linux-kernel - Re: [PATCH] videobuf2: Add missing lock held on vb2_fop

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Message-ID: <5262641C.5050406@gmail.com>
Date:	Sat, 19 Oct 2013 12:51:08 +0200
From:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>
To:	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
CC:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>,
	Pawel Osciak <pawel@...iak.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	linux-media <linux-media@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [PATCH] videobuf2: Add missing lock held on vb2_fop_relase

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