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>

1 files changed, 9 insertions, 2 deletions

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 12879f6c1ec3..a273183c37a0 100644
--- a/kernel/rtmutex.c
+++ b/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;
                 }