Re: SMP syncronization on AMD processors (broken?)

[Linus Torvalds]

On Thu, 6 Oct 2005, Andrey Savochkin wrote:
> 
> I start to wonder about existing mainstream code, presumably bug-free, that
> uses spinlocks without any problematic restart.

Actually, a number of cases where we found things to be a problem have 
been converted.

It _is_ a real-life issue, although with "normal" code it only happens in 
extreme machines (NUMA with tons of nodes). It's fairly fundamental in 
NUMA, and in many ways you do absolutely _not_ want fairness, because it's 
much better to take the lock locally a hundred times (almost free) than it 
is to bounce it back-and-forth between two nodes a hundred times (very 
expensive). 

Fairness is often very expensive indeed.

But places where unfairness can be a problem have been converted to things 
like RCU, which allows concurrent operations more gracefully.

And sometimes the answer is just "don't do that then".

		Linus
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