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

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Message-Id: <20090324.191134.05205089.davem@davemloft.net>
Date:	Tue, 24 Mar 2009 19:11:34 -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: Herbert Xu <herbert@...dor.apana.org.au>
Date: Wed, 25 Mar 2009 08:23:03 +0800

> On Tue, Mar 24, 2009 at 02:36:22PM -0700, David Miller wrote:
> >
> > I think the problem is that we need to do the GRO flush before the
> > list delete and clearing the NAPI_STATE_SCHED bit.
> 
> Well first of all GRO shouldn't even be on in Ingo's case, unless
> he enabled it by hand with ethtool.  Secondly the only thing that
> touches the GRO state for the legacy path is process_backlog, and
> since this is per-cpu, I can't see how another instance can run
> while the first is still going.

Right.

I think the conditions Ingo is running under is that both
loopback (using legacy paths) and his NAPI based device
(forcedeth) are processing a lot of packets at the same
time.

Another thing that seems to be critical is he can only trigger this on
UP, which means that we don't have the damn APIC potentially moving
the cpu target of the forcedeth interrupts around.  And this means
also that all the processing will be on one cpu's backlog queue only.
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