linux-kernel - Re: Revert "gro: Fix legacy path napi_complete crash", (was: Re: Linux 2.6.29)

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Date:	Tue, 24 Mar 2009 16:29:03 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Robert Schwebel <r.schwebel@...gutronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frank Blaschka <blaschka@...ux.vnet.ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernel@...gutronix.de
Subject: Re: Revert "gro: Fix legacy path napi_complete crash", (was: Re:
	Linux 2.6.29)

On Tue, Mar 24, 2009 at 11:09:28PM +0800, Herbert Xu wrote:
> On Tue, Mar 24, 2009 at 03:39:42PM +0100, Ingo Molnar wrote:
> >
> > Subject: [PATCH] net: Fix netpoll lockup in legacy receive path
> 
> Actually, this patch is still racy.  If some interrupt comes in
> and we suddenly get the maximum amount of backlog we can still
> hang when we call __napi_complete incorrectly.  It's unlikely
> but we certainly shouldn't allow that.  Here's a better version.

Yes, this one works. I always wanted to give a

Tested-by: Sascha Hauer <s.hauer@...gutronix.de>

Thanks
  Sascha

> 
> 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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