20.04.1 hang/shutdown failure when changing SSID

[Chris Green]

A couple of times recently when I have tried to change SSID (moving my
laptop around the house) my Lenovo T470 running 20.04.1 has partially
hung and can't be shut down.

It continues to work locally, i.e. I can use the terminal windows to
run user programs but I can't shut it down and I can't become root.

Any attempt to use sudo just hangs, even from console terminals (i.e.
outside the GUI).  Doing a shutdown from the GUI also hangs leaving
the screen spinner running for ever.

It recovers fine if I just power down (long hold the power button) and
reboots without any issues.

A series of messages like this one (but for a number of different
processes) appear on the console log:-

Aug 13 08:40:54 t470 kernel: [166747.511815] INFO: task NetworkManager:529 blocked for more than 120 seconds.
Aug 13 08:40:54 t470 kernel: [166747.511855]       Tainted: G W         5.4.0-42-generic ✔ 6-Ubuntu
Aug 13 08:40:54 t470 kernel: [166747.511886] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Aug 13 08:40:54 t470 kernel: [166747.511921] NetworkManager  D    0 529      1 0x00000000

-- 
Chris Green