Re: "movb" for spin-unlock

From: Jamie Lokier (
Date: Thu Apr 27 2000 - 16:44:19 EST

Gérard Roudier wrote:
> I was not trying to defend Linus, but had questions in mind given than
> this topic had been discussed in _full_ details about one year ago (or
> more) and Linus had explained _clearly_ the reasons that let him take his
> decision at this time.

Linus has changed his mind.

It didn't happen until we restarted this thread, got some new tests done
and input from the right person at Intel.

A small piece of code that occurs extremely often in the kernel just got
more than 20 times faster. Apparently it shows up in application

But even more importantly:

We understand what's going on now!

The older thread petered out with some loose ends. There were
conflicting conclusions and misunderstandings.

Now, we understand that the faster code works with all Intel-style SMP
systems from Pentiums up, but may fail for some 386 or 486 SMP systems.
And we understand why.

This is complex stuff and the folks writing the most basic code really
need to understand it. Why, only today, someone came to my office and
ask for an explanation of memory ordering problems between threads on an
SMP system. And I was able to refer to this thread. :-)

have a nice day,
-- Jamie

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