linux-kernel - Re: Intel Memory Ordering White Paper

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Date:	Sat, 8 Sep 2007 06:32:05 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andi Kleen <ak@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Intel Memory Ordering White Paper

On Saturday 08 September 2007 20:19, Andi Kleen wrote:
> On Friday 07 September 2007 21:57:35 Nick Piggin wrote:
> > > > Anyway, the lfence should be able to go away without so much trouble.
> > >
> > > You mean sfence? lfence in rmb is definitely needed.
> >
> > I mean lfence in smp_rmb().
>
> One point of rmb is to stop speculative loads and I don't think we
> can get that without lfence.

smp_rmb() should not need to do anything because loads are done
in order anyway. Both AMD and Intel have committed to this now.

The important point is that they *appear* to be done in order. AFAIK,
the CPUs can still do speculative and out of order loads, but throw
out the results if they could be wrong.
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