svn commit: r197643 - in head/sys: kern sys

[Stanislav Sedov]

On Thu, 1 Oct 2009 15:21:34 +0200
Attilio Rao <attilio at freebsd.org> mentioned:

> 2009/9/30 Robert Watson <rwatson at freebsd.org>:
> > On Wed, 30 Sep 2009, Attilio Rao wrote:
> >
> >>  When releasing a read/shared lock we need to use a write memory barrier
> >>  in order to avoid, on architectures which doesn't have strong ordered
> >>  writes, CPU instructions reordering.
> >
> > Hi Attilio (Fabio, et al),
> >
> > Nice catch!  Are we aware of specific reported problems that can be laid at
> > the feet of this bug, or was this more of a "wait a moment, shouldn't there
> > be a barrier there?".  Could you comment on the scope of this problem across
> > architectures we support?
> 
> A possible problem related to that would be MD specific and not on
> ia32/amd64 because there the barriers and simple atomics are the same.
> Given that sun4v suffers of serveral other problems, that MIPS has no
> SMP support, you would find it only for arm, ia64 and sparc
> eventually. Thus I'm not aware of any problem which can be reconducted
> to that.
> 

Actually, MIPS is going to grow SMP support really soon, and ARM cpus
do not do reordering until ARMv7 afaik.  So this should not result in
any real problems on ARM too.

OTOH, I suspect powerpc may be affected.  I'm not sure if any of the models
we support perform OOO, though.

-- 
Stanislav Sedov
ST4096-RIPE
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