kern/54786: Panic triggered by posix_spawn_kill_spawner test case

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

>Number:         54786
>Category:       kern
>Synopsis:       Panic triggered by posix_spawn_kill_spawner test case
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Dec 19 08:30:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

On the TNF i386 testbed, the ATF test runs have been randomly failing
to complete due to a panic triggered by the posix_spawn_kill_spawner
test case:

  i386/results/2019/2019.10.29.22.21.53/test.log.gz:    posix_spawn_kill_spawner: [ 8372.7092110] uvm_fault(0xc243ed68, 0, 1) -> 0xe
  i386/results/2019/2019.11.04.12.45.10/test.log.gz:    posix_spawn_kill_spawner: [ 7857.2869663] uvm_fault(0xc22e1f0c, 0, 1) -> 0xe
  i386/results/2019/2019.11.06.11.55.18/test.log.gz:    posix_spawn_kill_spawner: [ 7668.6998710] uvm_fault(0xc2441378, 0, 1) -> 0xe
  i386/results/2019/2019.11.13.07.56.10/test.log.gz:    posix_spawn_kill_spawner: [ 7276.8763671] uvm_fault(0xc2432870, 0, 1) -> 0xe
  i386/results/2019/2019.11.14.06.00.16/test.log.gz:    posix_spawn_kill_spawner: [ 7601.2529275] uvm_fault(0xc22d051c, 0, 1) -> 0xe
  i386/results/2019/2019.11.29.00.36.22/test.log.gz:    posix_spawn_kill_spawner: [ 7501.2478758] uvm_fault(0xc22d9a14, 0, 1) -> 0xe
  i386/results/2019/2019.12.08.19.52.37/test.log.gz:    posix_spawn_kill_spawner: [ 8422.8664408] uvm_fault(0xc237a0f8, 0, 1) -> 0xe
  i386/results/2019/2019.12.16.13.48.44/test.log.gz:    posix_spawn_kill_spawner: [ 7705.1209098] uvm_fault(0xc251d520, 0, 1) -> 0xe

The full console output from the latest failure is at:

  http://releng.netbsd.org/b5reports/i386/2019/2019.12.16.13.48.44/test.log

>How-To-Repeat:

Boot -current/i386 in qemu, log in as root, and run:

cd /usr/tests/lib/libc/sys/
while ./t_ptrace_waitpid posix_spawn_kill_spawner
do true
done

>Fix: