Re: [PATCH] videobuf2: Add missing lock held on vb2_fop_relase

[Sylwester Nawrocki]

On 10/19/2013 12:22 PM, Ricardo Ribalda Delgado wrote:
> On Sat, Oct 19, 2013 at 11:55 AM, Sylwester Nawrocki
> <sylvester.nawrocki@xxxxxxxxx>  wrote:
> > >  On 10/14/2013 09:41 AM, Ricardo Ribalda Delgado wrote:
> > > >>
> > > >>  vb2_fop_relase does not held the lock although it is modifying the
> > > >>  queue->owner field.
> > >  [...]
> > > >>  diff --git a/drivers/media/v4l2-core/videobuf2-core.c
> > > >>  b/drivers/media/v4l2-core/videobuf2-core.c
> > > >>  index 9fc4bab..3a961ee 100644
> > > >>  --- a/drivers/media/v4l2-core/videobuf2-core.c
> > > >>  +++ b/drivers/media/v4l2-core/videobuf2-core.c
> > > >>  @@ -2588,8 +2588,15 @@ int vb2_fop_release(struct file *file)
> > > >>           struct video_device *vdev = video_devdata(file);
> > > >>
> > > >>           if (file->private_data == vdev->queue->owner) {
> > > >>  +               struct mutex *lock;
> > > >>  +
> > > >>  +               lock = vdev->queue->lock ? vdev->queue->lock : vdev->lock;
> > > >>  +               if (lock)
> > > >>  +                       mutex_lock(lock);
> > > >>                   vb2_queue_release(vdev->queue);
> > > >>                   vdev->queue->owner = NULL;
> > > >>  +               if (lock)
> > > >>  +                       mutex_unlock(lock);
> > > >>           }
> > > >>           return v4l2_fh_release(file);
> > > >>     }
> > >
> > >
> > >  It seems you didn't inspect all users of vb2_fop_release(). There are 3
> > >  drivers
> > >  that don't assign vb2_fop_release() to struct v4l2_file_operations directly
> > >  but
> > >  instead call it from within its own release() handler. Two of them do call
> > >  vb2_fop_release() with the video queue lock already held.
> > >
> > >  $ git grep -n vb2_fop_rel -- drivers/media/
> > >
> > >  drivers/media/platform/exynos4-is/fimc-capture.c:552:   ret =
> > >  vb2_fop_release(file);
> > >  drivers/media/platform/exynos4-is/fimc-lite.c:549: vb2_fop_release(file);
> > >
>
> Very good catch, thanks!
>
> > >  A rather ugly solution would be to open code the vb2_fop_release() function
> > >  in those driver, like in below patch (untested). Unless there are better
> > >  proposals I would queue the patch as below together with the $subject patch
> > >  upstream.
>
> IMHO this will lead to the same type of mistakes in the future.
>
>   What about creating a function __vb2_fop_release that does exactly
> the same as the original function but with an extra parameter bool
> lock_held
>
> vb2_fop_release will be a wrapper for that funtion with lock_held== false

Hmm, the parameter would be telling whether the lock is already held ? 
Perhaps
we should inverse its meaning and it should indicate whether 
vb2_fop_release()
should be taking the lock internally ? It seems to me more straightforward.

> drivers that overload the fop_release and need to hold the lock will
> call the __ function with lock_held= true
>
> What do you think?

I was also considering this, it's probably better. I'm not sure about 
exporting
functions prefixed with __ though. And the locking becomes less clear 
with such
functions proliferation.

Anyway, I'm in general personally OK with having an additional version like:

__vb2_fop_release(struct file *filp, bool lock);


Regards,
Sylwester
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