netdev - RE: [PATCH v2] l2tp: use per-cpu variables for u64

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Message-ID: <1340907451.2577.3.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Thu, 28 Jun 2012 19:17:31 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Rick Jones <rick.jones2@...com>,
	Ben Greear <greearb@...delatech.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Tom Parkin <tparkin@...alix.com>, <netdev@...r.kernel.org>,
	James Chapman <jchapman@...alix.com>
Subject: RE: [PATCH v2] l2tp: use per-cpu variables for u64_stats updates

On Thu, 2012-06-28 at 09:46 +0100, David Laight wrote:
> > [1] : LLTX drivers case 
> >    since ndo_start_xmit() can be run concurrently by many cpus, safely
> > updating an "unsigned long" requires additional hassle :
> > 
> >    1) Use of a spinlock to protect the update.
> >    2) Use atomic_long_t instead of "unsigned long"
> >    3) Use percpu data
> 
> 4) These are statistics so it shouldn't really matter if
>    they are out by a small number.

You might be surprised just how much end users care about statistics.

>    So the errors
>    introduced by unlocked updates can probably be ignored.
>    So just use 'unsigned long'.

This can result in statistics being decremented in case of a race.  That
should never be allowed to happen.

Ben.

>    Might be worth putting in gcc barriers so that the
>    load and store instructions aren't separated too far.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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