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

Hugh Dickins
2006-09-23T15:34:40Z

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