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

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Message-Id: <20090325.171019.56283101.davem@davemloft.net>
Date:	Wed, 25 Mar 2009 17:10:19 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jarkao2@...il.com
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",

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!

> 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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