linux-kernel - Re: [PATCH 1/4] cgroup: change locking order in attach_task_by

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Date:	Mon, 10 Oct 2011 10:34:26 -0700
From:	Tejun Heo <htejun@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	rjw@...k.pl, paul@...lmenage.org, lizf@...fujitsu.com,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, fweisbec@...il.com,
	matthltc@...ibm.com, akpm@...ux-foundation.org,
	Paul Menage <menage@...gle.com>
Subject: Re: [PATCH 1/4] cgroup: change locking order in
 attach_task_by_pid()

Hello,

On Sun, Sep 18, 2011 at 08:56:21PM +0200, Oleg Nesterov wrote:
> On 09/05, Tejun Heo wrote:
> >
> > cgroup_mutex is updated to nest inside threadgroup_fork_lock instead
> > of the other way around.  threadgroup locking is scheduled to be
> > updated to cover all threadgroup altering operations and nesting it
> > inside cgroup_mutex complicates locking dependency unnecessarily.
> 
> Could you please explain this change?
> 
> I can't understand why threadgroup_lock() under cgroup_mutex is bad.
> Do you mean we can deadlock otherwise?

Yeah, I forgot the chain but it triggers a valid lockdep deadlock
warning.

> And, this means we can't change cpuset_do_move_task() to do
> threadgroup_lock().

Would that be problematic?

Thanks.

-- 
tejun
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