linux-kernel - Re: [PATCH 1/2] cgroup: Fix incorrect warning from cgroup_apply_control

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Date:   Mon, 13 Sep 2021 08:05:55 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
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()

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?

Thanks.

-- 
tejun