media: uvcvideo: Stop stream during unregister

uvc_unregister_video() can be called asynchronously from
uvc_disconnect(). If the device is still streaming when that happens, a
plethora of race conditions can occur.

Make sure that the device has stopped streaming before exiting this
function.

If the user still holds handles to the driver's file descriptors, any
ioctl will return -ENODEV from the v4l2 core.

This change makes uvc more consistent with the rest of the v4l2 drivers
using the vb2_fop_* and vb2_ioctl_* helpers.

This driver (and many other usb drivers) always had this problem, but it
wasn't possible to easily fix this until the vb2_video_unregister_device()
helper was added. So the Fixes tag points to the creation of that helper.

Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Suggested-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Fixes: f729ef5 ("media: videobuf2-v4l2.c: add vb2_video_unregister_device helper function")
Cc: stable@vger.kernel.org # 5.10.x
[hverkuil: add note regarding Fixes version]
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>

drivers/media/usb/uvc/uvc_driver.c

@@ -1935,11 +1935,41 @@ static void uvc_unregister_video(struct uvc_device *dev)
 	struct uvc_streaming *stream;
 
 	list_for_each_entry(stream, &dev->streams, list) {
+		/* Nothing to do here, continue. */
 		if (!video_is_registered(&stream->vdev))
 			continue;
 
+		/*
+		 * For stream->vdev we follow the same logic as:
+		 * vb2_video_unregister_device().
+		 */
+
+		/* 1. Take a reference to vdev */
+		get_device(&stream->vdev.dev);
+
+		/* 2. Ensure that no new ioctls can be called. */
 		video_unregister_device(&stream->vdev);
-		video_unregister_device(&stream->meta.vdev);
+
+		/* 3. Wait for old ioctls to finish. */
+		mutex_lock(&stream->mutex);
+
+		/* 4. Stop streaming. */
+		uvc_queue_release(&stream->queue);
+
+		mutex_unlock(&stream->mutex);
+
+		put_device(&stream->vdev.dev);
+
+		/*
+		 * For stream->meta.vdev we can directly call:
+		 * vb2_video_unregister_device().
+		 */
+		vb2_video_unregister_device(&stream->meta.vdev);
+
+		/*
+		 * Now both vdevs are not streaming and all the ioctls will
+		 * return -ENODEV.
+		 */
 
 		uvc_debugfs_cleanup_stream(stream);
 	}