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Subject: Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops
From: Peter Zijlstra <peterz@infradead.org>
To: Andi Kleen <andi@firstfloor.org>
Cc: Huang Ying <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 12 Sep 2011 16:23:53 +0200
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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@chello.nl>
> > 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.