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

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Date:	Wed, 25 Mar 2009 23:54:56 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Ingo Molnar <mingo@...e.hu>, David Miller <davem@...emloft.net>,
	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",

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...

Jarek P.
--- (2.6.29)
 net/core/dev.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index e3fe5c7..cf53c24 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2589,7 +2589,11 @@ static int process_backlog(struct napi_struct *napi, int quota)
 		skb = __skb_dequeue(&queue->input_pkt_queue);
 		if (!skb) {
 			local_irq_enable();
-			napi_complete(napi);
+			napi_gro_flush(napi);
+			local_irq_disable();
+			if (skb_queue_empty(&queue->input_pkt_queue))
+				__napi_complete(napi);
+			local_irq_enable();
 			goto out;
 		}
 		local_irq_enable();
--
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