[all-commits] [llvm/llvm-project] 70f716: [libc][rwlock] fix the race condition in waiter qu...

[Schrodinger ZHU Yifan via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 70f7167ce2622a610a6d6dca4d5934ea8a07a056
      https://github.com/llvm/llvm-project/commit/70f7167ce2622a610a6d6dca4d5934ea8a07a056
  Author: Schrodinger ZHU Yifan <yfzhu at google.com>
  Date:   2026-06-04 (Thu, 04 Jun 2026)

  Changed paths:
    M libc/src/__support/threads/raw_rwlock.h

  Log Message:
  -----------
  [libc][rwlock] fix the race condition in waiter queue (#201629)

Fix #201615.

Fix the issue that non atomic operations race in waiting queue, which
causes missed futex wakeup signals.

Confirmed by TSAN:

```
==================
WARNING: ThreadSanitizer: data race (pid=388518)
  Write of size 4 at 0x7ffd21cf98e4 by thread T23:
    #0 __llvm_libc_23_0_0_git::RawRwLock::notify_pending_threads() ./libc/src/__support/threads/raw_rwlock.h:443:44
    #1 __llvm_libc_23_0_0_git::RawRwLock::unlock() ./libc/src/__support/threads/raw_rwlock.h:520:5
    #2 randomized_thread_operation(SharedData*) ./libc/test/integration/src/__support/threads/tsan_full_rwlock.cpp:104:18
    #3 thread_runner(void*) ./libc/test/integration/src/__support/threads/tsan_full_rwlock.cpp:148:5

  Previous atomic read of size 4 at 0x7ffd21cf98e4 by thread T4:
    #0 __llvm_libc_23_0_0_git::cpp::Atomic<unsigned int>::load(...) ./libc/src/__support/CPP/atomic.h:115:5
    #1 __llvm_libc_23_0_0_git::Futex::wait(...) ./libc/src/__support/threads/linux/futex_utils.h:43:17
    #2 __llvm_libc_23_0_0_git::cpp::expected<int, int> __llvm_libc_23_0_0_git::rwlock::WaitingQueue::wait<Role::Reader>(...) ./libc/src/__support/threads/raw_rwlock.h:101:35
    #3 __llvm_libc_23_0_0_git::rwlock::LockResult __llvm_libc_23_0_0_git::RawRwLock::lock_slow<Role::Reader>(...) ./libc/src/__support/threads/raw_rwlock.h:402:34
    #4 __llvm_libc_23_0_0_git::RawRwLock::read_lock(...) ./libc/src/__support/threads/raw_rwlock.h:485:12
    #5 randomized_thread_operation(SharedData*) ./libc/test/integration/src/__support/threads/tsan_full_rwlock.cpp:79:16
    #6 thread_runner(void*) ./libc/test/integration/src/__support/threads/tsan_full_rwlock.cpp:148:5

  Thread T23 (tid=388553, running) created by main thread at:
    #0 pthread_create ...
    #1 main ./libc/test/integration/src/__support/threads/tsan_full_rwlock.cpp:166:5

  Thread T4 (tid=388533, running) created by main thread at:
    #0 pthread_create ...
    #1 main ./libc/test/integration/src/__support/threads/tsan_full_rwlock.cpp:166:5

SUMMARY: ThreadSanitizer: data race ./libc/src/__support/threads/raw_rwlock.h:443:44 in __llvm_libc_23_0_0_git::RawRwLock::notify_pending_threads()
==================
```

AI wrote the detection script. Manually fixed.



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