[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

1 files changed, 12 insertions, 0 deletions

diff --git a/libcxx/include/memory b/libcxx/include/memory
index 31f58b7c0a4..8cb094e6b6a 100644
--- a/libcxx/include/memory
+++ b/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;