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

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Date:	Thu, 26 Mar 2009 07:39:09 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Ingo Molnar <mingo@...e.hu>, David Miller <davem@...emloft.net>,
	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 Thu, Mar 26, 2009 at 10:41:29AM +0800, Herbert Xu wrote:
...
> Yes this is why my original patch that started all this is broken.
> However, this doesn't apply to my patch that open-codes __napi_complete.
...
> This should work too.  However, the fact that the following patch
> is broken means that we have bigger problems.

I agree. This should be effectively equal to the revert, so I guess
Ingo probably hit some other bug btw.

Jarek P.

> 
> net: Fix netpoll lockup in legacy receive path
> 
> When I fixed the GRO crash in the legacy receive path I used
> napi_complete to replace __napi_complete.  Unfortunately they're
> not the same when NETPOLL is enabled, which may result in us
> not calling __napi_complete at all.
> 
> What's more, we really do need to keep the __napi_complete call
> within the IRQ-off section since in theory an IRQ can occur in
> between and fill up the backlog to the maximum, causing us to
> lock up.
> 
> This patch fixes this by essentially open-coding __napi_complete.
> 
> Note we no longer need the memory barrier because this function
> is per-cpu.
> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index e3fe5c7..2a7f6b3 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2588,9 +2588,10 @@ static int process_backlog(struct napi_struct *napi, int quota)
>  		local_irq_disable();
>  		skb = __skb_dequeue(&queue->input_pkt_queue);
>  		if (!skb) {
> +			list_del(&napi->poll_list);
> +			clear_bit(NAPI_STATE_SCHED, &napi->state);
>  			local_irq_enable();
> -			napi_complete(napi);
> -			goto out;
> +			break;
>  		}
>  		local_irq_enable();
>  
> @@ -2599,7 +2600,6 @@ static int process_backlog(struct napi_struct *napi, int quota)
>  
>  	napi_gro_flush(napi);
>  
> -out:
>  	return work;
>  }
> 
> Thanks,
> -- 
> Visit Openswan at http://www.openswan.org/
> Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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