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SubjectRe: rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages
Date
> 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







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