linux-kernel - Re: [PATCH cgroup/for-next 1/3] cgroup: remove redundancy online

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Message-ID: <zumtn2pez376u2xacibdd6eueeueh6ole5w6pqbgccg2bj7fgd@o2szr644a3om>
Date: Tue, 26 Aug 2025 16:14:47 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Chen Ridong <chenridong@...weicloud.com>
Cc: tj@...nel.org, hannes@...xchg.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, lujialin4@...wei.com, chenridong@...wei.com
Subject: Re: [PATCH cgroup/for-next 1/3] cgroup: remove redundancy online_cnt

Hello Ridong.

On Tue, Aug 26, 2025 at 03:40:20AM +0000, Chen Ridong <chenridong@...weicloud.com> wrote:
> @@ -5949,7 +5944,7 @@ static void css_killed_work_fn(struct work_struct *work)
>  		css_put(css);
>  		/* @css can't go away while we're holding cgroup_mutex */
>  		css = css->parent;
> -	} while (css && atomic_dec_and_test(&css->online_cnt));
> +	} while (css && css_is_dying(css) && !css->nr_descendants);

Here it's OK...

>  
>  	cgroup_unlock();
>  }
> @@ -5960,7 +5955,7 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
>  	struct cgroup_subsys_state *css =
>  		container_of(ref, struct cgroup_subsys_state, refcnt);
>  
> -	if (atomic_dec_and_test(&css->online_cnt)) {
> +	if (!css->nr_descendants) {
>  		INIT_WORK(&css->destroy_work, css_killed_work_fn);
>  		queue_work(cgroup_offline_wq, &css->destroy_work);
>  	}

... but here in percpu_ref's confirm callback you're accessing
nr_descendants without cgroup_mutex where the atomic would have
prevented the data race.

Also the semantics of online_cnt and nr_descendants is slightly
different -- killed vs offlined. Or can you add a description why
they're same (after workqueue split)?

Thanks,
Michal

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