Re: [PATCH v2] [media] uvcvideo: Refactor teardown of uvc on USB disconnect

From: Greg KH
Date: Sun Apr 30 2017 - 12:22:21 EST


On Sun, Apr 23, 2017 at 02:53:49PM +1000, 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@xxxxxxxxxxxxxxxx>
> Cc: Dave Stevenson <linux-media@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
> Cc: Greg KH <greg@xxxxxxxxx>
> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>


Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Very nice work, what a tangled web this is...

greg k-h