linux-kernel - Re: Revert "gro: Fix legacy path napi

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Message-ID: <20090326064317.GA4204@ff.dom.local>
Date:	Thu, 26 Mar 2009 06:43:17 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	herbert@...dor.apana.org.au, mingo@...e.hu,
	r.schwebel@...gutronix.de, torvalds@...ux-foundation.org,
	blaschka@...ux.vnet.ibm.com, tglx@...utronix.de,
	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org,
	kernel@...gutronix.de
Subject: Re: Revert "gro: Fix legacy path napi_complete crash",

On Wed, Mar 25, 2009 at 05:10:19PM -0700, David Miller wrote:
> From: Jarek Poplawski <jarkao2@...il.com>
> Date: Wed, 25 Mar 2009 23:54:56 +0100
> 
> Ingo, in case it isn't completely obvious, it would be
> wonderful if you could try Jarek's patch below with your
> test case.
> 
> Thanks!

David, of course testing my patch would be very nice, but I think we
should definitely & immediately have this tested already Herbert's
revert patch in -stable now. 

Thanks,
Jarek P.

> 
> > Herbert Xu wrote, On 03/25/2009 01:26 PM:
> > 
> > > On Wed, Mar 25, 2009 at 01:20:46PM +0100, Ingo Molnar wrote:
> > >> ok - i have started testing the delta below, on top of the plain 
> > >> revert.
> > > 
> > > Thanks! BTW Ingo, any chance you could help us identify the problem
> > > with the previous patch? I don't have a forcedeth machine here
> > > and the hang you had with my patch that open-coded __napi_complete
> > > appears intimately connected to forcedeth (with NAPI enabled).
> > 
> > Of course it's too late for verifying this now, but (for the future)
> > I think, this scenario could be considered:
> > 
> > process_backlog()			netif_rx()
> > 
> > if (!skb)
> > local_irq_enable()
> > 					if (queue.qlen) //NO
> > 					napi_schedule() //NOTHING
> > 					__skb_queue_tail() //qlen > 0
> > napi_complete()
> > ...					...
> > 					Every next netif_rx() sees
> > 					qlen > 0, so napi is never
> > 					scheduled again.
> > 
> > Then, something like this might work...
> > 
> > Jarek P.
> > --- (2.6.29)
> >  net/core/dev.c |    6 +++++-
> >  1 files changed, 5 insertions(+), 1 deletions(-)
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index e3fe5c7..cf53c24 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -2589,7 +2589,11 @@ static int process_backlog(struct napi_struct *napi, int quota)
> >  		skb = __skb_dequeue(&queue->input_pkt_queue);
> >  		if (!skb) {
> >  			local_irq_enable();
> > -			napi_complete(napi);
> > +			napi_gro_flush(napi);
> > +			local_irq_disable();
> > +			if (skb_queue_empty(&queue->input_pkt_queue))
> > +				__napi_complete(napi);
> > +			local_irq_enable();
> >  			goto out;
> >  		}
> >  		local_irq_enable();
--
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