[libcxx-commits] [libcxx] [libc++] fix atomic::wait memory order (PR #146267)

[via libcxx-commits]

https://github.com/huixie90 updated https://github.com/llvm/llvm-project/pull/146267

>From 77d8844aded96b9ca5cbb9a04270eca7ef3ddadb Mon Sep 17 00:00:00 2001
From: Hui Xie <hui.xie1990 at gmail.com>
Date: Sun, 29 Jun 2025 11:44:15 +0100
Subject: [PATCH 1/2] [libc++] fix atomic::wait memory order

---
 libcxx/src/atomic.cpp | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libcxx/src/atomic.cpp b/libcxx/src/atomic.cpp
index c1af8d6f95aae..585fd1f538e46 100644
--- a/libcxx/src/atomic.cpp
+++ b/libcxx/src/atomic.cpp
@@ -151,8 +151,9 @@ __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* /*__con
 static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state,
                                      __cxx_atomic_contention_t const volatile* __platform_state,
                                      __cxx_contention_t __old_value) {
-  __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst);
+  __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_relaxed);
   // We sleep as long as the monitored value hasn't changed.
+  __cxx_atomic_thread_fence(memory_order_seq_cst);
   __libcpp_platform_wait_on_address(__platform_state, __old_value);
   __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
 }
@@ -163,7 +164,7 @@ static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __conte
 static void __libcpp_atomic_notify(void const volatile* __location) {
   auto const __entry = __libcpp_contention_state(__location);
   // The value sequence laundering happens on the next line below.
-  __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release);
+  __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_seq_cst);
   __libcpp_contention_notify(
       &__entry->__contention_state,
       &__entry->__platform_state,

>From 64d28443a78155c1504566875268032c903c4d6f Mon Sep 17 00:00:00 2001
From: Hui Xie <hui.xie1990 at gmail.com>
Date: Sat, 5 Jul 2025 09:15:40 +0100
Subject: [PATCH 2/2] add comment

---
 libcxx/src/atomic.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libcxx/src/atomic.cpp b/libcxx/src/atomic.cpp
index 585fd1f538e46..903084da053a1 100644
--- a/libcxx/src/atomic.cpp
+++ b/libcxx/src/atomic.cpp
@@ -152,8 +152,10 @@ static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __conte
                                      __cxx_atomic_contention_t const volatile* __platform_state,
                                      __cxx_contention_t __old_value) {
   __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_relaxed);
-  // We sleep as long as the monitored value hasn't changed.
+  // https://github.com/llvm/llvm-project/issues/109290
+  // There are no platform guarantees of a memory barrier in the platform wait implementation
   __cxx_atomic_thread_fence(memory_order_seq_cst);
+  // We sleep as long as the monitored value hasn't changed.
   __libcpp_platform_wait_on_address(__platform_state, __old_value);
   __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
 }