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Message-ID: <20110407164245.GA21838@one.firstfloor.org>
Date:	Thu, 7 Apr 2011 18:42:45 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@....edu>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFT/PATCH v2 2/6] x86-64: Optimize vread_tsc's barriers

> But before that, I would check that the second lfence is needed AT
> ALL. I don't know whether we ever tried the "only barrier before"
> version, because quite frankly, if you call this thing in a loop, then

> a single barrier will still mean that there are barriers in between
> iterations. So...

The testers will call it in a loop, but real users (e.g. using it for
a logging timestamp) may not.

I'm sure a single barrier would have fixed the testers, as you point out,
but the goal wasn't to only fix testers.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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