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

[Doug Rabson]

On Thu, 2003-10-16 at 12:16, Bruce Evans wrote:
> On Thu, 16 Oct 2003, Doug Rabson wrote:
> 
> > dfr         2003/10/16 02:16:28 PDT
> >
> >   FreeBSD src repository
> >
> >   Modified files:
> >     sys/sys              bus.h kobj.h param.h
> >     sys/kern             subr_bus.c subr_kobj.c
> >   Log:
> >   * Add multiple inheritance to kobj. Each class can have zero or more base
> >     ...
> >   * 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.

Without using something like <type>, how can you know how much to read
(without abusing some kind of GCC language extension)? I don't want to
use atomic_load_acq_* since I don't care which order values are written
to the cache.