Re: rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2010-11-25 at 09:50 +0900, KOSAKI Motohiro wrote:
> Hi
> 
> > Signed-off-by: Dave Jones <davej@redhat.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?