--- a/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Fri Apr 02 12:10:08 2010 -0400
+++ b/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Mon Apr 05 12:19:22 2010 -0400
@@ -297,6 +297,11 @@
   }
 }
 
+// Currently we do not call this at all. Normally we would call it
+// during the concurrent marking / remark phases but we now call
+// the lock-based version instead. But we might want to resurrect this
+// code in the future. So, we'll leave it here commented out.
+#if 0
 MemRegion CMRegionStack::pop() {
   while (true) {
     // Otherwise...
@@ -321,6 +326,41 @@
     // Otherwise, we need to try again.
   }
 }
+#endif // 0
+
+void CMRegionStack::push_with_lock(MemRegion mr) {
+  assert(mr.word_size() > 0, "Precondition");
+  MutexLockerEx x(CMRegionStack_lock, Mutex::_no_safepoint_check_flag);
+
+  if (isFull()) {
+    _overflow = true;
+    return;
+  }
+
+  _base[_index] = mr;
+  _index += 1;
+}
+
+MemRegion CMRegionStack::pop_with_lock() {
+  MutexLockerEx x(CMRegionStack_lock, Mutex::_no_safepoint_check_flag);
+
+  while (true) {
+    if (_index == 0) {
+      return MemRegion();
+    }
+    _index -= 1;
+
+    MemRegion mr = _base[_index];
+    if (mr.start() != NULL) {
+      assert(mr.end() != NULL, "invariant");
+      assert(mr.word_size() > 0, "invariant");
+      return mr;
+    } else {
+      // that entry was invalidated... let's skip it
+      assert(mr.end() == NULL, "invariant");
+    }
+  }
+}
 
 bool CMRegionStack::invalidate_entries_into_cset() {
   bool result = false;
@@ -3363,7 +3403,7 @@
       gclog_or_tty->print_cr("[%d] draining region stack, size = %d",
                              _task_id, _cm->region_stack_size());
 
-    MemRegion mr = _cm->region_stack_pop();
+    MemRegion mr = _cm->region_stack_pop_with_lock();
     // it returns MemRegion() if the pop fails
     statsOnly(if (mr.start() != NULL) ++_region_stack_pops );
 
@@ -3384,7 +3424,7 @@
         if (has_aborted())
           mr = MemRegion();
         else {
-          mr = _cm->region_stack_pop();
+          mr = _cm->region_stack_pop_with_lock();
           // it returns MemRegion() if the pop fails
           statsOnly(if (mr.start() != NULL) ++_region_stack_pops );
         }
@@ -3417,7 +3457,7 @@
           }
           // Now push the part of the region we didn't scan on the
           // region stack to make sure a task scans it later.
-          _cm->region_stack_push(newRegion);
+          _cm->region_stack_push_with_lock(newRegion);
         }
         // break from while
         mr = MemRegion();

--- a/src/share/vm/gc_implementation/g1/concurrentMark.hpp	Fri Apr 02 12:10:08 2010 -0400
+++ b/src/share/vm/gc_implementation/g1/concurrentMark.hpp	Mon Apr 05 12:19:22 2010 -0400
@@ -252,9 +252,19 @@
   // with other "push" operations (no pops).
   void push(MemRegion mr);
 
+#if 0
+  // This is currently not used. See the comment in the .cpp file.
+
   // Lock-free; assumes that it will only be called in parallel
   // with other "pop" operations (no pushes).
   MemRegion pop();
+#endif // 0
+
+  // These two are the implementations that use a lock. They can be
+  // called concurrently with each other but they should not be called
+  // concurrently with the lock-free versions (push() / pop()).
+  void push_with_lock(MemRegion mr);
+  MemRegion pop_with_lock();
 
   bool isEmpty()    { return _index == 0; }
   bool isFull()     { return _index == _capacity; }
@@ -540,6 +550,10 @@
 
   // Manipulation of the region stack
   bool region_stack_push(MemRegion mr) {
+    // Currently we only call the lock-free version during evacuation
+    // pauses.
+    assert(SafepointSynchronize::is_at_safepoint(), "world should be stopped");
+
     _regionStack.push(mr);
     if (_regionStack.overflow()) {
       set_has_overflown();
@@ -547,7 +561,33 @@
     }
     return true;
   }
-  MemRegion region_stack_pop()          { return _regionStack.pop(); }
+#if 0
+  // Currently this is not used. See the comment in the .cpp file.
+  MemRegion region_stack_pop() { return _regionStack.pop(); }
+#endif // 0
+
+  bool region_stack_push_with_lock(MemRegion mr) {
+    // Currently we only call the lock-based version during either
+    // concurrent marking or remark.
+    assert(!SafepointSynchronize::is_at_safepoint() || !concurrent(),
+           "if we are at a safepoint it should be the remark safepoint");
+
+    _regionStack.push_with_lock(mr);
+    if (_regionStack.overflow()) {
+      set_has_overflown();
+      return false;
+    }
+    return true;
+  }
+  MemRegion region_stack_pop_with_lock() {
+    // Currently we only call the lock-based version during either
+    // concurrent marking or remark.
+    assert(!SafepointSynchronize::is_at_safepoint() || !concurrent(),
+           "if we are at a safepoint it should be the remark safepoint");
+
+    return _regionStack.pop_with_lock();
+  }
+
   int region_stack_size()               { return _regionStack.size(); }
   bool region_stack_overflow()          { return _regionStack.overflow(); }
   bool region_stack_empty()             { return _regionStack.isEmpty(); }

--- a/src/share/vm/runtime/mutexLocker.cpp	Fri Apr 02 12:10:08 2010 -0400
+++ b/src/share/vm/runtime/mutexLocker.cpp	Mon Apr 05 12:19:22 2010 -0400
@@ -70,6 +70,7 @@
 Monitor* CMark_lock                   = NULL;
 Monitor* ZF_mon                       = NULL;
 Monitor* Cleanup_mon                  = NULL;
+Mutex*   CMRegionStack_lock           = NULL;
 Mutex*   SATB_Q_FL_lock               = NULL;
 Monitor* SATB_Q_CBL_mon               = NULL;
 Mutex*   Shared_SATB_Q_lock           = NULL;
@@ -167,6 +168,7 @@
     def(CMark_lock                 , Monitor, nonleaf,     true ); // coordinate concurrent mark thread
     def(ZF_mon                     , Monitor, leaf,        true );
     def(Cleanup_mon                , Monitor, nonleaf,     true );
+    def(CMRegionStack_lock         , Mutex,   leaf,        true );
     def(SATB_Q_FL_lock             , Mutex  , special,     true );
     def(SATB_Q_CBL_mon             , Monitor, nonleaf,     true );
     def(Shared_SATB_Q_lock         , Mutex,   nonleaf,     true );

--- a/src/share/vm/runtime/mutexLocker.hpp	Fri Apr 02 12:10:08 2010 -0400
+++ b/src/share/vm/runtime/mutexLocker.hpp	Mon Apr 05 12:19:22 2010 -0400
@@ -63,6 +63,7 @@
 extern Monitor* CMark_lock;                      // used for concurrent mark thread coordination
 extern Monitor* ZF_mon;                          // used for G1 conc zero-fill.
 extern Monitor* Cleanup_mon;                     // used for G1 conc cleanup.
+extern Mutex*   CMRegionStack_lock;              // used for protecting accesses to the CM region stack
 extern Mutex*   SATB_Q_FL_lock;                  // Protects SATB Q
                                                  // buffer free list.
 extern Monitor* SATB_Q_CBL_mon;                  // Protects SATB Q