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Releasing the mutex before the call to notify_all is an optimization. This optimization cannot be used here. The thread waiting on the condition might destroy the associated resources — mutex + condition variable — and the notifier thread will access an destroyed variable — the condition variable. In fact, notify_all_at_thread_exit is meant exactly to join on detached threads, and the waiting thread doesn't expect for the notifier thread to access any further shared resources, making this scenario very likely to happen. The waiting thread might awake spuriously on the release of the mutex lock. The reorder is necessary to prevent this race. Further details can be found at https://cplusplus.github.io/LWG/issue3343. Differential Revision: https://reviews.llvm.org/D105758