linux-kernel - Re: + cgroups-more-safe-tasklist-locking-in-cgroup_attach

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Date:	Fri, 2 Sep 2011 10:15:50 -0400
From:	Ben Blum <bblum@...rew.cmu.edu>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	bblum@...rew.cmu.edu, fweisbec@...il.com, neilb@...e.de,
	paul@...lmenage.org, paulmck@...ux.vnet.ibm.com
Subject: Re: +
 cgroups-more-safe-tasklist-locking-in-cgroup_attach_proc.patch added to
 -mm tree

On Fri, Sep 02, 2011 at 04:00:15PM +0200, Oleg Nesterov wrote:
> Forgot to mention, sorry...
> 
> That said, I believe the patch is correct and should fix the problem.

Thanks!

But I don't think the check becomes pointless? If a sub-thread execs
right before read_lock(&tasklist_lock) (but after the find_task_by_vpid
in attach_task_by_pid), that causes the case that the comment refers to.

-- Ben

> 
> On 09/02, Oleg Nesterov wrote:
> >
> > > From: Ben Blum <bblum@...rew.cmu.edu>
> > >
> > > Fix unstable tasklist locking in cgroup_attach_proc.
> > >
> > > According to this thread - https://lkml.org/lkml/2011/7/27/243 - RCU is
> > > not sufficient to guarantee the tasklist is stable w.r.t.  de_thread and
> > > exit.  Taking tasklist_lock for reading, instead of rcu_read_lock, ensures
> > > proper exclusion.
> >
> > I still think we should avoid the global lock.
> >
> > In any case, with tasklist or siglock,
> >
> > > -	rcu_read_lock();
> > > +	read_lock(&tasklist_lock);
> > >  	if (!thread_group_leader(leader)) {
> > >  		/*
> > >  		 * a race with de_thread from another thread's exec() may strip
> > > @@ -2036,7 +2036,7 @@ int cgroup_attach_proc(struct cgroup *cg
> > >  		 * throw this task away and try again (from cgroup_procs_write);
> > >  		 * this is "double-double-toil-and-trouble-check locking".
> > >  		 */
> > > -		rcu_read_unlock();
> > > +		read_unlock(&tasklist_lock);
> > >  		retval = -EAGAIN;
> >
> > this check+comment becomes completely pointless and imho very confusing.
> >
> > Oleg.
> 
> 
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