On Thu, February 13, 2020 8:02 am, Will Godfrey wrote:
  On running Jack into a USB soundcard, there have been
a few occasions
 where the USB lead was snagged and pulled out.
 On each occasion the computer locked up, 
On similar occurrence I have been able to run "killall jackd" and recover.
In your case the entire machine became unresponsive?
Are you running an RT kernel?  I wonder if with RT and a high enough
priority the jackd process could consume so much processor time that the
system was not usable.  In my case I have a kernel configured with
preempt, but not the full preempt-RT patches.
The only suggestion I would have is check whether the control-alt-F3 key
combo can switch to a different virtual terminal, perhaps some process in
the desktop environment locked up, but if you can get to a shell again you
could  kill jackd.
--
Chris Caudle