linux-kernel - Re: [RFC][PATCH] cgroup: fix race between fork and cgroup freezing

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Date:	Mon, 12 Mar 2012 09:10:40 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	缪 勰 <miaox@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH] cgroup: fix race between fork and cgroup freezing

Hello,

On Mon, Mar 12, 2012 at 05:02:04PM +0800, Li Zefan wrote:
> - We still need some kind of locking to syncronize fork and the traverser.
> fork side is protected by tasklist_lock, while the traverser takes
> css_set_lock.

Can't we do both after tasklist_lock is released under css_set_lock?

> - After linking the new task to css set list, the task is visible and thus
> can be moved to another cgroup, which makes things more complicated and
> the subsystem callbacks may have to acquire cgroup_mutex.

Hmmm... freezer currently doesn't allow migrating in and out of frozen
cgroup and even when it does callbacks in the migration path should
synchronize against freezer->lock.  I *think* that should be enough
and can't see why this will be simpler or more complex depending on
when fork callback is called.

> - The task_counter subsystem wants to get notified before the new task
> is linked, so it's able to abort the fork.

This one maybe but for this cgroup_fork_callbacks() is already too
late, isn't it?  We better have pre-fork callbacks instead, no?

Thanks.

-- 
tejun
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