kern/55753: Panic when detaching ucom(4) while in use

[jmcneill%invisible.ca@localhost]

>Number:         55753
>Category:       kern
>Synopsis:       Panic when detaching ucom(4) while in use
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Oct 26 11:20:00 +0000 2020
>Originator:     Jared McNeill
>Release:        9.99.73
>Organization:
>Environment:
NetBSD 9.99.73 (GENERIC) #0: Sat Oct  3 21:43:41 UTC 2020
>Description:
Kernel crashes when unplugging a ucom (in this case uftdi) adapter while in use:

panic: kernel diagnostic assertion "!cv_has_waiters(cv)" failed: file ".../kern/kern_condvar.c", line 107

Stack trace is vpanic() -> __x86_indirect_thunk_rax() -> cv_destroy() -> tty_free() -> ucom_detach() -> config_detach() -> uftdi_detach() -> config_detach() -> usb_disconnect_port() -> uhub_explore() -> usb_discover.isra.0() -> usb_event_thread()
>How-To-Repeat:
1) # screen /dev/ttyU0 115200
2) wait for some serial activity
3) unplug USB cable
>Fix: