From owner-freebsd-current  Wed Jan 17  8:15:38 2001
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Date: Wed, 17 Jan 2001 11:14:49 -0500 (EST)
From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
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To: Alfred Perlstein <bright@wintelcom.net>
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Subject: Re: Atomic breakage?
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<<On Tue, 16 Jan 2001 19:10:10 -0800, Alfred Perlstein <bright@wintelcom.net> said:

> Just wondering, can't you use 'LOCK addl' and then use 'LOCK addc'?
> add longword, add longword with carry?  I know it would be pretty
> ugly, but it should work, no?

The two bus cycles are independent, so there is a race condition.

OTOH, it's a fairly *unlikely* race condition, and the worst thing
that can happen is statistics that are obviously off by four billion.
(The race only occurs when there is a carry out of the low-order
longword, which is to say, once in every 2**32 operations.)

-GAWollman



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