In D20293#443814, @hselasky wrote:

I agree about that, but as you know the TTY layer holds a mutex while calling us, so sleeping is not allowed or am I wrong? You don't solve mutex problems by just dropping the TTY lock, that leaves races wide open.

I suggest a new method in the ttydevsw which drain any pending commands off the underlying layer, which can be called outside the tty_lock() so we don't mess with the TTY's internal state.

In D20293#443792, @hselasky wrote:

I understand you want synchronous behaviour, but how about implementing a drain command, which is called unlocked from the TTY layer, just before the end of the IOCTL return. Won't that fix the problems you see?

I was going through the code putting "I think this one needs to be sync too" comments here and there. Then I realized I was pretty tagging every call to ucom_queue_command() as "needs to be sync".

I think this is a good idea, especially if these turn into KASSERT after some initial period for flushing out existing offenders.

Looks good.

Eventually committed as r348164.

Looks good, and "make lintkernels" is happy with it.