kern/53884: fs/vfs/t_rmdirrace:lfs_race test case fails randomly on real hardware

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

>Number:         53884
>Category:       kern
>Synopsis:       fs/vfs/t_rmdirrace:lfs_race test case fails randomly on real hardware
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jan 16 11:40:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2015.10.13.20.49.39
>Organization:

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

Running the ATF tests on real amd64 hardware (an 8-core HP DL360 G7), the
lfs_race test case of the fs/vfs/t_rmdirrace test program randomly fails
in about half the runs, with

  panic: kernel diagnostic assertion "fs->lfs_cleaner_thread == curlwp" failed

Log output from the latest failure:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2019/2019.01.15.14.23.56/test.html#fs_vfs_t_rmdirrace_lfs_race

The first recorded failure was on 2015.10.13.20.49.39:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2015/2015.10.13.20.49.39/test.html#fs_vfs_t_rmdirrace_lfs_race

The test case is not failing on the qemu-based TNF testbed, which
emulates a uniprocessor system.

>How-To-Repeat:

>Fix: