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

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Date:	Wed, 25 Mar 2009 17:03:52 -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

> 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...

Excellent detective work, I would have never figured this one
out.

Herbert can you take a good look at this can confirm Jarek's
findings?

Thanks!
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