Re: spin_unlock optimization(i386)

Erich Boleyn (esboleyn@ichips.intel.com)
Wed, 24 Nov 1999 12:58:04 -0800

> >The only thing you need is to make sure there is a store in "spin_unlock()",
>
> So we could reimplement a rmb() (and in turn an mb()) that scales in SMP?
> Something like:
>
> #define rmb() ... "movb $0, ZERO_PAGE+32*smp_processor_id()"
>
> This because the spin_unlock really is:
>
> mb();
> spin_lock.lock = 0;
>
> If spin_unlock() doesn't need the lock, mb() doesn't need the lock
> on the bus either.

No. Any aligned store (which you get from the compiler) will be atomic
even in an SMP system.

> Am I missing something?

I don't think so.

What do you want the memory barrier for other than to prevent reads
from crossing the unlock? (apologies if I missed this in an earlier
message)

Erich Boleyn
PMD IA32 Architecture
Intel

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