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

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Date:	Tue, 20 Jan 2009 11:58:32 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Paul Menage <menage@...gle.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, 19 Jan 2009 18:23:03 -0800
Paul Menage <menage@...gle.com> wrote:

> 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.
> 

Hmm, moving this call to after populate is not good because
id > max_id cannot be handled ;)

> +               if (ss->use_id)
> +                       if (alloc_css_id(ss, parent, cgrp))
> +                               goto err_destroy;
> +               /* At error, ->destroy() callback has to free assigned ID. */
>        }

Should I delay to set css_id->css pointer to valid value until the end of
populate() ? (add populage_css_id() call after cgroup_populate_dir()).

I'd like to write add-on patch to the patch [1/4]. (or update it.)
css_id->css == NULL case is handled now, anyway.



> 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.
> 
yes ;)

> 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.
> 
> 

Thanks, I'll try some other patch. Maybe it's ok to delay updating css_id->css
pointer.

-Kame

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