From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction
 abstraction
Date: Wed, 18 Jun 2008 17:49:51 -0700 (PDT)
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To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: benh@kernel.crashing.org, xen-devel <xen-devel@lists.xensource.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, kvm-devel <kvm-devel@lists.sourceforge.net>, x86@kernel.org, LKML <linux-kernel@vger.kernel.org>, Virtualization Mailing List <virtualization@lists.osdl.org>, Hugh Dickins <hugh@veritas.com>, Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>
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On Wed, 18 Jun 2008, Jeremy Fitzhardinge wrote:
>
> It's not cmpxchg, just xchg. 
> In other words, is:
> 
> 	lock btr $_PAGE_BIT_RW, (%rbx)

Well, we can actually do it as

	lock andl $~_PAGE_RW,(%rbx)

although we haven't bothered (I've wanted several times to make 
clear_bit() do that, but have never gotten around to it - mainly because 
old gcc versions didn't work with __builtin_constant_p() in inline 
functions - so you have to do the macro from hell)

And yes, the "lock andl" should be noticeably faster than the xchgl.

			Linus