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