Re: rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 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?

(cc to Christoph and Oleg)

Good spotting.

As far as I understand, find_task_by_pid() did required tasklist_lock in
old days. but It isn't now.

So I think we can remove tasklist_lock from here.

===========================================================================
>From 9de9f70f74e55d92b5e9057e22fc629405f63295 Mon Sep 17 00:00:00 2001
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Date: Thu, 16 Dec 2010 17:49:23 +0900
Subject: [PATCH] mempolicy: remove tasklist_lock from migrate_pages

Today, tasklist_lock in migrate_pages doesn't protect anything. 
rcu_read_lock() provide enough protection from pid hash walk.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 mm/mempolicy.c |    3 ---
 1 files changed, 0 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 11ff260..9064945 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1308,16 +1308,13 @@ SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,
 
 	/* Find the mm_struct */
 	rcu_read_lock();
-	read_lock(&tasklist_lock);
 	task = pid ? find_task_by_vpid(pid) : current;
 	if (!task) {
-		read_unlock(&tasklist_lock);
 		rcu_read_unlock();
 		err = -ESRCH;
 		goto out;
 	}
 	mm = get_task_mm(task);
-	read_unlock(&tasklist_lock);
 	rcu_read_unlock();
 
 	err = -EINVAL;
-- 
1.6.5.2