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| author | Hui <hui.xie1990@gmail.com> | 2025-07-06 09:23:19 +0100 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2025-07-06 09:23:19 +0100 |
| commit | c358979d7f45fc7624abc2f14e6f36f25ac9acc0 (patch) | |
| tree | e8c9559a3cc3e6a398d047fbf5a9e6ff1739eab3 /libcxx/src | |
| parent | ba7d78ac4597c99e1815646d711cf736ead4f54a (diff) | |
| download | llvm-c358979d7f45fc7624abc2f14e6f36f25ac9acc0.zip llvm-c358979d7f45fc7624abc2f14e6f36f25ac9acc0.tar.gz llvm-c358979d7f45fc7624abc2f14e6f36f25ac9acc0.tar.bz2 | |
[libc++] fix atomic::wait memory order (#146267)
Fixes #109290
See the GH issue for the details. In conclusion, we have two issues in
the `atomic<T>::wait` when `T` does not match our `contention_t`:
- We don't have a total single order which can leads to missed
notification based on the Herd7 simulation on relaxed architectural like
PowerPC
- We assumed the platform wait (`futex_wait`/`__ulock_wait`) has seq_cst
barrier internally but there is no such guarantee
Diffstat (limited to 'libcxx/src')
| -rw-r--r-- | libcxx/src/atomic.cpp | 7 |
1 files changed, 5 insertions, 2 deletions
diff --git a/libcxx/src/atomic.cpp b/libcxx/src/atomic.cpp index c1af8d6..903084d 100644 --- a/libcxx/src/atomic.cpp +++ b/libcxx/src/atomic.cpp @@ -151,7 +151,10 @@ __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); + // 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); @@ -163,7 +166,7 @@ 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, |