From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Daniel Axtens <dja@axtens.net>
Cc: linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        Dave Stevenson <linux-media@destevenson.freeserve.co.uk>,
        Greg KH <greg@kroah.com>
Subject: Re: [PATCH 1/2] [media] uvcvideo: Refactor teardown of uvc on USB disconnect
Date: Mon, 17 Apr 2017 15:23:55 +0300
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Hi Daniel,

Thank you for the patch (and the investigation).

On Monday 17 Apr 2017 18:52:39 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 media_device_unregister() into the disconnect path.

While the patch looks reasonable to me (with one comment below though), isn't 
this an issue with the USB core, or possibly the device core ? It's a common 
practice to create device nodes as children of physical devices. Does the 
device core really require all device nodes to be unregistered synchronously 
with physical device hot-unplug ? If so, shouldn't it warn somehow when a 
device is deleted and still has children, instead of accepting that silently 
and later complaining due to sysfs issues ?

> [0]: https://lkml.org/lkml/2016/12/8/657
> 
> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Cc: Dave Stevenson <linux-media@destevenson.freeserve.co.uk>
> Cc: Greg KH <greg@kroah.com>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> 
> ---
> 
> Tested with cheese and yavta.
> ---
>  drivers/media/usb/uvc/uvc_driver.c | 8 ++++++--
>  drivers/media/usb/uvc/uvc_status.c | 8 ++++++--
>  drivers/media/usb/uvc/uvcvideo.h   | 1 +
>  3 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_driver.c
> b/drivers/media/usb/uvc/uvc_driver.c index 46d6be0bb316..2390592f78e0
> 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -1815,8 +1815,6 @@ static void uvc_delete(struct uvc_device *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);

media_device_unregister() will now be called before v4l2_device_unregister() 
which, unless I'm mistaken, will now result in 
media_device_unregister_entity() being called twice for every entity, the 
first time by media_device_unregister(), and the second time by 
v4l2_device_unregister_subdev() through v4l2_device_unregister(). Looking at 
media_device_unregister() I don't think that's safe.

We could move to v4l2_device_unregister() call to uvc_unregister_video(), but 
that worries me (perhaps unnecessarily though) due to the race conditions it 
could introduce. Would you still be able to give it a try ?

Note that your patch isn't really at fault here, the media controller and V4L2 
core code have been broken for a long time when it comes to entity lifetime 
management. That might be fixed some day, but I won't hold my breath given the 
bad track record of the previous year and a half.

>  	media_device_cleanup(&dev->mdev);
>  #endif
> 
> @@ -1884,6 +1882,12 @@ static void uvc_unregister_video(struct uvc_device
> *dev) uvc_debugfs_cleanup_stream(stream);
>  	}
> 
> +	uvc_status_unregister(dev);
> +#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);
> +}
> +
> +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)
> diff --git a/drivers/media/usb/uvc/uvcvideo.h
> b/drivers/media/usb/uvc/uvcvideo.h index 15e415e32c7f..4b4814df35cd 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -712,6 +712,7 @@ void uvc_video_clock_update(struct uvc_streaming
> *stream,
> 
>  /* Status */
>  extern int uvc_status_init(struct uvc_device *dev);
> +extern void uvc_status_unregister(struct uvc_device *dev);
>  extern void uvc_status_cleanup(struct uvc_device *dev);
>  extern int uvc_status_start(struct uvc_device *dev, gfp_t flags);
>  extern void uvc_status_stop(struct uvc_device *dev);

-- 
Regards,

Laurent Pinchart