linux-kernel - Re: [PATCH 2/4] cgroup:add css_is

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Date:	Mon, 19 Jan 2009 18:23:03 -0800
From:	Paul Menage <menage@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/4] cgroup:add css_is_populated

On Mon, Jan 19, 2009 at 6:02 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> Ah, this is related to CSS ID scanning. No real problem to current codes.
>
> Now, in my patch, CSS ID is attached just after create().
>
> Then, "scan by CSS ID" can find a cgroup which is not populated yet.
> I just wanted to skip them for avoiding mess.
>
> For example, css_tryget() can succeed against css which belongs to not-populated
> cgroup. If creation of cgroup fails, it's destroyed and freed without RCU synchronize.
> This may breaks CSS ID scanning's assumption that "we're safe under rcu_read_lock".
> And allows destroy css while css->refcnt > 1.

So for the CSS ID case, we could solve this by not populating
css_id->css until creation is guaranteed to have succeeded? (We'd
still allocated the css_id along with the subsystem, just not complete
its initialization). cgroup.c already knows about and hides the
details of css_id, so this wouldn't be hard.

The question is whether other users of css_tryget() might run into
this problem, without using CSS IDs. But currently no-one's using
css_tryget() apart from you, so that's a problem we can solve as it
arises.

I think we're safe from css_get() being used in this case, since
css_get() can only be used on css references obtained from a locked
task, or from other pointers that are known to be ref-counted, which
would be impossible if css_tryget() can't succeed on a css in the
partially-completed state.

Paul
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