linux-kernel - Re: [PATCH 2/2] cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg

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Message-ID: <20120227185722.GD30232@somewhere.redhat.com>
Date:	Mon, 27 Feb 2012 19:57:25 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mandeep Singh Baines <msb@...omium.org>,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] cgroup: Walk task list under tasklist_lock in
 cgroup_enable_task_cg_list

On Wed, Feb 22, 2012 at 07:03:27PM +0100, Oleg Nesterov wrote:
> Hi.
> 
> Sorry, currently I do not have the time to even read this discussion,
> just one note,
> 
> On 02/22, Frederic Weisbecker wrote:
> >
> > On Tue, Feb 21, 2012 at 05:19:34PM -0800, Paul E. McKenney wrote:
> > > On Wed, Feb 22, 2012 at 01:55:28AM +0100, Frederic Weisbecker wrote:
> > > > On Tue, Feb 21, 2012 at 02:23:43PM -0800, Mandeep Singh Baines wrote:
> > > > > Frederic Weisbecker (fweisbec@...il.com) wrote:
> > > > > > Walking through the tasklist in cgroup_enable_task_cg_list() inside
> > > > > > an RCU read side critical section is not enough because:
> > > > > >
> > > > > > - RCU is not (yet) safe against while_each_thread()
> 
> Yes. Except I'd say it is while_each_thread() who is not safe ;)

Right.

> 
> Please do not take this into account. This should be fixed, I hope
> to send the fix "soon".

Sure, but anyway we need the read_lock here for ordering/commiting reasons.
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