drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES
authorChris Wilson <chris@chris-wilson.co.uk>
Fri, 2 Jan 2015 09:47:10 +0000 (09:47 +0000)
committerJani Nikula <jani.nikula@intel.com>
Mon, 12 Jan 2015 08:53:02 +0000 (10:53 +0200)
commit226e5ae9e5f9108beb0bde4ac69f68fe6210fed9
tree3ef710700347c707757b89ea3f1e8f9c5228ddb7
parent48bf5b2d00bfeb681f6500c626189c7cd2c964d2
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 <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
drivers/gpu/drm/i915/i915_gem.c