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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 5cf6731b98e9ecf1ffffa754371701613cc64bcb..3ef3736002d895854794a4d940adcd96288640f2 100644 (file)
--- a/kernel/locking/mutex-debug.c
+++ b/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);
 }