https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110016
--- Comment #16 from Jonathan Wakely <redi at gcc dot gnu.org> --- The condition variable uses that mutex to synchronize. If you just modify the atomic variable _without locking and unlocking the mutex_ then you are not synchronizing with the condition variable. If you don't use the mutex for the producer thread, then (as Andrew described) the consumer can check the atomic, see that the condition is not yet true, and decide it needs to wait. Then the producer makes the condition true and signals using notify_all but that is missed because the consumer thread isn't waiting yet, then the consumer starts to wait. But because it already missed the notify_all call, it waits forever. Using the mutex in the producer does not just hide the bug, it fixes it properly. If the producer updates the atomic while the mutex is held then it's impossible for the condition to become true between the consumer checking it and starting to wait. Those two things (checking the condition and starting to wait) become atomic w.r.t the producer. It's not possible to get this interleaving if you use the mutex correctly: consumer checks condition producer makes condition true producer notifies consumer waits Instead you either get: consumer checks condition and waits producer makes condition true and notifies, waking the consumer or: producer makes condition true (and notifies, but consumer isn't waiting) consumer sees condition is true and never waits
