drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES
drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES

If CONFIG_DEBUG_MUTEXES is set, the mutex->owner field is only cleared
if the mutex debugging is enabled which introduces a race in our
mutex_is_locked_by() - i.e. we may inspect the old owner value before it
is acquired by the new task.

This is the root cause of this error:

 diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
 index 5cf6731..3ef3736 100644
 --- 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);
  }

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87955
Signed-off-by: Chris Wilson <711c73f64afdce07b7e38039a96d2224209e9a6c@chris-wilson.co.uk>
Cc: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org
Reviewed-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>
Signed-off-by: Jani Nikula <ba783f3beccaedfda693f41a15407d612a629408@intel.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs