Re: SMP syncronization on AMD processors (broken?)

[Andrey Savochkin <saw@sawoct.com>]

On Thu, Oct 06, 2005 at 03:19:07PM +0200, Arjan van de Ven wrote:
> On Thu, 2005-10-06 at 17:05 +0400, Kirill Korotaev wrote:
> > Hello Linus, Andrew and others,
> > 
> > Please help with a not simple question about spin_lock/spin_unlock on 
> > SMP archs. The question is whether concurrent spin_lock()'s should 
> > acquire it in more or less "fair" fashinon or one of CPUs can starve any 
> > arbitrary time while others do reacquire it in a loop.
> 
> spinlocks are designed to not be fair. or rather are allowed to not be.
> If you want them to be fair on x86 you need at minimum to put a
> cpu_relax() in your busy loop...

The question was raised exactly because cpu_relax() doesn't help on these AMD
CPUs.

Some Pentiums do more than expected from them, and the programs works in a
very fair manner even without cpu_relax(), so the question boils down to
whether there are some new AMD rules how to write such loops, is it a defect
of the CPU, or if we are missing something else.

	Andrey
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