Re: 64 bit netdev stats counter

[Chris Friesen <cfriesen@nortelnetworks.com>]

Alan Cox wrote:
> 
> 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

Isn't this the same as 32-bit counters on a machine that doesn't do atomic
32-bit ops?  Although in that case you could only be 2^16 off...

Chris

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