netdev - Re: [PATCH RFC]: napi

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Date:	Sat, 28 Jul 2007 22:32:06 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rdreier@...co.com
Cc:	netdev@...r.kernel.org, shemminger@...ux-foundation.org,
	jgarzik@...ox.com, hadi@...erus.ca, rusty@...tcorp.com.au
Subject: Re: [PATCH RFC]: napi_struct V4

From: Roland Dreier <rdreier@...co.com>
Date: Sat, 28 Jul 2007 08:27:18 -0700

>  > Most drivers are in good shape, although some still have very
>  > questionable netif_rx_complete() handling, in that racy area that
>  > Rusty and myself were discussing today.
>  > 
>  > My inclination is to wrap those sequences around with an IRQ
>  > safe spinlock to fix the race provably, and then if driver
>  > authors want to optimize that away with techniques like those
>  > that tg3, bnx2, sky2, skge et al. use, that's fine but can
>  > be done later.
> 
> Ouch... that extra lock seems pretty expensive.  Also I'm having a
> hard time understanding how the techniques you're alluding to apply to
> devices that may miss events when enabling interrupts; the drivers you
> mention all seem to be for devices that didn't have the race and
> didn't use netif_rx_reschedule() in the old NAPI world.  Can you
> provide a little more detail on how the lock could be optimized away?

If you have a means in the device (like tg3, bnx2, e1000, and a score
of others do) to force the device to trigger a HW interrupt, that's
what you do if you detect that events are pending after re-enabling
interrupt in the ->poll() handler.

Frankly I don't think the lock is a big deal and you need something
like it anyways typically.
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