netdev - Re: [PATCH 3/9] veth: convert to 64 bit statistics

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Date:	Fri, 10 Jun 2011 17:34:19 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 3/9] veth: convert to 64 bit statistics

On Wed, 2011-06-08 at 20:22 -0700, Stephen Hemminger wrote:
> On Thu, 09 Jun 2011 03:00:41 +0100
> Ben Hutchings <bhutchings@...arflare.com> wrote:
> 
> > On Wed, 2011-06-08 at 17:53 -0700, Stephen Hemminger wrote:
> > > Not much change, device was already keeping per cpu statistics.
> > > Use recent 64 statistics interface.
> > [...]
> > 
> > It's also going to need to use u64_stats_sync functions.
> > 
> > Ben.
> > 
> 
> No veth is doing per-cpu update therefore the new code has the same guarantee
> as the old code.

Per-cpu statistics avoid the problem of races between writers without
using expensive atomic operations.  They don't solve the problem of
races between writer and reader (on 32-bit systems).

For example, if veth_get_stats() races with a receive or transmit that
increments a value from 0x00000001ffffffff to 0x0000000200000000, it may
see the value as 0x00000002ffffffff.

Ben.

-- 
Ben Hutchings, Senior Software 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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