Hi Michael, On 01/18/18 10:57, Michael Walz wrote: > Hi there, > > I am currently developing a USB/UVC camera and I think I discovered an > issue in the V4L2 event system. In userland I have implemented a thread > for the VIDIOC_DQEVENT ioctl, which blocks until there is a new event. > This works perfectly until the camera is disconnected during operation: > > > In v4l2_event_dequeue() the thread is put to sleep with > > wait_event_interruptible(fh->wait, fh->navailable != 0) > > > and woken up only from __v4l2_event_queue_fh() > > wake_up_all(&fh->wait); > > > So the event-dq-thread will only wake up when there is really a new > event queued. It will not wake up when the event is unsubscribed in > v4l2_event_unsubscribe() and it will also not wake up when > video_unregister_device() and and consequently v4l2_fh_exit() is called. > > In userland, when the device is disconnected while streaming video, > VIDIOC_DQBUF and all other ioctls return with proper error codes, but > VIDIOC_DQEVENT blocks until doomsday. > The only (dirty) workaround I found is sending a signal to the userland > thread to force the wait_event_interruptible() to return. > > I think this issue could be solved. I discovered that in videobuf2 a > similar issue is solved by checking the flag q->streaming in > wait_event_interruptible() in __vb2_wait_for_done_vb(). In > __vb2_queue_cancel() this flag is deasserted and all threads are woken > up with wake_up_all. > I suggest that the problem could be solved by waking the > event-dq-threads in v4l2_event_unsubscribe_all() or > v4l2_event_unsubscribe() so that v4l2_event_dequeue() can return. > > Problem is that I have almost no experience in kernel development. Can > somebody with more knowledge in these matters please have a look at this > issue and confirm or correct my observations? You're absolutely correct, this is a bug. There are two problems: the first is that v4l2_event_unsubscribe doesn't call wake_up_all, the second is that __v4l2_event_dequeue doesn't detect if the device was unplugged. Are you able to compile the kernel if I give you a patch to test? It should be a fairly easy fix. Regards, Hans
