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@xxxxxxxxxxxxxxxx> > Cc: Dave Stevenson <linux-media@xxxxxxxxxxxxxxxxxxxxxxxxxxx> > Cc: Greg KH <greg@xxxxxxxxx> > Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx> > > --- > > 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
