Fix data race in v8::internal::UnboundQueue

src/unbound-queue-inl.h

Index: src/unbound-queue-inl.h
diff --git a/src/unbound-queue-inl.h b/src/unbound-queue-inl.h
index 86722f3a7cd18b24bbe44f5de9abbbdb51fcf7b4..796ba401d58664faccf1f8432dc5b7fdd073480d 100644
--- a/src/unbound-queue-inl.h
+++ b/src/unbound-queue-inl.h
@@ -68,11 +68,12 @@ void UnboundQueue<Record>::DeleteFirst() {
 
 
 template<typename Record>
-void UnboundQueue<Record>::Dequeue(Record* rec) {
-  ASSERT(divider_ != last_);
+bool UnboundQueue<Record>::Dequeue(Record* rec) {
+  if (divider_ == Acquire_Load(&last_)) return false;
   Node* next = reinterpret_cast<Node*>(divider_)->next;
   *rec = next->value;
   Release_Store(&divider_, reinterpret_cast<AtomicWord>(next));
+  return true;
 }
 
 
@@ -81,13 +82,22 @@ void UnboundQueue<Record>::Enqueue(const Record& rec) {
   Node*& next = reinterpret_cast<Node*>(last_)->next;
   next = new Node(rec);
   Release_Store(&last_, reinterpret_cast<AtomicWord>(next));
-  while (first_ != reinterpret_cast<Node*>(divider_)) DeleteFirst();
+
+  while (first_ != reinterpret_cast<Node*>(Acquire_Load(&divider_))) {
+    DeleteFirst();
+  }
+}
+
+
+template<typename Record>
+bool UnboundQueue<Record>::IsEmpty() const {
+  return NoBarrier_Load(&divider_) == NoBarrier_Load(&last_);
 }
 
 
 template<typename Record>
-Record* UnboundQueue<Record>::Peek() {
-  ASSERT(divider_ != last_);
+Record* UnboundQueue<Record>::Peek() const {
+  if (divider_ == Acquire_Load(&last_)) return NULL;
   Node* next = reinterpret_cast<Node*>(divider_)->next;
   return &next->value;
 }