Re: [patch] Re: spin_unlock optimization(i386)

Linus Torvalds (
Sun, 21 Nov 1999 12:04:59 -0800 (PST)

On Sun, 21 Nov 1999, Manfred Spraul wrote:
> You have reordered write _ahead_ of read, and the effects were
> externally visible.
> This cannot happen:

Not on a Pentium.

But I quote (from the PPro manual):

"The only enhancement in the PentiumPro processor is the added support
for speculative reads and store-buffer forwarding, .."

A Pentium is a in-order machine, without any of the interesting
speculation wrt reads etc. So on a Pentium you'll never see the problem.

But a Pentium is also very uninteresting from a SMP standpoint these days.
It's just too weak with too little per-CPU cache etc..

This is why the PPro has the MTRR's - exactly to let the core do
speculation (a Pentium doesn't need MTRR's, as it won't re-order anything
external to the CPU anyway, and in fact won't even re-order things


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