Re: [PATCH] media: uvcvideo: Stop stream during unregister

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Mon, Oct 07, 2024 at 04:53:30PM +0200, Hans Verkuil wrote:
> On 07/10/2024 16:44, Laurent Pinchart wrote:
> > On Mon, Oct 07, 2024 at 09:46:47AM +0200, Hans Verkuil wrote:
> >> Hi Laurent,
> >>
> >> Just a reminder: I have extensively reviewed this patch here:
> >>
> >> https://lore.kernel.org/linux-media/f4c49ccf-9dc9-475a-8fc9-4ef4c85a729a@xs4all.nl/
> >>
> >> and here (specifically checking for mmap() races):
> >>
> >> https://lore.kernel.org/linux-media/1a10530f-b4bb-4244-84ff-1f2365ae9b23@xs4all.nl/
> >>
> >> To the best of my ability I believe this patch is correct.
> >>
> >> Unless you have any additional concerns I plan to take this patch as a fix for
> >> v6.12 on Monday next week.
> > 
> > I thought we had an agreement that I could submit an alternative fix for
> > v6.12. Can you therefore delay merging this patch until v6.12-rc6 ?
> 
> Correct, if there is indeed something wrong with this patch and an alternative
> fix is needed (or at least should be considered).
> 
> But I see nothing wrong with this patch after careful analysis. If you disagree
> with my analysis, and you think I missed a possible race condition, then that's
> a reason to wait for a better fix. Otherwise there is no point in waiting any longer.

I'm in Montréal this week for the GStreamer conference and XDC. I'll
reply to your last e-mail early next week, let's make a decision then.
Surely this can wait until -rc4 before being merged ?

> >> Alternatively, you can make a PR for 6.12 with this patch that I can pull from.
> >>
> >> Regards,
> >>
> >> 	Hans
> >>
> >> On 26/09/2024 07:59, 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.
> >>>
> >>> Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> >>> Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> >>> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> >>> ---
> >>> This patch was part of the series:
> >>> https://patchwork.linuxtv.org/project/linux-media/list/?series=13064
> >>>
> >>> Moved out from it to ease the review.
> >>> ---
> >>>  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 f0febdc08c2d..bee150b852e4 100644
> >>> --- a/drivers/media/usb/uvc/uvc_driver.c
> >>> +++ b/drivers/media/usb/uvc/uvc_driver.c
> >>> @@ -1919,11 +1919,41 @@ 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);
> >>>  	}
> >>>
> >>> ---
> >>> base-commit: 81ee62e8d09ee3c7107d11c8bbfd64073ab601ad
> >>> change-id: 20240926-uvc_stop_streaming-6e9fd20e97bc

-- 
Regards,

Laurent Pinchart