linux-kernel - Re: Intel Memory Ordering White Paper

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Message-Id: <200709080646.56200.nickpiggin@yahoo.com.au>
Date:	Sat, 8 Sep 2007 06:46:56 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Jesse Barnes <jesse.barnes@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: Intel Memory Ordering White Paper

On Saturday 08 September 2007 20:30, Alan Cox wrote:
> On Sat, 8 Sep 2007 18:54:57 +1000
>
> Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > On Saturday 08 September 2007 08:26, Jesse Barnes wrote:
> > > FYI, we just released a new white paper describing memory ordering for
> > > Intel processors:
> > > http://developer.intel.com/products/processor/manuals/index.htm
> > >
> > > Should help answer some questions about some of the ordering primitives
> > > we use on i386 and x86_64.
> >
> > So, can we finally noop smp_rmb and smp_wmb on x86?
>
> Nakked-by: Alan Cox <alan@...hat.com>
>
> You can only no-op it on 64bit Intel processors. On 32bit it needs to be
> conditional on whether your processor family (or back compat for it) as
> the Pentium Pro has some serious store ordering errata (hence the way it
> needs lock decb for spin_unlock)

We already noop smp_wmb on i386 even when CONFIG_X86_PPRO_FENCE.

I'm not sure if either errata can be solved completely by adding lock ops
in barrier instructions anyway: they both seem to involve situations where
there is just a single problematic cacheline in question.
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