Comment 37 for bug 1640518

from "man pthread_mutex_unlock":
       If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking
       shall be provided. If a thread attempts to relock a mutex that it has
       already locked, an error shall be returned. If a thread attempts to
       unlock a mutex that it has not locked or a mutex which is unlocked, an
       error shall be returned.

That might be something to try.