The mutex is common across all the threads. It needn't be held across
the init_routine() call as long as the 'once' behaviour is guaranteed
Saw a deadlock case, where init_routine of one thread was waiting for
the completion of init_routine in another thread.
t2: wait for command
t1: pthread_once:
lock once_mux
init_routine:
inform thread t2
wait for signal from t2
t2: received command
pthread_once
lock once_mux (already held by t1)
---- Deadlock ----
