acm_port_shutdown hangs for 30 seconds

[Jookia <contact@jookia.org>]

Hello there,

I've been banging my head against this issue over the years but sat down
and started to debug it today.

When I try to quit GNU screen, minicom or another serial program it
sometimes hangs for around 30 seconds.

To reproduce I do this:

1. Connect an Arduino Micro with stock LED blink firmware
2. Run 'screen /dev/ttyACM1 9600'
3. Type some letters (no response from the board is given)
4. Quit in some way

If I skip step 3 (typing letters), the hang does not happen.

In userspace the hang happens at a call to close() on the TTY, and using
perf and ftrace it looks to be spending a lot of time poisoning urbs in
acm_port_shutdown.
This kind of makes sense to me as there is some in flight data to be
sent and there's no acknowledgement coming from the chip.

I've had this issue with embedded Linux boards too that for some reason
or another have stopped functioning.
The current solution I have is to forcefully unplug the board, but
that's not ideal.

My questions are:

- Is this a bug?
- Can I reduce the timeout somehow?

Thanks for your time,
Jookia.