linux-kernel - Re: [PATCH v6 1/4] media: uvcvideo: Stop stream during unregister

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Date: Mon, 17 Jun 2024 09:22:05 +0200
From: Ricardo Ribalda <ribalda@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, Guenter Roeck <linux@...ck-us.net>, 
	Tomasz Figa <tfiga@...omium.org>, Alan Stern <stern@...land.harvard.edu>, 
	Hans Verkuil <hverkuil-cisco@...all.nl>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Sean Paul <seanpaul@...omium.org>, 
	Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: [PATCH v6 1/4] media: uvcvideo: Stop stream during unregister

Hi Laurent

On Mon, 17 Jun 2024 at 01:54, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Ricardo,
>
> Thank you for the patch.
>
> On Fri, Jun 14, 2024 at 12:41:27PM +0000, Ricardo Ribalda wrote:
> > uvc_unregister_video() can be called asynchronously from
> > uvc_disconnect(). If the device is still streaming when that happens, a
> > plethora of race conditions can occur.
> >
> > Make sure that the device has stopped streaming before exiting this
> > function.
> >
> > If the user still holds handles to the driver's file descriptors, any
> > ioctl will return -ENODEV from the v4l2 core.
> >
> > This change makes uvc more consistent with the rest of the v4l2 drivers
> > using the vb2_fop_* and vb2_ioctl_* helpers.
>
> As I've said many times before, this issue needs a fix in the V4L2 core,
> ideally with support in the cdev core. It seems I'll have to do it
> myself ?

 vb2_video_unregister_device() already patched this issue. We are just
porting that solution to UVC, because uvc is not using the vb2
helpers.
I don't see why being more consistent with the rest of the v4l2 driver
makes it a bad thing.

I am reverting the ChromeOS solution for this race condition and
applying this patch instead:
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5632459/1
That is going to test this patch in some million devices in some days.

This change is self-contained, fixes a real problem, makes the driver
consistent and will be tested by lots of users. We could land this
patch and help our users until there is a solution in cdev.
I would argue that even with a solution in cdev this is not a bad patch.

> ideally with support in the cdev core. It seems I'll have to do it
> myself ?

I can help reviewing and testing ;)

Regards!


>
> > Reviewed-by: Hans Verkuil <hverkuil-cisco@...all.nl>
> > Suggested-by: Hans Verkuil <hverkuil-cisco@...all.nl>
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> >  drivers/media/usb/uvc/uvc_driver.c | 32 +++++++++++++++++++++++++++++++-
> >  1 file changed, 31 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> > index bbd90123a4e7..55132688e363 100644
> > --- a/drivers/media/usb/uvc/uvc_driver.c
> > +++ b/drivers/media/usb/uvc/uvc_driver.c
> > @@ -1908,11 +1908,41 @@ static void uvc_unregister_video(struct uvc_device *dev)
> >       struct uvc_streaming *stream;
> >
> >       list_for_each_entry(stream, &dev->streams, list) {
> > +             /* Nothing to do here, continue. */
> >               if (!video_is_registered(&stream->vdev))
> >                       continue;
> >
> > +             /*
> > +              * For stream->vdev we follow the same logic as:
> > +              * vb2_video_unregister_device().
> > +              */
> > +
> > +             /* 1. Take a reference to vdev */
> > +             get_device(&stream->vdev.dev);
> > +
> > +             /* 2. Ensure that no new ioctls can be called. */
> >               video_unregister_device(&stream->vdev);
> > -             video_unregister_device(&stream->meta.vdev);
> > +
> > +             /* 3. Wait for old ioctls to finish. */
> > +             mutex_lock(&stream->mutex);
> > +
> > +             /* 4. Stop streaming. */
> > +             uvc_queue_release(&stream->queue);
> > +
> > +             mutex_unlock(&stream->mutex);
> > +
> > +             put_device(&stream->vdev.dev);
> > +
> > +             /*
> > +              * For stream->meta.vdev we can directly call:
> > +              * vb2_video_unregister_device().
> > +              */
> > +             vb2_video_unregister_device(&stream->meta.vdev);
> > +
> > +             /*
> > +              * Now both vdevs are not streaming and all the ioctls will
> > +              * return -ENODEV.
> > +              */
> >
> >               uvc_debugfs_cleanup_stream(stream);
> >       }
>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda