linux-kernel - Re: [PATCH -mm v2 3/7] cgroup: release css->id after css

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Date:	Mon, 19 Jan 2015 09:30:01 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Dave Chinner <david@...morbit.com>,
	Al Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 3/7] cgroup: release css->id after css_free

On Mon, Jan 19, 2015 at 02:23:21PM +0300, Vladimir Davydov wrote:
> Currently, we release css->id in css_release_work_fn, right before
> calling css_free callback, so that when css_free is called, the id may
> have already been reused for a new cgroup.
> 
> I am going to use css->id to create unique names for per memcg kmem
> caches. Since kmem caches are destroyed only on css_free, I need css->id
> to be freed after css_free was called to avoid name clashes. This patch
> therefore moves css->id removal to css_free_work_fn. To prevent
> css_from_id from returning a pointer to a stale css, it makes
> css_release_work_fn replace the css ptr at css_idr:css->id with NULL.

I think it'd be better if you create a separate id for this purpose.
The requirement is pretty unusual and likely contradictory with other
usages.

Thanks.

-- 
tejun
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