mutex: Always clear owner field upon mutex_unlock()

Currently if DEBUG_MUTEXES is enabled, the mutex->owner field is only
cleared iff debug_locks is active. This exposes a race to other users of
the field where the mutex->owner may be still set to a stale value,
potentially upsetting mutex_spin_on_owner() among others.

References: https://bugs.freedesktop.org/show_bug.cgi?id=87955
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1420540175-30204-1-git-send-email-chris@chris-wilson.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/mutex-debug.c

@@ -80,13 +80,13 @@ void debug_mutex_unlock(struct mutex *lock)
 			DEBUG_LOCKS_WARN_ON(lock->owner != current);
 
 		DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
-		mutex_clear_owner(lock);
 	}
 
 	/*
 	 * __mutex_slowpath_needs_to_unlock() is explicitly 0 for debug
 	 * mutexes so that we can do it here after we've verified state.
 	 */
+	mutex_clear_owner(lock);
 	atomic_set(&lock->count, 1);
 }