Synchronize on concurrent store buffer entries.

src/heap/store-buffer.h

Index: src/heap/store-buffer.h
diff --git a/src/heap/store-buffer.h b/src/heap/store-buffer.h
index cb96fa9720a64909fbd1dcf04917f0018f095a08..37a78eb07528b212752828f885bba25d5358933b 100644
--- a/src/heap/store-buffer.h
+++ b/src/heap/store-buffer.h
@@ -33,9 +33,13 @@ class StoreBuffer {
   void SetUp();
   void TearDown();
 
-  // This is used by the mutator to enter addresses into the store buffer.
+  // This is used to add addresses to the store buffer non-concurrently.
   inline void Mark(Address addr);
 
+  // This is used to add addresses to the store buffer when multiple threads
+  // may operate on the store buffer.
+  inline void MarkSynchronized(Address addr);
+
   // This is used by the heap traversal to enter the addresses into the store
   // buffer that should still be in the store buffer after GC.  It enters
   // addresses directly into the old buffer because the GC starts by wiping the
@@ -129,6 +133,9 @@ class StoreBuffer {
   uintptr_t* hash_set_2_;
   bool hash_sets_are_empty_;
 
+  // Used for synchronization of concurrent store buffer access.
+  base::Mutex mutex_;
+
   void ClearFilteringHashSets();
 
   bool SpaceAvailable(intptr_t space_needed);