linux-kernel - Re: [PATCH] llist: add [READ|WRITE]_ONCE tags for llist

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Message-ID: <aF3ioBE_HX2_s-_o@slm.duckdns.org>
Date: Thu, 26 Jun 2025 14:15:28 -1000
From: Tejun Heo <tj@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: "Paul E . McKenney" <paulmck@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	JP Kobryn <inwardvessel@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Huang <huang.ying.caritas@...il.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Alexei Starovoitov <ast@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Michal Koutný <mkoutny@...e.com>,
	bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] llist: add [READ|WRITE]_ONCE tags for llist_node

Hello,

On Thu, Jun 26, 2025 at 04:38:18PM -0700, Shakeel Butt wrote:
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index c8a48cf83878..02258b43abb3 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -86,8 +86,12 @@ __bpf_kfunc void css_rstat_updated(struct cgroup_subsys_state *css, int cpu)
>  		return;
>  
>  	rstatc = css_rstat_cpu(css, cpu);
> -	/* If already on list return. */
> -	if (llist_on_list(&rstatc->lnode))
> +	/*
> +	 * If already on list return.
> +	 *
> +	 * TODO: add detailed comment on the potential race.
> +	 */
> +	if (data_race(llist_on_list(&rstatc->lnode)))
>  		return;

Yeah, also in the functdion comment, explain that if avoiding such races is
important enough to justify the overhead, the caller can put smp_mb() before
calling css_rstat_updated().

Thanks.

-- 
tejun