whether the call is done at a safepoint; this
+  //                   also determines whether a GC is permitted
+  //                   (at_safepoint == false) or not (at_safepoint == true)
+  // * do_dirtying   : whether the method should dirty the allocated
+  //                   block before returning
+  //
+  // They all return either the address of the block, if they
+  // successfully manage to allocate it, or NULL.
+
+  // It tries to satisfy an allocation request out of the current
+  // allocating region, which is passed as a parameter. It assumes
+  // that the caller has checked that the current allocating region is
+  // not NULL. Given that the caller has to check the current
+  // allocating region for at least NULL, it might as well pass it as
+  // the first parameter so that the method doesn't have to read it
+  // from the _cur_alloc_region field again.
+  inline HeapWord* allocate_from_cur_alloc_region(HeapRegion* cur_alloc_region,
+                                                  size_t word_size);
+
+  // It attempts to allocate out of the current alloc region. If that
+  // fails, it retires the current alloc region (if there is one),
+  // tries to get a new one and retries the allocation.
+  inline HeapWord* attempt_allocation(size_t word_size);
+
+  // It assumes that the current alloc region has been retired and
+  // tries to allocate a new one. If it's successful, it performs
+  // the allocation out of the new current alloc region and updates
+  // _cur_alloc_region.
+  HeapWord* replace_cur_alloc_region_and_allocate(size_t word_size,
+                                                  bool at_safepoint,
+                                                  bool do_dirtying);
+
+  // The slow path when we are unable to allocate a new current alloc
+  // region to satisfy an allocation request (i.e., when
+  // attempt_allocation() fails). It will try to do an evacuation
+  // pause, which might stall due to the GC locker, and retry the
+  // allocation attempt when appropriate.
+  HeapWord* attempt_allocation_slow(size_t word_size);
+
+  // The method that tries to satisfy a humongous allocation
+  // request. If it cannot satisfy it it will try to do an evacuation
+  // pause to perhaps reclaim enough space to be able to satisfy the
+  // allocation request afterwards.
+  HeapWord* attempt_allocation_humongous(size_t word_size,
+                                         bool at_safepoint);
+
+  // It does the common work when we are retiring the current alloc region.
+  inline void retire_cur_alloc_region_common(HeapRegion* cur_alloc_region);
+
+  // It retires the current alloc region, which is passed as a
+  // parameter (since, typically, the caller is already holding on to
+  // it). It sets _cur_alloc_region to NULL.
+  void retire_cur_alloc_region(HeapRegion* cur_alloc_region);
+
+  // It attempts to do an allocation immediately before or after an
+  // evacuation pause and can only be called by the VM thread. It has
+  // slightly different assumptions that the ones before (i.e.,
+  // assumes that the current alloc region has been retired).
+  HeapWord* attempt_allocation_at_safepoint(size_t word_size,
+                                            bool expect_null_cur_alloc_region);
+
+  // It dirties the cards that cover the block so that so that the post
+  // write barrier never queues anything when updating objects on this
+  // block. It is assumed (and in fact we assert) that the block
+  // belongs to a young region.
+  inline void dirty_young_block(HeapWord* start, size_t word_size);
 
   // Allocate blocks during garbage collection. Will ensure an
   // allocation region, either by picking one or expanding the
   // heap, and then allocate a block of the given size. The block
   // may not be a humongous - it must fit into a single heap region.
-  HeapWord* allocate_during_gc(GCAllocPurpose purpose, size_t word_size);
   HeapWord* par_allocate_during_gc(GCAllocPurpose purpose, size_t word_size);
 
   HeapWord* allocate_during_gc_slow(GCAllocPurpose purpose,
@@ -370,12 +558,14 @@
   void  retire_alloc_region(HeapRegion* alloc_region, bool par);
 
   // - if explicit_gc is true, the GC is for a System.gc() or a heap
-  // inspection request and should collect the entire heap
-  // - if clear_all_soft_refs is true, all soft references are cleared
-  // during the GC
+  //   inspection request and should collect the entire heap
+  // - if clear_all_soft_refs is true, all soft references should be
+  //   cleared during the GC
   // - if explicit_gc is false, word_size describes the allocation that
-  // the GC should attempt (at least) to satisfy
-  void do_collection(bool explicit_gc,
+  //   the GC should attempt (at least) to satisfy
+  // - it returns false if it is unable to do the collection due to the
+  //   GC locker being active, true otherwise
+  bool do_collection(bool explicit_gc,
                      bool clear_all_soft_refs,
                      size_t word_size);
 
@@ -391,13 +581,13 @@
   // Callback from VM_G1CollectForAllocation operation.
   // This function does everything necessary/possible to satisfy a
   // failed allocation request (including collection, expansion, etc.)
-  HeapWord* satisfy_failed_allocation(size_t word_size);
+  HeapWord* satisfy_failed_allocation(size_t word_size, bool* succeeded);
 
   // Attempting to expand the heap sufficiently
   // to support an allocation of the given "word_size".  If
   // successful, perform the allocation and return the address of the
   // allocated block, or else "NULL".
-  virtual HeapWord* expand_and_allocate(size_t word_size);
+  HeapWord* expand_and_allocate(size_t word_size);
 
 public:
   // Expand the garbage-first heap by at least the given size (in bytes!).
@@ -478,21 +668,27 @@
   void reset_taskqueue_stats();
   #endif // TASKQUEUE_STATS
 
-  // Do an incremental collection: identify a collection set, and evacuate
-  // its live objects elsewhere.
-  virtual void do_collection_pause();
+  // Schedule the VM operation that will do an evacuation pause to
+  // satisfy an allocation request of word_size. *succeeded will
+  // return whether the VM operation was successful (it did do an
+  // evacuation pause) or not (another thread beat us to it or the GC
+  // locker was active). Given that we should not be holding the
+  // Heap_lock when we enter this method, we will pass the
+  // gc_count_before (i.e., total_collections()) as a parameter since
+  // it has to be read while holding the Heap_lock. Currently, both
+  // methods that call do_collection_pause() release the Heap_lock
+  // before the call, so it's easy to read gc_count_before just before.
+  HeapWord* do_collection_pause(size_t       word_size,
+                                unsigned int gc_count_before,
+                                bool*        succeeded);
 
   // The guts of the incremental collection pause, executed by the vm
-  // thread.
-  virtual void do_collection_pause_at_safepoint(double target_pause_time_ms);
+  // thread. It returns false if it is unable to do the collection due
+  // to the GC locker being active, true otherwise
+  bool do_collection_pause_at_safepoint(double target_pause_time_ms);
 
   // Actually do the work of evacuating the collection set.
-  virtual void evacuate_collection_set();
-
-  // If this is an appropriate right time, do a collection pause.
-  // The "word_size" argument, if non-zero, indicates the size of an
-  // allocation request that is prompting this query.
-  void do_collection_pause_if_appropriate(size_t word_size);
+  void evacuate_collection_set();
 
   // The g1 remembered set of the heap.
   G1RemSet* _g1_rem_set;
@@ -762,11 +958,6 @@
 #endif // PRODUCT
 
   // These virtual functions do the actual allocation.
-  virtual HeapWord* mem_allocate(size_t word_size,
-                                 bool   is_noref,
-                                 bool   is_tlab,
-                                 bool* gc_overhead_limit_was_exceeded);
-
   // Some heaps may offer a contiguous region for shared non-blocking
   // allocation, via inlined code (by exporting the address of the top and
   // end fields defining the extent of the contiguous allocation region.)
@@ -1046,7 +1237,6 @@
   virtual bool supports_tlab_allocation() const;
   virtual size_t tlab_capacity(Thread* thr) const;
   virtual size_t unsafe_max_tlab_alloc(Thread* thr) const;
-  virtual HeapWord* allocate_new_tlab(size_t word_size);
 
   // Can a compiler initialize a new object without store barriers?
   // This permission only extends from the creation of a new object
@@ -1186,7 +1376,6 @@
   static G1CollectedHeap* heap();
 
   void empty_young_list();
-  bool should_set_young_locked();
 
   void set_region_short_lived_locked(HeapRegion* hr);
   // add appropriate methods for any other surv rate groups
@@ -1339,8 +1528,6 @@
 protected:
   size_t _max_heap_capacity;
 
-//  debug_only(static void check_for_valid_allocation_state();)
-
 public:
   // Temporary: call to mark things unimplemented for the G1 heap (e.g.,
   // MemoryService).  In productization, we can make this assert false
--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.inline.hpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.inline.hpp	Mon Dec 06 15:37:00 2010 -0500
@@ -27,6 +27,7 @@
 
 #include "gc_implementation/g1/concurrentMark.hpp"
 #include "gc_implementation/g1/g1CollectedHeap.hpp"
+#include "gc_implementation/g1/g1CollectorPolicy.hpp"
 #include "gc_implementation/g1/heapRegionSeq.hpp"
 #include "utilities/taskqueue.hpp"
 
@@ -58,37 +59,114 @@
   return r != NULL && r->in_collection_set();
 }
 
-inline HeapWord* G1CollectedHeap::attempt_allocation(size_t word_size,
-                                              bool permit_collection_pause) {
-  HeapWord* res = NULL;
+// See the comment in the .hpp file about the locking protocol and
+// assumptions of this method (and other related ones).
+inline HeapWord*
+G1CollectedHeap::allocate_from_cur_alloc_region(HeapRegion* cur_alloc_region,
+                                                size_t word_size) {
+  assert_heap_locked_and_not_at_safepoint();
+  assert(cur_alloc_region != NULL, "pre-condition of the method");
+  assert(cur_alloc_region == _cur_alloc_region, "pre-condition of the method");
+  assert(cur_alloc_region->is_young(),
+         "we only support young current alloc regions");
+  assert(!isHumongous(word_size), "allocate_from_cur_alloc_region() "
+         "should not be used for humongous allocations");
+  assert(!cur_alloc_region->isHumongous(), "Catch a regression of this bug.");
 
-  assert( SafepointSynchronize::is_at_safepoint() ||
-          Heap_lock->owned_by_self(), "pre-condition of the call" );
+  assert(!cur_alloc_region->is_empty(),
+         err_msg("region ["PTR_FORMAT","PTR_FORMAT"] should not be empty",
+                 cur_alloc_region->bottom(), cur_alloc_region->end()));
+  // This allocate method does BOT updates and we don't need them in
+  // the young generation. This will be fixed in the near future by
+  // CR 6994297.
+  HeapWord* result = cur_alloc_region->allocate(word_size);
+  if (result != NULL) {
+    assert(is_in(result), "result should be in the heap");
+    Heap_lock->unlock();
 
-  // All humongous allocation requests should go through the slow path in
-  // attempt_allocation_slow().
-  if (!isHumongous(word_size) && _cur_alloc_region != NULL) {
-    // If this allocation causes a region to become non empty,
-    // then we need to update our free_regions count.
+    // Do the dirtying after we release the Heap_lock.
+    dirty_young_block(result, word_size);
+    return result;
+  }
 
-    if (_cur_alloc_region->is_empty()) {
-      res = _cur_alloc_region->allocate(word_size);
-      if (res != NULL)
-        _free_regions--;
-    } else {
-      res = _cur_alloc_region->allocate(word_size);
+  assert_heap_locked();
+  return NULL;
+}
+
+// See the comment in the .hpp file about the locking protocol and
+// assumptions of this method (and other related ones).
+inline HeapWord*
+G1CollectedHeap::attempt_allocation(size_t word_size) {
+  assert_heap_locked_and_not_at_safepoint();
+  assert(!isHumongous(word_size), "attempt_allocation() should not be called "
+         "for humongous allocation requests");
+
+  HeapRegion* cur_alloc_region = _cur_alloc_region;
+  if (cur_alloc_region != NULL) {
+    HeapWord* result = allocate_from_cur_alloc_region(cur_alloc_region,
+                                                      word_size);
+    if (result != NULL) {
+      assert_heap_not_locked();
+      return result;
     }
 
-    if (res != NULL) {
-      if (!SafepointSynchronize::is_at_safepoint()) {
-        assert( Heap_lock->owned_by_self(), "invariant" );
-        Heap_lock->unlock();
-      }
-      return res;
-    }
+    assert_heap_locked();
+
+    // Since we couldn't successfully allocate into it, retire the
+    // current alloc region.
+    retire_cur_alloc_region(cur_alloc_region);
   }
-  // attempt_allocation_slow will also unlock the heap lock when appropriate.
-  return attempt_allocation_slow(word_size, permit_collection_pause);
+
+  // Try to get a new region and allocate out of it
+  HeapWord* result = replace_cur_alloc_region_and_allocate(word_size,
+                                                      false, /* at safepoint */
+                                                      true   /* do_dirtying */);
+  if (result != NULL) {
+    assert_heap_not_locked();
+    return result;
+  }
+
+  assert_heap_locked();
+  return NULL;
+}
+
+inline void
+G1CollectedHeap::retire_cur_alloc_region_common(HeapRegion* cur_alloc_region) {
+  assert_heap_locked_or_at_safepoint();
+  assert(cur_alloc_region != NULL && cur_alloc_region == _cur_alloc_region,
+         "pre-condition of the call");
+  assert(cur_alloc_region->is_young(),
+         "we only support young current alloc regions");
+
+  // The region is guaranteed to be young
+  g1_policy()->add_region_to_incremental_cset_lhs(cur_alloc_region);
+  _summary_bytes_used += cur_alloc_region->used();
+  _cur_alloc_region = NULL;
+}
+
+// It dirties the cards that cover the block so that so that the post
+// write barrier never queues anything when updating objects on this
+// block. It is assumed (and in fact we assert) that the block
+// belongs to a young region.
+inline void
+G1CollectedHeap::dirty_young_block(HeapWord* start, size_t word_size) {
+  assert_heap_not_locked();
+
+  // Assign the containing region to containing_hr so that we don't
+  // have to keep calling heap_region_containing_raw() in the
+  // asserts below.
+  DEBUG_ONLY(HeapRegion* containing_hr = heap_region_containing_raw(start);)
+  assert(containing_hr != NULL && start != NULL && word_size > 0,
+         "pre-condition");
+  assert(containing_hr->is_in(start), "it should contain start");
+  assert(containing_hr->is_young(), "it should be young");
+  assert(!containing_hr->isHumongous(), "it should not be humongous");
+
+  HeapWord* end = start + word_size;
+  assert(containing_hr->is_in(end - 1), "it should also contain end - 1");
+
+  MemRegion mr(start, end);
+  ((CardTableModRefBS*)_g1h->barrier_set())->dirty(mr);
 }
 
 inline RefToScanQueue* G1CollectedHeap::task_queue(int i) const {
--- a/src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp	Mon Dec 06 15:37:00 2010 -0500
@@ -458,8 +458,8 @@
     double now_sec = os::elapsedTime();
     double when_ms = _mmu_tracker->when_max_gc_sec(now_sec) * 1000.0;
     double alloc_rate_ms = predict_alloc_rate_ms();
-    int min_regions = (int) ceil(alloc_rate_ms * when_ms);
-    int current_region_num = (int) _g1->young_list()->length();
+    size_t min_regions = (size_t) ceil(alloc_rate_ms * when_ms);
+    size_t current_region_num = _g1->young_list()->length();
     _young_list_min_length = min_regions + current_region_num;
   }
 }
@@ -473,9 +473,12 @@
       _young_list_target_length = _young_list_fixed_length;
     else
       _young_list_target_length = _young_list_fixed_length / 2;
-
-    _young_list_target_length = MAX2(_young_list_target_length, (size_t)1);
   }
+
+  // Make sure we allow the application to allocate at least one
+  // region before we need to do a collection again.
+  size_t min_length = _g1->young_list()->length() + 1;
+  _young_list_target_length = MAX2(_young_list_target_length, min_length);
   calculate_survivors_policy();
 }
 
@@ -568,7 +571,7 @@
 
     // we should have at least one region in the target young length
     _young_list_target_length =
-        MAX2((size_t) 1, final_young_length + _recorded_survivor_regions);
+                              final_young_length + _recorded_survivor_regions;
 
     // let's keep an eye of how long we spend on this calculation
     // right now, I assume that we'll print it when we need it; we
@@ -617,8 +620,7 @@
                            _young_list_min_length);
 #endif // TRACE_CALC_YOUNG_LENGTH
     // we'll do the pause as soon as possible by choosing the minimum
-    _young_list_target_length =
-      MAX2(_young_list_min_length, (size_t) 1);
+    _young_list_target_length = _young_list_min_length;
   }
 
   _rs_lengths_prediction = rs_lengths;
@@ -801,7 +803,7 @@
   _survivor_surv_rate_group->reset();
   calculate_young_list_min_length();
   calculate_young_list_target_length();
- }
+}
 
 void G1CollectorPolicy::record_before_bytes(size_t bytes) {
   _bytes_in_to_space_before_gc += bytes;
@@ -824,9 +826,9 @@
       gclog_or_tty->print(" (%s)", full_young_gcs() ? "young" : "partial");
   }
 
-  assert(_g1->used_regions() == _g1->recalculate_used_regions(),
-         "sanity");
-  assert(_g1->used() == _g1->recalculate_used(), "sanity");
+  assert(_g1->used() == _g1->recalculate_used(),
+         err_msg("sanity, used: "SIZE_FORMAT" recalculate_used: "SIZE_FORMAT,
+                 _g1->used(), _g1->recalculate_used()));
 
   double s_w_t_ms = (start_time_sec - _stop_world_start) * 1000.0;
   _all_stop_world_times_ms->add(s_w_t_ms);
@@ -2266,24 +2268,13 @@
 #endif // PRODUCT
 }
 
-bool
-G1CollectorPolicy::should_add_next_region_to_young_list() {
-  assert(in_young_gc_mode(), "should be in young GC mode");
-  bool ret;
-  size_t young_list_length = _g1->young_list()->length();
-  size_t young_list_max_length = _young_list_target_length;
-  if (G1FixedEdenSize) {
-    young_list_max_length -= _max_survivor_regions;
-  }
-  if (young_list_length < young_list_max_length) {
-    ret = true;
+void
+G1CollectorPolicy::update_region_num(bool young) {
+  if (young) {
     ++_region_num_young;
   } else {
-    ret = false;
     ++_region_num_tenured;
   }
-
-  return ret;
 }
 
 #ifndef PRODUCT
@@ -2327,32 +2318,6 @@
   }
 }
 
-bool
-G1CollectorPolicy_BestRegionsFirst::should_do_collection_pause(size_t
-                                                               word_size) {
-  assert(_g1->regions_accounted_for(), "Region leakage!");
-  double max_pause_time_ms = _mmu_tracker->max_gc_time() * 1000.0;
-
-  size_t young_list_length = _g1->young_list()->length();
-  size_t young_list_max_length = _young_list_target_length;
-  if (G1FixedEdenSize) {
-    young_list_max_length -= _max_survivor_regions;
-  }
-  bool reached_target_length = young_list_length >= young_list_max_length;
-
-  if (in_young_gc_mode()) {
-    if (reached_target_length) {
-      assert( young_list_length > 0 && _g1->young_list()->length() > 0,
-              "invariant" );
-      return true;
-    }
-  } else {
-    guarantee( false, "should not reach here" );
-  }
-
-  return false;
-}
-
 #ifndef PRODUCT
 class HRSortIndexIsOKClosure: public HeapRegionClosure {
   CollectionSetChooser* _chooser;
--- a/src/share/vm/gc_implementation/g1/g1CollectorPolicy.hpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/g1CollectorPolicy.hpp	Mon Dec 06 15:37:00 2010 -0500
@@ -993,11 +993,6 @@
   void record_before_bytes(size_t bytes);
   void record_after_bytes(size_t bytes);
 
-  // Returns "true" if this is a good time to do a collection pause.
-  // The "word_size" argument, if non-zero, indicates the size of an
-  // allocation request that is prompting this query.
-  virtual bool should_do_collection_pause(size_t word_size) = 0;
-
   // Choose a new collection set.  Marks the chosen regions as being
   // "in_collection_set", and links them together.  The head and number of
   // the collection set are available via access methods.
@@ -1116,7 +1111,16 @@
     // do that for any other surv rate groups
   }
 
-  bool should_add_next_region_to_young_list();
+  bool is_young_list_full() {
+    size_t young_list_length = _g1->young_list()->length();
+    size_t young_list_max_length = _young_list_target_length;
+    if (G1FixedEdenSize) {
+      young_list_max_length -= _max_survivor_regions;
+    }
+
+    return young_list_length >= young_list_max_length;
+  }
+  void update_region_num(bool young);
 
   bool in_young_gc_mode() {
     return _in_young_gc_mode;
@@ -1270,7 +1274,6 @@
     _collectionSetChooser = new CollectionSetChooser();
   }
   void record_collection_pause_end();
-  bool should_do_collection_pause(size_t word_size);
   // This is not needed any more, after the CSet choosing code was
   // changed to use the pause prediction work. But let's leave the
   // hook in just in case.
--- a/src/share/vm/gc_implementation/g1/vm_operations_g1.cpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/vm_operations_g1.cpp	Mon Dec 06 15:37:00 2010 -0500
@@ -27,13 +27,22 @@
 #include "gc_implementation/g1/g1CollectorPolicy.hpp"
 #include "gc_implementation/g1/vm_operations_g1.hpp"
 #include "gc_implementation/shared/isGCActiveMark.hpp"
+#include "gc_implementation/g1/vm_operations_g1.hpp"
 #include "runtime/interfaceSupport.hpp"
 
+VM_G1CollectForAllocation::VM_G1CollectForAllocation(
+                                                  unsigned int gc_count_before,
+                                                  size_t word_size)
+  : VM_G1OperationWithAllocRequest(gc_count_before, word_size) {
+  guarantee(word_size > 0, "an allocation should always be requested");
+}
+
 void VM_G1CollectForAllocation::doit() {
   JvmtiGCForAllocationMarker jgcm;
   G1CollectedHeap* g1h = G1CollectedHeap::heap();
-  _res = g1h->satisfy_failed_allocation(_size);
-  assert(g1h->is_in_or_null(_res), "result not in heap");
+  _result = g1h->satisfy_failed_allocation(_word_size, &_pause_succeeded);
+  assert(_result == NULL || _pause_succeeded,
+         "if we get back a result, the pause should have succeeded");
 }
 
 void VM_G1CollectFull::doit() {
@@ -43,6 +52,25 @@
   g1h->do_full_collection(false /* clear_all_soft_refs */);
 }
 
+VM_G1IncCollectionPause::VM_G1IncCollectionPause(
+                                      unsigned int   gc_count_before,
+                                      size_t         word_size,
+                                      bool           should_initiate_conc_mark,
+                                      double         target_pause_time_ms,
+                                      GCCause::Cause gc_cause)
+  : VM_G1OperationWithAllocRequest(gc_count_before, word_size),
+    _should_initiate_conc_mark(should_initiate_conc_mark),
+    _target_pause_time_ms(target_pause_time_ms),
+    _full_collections_completed_before(0) {
+  guarantee(target_pause_time_ms > 0.0,
+            err_msg("target_pause_time_ms = %1.6lf should be positive",
+                    target_pause_time_ms));
+  guarantee(word_size == 0 || gc_cause == GCCause::_g1_inc_collection_pause,
+            "we can only request an allocation if the GC cause is for "
+            "an incremental GC pause");
+  _gc_cause = gc_cause;
+}
+
 void VM_G1IncCollectionPause::doit() {
   JvmtiGCForAllocationMarker jgcm;
   G1CollectedHeap* g1h = G1CollectedHeap::heap();
@@ -51,6 +79,18 @@
    (_gc_cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent)),
          "only a GC locker or a System.gc() induced GC should start a cycle");
 
+  if (_word_size > 0) {
+    // An allocation has been requested. So, try to do that first.
+    _result = g1h->attempt_allocation_at_safepoint(_word_size,
+                                     false /* expect_null_cur_alloc_region */);
+    if (_result != NULL) {
+      // If we can successfully allocate before we actually do the
+      // pause then we will consider this pause successful.
+      _pause_succeeded = true;
+      return;
+    }
+  }
+
   GCCauseSetter x(g1h, _gc_cause);
   if (_should_initiate_conc_mark) {
     // It's safer to read full_collections_completed() here, given
@@ -63,7 +103,16 @@
     // will do so if one is not already in progress.
     bool res = g1h->g1_policy()->force_initial_mark_if_outside_cycle();
   }
-  g1h->do_collection_pause_at_safepoint(_target_pause_time_ms);
+
+  _pause_succeeded =
+    g1h->do_collection_pause_at_safepoint(_target_pause_time_ms);
+  if (_pause_succeeded && _word_size > 0) {
+    // An allocation had been requested.
+    _result = g1h->attempt_allocation_at_safepoint(_word_size,
+                                      true /* expect_null_cur_alloc_region */);
+  } else {
+    assert(_result == NULL, "invariant");
+  }
 }
 
 void VM_G1IncCollectionPause::doit_epilogue() {
--- a/src/share/vm/gc_implementation/g1/vm_operations_g1.hpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/gc_implementation/g1/vm_operations_g1.hpp	Mon Dec 06 15:37:00 2010 -0500
@@ -31,19 +31,33 @@
 // VM_GC_Operation:
 //   - VM_CGC_Operation
 //   - VM_G1CollectFull
-//   - VM_G1CollectForAllocation
-//   - VM_G1IncCollectionPause
-//   - VM_G1PopRegionCollectionPause
+//   - VM_G1OperationWithAllocRequest
+//     - VM_G1CollectForAllocation
+//     - VM_G1IncCollectionPause
+
+class VM_G1OperationWithAllocRequest: public VM_GC_Operation {
+protected:
+  size_t    _word_size;
+  HeapWord* _result;
+  bool      _pause_succeeded;
+
+public:
+  VM_G1OperationWithAllocRequest(unsigned int gc_count_before,
+                                 size_t       word_size)
+    : VM_GC_Operation(gc_count_before),
+      _word_size(word_size), _result(NULL), _pause_succeeded(false) { }
+  HeapWord* result() { return _result; }
+  bool pause_succeeded() { return _pause_succeeded; }
+};
 
 class VM_G1CollectFull: public VM_GC_Operation {
- public:
+public:
   VM_G1CollectFull(unsigned int gc_count_before,
                    unsigned int full_gc_count_before,
                    GCCause::Cause cause)
     : VM_GC_Operation(gc_count_before, full_gc_count_before) {
     _gc_cause = cause;
   }
-  ~VM_G1CollectFull() {}
   virtual VMOp_Type type() const { return VMOp_G1CollectFull; }
   virtual void doit();
   virtual const char* name() const {
@@ -51,45 +65,28 @@
   }
 };
 
-class VM_G1CollectForAllocation: public VM_GC_Operation {
- private:
-  HeapWord*   _res;
-  size_t      _size;                       // size of object to be allocated
- public:
-  VM_G1CollectForAllocation(size_t size, int gc_count_before)
-    : VM_GC_Operation(gc_count_before) {
-    _size        = size;
-    _res         = NULL;
-  }
-  ~VM_G1CollectForAllocation()        {}
+class VM_G1CollectForAllocation: public VM_G1OperationWithAllocRequest {
+public:
+  VM_G1CollectForAllocation(unsigned int gc_count_before,
+                            size_t       word_size);
   virtual VMOp_Type type() const { return VMOp_G1CollectForAllocation; }
   virtual void doit();
   virtual const char* name() const {
     return "garbage-first collection to satisfy allocation";
   }
-  HeapWord* result() { return _res; }
 };
 
-class VM_G1IncCollectionPause: public VM_GC_Operation {
+class VM_G1IncCollectionPause: public VM_G1OperationWithAllocRequest {
 private:
-  bool _should_initiate_conc_mark;
-  double _target_pause_time_ms;
+  bool         _should_initiate_conc_mark;
+  double       _target_pause_time_ms;
   unsigned int _full_collections_completed_before;
 public:
   VM_G1IncCollectionPause(unsigned int   gc_count_before,
+                          size_t         word_size,
                           bool           should_initiate_conc_mark,
                           double         target_pause_time_ms,
-                          GCCause::Cause cause)
-    : VM_GC_Operation(gc_count_before),
-      _full_collections_completed_before(0),
-      _should_initiate_conc_mark(should_initiate_conc_mark),
-      _target_pause_time_ms(target_pause_time_ms) {
-    guarantee(target_pause_time_ms > 0.0,
-              err_msg("target_pause_time_ms = %1.6lf should be positive",
-                      target_pause_time_ms));
-
-    _gc_cause = cause;
-  }
+                          GCCause::Cause gc_cause);
   virtual VMOp_Type type() const { return VMOp_G1IncCollectionPause; }
   virtual void doit();
   virtual void doit_epilogue();
@@ -103,14 +100,9 @@
 class VM_CGC_Operation: public VM_Operation {
   VoidClosure* _cl;
   const char* _printGCMessage;
- public:
-  VM_CGC_Operation(VoidClosure* cl, const char *printGCMsg) :
-    _cl(cl),
-    _printGCMessage(printGCMsg)
-    {}
-
-  ~VM_CGC_Operation() {}
-
+public:
+  VM_CGC_Operation(VoidClosure* cl, const char *printGCMsg)
+    : _cl(cl), _printGCMessage(printGCMsg) { }
   virtual VMOp_Type type() const { return VMOp_CGC_Operation; }
   virtual void doit();
   virtual bool doit_prologue();
--- a/src/share/vm/memory/referenceProcessor.cpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/memory/referenceProcessor.cpp	Mon Dec 06 15:37:00 2010 -0500
@@ -770,9 +770,8 @@
   // loop over the lists
   for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
-      gclog_or_tty->print_cr(
-        "\nAbandoning %s discovered list",
-        list_name(i));
+      gclog_or_tty->print_cr("\nAbandoning %s discovered list",
+                             list_name(i));
     }
     abandon_partial_discovered_list(_discoveredSoftRefs[i]);
   }
@@ -1059,9 +1058,7 @@
     // During a multi-threaded discovery phase,
     // each thread saves to its "own" list.
     Thread* thr = Thread::current();
-    assert(thr->is_GC_task_thread(),
-           "Dubious cast from Thread* to WorkerThread*?");
-    id = ((WorkerThread*)thr)->id();
+    id = thr->as_Worker_thread()->id();
   } else {
     // single-threaded discovery, we save in round-robin
     // fashion to each of the lists.
@@ -1095,8 +1092,7 @@
       ShouldNotReachHere();
   }
   if (TraceReferenceGC && PrintGCDetails) {
-    gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT,
-      id, list);
+    gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
   }
   return list;
 }
@@ -1135,6 +1131,11 @@
     if (_discovered_list_needs_barrier) {
       _bs->write_ref_field((void*)discovered_addr, current_head);
     }
+
+    if (TraceReferenceGC) {
+      gclog_or_tty->print_cr("Enqueued reference (mt) (" INTPTR_FORMAT ": %s)",
+                             obj, obj->blueprint()->internal_name());
+    }
   } else {
     // If retest was non NULL, another thread beat us to it:
     // The reference has already been discovered...
@@ -1239,8 +1240,8 @@
       // Check assumption that an object is not potentially
       // discovered twice except by concurrent collectors that potentially
       // trace the same Reference object twice.
-      assert(UseConcMarkSweepGC,
-             "Only possible with an incremental-update concurrent collector");
+      assert(UseConcMarkSweepGC || UseG1GC,
+             "Only possible with a concurrent marking collector");
       return true;
     }
   }
@@ -1293,26 +1294,14 @@
     }
     list->set_head(obj);
     list->inc_length(1);
-  }
 
-  // In the MT discovery case, it is currently possible to see
-  // the following message multiple times if several threads
-  // discover a reference about the same time. Only one will
-  // however have actually added it to the disocvered queue.
-  // One could let add_to_discovered_list_mt() return an
-  // indication for success in queueing (by 1 thread) or
-  // failure (by all other threads), but I decided the extra
-  // code was not worth the effort for something that is
-  // only used for debugging support.
-  if (TraceReferenceGC) {
-    oop referent = java_lang_ref_Reference::referent(obj);
-    if (PrintGCDetails) {
+    if (TraceReferenceGC) {
       gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",
-                             obj, obj->blueprint()->internal_name());
+                                obj, obj->blueprint()->internal_name());
     }
-    assert(referent->is_oop(), "Enqueued a bad referent");
   }
   assert(obj->is_oop(), "Enqueued a bad reference");
+  assert(java_lang_ref_Reference::referent(obj)->is_oop(), "Enqueued a bad referent");
   return true;
 }
 
--- a/src/share/vm/runtime/thread.hpp	Sat Dec 04 00:09:05 2010 -0500
+++ b/src/share/vm/runtime/thread.hpp	Mon Dec 06 15:37:00 2010 -0500
@@ -78,6 +78,8 @@
 class ThreadClosure;
 class IdealGraphPrinter;
 
+class WorkerThread;
+
 // Class hierarchy
 // - Thread
 //   - NamedThread
@@ -289,6 +291,10 @@
   virtual bool is_Watcher_thread() const             { return false; }
   virtual bool is_ConcurrentGC_thread() const        { return false; }
   virtual bool is_Named_thread() const               { return false; }
+  virtual bool is_Worker_thread() const              { return false; }
+
+  // Casts
+  virtual WorkerThread* as_Worker_thread() const     { return NULL; }
 
   virtual char* name() const { return (char*)"Unknown thread"; }
 
@@ -628,9 +634,16 @@
 private:
   uint _id;
 public:
-  WorkerThread() : _id(0) { }
-  void set_id(uint work_id) { _id = work_id; }
-  uint id() const { return _id; }
+  WorkerThread() : _id(0)               { }
+  virtual bool is_Worker_thread() const { return true; }
+
+  virtual WorkerThread* as_Worker_thread() const {
+    assert(is_Worker_thread(), "Dubious cast to WorkerThread*?");
+    return (WorkerThread*) this;
+  }
+
+  void set_id(uint work_id)             { _id = work_id; }
+  uint id() const                       { return _id; }
 };
 
 // A single WatcherThread is used for simulating timer interrupts.