linux-kernel - Re: [PATCH] cgroup: fix a race condition in manipulating tsk->cg

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Message-ID: <6599ad830804162128g3d383bc6v606fcd14964bd21e@mail.gmail.com>
Date:	Wed, 16 Apr 2008 21:28:33 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Li Zefan" <lizf@...fujitsu.com>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Linux Containers" <containers@...ts.linux-foundation.org>,
	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Paul Jackson" <pj@....com>
Subject: Re: [PATCH] cgroup: fix a race condition in manipulating tsk->cg_list

On Wed, Apr 16, 2008 at 9:18 PM, Paul Menage <menage@...gle.com> wrote:
>
>  My only thought about the downside of this is that an exiting task
>  that gets stuck somewhere  between setting PF_EXITING and calling
>  cgroup_exit() won't show up in its cgroup's tasks file, since we'll
>  enable cgroup links but skip it. I guess that's not a big deal.
>

How about this as an alternative approach? We can take advantage of
the indirection in tsk->cgroups to create an additional distinguished
css_set that indicates the task has passed the point of checking
tsk->cg_list:

- create a new css_set, called exit_css_set; it has the same cgroup
pointer set as init_css_set.

- in cgroup_exit(), set current->cgroups to &exit_css_set rather than
&init_css_set

- in cgroup_enable_task_cg_list(), ignore any task where p->cgroups ==
&exit_css_set

That way we're synchronizing directly with the task_lock()-protected
section in cgroup_exit(), rather than with the setting of PF_EXITING
at the beginning of do_exit().

Paul
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