Date: Sat, 19 Dec 2009 12:27:12 +0100 From: Ulrich =?utf-8?B?U3DDtnJsZWlu?= <uqs@spoerlein.net> To: John Baldwin <jhb@freebsd.org> Cc: Harti Brandt <harti@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: network statistics in SMP Message-ID: <20091219112711.GR55913@acme.spoerlein.net> In-Reply-To: <200912151313.28326.jhb@freebsd.org> References: <20091215103759.P97203@beagle.kn.op.dlr.de> <200912150812.35521.jhb@freebsd.org> <20091215183859.S53283@beagle.kn.op.dlr.de> <200912151313.28326.jhb@freebsd.org>
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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
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