[libcxx] Fix a data race in call_once

call_once is using relaxed atomic load to perform double-checked locking, which contains a data race. The fast-path load has to be an acquire atomic load.

Differential Revision: https://reviews.llvm.org/D24028

llvm-svn: 280621

libcxx/include/memory

@@ -663,6 +663,18 @@ _ValueType __libcpp_relaxed_load(_ValueType const* __value) {
 #endif
 }
 
+template <class _ValueType>
+inline _LIBCPP_ALWAYS_INLINE
+_ValueType __libcpp_acquire_load(_ValueType const* __value) {
+#if !defined(_LIBCPP_HAS_NO_THREADS) && \
+    defined(__ATOMIC_ACQUIRE) &&        \
+    (__has_builtin(__atomic_load_n) || _GNUC_VER >= 407)
+    return __atomic_load_n(__value, __ATOMIC_ACQUIRE);
+#else
+    return *__value;
+#endif
+}
+
 // addressof moved to <__functional_base>
 
 template <class _Tp> class allocator;

libcxx/include/mutex

@@ -574,7 +574,7 @@ inline _LIBCPP_INLINE_VISIBILITY
 void
 call_once(once_flag& __flag, _Callable&& __func, _Args&&... __args)
 {
-    if (__libcpp_relaxed_load(&__flag.__state_) != ~0ul)
+    if (__libcpp_acquire_load(&__flag.__state_) != ~0ul)
     {
         typedef tuple<_Callable&&, _Args&&...> _Gp;
         _Gp __f(_VSTD::forward<_Callable>(__func), _VSTD::forward<_Args>(__args)...);
@@ -590,7 +590,7 @@ inline _LIBCPP_INLINE_VISIBILITY
 void
 call_once(once_flag& __flag, _Callable& __func)
 {
-    if (__libcpp_relaxed_load(&__flag.__state_) != ~0ul)
+    if (__libcpp_acquire_load(&__flag.__state_) != ~0ul)
     {
         __call_once_param<_Callable> __p(__func);
         __call_once(__flag.__state_, &__p, &__call_once_proxy<_Callable>);

libcxx/src/mutex.cpp

@@ -199,9 +199,6 @@ static __libcpp_mutex_t mut = _LIBCPP_MUTEX_INITIALIZER;
 static __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
 #endif
 
-/// NOTE: Changes to flag are done via relaxed atomic stores
-///       even though the accesses are protected by a mutex because threads
-///       just entering 'call_once` concurrently read from flag.
 void
 __call_once(volatile unsigned long& flag, void* arg, void(*func)(void*))
 {
@@ -238,7 +235,7 @@ __call_once(volatile unsigned long& flag, void* arg, void(*func)(void*))
             __libcpp_mutex_unlock(&mut);
             func(arg);
             __libcpp_mutex_lock(&mut);
-            __libcpp_relaxed_store(&flag, ~0ul);
+            __libcpp_atomic_store(&flag, ~0ul, _AO_Release);
             __libcpp_mutex_unlock(&mut);
             __libcpp_condvar_broadcast(&cv);
 #ifndef _LIBCPP_NO_EXCEPTIONS