--- a/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Thu Apr 28 15:29:18 2011 -0700
+++ b/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Fri Apr 29 12:40:49 2011 -0400
@@ -826,6 +826,14 @@
 void ConcurrentMark::checkpointRootsInitialPost() {
   G1CollectedHeap*   g1h = G1CollectedHeap::heap();
 
+  // If we force an overflow during remark, the remark operation will
+  // actually abort and we'll restart concurrent marking. If we always
+  // force an oveflow during remark we'll never actually complete the
+  // marking phase. So, we initilize this here, at the start of the
+  // cycle, so that at the remaining overflow number will decrease at
+  // every remark and we'll eventually not need to cause one.
+  force_overflow_stw()->init();
+
   // For each region note start of marking.
   NoteStartOfMarkHRClosure startcl;
   g1h->heap_region_iterate(&startcl);
@@ -893,27 +901,37 @@
 }
 
 /*
-   Notice that in the next two methods, we actually leave the STS
-   during the barrier sync and join it immediately afterwards. If we
-   do not do this, this then the following deadlock can occur: one
-   thread could be in the barrier sync code, waiting for the other
-   thread to also sync up, whereas another one could be trying to
-   yield, while also waiting for the other threads to sync up too.
-
-   Because the thread that does the sync barrier has left the STS, it
-   is possible to be suspended for a Full GC or an evacuation pause
-   could occur. This is actually safe, since the entering the sync
-   barrier is one of the last things do_marking_step() does, and it
-   doesn't manipulate any data structures afterwards.
-*/
+ * Notice that in the next two methods, we actually leave the STS
+ * during the barrier sync and join it immediately afterwards. If we
+ * do not do this, the following deadlock can occur: one thread could
+ * be in the barrier sync code, waiting for the other thread to also
+ * sync up, whereas another one could be trying to yield, while also
+ * waiting for the other threads to sync up too.
+ *
+ * Note, however, that this code is also used during remark and in
+ * this case we should not attempt to leave / enter the STS, otherwise
+ * we'll either hit an asseert (debug / fastdebug) or deadlock
+ * (product). So we should only leave / enter the STS if we are
+ * operating concurrently.
+ *
+ * Because the thread that does the sync barrier has left the STS, it
+ * is possible to be suspended for a Full GC or an evacuation pause
+ * could occur. This is actually safe, since the entering the sync
+ * barrier is one of the last things do_marking_step() does, and it
+ * doesn't manipulate any data structures afterwards.
+ */
 
 void ConcurrentMark::enter_first_sync_barrier(int task_num) {
   if (verbose_low())
     gclog_or_tty->print_cr("[%d] entering first barrier", task_num);
 
-  ConcurrentGCThread::stsLeave();
+  if (concurrent()) {
+    ConcurrentGCThread::stsLeave();
+  }
   _first_overflow_barrier_sync.enter();
-  ConcurrentGCThread::stsJoin();
+  if (concurrent()) {
+    ConcurrentGCThread::stsJoin();
+  }
   // at this point everyone should have synced up and not be doing any
   // more work
 
@@ -923,7 +941,12 @@
   // let task 0 do this
   if (task_num == 0) {
     // task 0 is responsible for clearing the global data structures
-    clear_marking_state();
+    // We should be here because of an overflow. During STW we should
+    // not clear the overflow flag since we rely on it being true when
+    // we exit this method to abort the pause and restart concurent
+    // marking.
+    clear_marking_state(concurrent() /* clear_overflow */);
+    force_overflow()->update();
 
     if (PrintGC) {
       gclog_or_tty->date_stamp(PrintGCDateStamps);
@@ -940,15 +963,45 @@
   if (verbose_low())
     gclog_or_tty->print_cr("[%d] entering second barrier", task_num);
 
-  ConcurrentGCThread::stsLeave();
+  if (concurrent()) {
+    ConcurrentGCThread::stsLeave();
+  }
   _second_overflow_barrier_sync.enter();
-  ConcurrentGCThread::stsJoin();
+  if (concurrent()) {
+    ConcurrentGCThread::stsJoin();
+  }
   // at this point everything should be re-initialised and ready to go
 
   if (verbose_low())
     gclog_or_tty->print_cr("[%d] leaving second barrier", task_num);
 }
 
+#ifndef PRODUCT
+void ForceOverflowSettings::init() {
+  _num_remaining = G1ConcMarkForceOverflow;
+  _force = false;
+  update();
+}
+
+void ForceOverflowSettings::update() {
+  if (_num_remaining > 0) {
+    _num_remaining -= 1;
+    _force = true;
+  } else {
+    _force = false;
+  }
+}
+
+bool ForceOverflowSettings::should_force() {
+  if (_force) {
+    _force = false;
+    return true;
+  } else {
+    return false;
+  }
+}
+#endif // !PRODUCT
+
 void ConcurrentMark::grayRoot(oop p) {
   HeapWord* addr = (HeapWord*) p;
   // We can't really check against _heap_start and _heap_end, since it
@@ -1117,6 +1170,7 @@
   _restart_for_overflow = false;
 
   size_t active_workers = MAX2((size_t) 1, parallel_marking_threads());
+  force_overflow_conc()->init();
   set_phase(active_workers, true /* concurrent */);
 
   CMConcurrentMarkingTask markingTask(this, cmThread());
@@ -2703,12 +2757,16 @@
 
 }
 
-void ConcurrentMark::clear_marking_state() {
+void ConcurrentMark::clear_marking_state(bool clear_overflow) {
   _markStack.setEmpty();
   _markStack.clear_overflow();
   _regionStack.setEmpty();
   _regionStack.clear_overflow();
-  clear_has_overflown();
+  if (clear_overflow) {
+    clear_has_overflown();
+  } else {
+    assert(has_overflown(), "pre-condition");
+  }
   _finger = _heap_start;
 
   for (int i = 0; i < (int)_max_task_num; ++i) {
@@ -4279,6 +4337,15 @@
     }
   }
 
+  // If we are about to wrap up and go into termination, check if we
+  // should raise the overflow flag.
+  if (do_termination && !has_aborted()) {
+    if (_cm->force_overflow()->should_force()) {
+      _cm->set_has_overflown();
+      regular_clock_call();
+    }
+  }
+
   // We still haven't aborted. Now, let's try to get into the
   // termination protocol.
   if (do_termination && !has_aborted()) {

--- a/src/share/vm/gc_implementation/g1/concurrentMark.hpp	Thu Apr 28 15:29:18 2011 -0700
+++ b/src/share/vm/gc_implementation/g1/concurrentMark.hpp	Fri Apr 29 12:40:49 2011 -0400
@@ -316,6 +316,19 @@
   void setEmpty()   { _index = 0; clear_overflow(); }
 };
 
+class ForceOverflowSettings VALUE_OBJ_CLASS_SPEC {
+private:
+#ifndef PRODUCT
+  uintx _num_remaining;
+  bool _force;
+#endif // !defined(PRODUCT)
+
+public:
+  void init() PRODUCT_RETURN;
+  void update() PRODUCT_RETURN;
+  bool should_force() PRODUCT_RETURN_( return false; );
+};
+
 // this will enable a variety of different statistics per GC task
 #define _MARKING_STATS_       0
 // this will enable the higher verbose levels
@@ -462,6 +475,9 @@
 
   WorkGang* _parallel_workers;
 
+  ForceOverflowSettings _force_overflow_conc;
+  ForceOverflowSettings _force_overflow_stw;
+
   void weakRefsWork(bool clear_all_soft_refs);
 
   void swapMarkBitMaps();
@@ -470,7 +486,7 @@
   // task local ones; should be called during initial mark.
   void reset();
   // It resets all the marking data structures.
-  void clear_marking_state();
+  void clear_marking_state(bool clear_overflow = true);
 
   // It should be called to indicate which phase we're in (concurrent
   // mark or remark) and how many threads are currently active.
@@ -547,6 +563,22 @@
   void enter_first_sync_barrier(int task_num);
   void enter_second_sync_barrier(int task_num);
 
+  ForceOverflowSettings* force_overflow_conc() {
+    return &_force_overflow_conc;
+  }
+
+  ForceOverflowSettings* force_overflow_stw() {
+    return &_force_overflow_stw;
+  }
+
+  ForceOverflowSettings* force_overflow() {
+    if (concurrent()) {
+      return force_overflow_conc();
+    } else {
+      return force_overflow_stw();
+    }
+  }
+
 public:
   // Manipulation of the global mark stack.
   // Notice that the first mark_stack_push is CAS-based, whereas the

--- a/src/share/vm/gc_implementation/g1/g1_globals.hpp	Thu Apr 28 15:29:18 2011 -0700
+++ b/src/share/vm/gc_implementation/g1/g1_globals.hpp	Fri Apr 29 12:40:49 2011 -0400
@@ -311,7 +311,11 @@
                                                                             \
   develop(bool, G1ExitOnExpansionFailure, false,                            \
           "Raise a fatal VM exit out of memory failure in the event "       \
-          " that heap expansion fails due to running out of swap.")
+          " that heap expansion fails due to running out of swap.")         \
+                                                                            \
+  develop(uintx, G1ConcMarkForceOverflow, 0,                                \
+          "The number of times we'll force an overflow during "             \
+          "concurrent marking")
 
 G1_FLAGS(DECLARE_DEVELOPER_FLAG, DECLARE_PD_DEVELOPER_FLAG, DECLARE_PRODUCT_FLAG, DECLARE_PD_PRODUCT_FLAG, DECLARE_DIAGNOSTIC_FLAG, DECLARE_EXPERIMENTAL_FLAG, DECLARE_NOTPRODUCT_FLAG, DECLARE_MANAGEABLE_FLAG, DECLARE_PRODUCT_RW_FLAG)