arm64 torture test hotplug failures (offlining causes -EBUSY)

[Joel Fernandes]

Hello,
I am seeing -EBUSY returned a lot during torture_onoff() when running
rcutorture on arm64. This causes hotplug failure 30% of the time. I am
also seeing this in 6.1-rc kernels. I believe see this only for CPU0.

This causes warnings in torture tests:
[  217.582290] rcu-torture:torture_onoff task: offline 0 failed: errno -16
[  221.866362] rcu-torture:torture_onoff task: offline 0 failed: errno -16

Full kernel log here:
http://box.joelfernandes.org:9080/job/rcutorture_stable_arm/job/linux-5.15.y/7/artifact/tools/testing/selftests/rcutorture/res/2023.01.15-14.51.11/TREE04/console.log

Any ideas on why this is happening and only for CPU 0 (presumably the
boot CPU)? I'd personally need these warnings to go away for my tests
as this causes rcutorture's tests to not cleanly pass for me. It
appears remove_cpu() -> device_offline() is what returns the error.

You can browse through all the torture test artifacts here which
contains everything (build logs, vmlinux, etc).
http://box.joelfernandes.org:9080/job/rcutorture_stable_arm/job/linux-5.15.y/7/artifact/tools/testing/selftests/rcutorture/res/2023.01.15-14.51.11/

Thanks for your help!

 - Joel