8035496: G1 ARM: missing remset entry noticed by VerifyAfterGC for...

8035496: G1 ARM: missing remset entry noticed by VerifyAfterGC for vm/gc/concurrent/lp50yp10rp70mr30st0
Summary: release_store used when registering a PerRegionTable. Also reviewed-by: vitalyd@gmail.com
Reviewed-by: jmasa, tschatzl, brutisso

src/share/vm/gc_implementation/g1/heapRegionRemSet.cpp

@@ -514,7 +514,15 @@ void OtherRegionsTable::add_reference(OopOrNarrowOopStar from, int tid) {
 
       PerRegionTable* first_prt = _fine_grain_regions[ind];
       prt->set_collision_list_next(first_prt);
-      _fine_grain_regions[ind] = prt;
+      // The assignment into _fine_grain_regions allows the prt to
+      // start being used concurrently. In addition to
+      // collision_list_next which must be visible (else concurrent
+      // parsing of the list, if any, may fail to see other entries),
+      // the content of the prt must be visible (else for instance
+      // some mark bits may not yet seem cleared or a 'later' update
+      // performed by a concurrent thread could be undone when the
+      // zeroing becomes visible). This requires store ordering.
+      OrderAccess::release_store_ptr((volatile PerRegionTable*)&_fine_grain_regions[ind], prt);
       _n_fine_entries++;
 
       if (G1HRRSUseSparseTable) {