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>

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);
 }