Date:   Thu, 08 Nov 2018 00:06:11 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Daniel Axtens <dja@...ens.net>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dave Stevenson <linux-media@...tevenson.freeserve.co.uk>,
        Greg KH <greg@...ah.com>
Subject: Re: [PATCH v2] [media] uvcvideo: Refactor teardown of uvc on USB disconnect

Hi Daniel,

Thank you for the patch.

On Sunday, 23 April 2017 07:53:49 EET Daniel Axtens wrote:
> Currently, disconnecting a USB webcam while it is in use prints out a
> number of warnings, such as:
> 
> WARNING: CPU: 2 PID: 3118 at
> /build/linux-ezBi1T/linux-4.8.0/fs/sysfs/group.c:237
> sysfs_remove_group+0x8b/0x90 sysfs group ffffffffa7cd0780 not found for
> kobject 'event13'
> 
> This has been noticed before. [0]
> 
> This is because of the order in which things are torn down.
> 
> If there are no streams active during a USB disconnect:
> 
>  - uvc_disconnect() is invoked via device_del() through the bus
>    notifier mechanism.
> 
>  - this calls uvc_unregister_video().
> 
>  - uvc_unregister_video() unregisters the video device for each
>    stream,
> 
>  - because there are no streams open, it calls uvc_delete()
> 
>  - uvc_delete() calls uvc_status_cleanup(), which cleans up the status
>    input device.
> 
>  - uvc_delete() calls media_device_unregister(), which cleans up the
>    media device
> 
>  - uvc_delete(), uvc_unregister_video() and uvc_disconnect() all
>    return, and we end up back in device_del().
> 
>  - device_del() then cleans up the sysfs folder for the camera with
>    dpm_sysfs_remove(). Because uvc_status_cleanup() and
>    media_device_unregister() have already been called, this all works
>    nicely.
> 
> If, on the other hand, there *are* streams active during a USB disconnect:
> 
>  - uvc_disconnect() is invoked
> 
>  - this calls uvc_unregister_video()
> 
>  - uvc_unregister_video() unregisters the video device for each
>    stream,
> 
>  - uvc_unregister_video() and uvc_disconnect() return, and we end up
>    back in device_del().
> 
>  - device_del() then cleans up the sysfs folder for the camera with
>    dpm_sysfs_remove(). Because the status input device and the media
>    device are children of the USB device, this also deletes their
>    sysfs folders.
> 
>  - Sometime later, the final stream is closed, invoking uvc_release().
> 
>  - uvc_release() calls uvc_delete()
> 
>  - uvc_delete() calls uvc_status_cleanup(), which cleans up the status
>    input device. Because the sysfs directory has already been removed,
>    this causes a WARNing.
> 
>  - uvc_delete() calls media_device_unregister(), which cleans up the
>    media device. Because the sysfs directory has already been removed,
>    this causes another WARNing.
> 
> To fix this, we need to make sure the devices are always unregistered
> before the end of uvc_disconnect(). To this, move the unregistration
> into the disconnect path:
> 
>  - split uvc_status_cleanup() into two parts, one on disconnect that
>    unregisters and one on delete that frees.
> 
>  - move v4l2_device_unregister() and media_device_unregister() into
>    the disconnect path.
> 
> [0]: https://lkml.org/lkml/2016/12/8/657
> 
> Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> Cc: Dave Stevenson <linux-media@...tevenson.freeserve.co.uk>
> Cc: Greg KH <greg@...ah.com>
> Signed-off-by: Daniel Axtens <dja@...ens.net>
> 
> ---
> 
> v2: Move v4l2_device_unregister() as well as media_device_unregister(),
>     thanks Laurent.
> 
> Tested with cheese and yavta.
> 
> Laurent, I know you were concerned about race conditions so I plugged
> and unplugged the camera over a dozen times, with lots of debugging
> turned on.
> 
> In particular:
>  - KASan was enabled and didn't trigger
>  - If I unplugged while yavta was running, and replugged *while yatva
>    was still running*, the video camera came up as video2, not video1.
>    This indicates that yatva was successfully holding a reference to
>    video1 - as it should.
>  - No other debugging triggered any warning.

Thank you for running those tests. I'm still a bit concerned but I'll live 
with it. The issues are in the MC and V4L2 core anyway, there isn't much we 
could do here to fix them.

> ---
>  drivers/media/usb/uvc/uvc_driver.c | 12 ++++++++----
>  drivers/media/usb/uvc/uvc_status.c |  8 ++++++--
>  drivers/media/usb/uvc/uvcvideo.h   |  1 +
>  3 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_driver.c
> b/drivers/media/usb/uvc/uvc_driver.c index 46d6be0bb316..0d0541054ce2
> 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -1812,11 +1812,7 @@ static void uvc_delete(struct uvc_device *dev)
>  	usb_put_intf(dev->intf);
>  	usb_put_dev(dev->udev);
> 
> -	if (dev->vdev.dev)
> -		v4l2_device_unregister(&dev->vdev);
>  #ifdef CONFIG_MEDIA_CONTROLLER
> -	if (media_devnode_is_registered(dev->mdev.devnode))
> -		media_device_unregister(&dev->mdev);
>  	media_device_cleanup(&dev->mdev);
>  #endif
> 
> @@ -1884,6 +1880,14 @@ static void uvc_unregister_video(struct uvc_device
> *dev) uvc_debugfs_cleanup_stream(stream);
>  	}
> 
> +	uvc_status_unregister(dev);
> +	if (dev->vdev.dev)
> +		v4l2_device_unregister(&dev->vdev);
> +#ifdef CONFIG_MEDIA_CONTROLLER
> +	if (media_devnode_is_registered(dev->mdev.devnode))
> +		media_device_unregister(&dev->mdev);
> +#endif
> +
>  	/* Decrement the stream count and call uvc_delete explicitly if there
>  	 * are no stream left.
>  	 */
> diff --git a/drivers/media/usb/uvc/uvc_status.c
> b/drivers/media/usb/uvc/uvc_status.c index f552ab997956..95709b23d3b4
> 100644
> --- a/drivers/media/usb/uvc/uvc_status.c
> +++ b/drivers/media/usb/uvc/uvc_status.c
> @@ -198,12 +198,16 @@ int uvc_status_init(struct uvc_device *dev)
>  	return 0;
>  }
> 
> -void uvc_status_cleanup(struct uvc_device *dev)
> +void uvc_status_unregister(struct uvc_device *dev)
>  {
>  	usb_kill_urb(dev->int_urb);
> +	uvc_input_cleanup(dev);

I think it would make sense to rename uvc_input_cleanup() to 
uvc_input_unregister(). I'll make the change locally so there's no need to 
resubmit this patch.

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

and applied to my tree.

> +}
> +
> +void uvc_status_cleanup(struct uvc_device *dev)
> +{
>  	usb_free_urb(dev->int_urb);
>  	kfree(dev->status);
> -	uvc_input_cleanup(dev);
>  }
> 
>  int uvc_status_start(struct uvc_device *dev, gfp_t flags)

[snip]

-- 
Regards,

Laurent Pinchart

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