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

[Tejun Heo <tj@kernel.org>]

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():

	kill_css_sync(css);
	if (!css_is_populated(css))
		kill_css_finish(css);

When the dying css is still populated, kill_css_finish() is deferred. The
walker in css_update_populated() fires kill_finish_work once the css's
hierarchical populated count drops to zero.

cgroup_lock_and_drain_offline()'s wait predicate switches from
percpu_ref_is_dying() to css_is_dying(). CSS_DYING is set by kill_css_sync()
and is a strict superset of percpu_ref_is_dying. Without this change, a +cpu
re-enable after a deferred -cpu disable would skip the drain (percpu_ref
isn't killed yet) and observe the still-CSS_DYING css through cgroup_css(),
treating it as live.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/cgroup/cgroup.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index fa24102535d9..bdc8deedb4f7 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3237,7 +3237,7 @@ void cgroup_lock_and_drain_offline(struct cgroup *cgrp)
 			struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
 			DEFINE_WAIT(wait);
 
-			if (!css || !percpu_ref_is_dying(&css->refcnt))
+			if (!css || !css_is_dying(css))
 				continue;
 
 			cgroup_get_live(dsct);
@@ -3405,7 +3405,8 @@ static void cgroup_apply_control_disable(struct cgroup *cgrp)
 			if (css->parent &&
 			    !(cgroup_ss_mask(dsct) & (1 << ss->id))) {
 				kill_css_sync(css);
-				kill_css_finish(css);
+				if (!css_is_populated(css))
+					kill_css_finish(css);
 			} else if (!css_visible(css)) {
 				css_clear_dir(css);
 				if (ss->css_reset)
-- 
2.54.0