rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

[Dave Jones]

===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
kernel/pid.c:419 invoked rcu_dereference_check() without protection!

other info that might help us debug this:


rcu_scheduler_active = 1, debug_locks = 1
1 lock held by scrashme/11079:
 #0:  (tasklist_lock){.?.?..}, at: [<ffffffff81112702>] sys_migrate_pages+0xd6/0x29d

stack backtrace:
Pid: 11079, comm: scrashme Not tainted 2.6.37-rc3+ #2
Call Trace:
 [<ffffffff8107d095>] lockdep_rcu_dereference+0x9d/0xa5
 [<ffffffff81069dc8>] find_task_by_pid_ns+0x44/0x5d
 [<ffffffff81069e03>] find_task_by_vpid+0x22/0x24
 [<ffffffff8111270f>] sys_migrate_pages+0xe3/0x29d
 [<ffffffff8107c7c6>] ? trace_hardirqs_off_caller+0xa3/0x10b
 [<ffffffff8147b635>] ? retint_swapgs+0x13/0x1b
 [<ffffffff8107e2e7>] ? trace_hardirqs_on_caller+0x13f/0x172
 [<ffffffff8147a95c>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b

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;

Re: rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

[KOSAKI Motohiro]

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.

Re: rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

[Dave Jones]

On Thu, Nov 25, 2010 at 09:50:37AM +0900, KOSAKI Motohiro wrote:
 
 > > @@ -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.

indeed. It's also better than mine, which missed the unlock on error path.

thanks,

	Dave

Re: rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

[Peter Zijlstra]

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?

Re: rcu_read_lock/unlock protect find_task_by_vpid call in migrate_pages

[KOSAKI Motohiro]

> 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