kern/55020: dbregs_dr?_dont_inherit_lwp test cases fail on real hardware

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

>Number:         55020
>Category:       kern
>Synopsis:       dbregs_dr?_dont_inherit_lwp test cases fail on real hardware
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Feb 26 10:25:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2020.02.21.02.04.40
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Running the ATF test on real amd64 hardware, ptrace test cases with
names matching the pattern "dbregs_dr?_dont_inherit_lwp" have been
failing infrequently (0 to 1 failures per run) for some time, and are
now failing frequently (8 to 16 failures per run):

  guido /bracket/amd64-baremetal/results/2020 $ zgrep -c 'dbregs_dr._dont_inherit.*Fail' $(zgrep -l test_completed_status=0 2020.02*/bracket.db | sed 's/bracket.db/test.log.gz/')
  [...]
  2020.02.18.17.50.32/test.log.gz:0
  2020.02.18.20.23.17/test.log.gz:0
  2020.02.18.21.29.39/test.log.gz:0
  2020.02.19.13.32.40/test.log.gz:0
  2020.02.19.21.51.21/test.log.gz:1
  2020.02.20.08.06.15/test.log.gz:1
  2020.02.20.08.31.17/test.log.gz:1
  2020.02.20.15.48.05/test.log.gz:1
  2020.02.21.02.04.40/test.log.gz:11
  2020.02.22.00.42.15/test.log.gz:12
  2020.02.22.02.28.06/test.log.gz:8
  2020.02.22.11.52.45/test.log.gz:8
  2020.02.22.19.09.51/test.log.gz:13
  2020.02.22.19.46.49/test.log.gz:8
  2020.02.22.19.54.35/test.log.gz:9
  2020.02.23.18.57.28/test.log.gz:11
  2020.02.23.20.08.35/test.log.gz:12
  2020.02.24.00.47.38/test.log.gz:16
  2020.02.25.10.27.01/test.log.gz:10
  2020.02.25.21.41.38/test.log.gz:10

Log output from one of the earlier infrequent failures is at:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.02.20.15.48.05/test.html#lib_libc_sys_t_ptrace_wait6_dbregs_dr0_dont_inherit_lwp

Log output from the latest run is at:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.02.25.21.41.38/test.html#failed-tcs-summary

>How-To-Repeat:

>Fix: