netdev - Re: [PATCH/RFC] net: Add __napi_sycnhronize() to sync with napi poll

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Date:	Tue, 16 Oct 2007 11:53:18 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	benh@...nel.crashing.org
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Roland Dreier <rdreier@...co.com>,
	linuxppc-dev list <linuxppc-dev@...abs.org>
Subject: Re: [PATCH/RFC] net: Add __napi_sycnhronize() to sync with napi
 poll

On Tue, 16 Oct 2007 15:49:52 +1000
Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> net: Add __napi_sycnhronize() to sync with napi poll
> 
> The EMAC driver which needs to handle multiple devices with one
> NAPI instance implements its own per-channel disable bit. However,
> when setting such a bit, it needs to synchronize with the poller
> (that is make sure that any pending poller instance has completed,
> or is started late enough to see that disable bit).
> 
> This implements a low level __napi_synchronize() function to acheive
> that. The underscores are to emphasis the low level aspect of it and
> to discourage driver writers who don't know what they are doing to
> use it (to please DaveM :-)
> 
> Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> ---
> 
> (Use correct address for Stephen this time)
> 
> If the approach is accepted, I would like to have this merged now
> so the EMAC patch to make it work again can follow :-)
> 
> Note: I use msleep_interruptible(1); just like napi_disable(). However
> I'm not too happy that the "hot" loop that results of a pending signal
> here will spin without even a cpu_relax ... what do you guys think would
> be the best way to handle this ?


So this is really just like synchronize_irq()?  Using msleep is bogus
because you want to spin, you are only waiting for a softirq on the other
cpu to finish. If you wait for a whole millisecond and sleep that
is far longer than the napi routine should take.

You could even optimize it like synchronize_irq() for the non-SMP case.



-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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