kern/56447: Random rump kernel panic: assertion "LIST_EMPTY(&p->p_lwps)" failed

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         56447
>Category:       kern
>Synopsis:       Random rump kernel panic: assertion "LIST_EMPTY(&p->p_lwps)" failed
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Oct 07 10:15:01 +0000 2021
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
  
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Running the ATF tests on real amd64 hardware, I'm seeing random test
failures where the rump kernel panics with the message

  panic: kernel diagnostic assertion "LIST_EMPTY(&p->p_lwps)" failed

It happened yesterday in net/ipsec/t_ipsec_tcp:ipsec_tcp_ipv6_ah_null:

  https://www.gson.org/netbsd/bugs/build/amd64-baremetal/2021/2021.10.05.21.05.12/test.html#net_ipsec_t_ipsec_tcp_ipsec_tcp_ipv6_ah_null

The previous occurrence involved the net/if_wg/t_misc:wg_malformed
test case:

  https://www.gson.org/netbsd/bugs/build/amd64-baremetal/2021/2021.09.14.20.45.19/test.html#net_if_wg_t_misc_wg_malformed

It's happened in 12 out of 207 full ATF test runs so far this year.
The first recorded failure is from sources dated 2020.06.15.20.29.29,
but since the failure is infrequent, the bug is likely older.

>How-To-Repeat:

>Fix: