From mboxrd@z Thu Jan  1 00:00:00 1970
From: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq
Date: Thu, 25 May 2023 11:25:05 -0400
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To: =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny-IBi9RG/b67k@public.gmane.org>, Ming Lei <ming.lei-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Linux-MM <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>, Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>, Muchun Song <muchun.song-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>, Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>, linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>


On 5/25/23 10:11, Michal Koutn=C3=BD wrote:
> On Wed, May 24, 2023 at 10:37:10AM +0800, Ming Lei <ming.lei-H+wXaHxf7aLQ=
T0dZR+AlfA@public.gmane.org> wrote:
>>> I am not at all familiar with blkcg, but does calling
>>> cgroup_rstat_flush() in offline_css() fix the problem?
>> Except for offline, this list needs to be flushed after the associated d=
isk
>> is deleted.
> Why the second flush trigger?
> a) To break another ref-dependency cycle (like on the blkcg side)?
> b) To avoid stale data upon device removal?
>
> (Because b) should be unnecessary, a future reader would flush when
> needed.)

Since the percpu blkg_iostat_set's that are linked in the lockless list=20
will be freed if the corresponding blkcg_gq is freed, we need to flush=20
the lockless list to avoid potential use-after-free in a future=20
cgroup_rstat_flush*() call.

Cheers,
Longman