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

[David Herrmann]

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.

Thanks
David

>         if (obj != NULL)
>                 kref_put(&obj->refcount, drm_gem_object_free);
>  }
> --
> 2.5.1
>
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