[PATCH 09/25] drm/gem: Check locking in drm_gem_object_unreference

[David Herrmann]

Hi

On Thu, Oct 15, 2015 at 10:55 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Thu, Oct 15, 2015 at 10:36:08AM +0200, David Herrmann wrote:
>> Hi
>>
>> On Thu, Oct 15, 2015 at 9:36 AM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>> > Pretty soon only some drivers will need dev->struct_mutex in their
>> > gem_free_object callbacks. Hence it's really important to make sure
>> > everything still keeps getting this right.
>> >
>> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
>> > ---
>> >  include/drm/drm_gem.h | 2 ++
>> >  1 file changed, 2 insertions(+)
>> >
>> > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
>> > index 7a592d7e398b..5b3754864fb0 100644
>> > --- a/include/drm/drm_gem.h
>> > +++ b/include/drm/drm_gem.h
>> > @@ -142,6 +142,8 @@ drm_gem_object_reference(struct drm_gem_object *obj)
>> >  static inline void
>> >  drm_gem_object_unreference(struct drm_gem_object *obj)
>> >  {
>> > +       WARN_ON(!mutex_is_locked(&obj->dev->struct_mutex));
>> > +
>>
>> This doesn't work. mutex_is_locked() is not context-aware, so it does
>> not check whether the holder is the current thread or someone else.
>> You *have* to use lockdep if you want negative lock checks.
>>
>> In other words, if some other thread holds the mutex in parallel to
>> this being called, you will trigger the WARN_ON.
>
> It's the other way round: If another thread holds the lock, but the caller
> doesn't, then we won't WARN: It's a false negative, not a false positive.
>
> And since lockdep is a serious hog and no one runs it I prefer this way
> round than the "perfect" lockdep_assert_hold.
>
> Not that in the unlocked variant we can't afford this mistake, so there we
> do rely upon the perfect lockdep locking tracking and use might_lock.

Gnah, you're right! I read this as assert, not WARN_ON.. *sigh* Sorry
for the noise.

David