kern/54111: t_ptrace_wait* hangs and kills test runs

[martin%NetBSD.org@localhost]

>Number:         54111
>Category:       kern
>Synopsis:       t_ptrace_wait* hangs and kills test runs
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Apr 10 05:10:00 +0000 2019
>Originator:     Martin Husemann
>Release:        NetBSD 8.99.37
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD thirdstage.duskware.de 8.99.37 NetBSD 8.99.37 (MODULAR) #169: Tue Apr 9 18:41:05 CEST 2019 martin%thirdstage.duskware.de@localhost:/usr/src/sys/arch/sparc64/compile/MODULAR sparc64
Architecture: sparc64
Machine: sparc64
>Description:

Running the atf tests on a mulitprocessor sparc64 machine hangs (randomly,
but nearly every time) in one of the t_ptrace_wait* tests.

The exiting test program is not collected, any further test activity
is blocked. This kills the regular test runs on real hardware
and is a netbsd-9 branch blocker.

Excerpt from ps axwwwwd:

  PID TTY    STAT    TIME COMMAND
    0 ?      OKl  7:45.83 [system]
    1 ?      Is   0:00.06 - init 
 7955 ?      D    0:00.00 |-- t_ptrace_wait6 -r/tmp/atf-run.Aq6p7i/tcr -s/usr/tests/lib/libc/sys -vunprivileged-user clone_vfork_signalignored:body 
 3911 ?      Z    0:00.00 | `-- (t_ptrace_wait6)
   34 pts/0- I    0:00.02 |-- /bin/sh ./test.sh 
  435 pts/0- I    0:00.01 | `-- /bin/sh ./test.sh 
  425 pts/0- I    0:02.23 |   |-- atf-report -oxml:/test-bed/work/atf.xml -oticker:- 
  430 pts/0- I    0:18.51 |   |-- atf-run 
 4954 ?      Z    0:00.00 |   | `-- (t_ptrace_wait6)
  431 pts/0- I    0:00.86 |   |-- tee /test-bed/work/atf.raw 
  445 pts/0- I    0:00.22 |   `-- tee /test-bed/work/atf.log 

PID 4954 is showing the issue.

Test machine dmesg at: https://www.NetBSD.org/~martin/sparc64-atf/dmesg.txt

I have also seen similar hangs with other variants (ISTR t_ptrace_wait4
at least).

>How-To-Repeat:
s/a

>Fix:
n/a