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
