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Date: Sat, 19 Dec 2009 12:27:12 +0100
From: Ulrich =?utf-8?B?U3DDtnJsZWlu?= <uqs@spoerlein.net>
To: John Baldwin <jhb@freebsd.org>
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Cc: Harti Brandt <harti@freebsd.org>, freebsd-arch@freebsd.org
Subject: Re: network statistics in SMP
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On Tue, 15.12.2009 at 13:13:28 -0500, John Baldwin wrote:
> On Tuesday 15 December 2009 12:45:13 pm Harti Brandt wrote:
> > I see. I was also thinking along these lines, but was not sure whether it 
> > is worth the trouble. I suppose this does not help to implement 64-bit 
> > counters on 32-bit architectures, though, because you cannot read them 
> > reliably without locking to sum them up, right?
> 
> Either that or you just accept that you have a small race since it is only stats. :)

This might be stupid, but can we not easily *read* 64bit counters
on 32bit machines like this:

do {
    h1 = read_upper_32bits;
    l1 = read_lower_32bits;
    h2 = read_upper_32bits;
    l2 = read_lower_32bits; /* not needed */
} while (h1 != h2);

sum64 = (h1<<32) + l1;

or something like that? If h2 does not change between readings, no
wrap-around has occured. If l1 was read in between the readings of h1
and h2, the code above is sound. Right?

Regards,
Uli