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

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Date:	Sat, 19 Oct 2013 18:10:24 +0200
From:	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
To:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>
Cc:	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

Hello Sylwester

I have just posted a new version. Please take a look to it, it should
fix your issue.

I havent tried it in hw because I am out of the office.

Regards!

On Sat, Oct 19, 2013 at 12:51 PM, Sylwester Nawrocki
<sylvester.nawrocki@...il.com> wrote:
> 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



-- 
Ricardo Ribalda
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