linux-kernel - Re: [PATCH 11/11] cgroup: use percpu refcnt for cgroup_subsys

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Date:	Fri, 14 Jun 2013 14:55:39 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...il.com>
Cc:	lizefan@...wei.com, containers@...ts.linux-foundation.org,
	cgroups@...r.kernel.org, koverstreet@...gle.com,
	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	Mike Snitzer <snitzer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"Alasdair G. Kergon" <agk@...hat.com>,
	Jens Axboe <axboe@...nel.dk>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Glauber Costa <glommer@...il.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 11/11] cgroup: use percpu refcnt for cgroup_subsys_states

On Wed 12-06-13 21:04:58, Tejun Heo wrote:
[...]
> +/**
> + * cgroup_destroy_locked - the first stage of cgroup destruction
> + * @cgrp: cgroup to be destroyed
> + *
> + * css's make use of percpu refcnts whose killing latency shouldn't be
> + * exposed to userland and are RCU protected.  Also, cgroup core needs to
> + * guarantee that css_tryget() won't succeed by the time ->css_offline() is
> + * invoked.  To satisfy all the requirements, destruction is implemented in
> + * the following two steps.
> + *
> + * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
> + *     userland visible parts and start killing the percpu refcnts of
> + *     css's.  Set up so that the next stage will be kicked off once all
> + *     the percpu refcnts are confirmed to be killed.
> + *
> + * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
> + *     rest of destruction.  Once all cgroup references are gone, the
> + *     cgroup is RCU-freed.
> + *
> + * This function implements s1.  After this step, @cgrp is gone as far as
> + * the userland is concerned and a new cgroup with the same name may be
> + * created.  As cgroup doesn't care about the names internally, this
> + * doesn't cause any problem.

Glauber is this asumption correct for kmem caches naming scheme?
I guess it should, but I would rather be sure this won't blow up later
specially when caches might live longer than css_offline.

> + */
>  static int cgroup_destroy_locked(struct cgroup *cgrp)
>  	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
>  {
-- 
Michal Hocko
SUSE Labs
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