Right now, the main loop in semlock_acquire looks like this:    do {        Py_BEGIN_ALLOW_THREADS        if (blocking && timeout_obj == Py_None)            res = sem_wait(self->handle);        else if (!blocking)            res = sem_trywait(self->handle);        else            res = sem_timedwait(self->handle, &deadline);        Py_END_ALLOW_THREADS        err = errno;        if (res == MP_EXCEPTION_HAS_BEEN_SET)            break;    } while (res < 0 && errno == EINTR && !PyErr_CheckSignals());Here, we unconditionally release the GIL even we could acquire the mutex without blocking! As a result, we could end up switching to another thread in the process and greatly increasing the latency of operations that lock and release multiple shared data structures.Instead, we should unconditionally try sem_trywait, and only then, if we want to block, release the GIL and try sem_wait or sem_timedwait. This way, we'll churn only when we need to.Note that threading.Lock works the way I propose.

Looks reasonable to me. Do you want to provide a patch Daniel?

New changesetc872d39d324cd6f1a71b73e10406bbaed192d35f by Antoine Pitrou in branch 'master':bpo-31653: Don't release the GIL if we can acquire a multiprocessing semaphore immediately (#4078)https://github.com/python/cpython/commit/c872d39d324cd6f1a71b73e10406bbaed192d35f

New changeset828ca59208af0b1b52a328676c5cc0c5e2e999b0 by Victor Stinner in branch 'master':bpo-31653: Remove deadcode in semlock_acquire() (#4091)https://github.com/python/cpython/commit/828ca59208af0b1b52a328676c5cc0c5e2e999b0