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Subject: Re: 64 bit netdev stats counter
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "David S. Miller" <davem@redhat.com>
Cc: matts@ksu.edu, shemminger@osdl.org, linux-kernel@vger.kernel.org
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On Fri, 2002-07-12 at 22:58, David S. Miller wrote:
> 32-bit values aren't atomic either, what is the issue?
> We don't use atomic_t ops on these counters so they aren't
> guarenteed in any way right now even.  GCC is going to
> output "incl MEM" or similar for net_stats->counter++, since
> it lacks the 'lock;' prefix it is not atomic.

The behaviour is quite different though. On a 32bit counter the worst we
do is lose a few counts. On a 64bit one on 32bit cpus its quite likely
gcc will output

		increment low 32bit
		if zero
			increment high

Which means we can rapidly get 2^32 out of sync

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