If a thread is cancelled, it may have already consumed a umtx_wake,

check waiter and semphore counter to see if we may wake up next thread.

lib/libthr/thread/thr_sem.c

@@ -178,6 +178,8 @@ sem_cancel_handler(void *arg)
 	sem_t *sem = arg;
 
 	atomic_add_int(&(*sem)->nwaiters, -1);
+	if ((*sem)->nwaiters && (*sem)->count)
+		_thr_umtx_wake(&(*sem)->count, 1);
 }
 
 int