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

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Message-ID: <20120222010431.GD13403@somewhere.redhat.com>
Date:	Wed, 22 Feb 2012 02:04:34 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Mandeep Singh Baines <msb@...omium.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	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 Tue, Feb 21, 2012 at 05:00:15PM -0800, Tejun Heo wrote:
> On Wed, Feb 22, 2012 at 01:55:28AM +0100, Frederic Weisbecker wrote:
> > > > +	 * We need tasklist_lock because RCU is not safe against
> > > > +	 * while_each_thread(). Besides, a forking task that has passed
> > > > +	 * cgroup_post_fork() without seeing use_task_css_set_links = 1
> > > > +	 * is not guaranteed to have its child immediately visible in the
> > > > +	 * tasklist if we walk through it with RCU.
> > > > +	 */
> > > 
> > > Maybe add TODO to remove the lock once do_each_thread()/while_each_thread()
> > > is made rcu safe. On a large system, it could take a while to iterate
> > > over every thread in the system. Thats a long time to hold a spinlock.
> > > But it only happens once so probably not that big a deal.
> > 
> > I think that even if while_each_thread() was RCU safe, that wouldn't
> > work here.
> 
> Guys, this is one time thing.  It happens *once* after boot and we're
> already holding exclusive lock.  There's no reason to optimize this at
> all.  Let's just keep it simple.

For now I agree. But one day the real time guys might eye that thing.
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