BTW, looking at the spec for pthread_cond_wait, there's actually no mention about a limitation regarding using pthread_cond_signal invoked from within a signal handler to unblock a pthread_cond_wait. The restriction is elsewhere. pthread_cond_signal() is not in the list of async-signal-safe functions at
https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04 Therefore, portable programs may not call it from a signal handler and except it to work. Asynchronous signal handler can get called between any two instructions so safe synchronization with condition variables might be hard to implement. Use semaphores and sem_post() instead. -Juha ________________________________ From: Gregory Nutt <spudan...@gmail.com> Sent: Wednesday, January 13, 2021 4:24 AM To: dev@nuttx.apache.org <dev@nuttx.apache.org> Subject: Re: condition variables and signals BTW, looking at the spec for pthread_cond_wait, there's actually no mention about a limitation regarding using pthread_cond_signal invoked from within a signal handler to unblock a pthread_cond_wait. However, this does not work either for me. If you follow the signal operation, I can see that when it reaches the nxsem_get_value() on the cond->sem, the semaphore value is zero and the sem_give is not done. [cid:part1.02F16F75.CA50FB6D@gmail.com] Should this work? Yes, calling pthread_cond_signal() should work, however, no signal should be delivered. The word "signal" is overloaded: Here the condition is signaled but no signal should be sent so I am not quite sure what I am looking at. Apparently some other logic is signaling the task group asynchronously? I am thinking that there must be some race condition: When the condition variable is created the cond->sem value will be set to zero. While waiting for cond->sem, it will be set to -1. If you see cond->sem equal to zero, my guess would be that the signal was received and cond->sem was incremented asychronously by the signal delivery logic. But that is only a guess. It would be good to check the behavior without the signal interfering.
