linux-kernel - Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat

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Message-ID: <YpjsTNZx4DG+kot4@slm.duckdns.org>
Date:   Thu, 2 Jun 2022 06:58:52 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat_flush()

Hello,

On Thu, Jun 02, 2022 at 09:35:43AM -0400, Waiman Long wrote:
> @@ -2011,9 +2076,16 @@ void blk_cgroup_bio_start(struct bio *bio)
>  	}
>  	bis->cur.ios[rwd]++;
>  
> +	if (!READ_ONCE(bis->lnode.next)) {
> +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
> +
> +		llist_add(&bis->lnode, lhead);
> +		percpu_ref_get(&bis->blkg->refcnt);

Hmm... what guarantees that more than one threads race here? llist assumes
that there's a single writer for a given llist_node and the ref count would
be off too, right?

Thanks.

-- 
tejun