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

Ricardo Ribalda Delgado
2013-10-19T16:10:45Z

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@gmail.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@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

--
Ricardo Ribalda