linux-kernel - Re: [PATCH 04/10] cgroup: always lock threadgroup during migration

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Message-ID: <20111104152150.GX4417@google.com>
Date:	Fri, 4 Nov 2011 08:21:50 -0700
From:	Tejun Heo <tj@...nel.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	paul@...lmenage.org, rjw@...k.pl, 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, oleg@...hat.com
Subject: Re: [PATCH 04/10] cgroup: always lock threadgroup during migration

Hello,

On Fri, Nov 04, 2011 at 05:54:13PM +0900, KAMEZAWA Hiroyuki wrote:
> > -	/* if PF_EXITING is set, the tsk->cgroups pointer is no longer safe. */
> > +	/* @tsk can't exit as its threadgroup is locked */
> >  	task_lock(tsk);
> > -	if (tsk->flags & PF_EXITING) {
> > -		task_unlock(tsk);
> > -		put_css_set(newcg);
> > -		return -ESRCH;
> > -	}
> > +	WARN_ON_ONCE(tsk->flags & PF_EXITING);
> >  	rcu_assign_pointer(tsk->cgroups, newcg);
> >  	task_unlock(tsk);
> 
> Is this task_lock/unlock is required ?

For put, I don't think it's necessary.

> > @@ -2116,11 +2120,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
> >  			continue;
> >  		/* get old css_set pointer */
> >  		task_lock(tsk);
> > -		if (tsk->flags & PF_EXITING) {
> > -			/* ignore this task if it's going away */
> > -			task_unlock(tsk);
> > -			continue;
> > -		}
> >  		oldcg = tsk->cgroups;
> >  		get_css_set(oldcg);
> >  		task_unlock(tsk);

For get, I think it is; otherwise, nothing prevents someone else from
changing tsk->cgroups between load and get, and destroying it.

Thanks.

-- 
tejun
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