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Message-ID: <3F83F62D.5090805@draigBrady.com>
Date: Wed, 08 Oct 2003 12:34:05 +0100
From: P@draigBrady.com
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To: Marko Rauhamaa <marko@pacujo.net>
CC: linux-kernel@vger.kernel.org, Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
       Jamal Hadi Salim <hadi@cyberus.ca>,
       Robert Olsson <Robert.Olsson@data.slu.se>
Subject: Re: NAPI Race?
References: <m3smm4qvf0.fsf@lumo.pacujo.net>
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Marko Rauhamaa wrote:
> It looks to me like net_rx_action() might suffer from a race, which in
> turn might explain some weirdness in my driver test results.
> 
> Here's the essence of the function from net/core/dev.c:
> 
> net_rx_action()
> {
>         local_irq_disable();
>         while (!list_empty(&queue->poll_list)) {
>                 local_irq_enable();
>                 /* do stuff */
>                 local_irq_disable();
>         }
>         local_irq_enable();
> }
> 
> Say I receive a packet. net_rx_action() processes it in the while loop
> and reenables interrupts. But just before net_rx_action() returns, I
> receive another packet, and __netif_rx_schedule() gets called from the
> driver. Then the soft irq is raised from within itself. If I'm not
> interrupted for some other reason, the packet will get processed only at
> the next jiffie when the soft irq is invoked again.
> 
> Am I mistaken?

Probably not, as I tested the reception timing
accuracy against an independent hardware "packet
timestamper", and out of 2 million packets,
3 were delayed by up to 5ms on the linux box
(e100 NAPI). There were about 10 packets delayed
between 1ms and 5ms.

Pádraig.