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[libc++] fix atomic::wait memory order #146267

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Merged
merged 2 commits into from
Jul 6, 2025
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[libc++] fix atomic::wait memory order #146267

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[libc++] fix atomic::wait memory order
huixie90 Jun 29, 2025
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huixie90 Jul 5, 2025
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7 changes: 5 additions & 2 deletions libcxx/src/atomic.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
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@gonzalobg gonzalobg Aug 25, 2025 •
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@huixie90 why seq_cst? Could we add a comment explaining that? e.g. why doesn't acquire or acq_rel suffice.

// 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);
Expand All @@ -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);
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@gonzalobg gonzalobg Aug 25, 2025 •
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Similar as above, why is seq_cst required? e.g. why doesn't release or acq_rel suffice.

__libcpp_contention_notify(
&__entry->__contention_state,
&__entry->__platform_state,
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