linux-kernel - Re: [PATCH 04/17] cgroup: create directory before linking while creating a new cgroup

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Message-ID: <20121114190407.GI21185@mtj.dyndns.org>
Date:	Wed, 14 Nov 2012 11:04:07 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizefan@...wei.com>
Cc:	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, mhocko@...e.cz, glommer@...allels.com
Subject: Re: [PATCH 04/17] cgroup: create directory before linking while
 creating a new cgroup

Hello, Li.

On Wed, Nov 14, 2012 at 11:20:47AM +0800, Li Zefan wrote:
> > It also removes the need to check whether cgroup->dentry is %NULL in
> > cgroup_path.  If a cgroup is visible, its dentry is guaranteed to be
> > visible too.
> 
> I'm afraid this isn't safe.
> 
> The cgroup is visible to a controller soon after ss->create(), and then
> the controller might call cgroup_path() while cgrp->dentry is still NULL.

Hmmm... I can't find any case where ss->create() is calling
cgroup_path().  Do you remember which one that was?

> This can happen for cpu cgroup I think, and in fact the NULL check was
> added for this reason IIRC.

Then, I think we better move the dentry assignment further up.  It's
really silly to be unable to answer "what's this new cgroup's name?"
for any callback really.  Will update accordingly.

Thanks.

-- 
tejun
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