From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH v2] blk-throttle: fix race between blkcg_bio_issue_check
 and cgroup_rmdir
Date: Mon, 12 Feb 2018 09:11:43 -0800
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To: Joseph Qi <joseph.qi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>
Cc: Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>, xuejiufei <jiufei.xue-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>, Caspar Zhang <caspar-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>, linux-block <linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org

Hello, Joseph.

On Fri, Feb 09, 2018 at 10:15:19AM +0800, Joseph Qi wrote:
> IIUC, we have to identify it is in blkcg_css_offline now which will
> blkg_put. Since percpu_ref_kill_and_confirm in kill_css will set flag
> __PERCPU_REF_DEAD, so we can use this to avoid the race. IOW, if
> __PERCPU_REF_DEAD is set now, we know blkcg css is in offline and
> continue access blkg may risk double free. Thus we choose to skip these
> ios.
> I don't get how css_tryget works since it doesn't care the flag
> __PERCPU_REF_DEAD. Also css_tryget can't prevent blkcg_css from
> offlining since the race happens blkcg_css_offline is in progress.
> Am I missing something here?

Once marked dead, the ref is in atomic mode and css_tryget() would hit
the atomic counter.  Here, we don't care about the offlining and
draining.  A draining memcg can still have a lot of memory to be
written back attached to it and we don't want punt all of them to the
root cgroup.

Thanks.

-- 
tejun