cvs commit: src/sys/sys bus.h kobj.h param.h src/sys/kern subr_bus.c subr_kobj.c

[Poul-Henning Kamp]

In message <20031016210328.T930 at 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 at 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.