linux-kernel - score-boarding [was Re: [PATCH] Linux Kernel Markers]

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Message-ID: <Pine.LNX.4.64.0609231619220.27459@blonde.wat.veritas.com>
Date:	Sat, 23 Sep 2006 16:34:33 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Richard J Moore <richardj_moore@...ibm.com>
cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: score-boarding [was Re: [PATCH] Linux Kernel Markers]

On Thu, 21 Sep 2006, Richard J Moore wrote:
> 
> It can for another reason - score-boarding: that's where a byte being
> stored assumes intermediate values due to the bits not being set
> simultaneously. Generally this doesn't cause a problem because data across
> processors is serialised for update by mutexes. However, when applied to
> code all sorts of interesting instructions can execute before the bits
> settle down. I haven't heard of this troubling Intel, but it does occur on
> some current architectures.

I'd not heard of this phenomenon, and it worries me.  There are places
in kernel code where we peek at some volatile variable (perhaps a long)
without locking, and expect to see it in any one of several well-defined
states.  Are you saying that there are architectures supported by Linux,
on which we might see an "impossible" mix of states, due to score-boarding?

Hugh
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