[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
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 224264ade0674f8ce120432614abfd880323f105
diff --git a/include/memory b/include/memory
index 31f58b7..8cb094e 100644
--- a/include/memory
+++ b/include/memory
@@ -663,6 +663,18 @@
#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;