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Date: Mon, 13 Sep 2021 14:43:44 -0400 From: Waiman Long <llong@...hat.com> To: Tejun Heo <tj@...nel.org> Cc: Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>, Juri Lelli <juri.lelli@...hat.com>, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 1/2] cgroup: Fix incorrect warning from cgroup_apply_control_disable() On 9/13/21 2:35 PM, Waiman Long wrote: > On 9/13/21 2:05 PM, Tejun Heo wrote: >> Hello, >> >> On Thu, Sep 09, 2021 at 10:42:55PM -0400, Waiman Long wrote: >>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c >>> index 881ce1470beb..e31bca9fcd46 100644 >>> --- a/kernel/cgroup/cgroup.c >>> +++ b/kernel/cgroup/cgroup.c >>> @@ -3140,7 +3140,16 @@ static void >>> cgroup_apply_control_disable(struct cgroup *cgrp) >>> if (!css) >>> continue; >>> - WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt)); >>> + /* >>> + * A kill_css() might have been called previously, but >>> + * the css may still linger for a while before being >>> + * removed. Skip it in this case. >>> + */ >>> + if (percpu_ref_is_dying(&css->refcnt)) { >>> + WARN_ON_ONCE(css->parent && >>> + cgroup_ss_mask(dsct) & (1 << ss->id)); >>> + continue; >>> + } >> This warning did help me catch some gnarly bugs. Any chance we can >> keep it >> for normal cases and elide it just for remounting? > > The problem with percpu_ref_is_dying() is the fact that it becomes > true after percpu_ref_exit() is called in css_free_rwork_fn() which > has an RCU delay. If you want to catch the fact that kill_css() has > been called, we can check the CSS_DYING flag which is set in > kill_css() by commit 33c35aa481786 ("cgroup: Prevent kill_css() from > being called more than once"). Will that be an acceptable alternative? Something like diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 881ce1470beb..851e54800ad8 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -3140,6 +3140,9 @@ static void cgroup_apply_control_disable(struct cgroup *cg if (!css) continue; + if (css->flags & CSS_DYING) + continue; + WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt)); if (css->parent && Cheers, Longman
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