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[libc][rwlock] fix the race condition in waiter queue #201629

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Merged
SchrodingerZhu merged 2 commits into from
Jun 4, 2026
Merged

[libc][rwlock] fix the race condition in waiter queue #201629

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[libc][rwlock] fix the race condition in waiter queue
SchrodingerZhu Jun 4, 2026
de29f59
fmt
SchrodingerZhu Jun 4, 2026
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17 changes: 10 additions & 7 deletions libc/src/__support/threads/raw_rwlock.h
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Original file line number Diff line number Diff line change
Expand Up @@ -75,11 +75,11 @@ class WaitingQueue final : private RawMutex {
else
return queue.pending_writers;
}
template <Role role> LIBC_INLINE FutexWordType &serialization() {
template <Role role> LIBC_INLINE Futex &serialization() {
if constexpr (role == Role::Reader)
return queue.reader_serialization.val;
return queue.reader_serialization;
else
return queue.writer_serialization.val;
return queue.writer_serialization;
}
friend WaitingQueue;
};
Expand Down Expand Up @@ -391,8 +391,9 @@ class RawRwLock {
// sleep on the futex, we can avoid such waiting.
old = RwState::fetch_set_pending_bit<role>(state,
cpp::MemoryOrder::RELAXED);
// no need to use atomic since it is already protected by the mutex.
serial_number = guard.serialization<role>();
// relaxed atomic since it is already protected by the mutex.
serial_number =
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guard.serialization<role>().load(cpp::MemoryOrder::RELAXED);
}

// Phase 6: do futex wait until the lock is available or timeout is
Expand Down Expand Up @@ -437,10 +438,12 @@ class RawRwLock {
{
WaitingQueue::Guard guard = queue.acquire(is_pshared);
if (guard.pending_count<Role::Writer>() != 0) {
guard.serialization<Role::Writer>()++;
guard.serialization<Role::Writer>().fetch_add(
1, cpp::MemoryOrder::RELEASE);
status = WakeTarget::Writers;
} else if (guard.pending_count<Role::Reader>() != 0) {
guard.serialization<Role::Reader>()++;
guard.serialization<Role::Reader>().fetch_add(
1, cpp::MemoryOrder::RELEASE);
status = WakeTarget::Readers;
} else {
status = WakeTarget::None;
Expand Down
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