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

From: David Herrmann <dh.herrmann@gmail.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 09/25] drm/gem: Check locking in drm_gem_object_unreference
Date: Thu, 15 Oct 2015 10:56:07 +0200	[thread overview]
Message-ID: <CANq1E4T+o_9kJtd4TF5Hj0zvdtSXEJc8jS_Ro3VrW7Zp5c8sjQ@mail.gmail.com> (raw)
In-Reply-To: <20151015085510.GU26718@phenom.ffwll.local>

Hi

On Thu, Oct 15, 2015 at 10:55 AM, Daniel Vetter <daniel@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@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@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
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