Author: kuba.brecka Date: Sun Sep 4 04:55:12 2016 New Revision: 280621 URL: http://llvm.org/viewvc/llvm-project?rev=280621&view=rev Log: [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 Modified: libcxx/trunk/include/memory libcxx/trunk/include/mutex libcxx/trunk/src/mutex.cpp Modified: libcxx/trunk/include/memory URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/memory?rev=280621&r1=280620&r2=280621&view=diff ============================================================================== --- libcxx/trunk/include/memory (original) +++ libcxx/trunk/include/memory Sun Sep 4 04:55:12 2016 @@ -663,6 +663,18 @@ _ValueType __libcpp_relaxed_load(_ValueT #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; Modified: libcxx/trunk/include/mutex URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/mutex?rev=280621&r1=280620&r2=280621&view=diff ============================================================================== --- libcxx/trunk/include/mutex (original) +++ libcxx/trunk/include/mutex Sun Sep 4 04:55:12 2016 @@ -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>); Modified: libcxx/trunk/src/mutex.cpp URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/mutex.cpp?rev=280621&r1=280620&r2=280621&view=diff ============================================================================== --- libcxx/trunk/src/mutex.cpp (original) +++ libcxx/trunk/src/mutex.cpp Sun Sep 4 04:55:12 2016 @@ -199,9 +199,6 @@ static __libcpp_mutex_t mut = _LIBCPP_MU 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 __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 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
