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

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Message-Id: <20090324.154743.200977253.davem@davemloft.net>
Date:	Tue, 24 Mar 2009 15:47:43 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	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: David Miller <davem@...emloft.net>
Date: Tue, 24 Mar 2009 14:36:22 -0700 (PDT)

> From: Herbert Xu <herbert@...dor.apana.org.au>
> Date: Tue, 24 Mar 2009 23:09:28 +0800
> 
> > @@ -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();
> 
> I think the problem is that we need to do the GRO flush before the
> list delete and clearing the NAPI_STATE_SCHED bit.

Ok Herbert, I'm even more sure of this because in your original commit
log message you mention:

	This simply doesn't work since we need to flush the held
	GRO packets first.

We are certainly in a pickle here, actually.

We can't run the GRO flush until we re-enable interrupts.  But if we
re-enable interrupts, more packets get queued to the input_pkt_queue
and we end up back where we started.
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