Re: [NETPOLL] netconsole: fix soft lockup when removing module

Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Jarek Poplawski]

On Sun, Jul 01, 2007 at 09:35:58PM +0400, Oleg Nesterov wrote:
> Jarek Poplawski wrote:
> >
> >    #1
> >    Until kernel ver. 2.6.21 (including) cancel_rearming_delayed_work()
> >    required a work function should always (unconditionally) rearm with
> >    delay > 0 - otherwise it would endlessly loop. This patch replaces
> >    this function with cancel_delayed_work(). Later kernel versions don't
> >    require this, so here it's only for uniformity.
> 
> But 2.6.22 doesn't need this change, why it was merged?

One bad reason is given above. Should I look for another one?

> 
> In fact, I suspect this change adds a race,

You are right!

> 
> > --- a/net/core/netpoll.c
> > +++ b/net/core/netpoll.c
> > @@ -72,7 +72,8 @@ static void queue_process(struct work_struct *work)
> >  			netif_tx_unlock(dev);
> >  			local_irq_restore(flags);
> >  
> > -			schedule_delayed_work(&npinfo->tx_work, HZ/10);
> > +			if (atomic_read(&npinfo->refcnt))
> > +				schedule_delayed_work(&npinfo->tx_work, HZ/10);
> >  			return;
> >  		}
> >  		netif_tx_unlock(dev);
> > @@ -785,9 +786,15 @@ void netpoll_cleanup(struct netpoll *np)
> >  			if (atomic_dec_and_test(&npinfo->refcnt)) {
> >  				skb_queue_purge(&npinfo->arp_tx);
> >  				skb_queue_purge(&npinfo->txq);
> > -				cancel_rearming_delayed_work(&npinfo->tx_work);
> > +				cancel_delayed_work(&npinfo->tx_work);
> >  				flush_scheduled_work();
> 
> Suppose that ->refcnt == 1, and queue_process() was preempted just after
> atomic_read(&npinfo->refcnt).
> 
> netpoll_cleanup() comes, cancel_delayed_work() does nothing, flush_scheduled_work()
> sleeps.
> 
> queue_process() gets CPU, re-schedules ->tx_work, and returns.
> 
> flush_scheduled_work() completes, netpoll_cleanup() frees npinfo and returns
> while ->tx_work is pending.
> 
> No?

No no. (Yes!)

I had some doubts about this, and you found very good reason
for this.

I'll soon send a patch to restore cancel_rearming_delayed_work
in 2.6.22.

So, 2.6.21 needs something better (maybe you've found it btw.?),
but they weren't too interested, anyway.

Thanks very much & regards,
Jarek P.

Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Oleg Nesterov]

On 07/02, Jarek Poplawski wrote:
>
> > > --- a/net/core/netpoll.c
> > > +++ b/net/core/netpoll.c
> > > @@ -72,7 +72,8 @@ static void queue_process(struct work_struct *work)
> > >  			netif_tx_unlock(dev);
> > >  			local_irq_restore(flags);
> > >  
> > > -			schedule_delayed_work(&npinfo->tx_work, HZ/10);
> > > +			if (atomic_read(&npinfo->refcnt))
> > > +				schedule_delayed_work(&npinfo->tx_work, HZ/10);
> > >  			return;
> > >  		}
>
> [...snip...]
> 
> So, 2.6.21 needs something better (maybe you've found it btw.?),
> but they weren't too interested, anyway.

We can do a double flush trick. If queue_process() checks ->refcnt before
schedule_delayed_work() like above, netpoll_cleanup() can do

	flush_scheduled_work();

	// the next invocation of queue_process()
	// must see ->refcnt == 0
	if (!cancel_delayed_work(&npinfo->tx_work)) {
		/* may be queued, wait for completion */
		flush_scheduled_work();
	}

Jarek, I don't understand net/, a silly question. Why do we need the #2 chunk?
Isn't it better to move skb_queue_purge(&npinfo->txq) after cancel_..._work()
instead?

Oleg.	

Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Jarek Poplawski]

On Mon, Jul 02, 2007 at 01:24:08PM +0400, Oleg Nesterov wrote:
> On 07/02, Jarek Poplawski wrote:
> >
> > > > --- a/net/core/netpoll.c
> > > > +++ b/net/core/netpoll.c
> > > > @@ -72,7 +72,8 @@ static void queue_process(struct work_struct *work)
> > > >  			netif_tx_unlock(dev);
> > > >  			local_irq_restore(flags);
> > > >  
> > > > -			schedule_delayed_work(&npinfo->tx_work, HZ/10);
> > > > +			if (atomic_read(&npinfo->refcnt))
> > > > +				schedule_delayed_work(&npinfo->tx_work, HZ/10);
> > > >  			return;
> > > >  		}
> >
> > [...snip...]
> > 
> > So, 2.6.21 needs something better (maybe you've found it btw.?),
> > but they weren't too interested, anyway.
> 
> We can do a double flush trick. If queue_process() checks ->refcnt before
> schedule_delayed_work() like above, netpoll_cleanup() can do
> 
> 	flush_scheduled_work();
> 
> 	// the next invocation of queue_process()
> 	// must see ->refcnt == 0
> 	if (!cancel_delayed_work(&npinfo->tx_work)) {
> 		/* may be queued, wait for completion */
> 		flush_scheduled_work();
> 	}

I'll try to think about it later, but I don't plan to do next patch,
so feel free to send this. I didn't plan to fix netpoll at all
(I never didn't use nor studied this before...). But couldn't stand
this stupid lockup stays in 2.6.21. Now, I see it probably doesn't
annoy more than 2 or 3 people around...

> 
> Jarek, I don't understand net/, a silly question. Why do we need the #2 chunk?
> Isn't it better to move skb_queue_purge(&npinfo->txq) after cancel_..._work()
> instead?

I've thought about this too, but because I don't know netpoll/netconsole
enough I didn't want to change more than minimum needed. 

skb_queue_purge() uses heavy locking (irqsave) and I don't remember now
if I've found the reason or simply believed somebody had to have a reason
to do this there, anyway, if moved after cancel_ it could be done without
this locking, and something like while () instead of my if () should be
enough.

But there was not much interest about this patch, and I'm not currently
interested to be the main netconsole expert too, so maybe you would
like to try...

Cheers,
Jarek P.

[PATCH] Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Jarek Poplawski]

>From my recent patch:

> >    #1
> >    Until kernel ver. 2.6.21 (including) cancel_rearming_delayed_work()
> >    required a work function should always (unconditionally) rearm with
> >    delay > 0 - otherwise it would endlessly loop. This patch replaces
> >    this function with cancel_delayed_work(). Later kernel versions don't
> >    require this, so here it's only for uniformity.

But Oleg Nesterov <oleg@tv-sign.ru> found:

> But 2.6.22 doesn't need this change, why it was merged?
> 
> In fact, I suspect this change adds a race,
...

His description was right (thanks), so this patch reverts #1.

Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>

---

diff -Nurp 2.6.22-rc7-/net/core/netpoll.c 2.6.22-rc7/net/core/netpoll.c
--- 2.6.22-rc7-/net/core/netpoll.c	2007-07-02 09:03:27.000000000 +0200
+++ 2.6.22-rc7/net/core/netpoll.c	2007-07-02 09:32:34.000000000 +0200
@@ -72,8 +72,7 @@ static void queue_process(struct work_st
 			netif_tx_unlock(dev);
 			local_irq_restore(flags);
 
-			if (atomic_read(&npinfo->refcnt))
-				schedule_delayed_work(&npinfo->tx_work, HZ/10);
+			schedule_delayed_work(&npinfo->tx_work, HZ/10);
 			return;
 		}
 		netif_tx_unlock(dev);
@@ -786,7 +785,7 @@ void netpoll_cleanup(struct netpoll *np)
 			if (atomic_dec_and_test(&npinfo->refcnt)) {
 				skb_queue_purge(&npinfo->arp_tx);
 				skb_queue_purge(&npinfo->txq);
-				cancel_delayed_work(&npinfo->tx_work);
+				cancel_rearming_delayed_work(&npinfo->tx_work);
 				flush_scheduled_work();
 
 				/* clean after last, unfinished work */

Re: [PATCH] Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Oleg Nesterov]

On 07/02, Jarek Poplawski wrote:
> 
> diff -Nurp 2.6.22-rc7-/net/core/netpoll.c 2.6.22-rc7/net/core/netpoll.c
> --- 2.6.22-rc7-/net/core/netpoll.c	2007-07-02 09:03:27.000000000 +0200
> +++ 2.6.22-rc7/net/core/netpoll.c	2007-07-02 09:32:34.000000000 +0200
> @@ -72,8 +72,7 @@ static void queue_process(struct work_st
>  			netif_tx_unlock(dev);
>  			local_irq_restore(flags);
>  
> -			if (atomic_read(&npinfo->refcnt))
> -				schedule_delayed_work(&npinfo->tx_work, HZ/10);
> +			schedule_delayed_work(&npinfo->tx_work, HZ/10);
>  			return;
>  		}
>  		netif_tx_unlock(dev);
> @@ -786,7 +785,7 @@ void netpoll_cleanup(struct netpoll *np)
>  			if (atomic_dec_and_test(&npinfo->refcnt)) {
>  				skb_queue_purge(&npinfo->arp_tx);
>  				skb_queue_purge(&npinfo->txq);
> -				cancel_delayed_work(&npinfo->tx_work);
> +				cancel_rearming_delayed_work(&npinfo->tx_work);
>  				flush_scheduled_work();

While you are here, could you also delete this flush_scheduled_work() ?
It is not needed any longer.

Oleg.

[PATCH 2/2][NETPOLL] netconsole: delete flush_scheduled_work

[Jarek Poplawski]

On Mon, Jul 02, 2007 at 12:59:49PM +0400, Oleg Nesterov wrote:
...
> While you are here, could you also delete this flush_scheduled_work() ?
> It is not needed any longer.

Yes. I've thought about this, and even planned to mention, but then
forgotten... Of course, you are right, but since it stayed so long
and doesn't seem to be dangerous, and there is -rc7 I wasn't so brave.
But now I have an explanation...

Jarek P.

---------->

Subject: [PATCH][NETPOLL] netconsole: delete flush_scheduled_work

flush_scheduled_work() isn't needed after cancel_rearming_delayed_work(),
so here it's removed from netpoll_cleanup().

PS: This patch was prepared on 2.6.22-rc7 with my other today's patch:
netconsole: fix soft lockup ...

Noticed-by: Oleg Nesterov <oleg@tv-sign.ru>

Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>

---

diff -Nurp 2.6.22-rc7-plus-revert1-/net/core/netpoll.c 2.6.22-rc7-plus-revert1/net/core/netpoll.c
--- 2.6.22-rc7-plus-revert1-/net/core/netpoll.c	2007-07-02 09:32:34.000000000 +0200
+++ 2.6.22-rc7-plus-revert1/net/core/netpoll.c	2007-07-02 11:43:29.000000000 +0200
@@ -786,7 +786,6 @@ void netpoll_cleanup(struct netpoll *np)
 				skb_queue_purge(&npinfo->arp_tx);
 				skb_queue_purge(&npinfo->txq);
 				cancel_rearming_delayed_work(&npinfo->tx_work);
-				flush_scheduled_work();
 
 				/* clean after last, unfinished work */
 				if (!skb_queue_empty(&npinfo->txq)) {

Re: [PATCH] Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Jarek Poplawski]

On Mon, Jul 02, 2007 at 09:52:26AM +0200, Jarek Poplawski wrote:
> 
> From my recent patch:
> 
> > >    #1
> > >    Until kernel ver. 2.6.21 (including) cancel_rearming_delayed_work()
> > >    required a work function should always (unconditionally) rearm with
> > >    delay > 0 - otherwise it would endlessly loop. This patch replaces
> > >    this function with cancel_delayed_work(). Later kernel versions don't
> > >    require this, so here it's only for uniformity.
> 
> But Oleg Nesterov <oleg@tv-sign.ru> found:
> 
> > But 2.6.22 doesn't need this change, why it was merged?
> > 
> > In fact, I suspect this change adds a race,
> ...
> 
> His description was right (thanks), so this patch reverts #1.
> 
> Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>

Oleg,

I think maybe you could ack these 2 netconsole patches...
They were done on your request but it looks like Andrew
is waiting on something...

Thanks,
Jarek P.

Re: [PATCH] Re: [NETPOLL] netconsole: fix soft lockup when removing module

[David Miller]

From: Jarek Poplawski <jarkao2@o2.pl>
Date: Wed, 4 Jul 2007 08:41:59 +0200

> On Mon, Jul 02, 2007 at 09:52:26AM +0200, Jarek Poplawski wrote:
> > 
> > From my recent patch:
> > 
> > > >    #1
> > > >    Until kernel ver. 2.6.21 (including) cancel_rearming_delayed_work()
> > > >    required a work function should always (unconditionally) rearm with
> > > >    delay > 0 - otherwise it would endlessly loop. This patch replaces
> > > >    this function with cancel_delayed_work(). Later kernel versions don't
> > > >    require this, so here it's only for uniformity.
> > 
> > But Oleg Nesterov <oleg@tv-sign.ru> found:
> > 
> > > But 2.6.22 doesn't need this change, why it was merged?
> > > 
> > > In fact, I suspect this change adds a race,
> > ...
> > 
> > His description was right (thanks), so this patch reverts #1.
> > 
> > Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
> 
> Oleg,
> 
> I think maybe you could ack these 2 netconsole patches...
> They were done on your request but it looks like Andrew
> is waiting on something...

I plan to apply this patch, don't worry about it :)

Re: [PATCH] Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Jarek Poplawski]

On Tue, Jul 03, 2007 at 11:47:18PM -0700, David Miller wrote:
...
> I plan to apply this patch, don't worry about it :)
 
Now I'm really worried! Don't you evere sleep?

Good night,
Jarek P.

Re: [PATCH] Re: [NETPOLL] netconsole: fix soft lockup when removing module

[Jarek Poplawski]

On Wed, Jul 04, 2007 at 08:41:59AM +0200, Jarek Poplawski wrote:
...
> They were done on your request but it looks like Andrew
> is waiting on something...

Andrew,

This time I'm not sorry for my English because I've just
found I could speak "Chiefly Midland and Southern U.S.".

Jarek P.