Fix concurrent access to VMState::current_state_.

src/vm-state.h

 // Copyright 2010 the V8 project authors. 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
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 26 matching lines @@
   inline VMState(StateTag state);
   inline ~VMState();
 
   StateTag state() { return state_; }
   void set_external_callback(Address external_callback) {
     external_callback_ = external_callback;
   }
 
   // Used for debug asserts.
   static bool is_outermost_external() {
-    return current_state_ == NULL;
+    return current_state_ == 0;
   }
 
   static StateTag current_state() {
-    return current_state_ ? current_state_->state() : EXTERNAL;
+    VMState* state = reinterpret_cast<VMState*>(current_state_);
+    return state ? state->state() : EXTERNAL;
   }
 
   static Address external_callback() {
-    return current_state_ ? current_state_->external_callback_ : NULL;
+    VMState* state = reinterpret_cast<VMState*>(current_state_);
+    return state ? state->external_callback_ : NULL;
   }
 
  private:
   bool disabled_;
   StateTag state_;
   VMState* previous_;
   Address external_callback_;
 
   // A stack of VM states.
-  static VMState* current_state_;
+  static AtomicWord current_state_;
 #else
  public:
   explicit VMState(StateTag state) {}
 #endif
 };
 
 } }  // namespace v8::internal
 
 
 #endif  // V8_VM_STATE_H_