Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops

[Peter Zijlstra]

On Mon, 2011-09-12 at 16:23 +0200, Andi Kleen wrote:
> On Mon, Sep 12, 2011 at 04:05:58PM +0200, Peter Zijlstra wrote:
> > Subject: llist: Remove cpu_relax() usage in cmpxchg loops
> > From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Date: Mon Sep 12 15:50:49 CEST 2011
> > 
> > Initial benchmarks show they're a net loss (2 socket wsm):
> > 
> 
> May still save power. however:
> 
> >  		entry = cmpxchg(&head->first, old_entry, new);
> >  		if (entry == old_entry)
> >  			break;
> > -		cpu_relax();
> 
> You need at least a barrier() then, otherwise the compiler could
> change the memory order in the loop.

the cmpxchg is a full memory barrier and compiler barrier.
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