Fix some potential data races based on output from tsan.

runtime/vm/class_table.cc

Index: runtime/vm/class_table.cc
diff --git a/runtime/vm/class_table.cc b/runtime/vm/class_table.cc
index 7129cf8f0ab3393ac35f09d913277797b0376e2c..8cb9fa192a50850752d0ab7c706c4aa1eadfcf4e 100644
--- a/runtime/vm/class_table.cc
+++ b/runtime/vm/class_table.cc
@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+#include "vm/atomic.h"
 #include "vm/class_table.h"
 #include "vm/flags.h"
 #include "vm/freelist.h"
@@ -106,11 +107,9 @@ void ClassTable::Register(const Class& cls) {
     // Add the vtable for this predefined class into the static vtable registry
     // if it has not been setup yet.
     cpp_vtable cls_vtable = cls.handle_vtable();
-    cpp_vtable table_entry = Object::builtin_vtables_[index];
-    ASSERT((table_entry == 0) || (table_entry == cls_vtable));
-    if (table_entry == 0) {
-      Object::builtin_vtables_[index] = cls_vtable;
-    }
+    AtomicOperations::CompareAndSwapWord(
+        &(Object::builtin_vtables_[index]), 0, cls_vtable);
+    ASSERT(Object::builtin_vtables_[index] == cls_vtable);

[zra, 2016/03/03 23:50:02]

Is this asserting that the compare and swap was successful? Doesn't
CompareAndSwap return something to indicate success?

[siva, 2016/03/04 20:53:29]

The ASSERT is to make sure the value of builtin_vtables_[index] is not 0 after
the CAS.

The CompareAndSwap function returns the old value in the location which could be
0 or cls_vtable depending on whether the CAS stored cls_vtable or it already had
cls_vtable in that location.

   } else {
     if (top_ == capacity_) {
       // Grow the capacity of the class table.