8138966: Intermittent SEGV running ParallelGC

Summary: Add necessary memory fences so that the parallel threads are unable to observe partially filled block tables.
Reviewed-by: tschatzl

src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.hpp

@@ -348,7 +348,7 @@ public:
     HeapWord*            _partial_obj_addr;
     region_sz_t          _partial_obj_size;
     region_sz_t volatile _dc_and_los;
-    bool                 _blocks_filled;
+    bool        volatile _blocks_filled;
 
 #ifdef ASSERT
     size_t               _blocks_filled_count;   // Number of block table fills.
@@ -499,7 +499,9 @@ ParallelCompactData::RegionData::destination_count() const
 inline bool
 ParallelCompactData::RegionData::blocks_filled() const
 {
-  return _blocks_filled;
+  bool result = _blocks_filled;
+  OrderAccess::acquire();
+  return result;
 }
 
 #ifdef ASSERT
@@ -513,6 +515,7 @@ ParallelCompactData::RegionData::blocks_filled_count() const
 inline void
 ParallelCompactData::RegionData::set_blocks_filled()
 {
+  OrderAccess::release();
   _blocks_filled = true;
   // Debug builds count the number of times the table was filled.
   DEBUG_ONLY(Atomic::inc_ptr(&_blocks_filled_count));