src/hotspot/share/gc/cms/compactibleFreeListSpace.cpp

rev 53367 : 8248851: CMS: Missing memory fences between free chunk check and klass read
Reviewed-by: aph, kbarrett, dholmes
Contributed-by: wangshuai94@huawei.com

@@ -1,7 +1,7 @@
 /*
- * Copyright (c) 2001, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.

@@ -1176,10 +1176,14 @@
       if (FreeChunk::indicatesFreeChunk(p)) {
         assert(res != 0, "Block size should not be 0");
         return res;
       }
     } else {
+      // The barrier is required to prevent reordering of the free chunk check
+      // and the klass read.
+      OrderAccess::loadload();
+
       // Ensure klass read before size.
       Klass* k = oop(p)->klass_or_null_acquire();
       if (k != NULL) {
         assert(k->is_klass(), "Should really be klass oop.");
         oop o = (oop)p;

@@ -1226,10 +1230,14 @@
         assert(res != 0, "Block size should not be 0");
         assert(loops == 0, "Should be 0");
         return res;
       }
     } else {
+      // The barrier is required to prevent reordering of the free chunk check
+      // and the klass read.
+      OrderAccess::loadload();
+
       // Ensure klass read before size.
       Klass* k = oop(p)->klass_or_null_acquire();
       if (k != NULL) {
         assert(k->is_klass(), "Should really be klass oop.");
         oop o = (oop)p;

@@ -1269,10 +1277,15 @@
 // promotion.)
 bool CompactibleFreeListSpace::block_is_obj(const HeapWord* p) const {
   FreeChunk* fc = (FreeChunk*)p;
   assert(is_in_reserved(p), "Should be in space");
   if (FreeChunk::indicatesFreeChunk(p)) return false;
+
+  // The barrier is required to prevent reordering of the free chunk check
+  // and the klass read.
+  OrderAccess::loadload();
+
   Klass* k = oop(p)->klass_or_null_acquire();
   if (k != NULL) {
     // Ignore mark word because it may have been used to
     // chain together promoted objects (the last one
     // would have a null value).