linux-kernel - Re: rcu_read_lock/unlock protect find_task_by_vpid call in migrate

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Date:	Thu, 25 Nov 2010 09:42:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	tglx <tglx@...utronix.de>, Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: rcu_read_lock/unlock protect find_task_by_vpid call in
 migrate_pages

On Thu, 2010-11-25 at 09:50 +0900, KOSAKI Motohiro wrote:
> Hi
> 
> > Signed-off-by: Dave Jones <davej@...hat.com>
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 4a57f13..2f0f55b 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -1308,6 +1308,7 @@ SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,
> >  
> >  	/* Find the mm_struct */
> >  	read_lock(&tasklist_lock);
> > +	rcu_read_lock();
> >  	task = pid ? find_task_by_vpid(pid) : current;
> >  	if (!task) {
> >  		read_unlock(&tasklist_lock);
> > @@ -1315,6 +1316,7 @@ SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,
> >  		goto out;
> >  	}
> >  	mm = get_task_mm(task);
> > +	rcu_read_unlock();
> >  	read_unlock(&tasklist_lock);
> >  
> >  	err = -EINVAL;
> 
> Thanks reporting. but mmotm seems already has the same patch.

Another one,... again:


Do we still need the tasklist_lock in this case?

Also, why is that think complaining, surely the tasklist_lock pins any
and all PID objects?
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