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To: Bruce Evans <bde@zeta.org.au>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Thu, 16 Oct 2003 21:16:17 +1000."
             <20031016210328.T930@gamplex.bde.org> 
Date: Fri, 17 Oct 2003 00:11:35 +0200
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cc: Doug Rabson <dfr@FreeBSD.org>
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Subject: Re: cvs commit: src/sys/sys bus.h kobj.h param.h src/sys/kern
	subr_bus.c subr_kobj.c 
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In message <20031016210328.T930@gamplex.bde.org>, Bruce Evans writes:

>>   * Change the kobj method lookup algorithm to one which is SMP-safe. This
>>     relies only on the constraint that an observer of a sequence of writes
>>     of pointer-sized values will see exactly one of those values, not a
>>     mixture of two or more values. This assumption holds for all processors
>>     which FreeBSD supports.
>
>This assumption should be avoided by using atomic_load() (and
>atomic_store_mumble()).  See a discussion of "atomicity of unlocked
>reads" last month.  First implement atomic_load().  There is currently
>only atomic_load_acq_<type>().  "acq" gives acquire semantics which
>is more than what is needed here and our implementations may do more
>than what is required anyway for some arches.  "<type>" is part of a
>bad API.

There are _so_ many things which explode if this assumption does not
hold that I think doing anything but gold-plating it would be a
total waste of time.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.