lib/55241: Many t_ptrace_wait* test cases fail since Apr 16

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

>Number:         55241
>Category:       lib
>Synopsis:       Many t_ptrace_wait* test cases fail since Apr 16
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu May 07 11:15:01 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2020.04.16.14.39.58
>Organization:
>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

On Apr 18, an automated report was sent to current-users, saying:

>  The newly failing test cases are:
>
>     lib/libc/sys/t_ptrace_wait4:fork10
>     lib/libc/sys/t_ptrace_wait4:fork2
>     lib/libc/sys/t_ptrace_wait4:fork4
>     lib/libc/sys/t_ptrace_wait4:fork6
>     lib/libc/sys/t_ptrace_wait4:fork8
>     lib/libc/sys/t_ptrace_wait4:fork_singalmasked
>     lib/libc/sys/t_ptrace_wait4:unrelated_tracer_fork16
>     lib/libc/sys/t_ptrace_wait6:fork14
>     lib/libc/sys/t_ptrace_wait6:fork4
>     lib/libc/sys/t_ptrace_wait6:fork_singalmasked
>     lib/libc/sys/t_ptrace_wait6:unrelated_tracer_fork14
>     lib/libc/sys/t_ptrace_wait6:unrelated_tracer_fork2
>     lib/libc/sys/t_ptrace_wait6:unrelated_tracer_fork4
>     lib/libc/sys/t_ptrace_waitid:fork14
>     lib/libc/sys/t_ptrace_waitid:fork2
>     lib/libc/sys/t_ptrace_waitid:fork4
>     lib/libc/sys/t_ptrace_waitid:fork6
>     lib/libc/sys/t_ptrace_waitid:fork8
>     lib/libc/sys/t_ptrace_waitid:fork_singalmasked
>     lib/libc/sys/t_ptrace_waitid:unrelated_tracer_fork10
>     lib/libc/sys/t_ptrace_waitid:unrelated_tracer_fork12
>     lib/libc/sys/t_ptrace_waitid:unrelated_tracer_fork8
>     lib/libc/sys/t_ptrace_waitpid:fork10
>     lib/libc/sys/t_ptrace_waitpid:fork12
>     lib/libc/sys/t_ptrace_waitpid:fork14
>     lib/libc/sys/t_ptrace_waitpid:fork16
>     lib/libc/sys/t_ptrace_waitpid:fork2
>     lib/libc/sys/t_ptrace_waitpid:fork4
>     lib/libc/sys/t_ptrace_waitpid:fork6
>     lib/libc/sys/t_ptrace_waitpid:fork8
>     lib/libc/sys/t_ptrace_waitpid:fork_singalignored
>     lib/libc/sys/t_ptrace_waitpid:fork_singalmasked
>     lib/libc/sys/t_ptrace_waitpid:unrelated_tracer_fork2
>     lib/libc/sys/t_ptrace_waitpid:unrelated_tracer_fork8
>
[...]
>
> The following commits were made between the last successful test and
> the failed test:
>
>     2020.04.16.14.39.58 joerg src/lib/libc/gen/pthread_atfork.c,v 1.13
>     2020.04.16.14.39.58 joerg src/libexec/ld.elf_so/rtld.c,v 1.204
>     2020.04.16.14.39.58 joerg src/libexec/ld.elf_so/rtld.h,v 1.139
>     2020.04.16.14.39.58 joerg src/libexec/ld.elf_so/symbols.map,v 1.3

The following day, Joerg wrote:
> AFAICT the tests are bad. They fail significantly more often when
> compiled with optimisations than without, strongly suggesting race
> conditions involved. The original change here certainly changes the
> timing and I am aware of one potential bug in it, but that bug is most
> definitely not exercised by the test cases.

These test cases (and other t_ptrace_wait* test cases that were reported
in separate mails) are still failing:

  http://releng.netbsd.org/b5reports/i386/2020/2020.05.06.09.18.10/test.html#failed-tcs-summary

Most of them fail only under qemu, but the fork_signalmasked test
cases are also failing on real hardware:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.05.06.20.40.33/test.html#failed-tcs-summary

>How-To-Repeat:

>Fix: