https://github.com/bminor/musl/blob/54ca677983d47529bab8752315ac1a2b49888870/src/thread/pthread_mutex_unlock.c#L34

In the case where a mutex:
is one of PTHREAD_MUTEX_ERRORCHECK or PTHREAD_MUTEX_RECURSIVE
and PTHREAD_PRIO_INHERIT

an uninitialized value of 'old' is used to check whether to futex.