Index: libcxx/trunk/include/mutex
===================================================================
--- libcxx/trunk/include/mutex
+++ libcxx/trunk/include/mutex
@@ -685,7 +685,7 @@
 void
 call_once(once_flag& __flag, const _Callable& __func)
 {
-    if (__flag.__state_ != ~0ul)
+    if (__libcpp_acquire_load(&__flag.__state_) != ~0ul)
     {
         __call_once_param<const _Callable> __p(__func);
         __call_once(__flag.__state_, &__p, &__call_once_proxy<const _Callable>);