[patch-stable 1/3] rt-mutex: Fix stale return value

[Thomas Gleixner <tglx@linutronix.de>]

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 woken up path.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>

---
 kernel/rtmutex.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

Index: linux-2.6.21/kernel/rtmutex.c
===================================================================
--- linux-2.6.21.orig/kernel/rtmutex.c	2007-06-08 11:56:10.000000000 +0200
+++ linux-2.6.21/kernel/rtmutex.c	2007-06-08 11:56:10.000000000 +0200
@@ -659,9 +659,16 @@ rt_mutex_slowlock(struct rt_mutex *lock,
 			 * 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;
 		}

-- 

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