netdev - Re: GRO with non napi driver: BUG in __napi

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Date:	Fri, 27 Mar 2009 12:05:30 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Frank Blaschka <blaschka@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: GRO with non napi driver: BUG in __napi_complete

> On the legacy netif_rx path, I incorrectly tried to optimise
>
> the napi_complete call by using __napi_complete before we reenable
> IRQs.  This simply doesn't work since we need to flush the held
> GRO packets first.
>
> This patch fixes it by doing the obvious thing of reenabling
> IRQs first and then calling napi_complete.

Does this fix generate a race condition for a non-NAPI device?  If
netif_rx runs immediately after local_irq_enable it would queue a
packet on the backlog queue and try to schedule napi (the latter has
no effect because napi has not completed).  On return from interrupt,
napi_complete is done leaving a packet in the input queue but napi is
not scheduled to process it.

Thanks,
Tom

>
> Reported-by: Frank Blaschka <blaschka@...ux.vnet.ibm.com>
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index f112970..2565f6d 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2588,9 +2588,9 @@ static int process_backlog(struct napi_struct *napi, int quota)
>                local_irq_disable();
>                skb = __skb_dequeue(&queue->input_pkt_queue);
>                if (!skb) {
> -                       __napi_complete(napi);
>                        local_irq_enable();
> -                       break;
> +                       napi_complete(napi);
> +                       goto out;
>                }
>                local_irq_enable();
>
> @@ -2599,6 +2599,7 @@ 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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