6940310: G1: MT-unsafe calls to CM::region_stack_push() / CM::region_stack_pop()

Calling the methods region_stack_push() and region_stack_pop() concurrent is not MT-safe. The assumption is that we will only call region_stack_push() during a GC pause and region_stack_pop() during marking. Unfortunately, we also call region_stack_push() during marking which seems to be introducing subtle marking failures. This change introduces lock-based methods for pushing / popping to be called during marking.

Reviewed-by: iveresov, johnc

hotspot/src/share/vm/gc_implementation/g1/concurrentMark.cpp

@@ -297,6 +297,11 @@ void CMRegionStack::push(MemRegion mr) {
   }
 }
 
+// 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 @@ MemRegion CMRegionStack::pop() {
     // 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 @@ void CMTask::drain_region_stack(BitMapClosure* bc) {
       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 @@ void CMTask::drain_region_stack(BitMapClosure* bc) {
         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 @@ void CMTask::drain_region_stack(BitMapClosure* bc) {
           }
           // 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();