[media] v4l2-dev: fix race condition

The unregister function had a race condition with the v4l2_open
function. Ensure that both functions test and clear the REGISTER
flag from within a critical section.

Thanks to Laurent Pinchart for finding this race.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/v4l2-dev.c

@@ -309,7 +309,7 @@ static int v4l2_open(struct inode *inode, struct file *filp)
 	mutex_lock(&videodev_lock);
 	vdev = video_devdata(filp);
 	/* return ENODEV if the video device has already been removed. */
-	if (vdev == NULL) {
+	if (vdev == NULL || !video_is_registered(vdev)) {
 		mutex_unlock(&videodev_lock);
 		return -ENODEV;
 	}
@@ -624,7 +624,12 @@ void video_unregister_device(struct video_device *vdev)
 	if (!vdev || !video_is_registered(vdev))
 		return;
 
+	mutex_lock(&videodev_lock);
+	/* This must be in a critical section to prevent a race with v4l2_open.
+	 * Once this bit has been cleared video_get may never be called again.
+	 */
 	clear_bit(V4L2_FL_REGISTERED, &vdev->flags);
+	mutex_unlock(&videodev_lock);
 	device_unregister(&vdev->dev);
 }
 EXPORT_SYMBOL(video_unregister_device);