RE: [PATCH v2] l2tp: use per-cpu variables for u64_stats updates

[Ben Hutchings <bhutchings@solarflare.com>]

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.