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

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Message-Id: <1275675318.2095.30.camel@achroite.uk.solarflarecom.com>
Date:	Fri, 04 Jun 2010 19:15:18 +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 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.

-- 
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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