Re: SMP syncronization on AMD processors (broken?)

[Arjan van de Ven]

On Thu, 2005-10-06 at 17:32 +0400, Andrey Savochkin wrote:
> 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.

the rule basically is "don't write such a loop" though; this is only the
beginning; because if you have two such things on separate cores of a
dual core cpu you for sure starve anything outside of that core just the
same. Eg it goes one level up as well.

There is no spin_lock_yield() currently and until there is this is just
a code pattern you should avoid.


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