linux-kernel - Re: [patch] x86: improved memory barrier implementation

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Message-ID: <alpine.LFD.0.999.0709280911100.3579@woody.linux-foundation.org>
Date:	Fri, 28 Sep 2007 09:15:06 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [patch] x86: improved memory barrier implementation

On Fri, 28 Sep 2007, Alan Cox wrote:
> 
> However 
> - You've not shown the patch has any performance gain

It would be nice to see this.

> - You've probably broken Pentium Pro

Probably not a big deal, but yeah, we should have that broken-ppro option.

> - and for modern processors its still not remotely clear your patch is
> correct because of NT stores.

This one I disagree with. The *old* code doesn't honor NT stores *either*. 

The fact is, if you use NT stores and then depend on ordering, it has 
nothing what-so-ever to do with spinlocks or smp_[rw]mb. You need to use 
the DMA barriers (ie the non-smp ones).

The non-temporal stores should be basically considered to be "IO", not any 
normal memory operation.

		Linus
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