netdev - Re: [PATCH 1/2] net: Enable 64-bit net device statistics on 32-bit architectures

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Message-Id: <1275684425.2095.32.camel@achroite.uk.solarflarecom.com>
Date:	Fri, 04 Jun 2010 21:47:05 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Miller <davem@...emloft.net>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, linux-net-drivers@...arflare.com
Subject: Re: [PATCH 1/2] net: Enable 64-bit net device statistics on 32-bit
 architectures

On Fri, 2010-06-04 at 13:39 -0700, Stephen Hemminger wrote:
> On Fri, 04 Jun 2010 19:15:18 +0100
> Ben Hutchings <bhutchings@...arflare.com> wrote:
> 
> > On Fri, 2010-06-04 at 10:28 -0700, Stephen Hemminger wrote:
> > > On Thu, 03 Jun 2010 20:11:38 +0100
> > > Ben Hutchings <bhutchings@...arflare.com> wrote:
> > > 
> > > > static inline u64 rtnl_link_stats64_read(const u64 *field)
> > > > {
> > > > 	return ACCESS_ONCE(*field);
> > > > }
> > > > static inline u32 rtnl_link_stats64_read32(const u64 *field)
> > > > {
> > > > 	return ACCESS_ONCE(*field);
> > > > }
> > > 
> > > Do we really care if compiler reorders access. I think not.
> > > There was no order guarantee in the past.
> > 
> > Since these reads are potentially racing with writes, we want to ensure
> > that they are atomic.  Without the volatile-qualification, the compiler
> > can legitimately split or repeat the reads, though I don't see any
> > particular reason why this is a likely optimisation.
> > 
> > Ben.
> > 
> 
> But this part of the code is only being run on on 64 bit machines.
> Updates of basic types for the CPU are atomic, lots of other code
> already assumes this.
> Take off your tin hat, this is excessive paranoia.

I like my tin hat, I'm sure I saw more oopses before I put it on. ;-)

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
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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