Oilpan: avoid data race on gcInfo index initialization.

Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 16 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef Visitor_h
 #define Visitor_h
 
 #include "platform/PlatformExport.h"
 #include "platform/heap/ThreadState.h"
 #include "wtf/Assertions.h"
+#include "wtf/Atomics.h"
 #include "wtf/Deque.h"
 #include "wtf/Forward.h"
 #include "wtf/HashMap.h"
 #include "wtf/HashTraits.h"
 #include "wtf/InstanceCounter.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/RefPtr.h"
 #include "wtf/TypeTraits.h"
 #include "wtf/WeakPtr.h"
 #if ENABLE(GC_PROFILING)
@@ skipping 827 matching lines @@
 
     static int s_gcInfoIndex;
     static size_t s_gcInfoTableSize;
 };
 
 // This macro should be used when returning a unique 15 bit integer
 // for a given gcInfo.
 #define RETURN_GCINFO_INDEX()                                  \
     static size_t gcInfoIndex = 0;                             \
     ASSERT(s_gcInfoTable);                                     \
-    if (!gcInfoIndex)                                          \
+    if (!acquireLoad(&gcInfoIndex))                            \
         GCInfoTable::ensureGCInfoIndex(&gcInfo, &gcInfoIndex); \
     ASSERT(gcInfoIndex >= 1);                                  \
     ASSERT(gcInfoIndex < GCInfoTable::maxIndex);               \
     return gcInfoIndex;
 
 template<typename T>
 struct GCInfoAtBase {
     static size_t index()
     {
         static const GCInfo gcInfo = {
@@ skipping 25 matching lines @@
 struct GCInfoTrait {
     static size_t index()
     {
         return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::index();
     }
 };
 
 }
 
 #endif