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Date: Mon, 19 Jan 2015 09:30:01 -0500
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH -mm v2 3/7] cgroup: release css->id after css_free
Message-ID: <20150119143001.GH8140@htj.dyndns.org>
References: <cover.1421664712.git.vdavydov@parallels.com>
 <4d7447a920522c1085ff96c08b2be71e0eb5d896.1421664712.git.vdavydov@parallels.com>
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Sender: owner-linux-mm@kvack.org
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To: Vladimir Davydov <vdavydov@parallels.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>, Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Dave Chinner <david@fromorbit.com>, Al Viro <viro@zeniv.linux.org.uk>, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org

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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