rt-mutex: fix stale return value

Alexey Kuznetsov found some problems in the pi-futex code.

The major problem is a stale return value in rt_mutex_slowlock():

When the pi chain walk returns -EDEADLK, but the waiter was woken up during
the phases where the locks were dropped, the rtmutex could be acquired, but
due to the stale return value -EDEADLK returned to the caller.

Reset the return value in the retry path.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/rtmutex.c

@@ -636,9 +636,16 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 			 * all over without going into schedule to try
 			 * to get the lock now:
 			 */
-			if (unlikely(!waiter.task))
+			if (unlikely(!waiter.task)) {
+				/*
+				 * Reset the return value. We might
+				 * have returned with -EDEADLK and the
+				 * owner released the lock while we
+				 * were walking the pi chain.
+				 */
+				ret = 0;
 				continue;
-
+			}
 			if (unlikely(ret))
 				break;
 		}