linux-kernel - Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction path

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Date:   Mon, 12 Dec 2022 12:24:02 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Josef Bacik <josef@...icpanda.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Koutný <mkoutny@...e.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>
Subject: Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs
 destruction path

On Sun, Dec 11, 2022 at 05:20:58PM -0500, Waiman Long wrote:
> As noted by Michal, the blkg_iostat_set's in the lockless list
> hold reference to blkg's to protect against their removal. Those
> blkg's hold reference to blkcg. When a cgroup is being destroyed,
> cgroup_rstat_flush() is only called at css_release_work_fn() which is
> called when the blkcg reference count reaches 0. This circular dependency
> will prevent blkcg from being freed until some other events cause
> cgroup_rstat_flush() to be called to flush out the pending blkcg stats.
> 
> To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush()
> function to flush stats for a given css and cpu and call it at the blkgs
> destruction path, blkcg_destroy_blkgs(), whenever there are still some
> pending stats to be flushed. This will ensure that blkcg reference
> count can reach 0 ASAP.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

Acked-by: Tejun Heo <tj@...nel.org>

But a nit below

> +	/*
> +	 * Flush all the non-empty percpu lockless lists.
> +	 */

Can you please explain the deadlock that's being avoided in the above
comment? ie. it should say why this flush is necessary.

> +	for_each_possible_cpu(cpu) {
> +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
> +
> +		if (!llist_empty(lhead))
> +			cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);
> +	}

Thanks.

-- 
tejun