Re: [PATCH 5/5] cgroup: Defer kill_css_finish() in cgroup_apply_control_disable()

[Tejun Heo <tj@kernel.org>]

Hello, Mark.

On Wed, May 27, 2026 at 11:45:54AM +0100, Mark Brown wrote:
> On Mon, May 04, 2026 at 02:51:21PM -1000, Tejun Heo wrote:
> 
> > Same race shape as the rmdir path that 93618edf7538 ("cgroup: Defer css
> > percpu_ref kill on rmdir until cgroup is depopulated") fixed: a task past
> > exit_signals() whose cset subsys[ssid] still pins the disabled controller's
> > css can be touching subsys state while ->css_offline() runs. The earlier
> > patches in this series built up the per-subsys-css deferral machinery and
> > routed cgroup_destroy_locked() through it. Apply the same shape to
> > cgroup_apply_control_disable():
> 
> We've been seeing hangs during testing in our testing of -next on
> multiple arm64 platforms when running LTP test jobs which bisect to this
> patch, which is 1dffd95575eb05bc7e in -next.  It looks like we hit a
> deadlock running stress tests, the end of a typical log looks like this:
> 
> <12>[  181.849144] /opt/ltp/kirk[558]: cgroup_fj_stress_blkio_3_3_none: end (returncode: 0)
> <12>[  181.860375] /opt/ltp/kirk[558]: cgroup_fj_stress_blkio_3_3_one: start (command: cgroup_fj_stress.sh blkio 3 3 one)
> cgroup_fj_stress_blkio_3_3_one: pass  (1.166s)
> <12>[  183.053379] /opt/ltp/kirk[558]: cgroup_fj_stress_blkio_3_3_one: end (returncode: 0)
> <12>[  183.064884] /opt/ltp/kirk[558]: cgroup_fj_stress_blkio_4_4_each: start (command: cgroup_fj_stress.sh blkio 4 4 each)
> cgroup_fj_stress_blkio_4_4_each: pass  (8.183s)
> <12>[  191.275815] /opt/ltp/kirk[558]: cgroup_fj_stress_blkio_4_4_each: end (returncode: 0)
> <12>[  191.287614] /opt/ltp/kirk[558]: cgroup_fj_stress_blkio_4_4_none: start (command: cgroup_fj_stress.sh blkio 4 4 none)
> cgroup_fj_stress_blkio_4_4_none: pass  (3.570s)
> <12>[  194.884173] /opt/ltp/kirk[558]: cgroup_fj_stress_blkio_4_4_none: end (returncode: 0)
> <12>[  194.895255] /opt/ltp/kirk[558]: cgroup_fj_stress_cpu_1_200_each: start (command: cgroup_fj_stress.sh cpu 1 200 each)
> 
> with no further output and given that this is a cgroup locking change
> this does seem like a plausible commmit, though I didn't look into it in
> detail.  Bisect log and the list of LTP tests we're running in our test
> job below.  We are running multuple tests in parallel.

Unfortunately, I can't reproduce this in my environment. Any chance you can
try testing on x86 tooa nd see whether it produces there?

Thanks.

-- 
tejun