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

Sylwester Nawrocki
2013-10-19T10:51:14Z

On 10/19/2013 12:22 PM, Ricardo Ribalda Delgado wrote:
> On Sat, Oct 19, 2013 at 11:55 AM, Sylwester Nawrocki
> <sylvester.nawrocki@gmail.com> 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