--- a/hotspot/src/share/vm/gc_implementation/g1/concurrentMarkThread.cpp	Wed Dec 15 23:38:10 2010 -0800
+++ b/hotspot/src/share/vm/gc_implementation/g1/concurrentMarkThread.cpp	Thu Dec 16 09:14:34 2010 -0800
@@ -277,7 +277,9 @@
     // completed. This will also notify the FullGCCount_lock in case a
     // Java thread is waiting for a full GC to happen (e.g., it
     // called System.gc() with +ExplicitGCInvokesConcurrent).
-    g1->increment_full_collections_completed(true /* outer */);
+    _sts.join();
+    g1->increment_full_collections_completed(true /* concurrent */);
+    _sts.leave();
   }
   assert(_should_terminate, "just checking");
 

--- a/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Wed Dec 15 23:38:10 2010 -0800
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Thu Dec 16 09:14:34 2010 -0800
@@ -1389,7 +1389,7 @@
   }
 
   // Update the number of full collections that have been completed.
-  increment_full_collections_completed(false /* outer */);
+  increment_full_collections_completed(false /* concurrent */);
 
   if (PrintHeapAtGC) {
     Universe::print_heap_after_gc();
@@ -2176,9 +2176,14 @@
      (cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent));
 }
 
-void G1CollectedHeap::increment_full_collections_completed(bool outer) {
+void G1CollectedHeap::increment_full_collections_completed(bool concurrent) {
   MonitorLockerEx x(FullGCCount_lock, Mutex::_no_safepoint_check_flag);
 
+  // We assume that if concurrent == true, then the caller is a
+  // concurrent thread that was joined the Suspendible Thread
+  // Set. If there's ever a cheap way to check this, we should add an
+  // assert here.
+
   // We have already incremented _total_full_collections at the start
   // of the GC, so total_full_collections() represents how many full
   // collections have been started.
@@ -2192,17 +2197,18 @@
   // behind the number of full collections started.
 
   // This is the case for the inner caller, i.e. a Full GC.
-  assert(outer ||
+  assert(concurrent ||
          (full_collections_started == _full_collections_completed + 1) ||
          (full_collections_started == _full_collections_completed + 2),
-         err_msg("for inner caller: full_collections_started = %u "
+         err_msg("for inner caller (Full GC): full_collections_started = %u "
                  "is inconsistent with _full_collections_completed = %u",
                  full_collections_started, _full_collections_completed));
 
   // This is the case for the outer caller, i.e. the concurrent cycle.
-  assert(!outer ||
+  assert(!concurrent ||
          (full_collections_started == _full_collections_completed + 1),
-         err_msg("for outer caller: full_collections_started = %u "
+         err_msg("for outer caller (concurrent cycle): "
+                 "full_collections_started = %u "
                  "is inconsistent with _full_collections_completed = %u",
                  full_collections_started, _full_collections_completed));
 
@@ -2212,7 +2218,7 @@
   // we wake up any waiters (especially when ExplicitInvokesConcurrent
   // is set) so that if a waiter requests another System.gc() it doesn't
   // incorrectly see that a marking cyle is still in progress.
-  if (outer) {
+  if (concurrent) {
     _cmThread->clear_in_progress();
   }
 

--- a/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp	Wed Dec 15 23:38:10 2010 -0800
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp	Thu Dec 16 09:14:34 2010 -0800
@@ -643,16 +643,16 @@
   // can happen in a nested fashion, i.e., we start a concurrent
   // cycle, a Full GC happens half-way through it which ends first,
   // and then the cycle notices that a Full GC happened and ends
-  // too. The outer parameter is a boolean to help us do a bit tighter
-  // consistency checking in the method. If outer is false, the caller
-  // is the inner caller in the nesting (i.e., the Full GC). If outer
-  // is true, the caller is the outer caller in this nesting (i.e.,
-  // the concurrent cycle). Further nesting is not currently
-  // supported. The end of the this call also notifies the
-  // FullGCCount_lock in case a Java thread is waiting for a full GC
-  // to happen (e.g., it called System.gc() with
+  // too. The concurrent parameter is a boolean to help us do a bit
+  // tighter consistency checking in the method. If concurrent is
+  // false, the caller is the inner caller in the nesting (i.e., the
+  // Full GC). If concurrent is true, the caller is the outer caller
+  // in this nesting (i.e., the concurrent cycle). Further nesting is
+  // not currently supported. The end of the this call also notifies
+  // the FullGCCount_lock in case a Java thread is waiting for a full
+  // GC to happen (e.g., it called System.gc() with
   // +ExplicitGCInvokesConcurrent).
-  void increment_full_collections_completed(bool outer);
+  void increment_full_collections_completed(bool concurrent);
 
   unsigned int full_collections_completed() {
     return _full_collections_completed;