linux-kernel - Re: [PATCH 1 of 4] mm: add a ptep_modify

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Date:	Wed, 18 Jun 2008 17:37:27 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	benh@...nel.crashing.org,
	xen-devel <xen-devel@...ts.xensource.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Hugh Dickins <hugh@...itas.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction abstraction

Linus Torvalds wrote:
> On Wed, 18 Jun 2008, Jeremy Fitzhardinge wrote:
>   
>> Along the lines of:
>>     
>
> Hell no. There's a reason we have a special set_wrprotect() thing. We can 
> do it more efficiently on native hardware by just clearing the bit 
> atomically. No need to do the cmpxchg games.
>   

It's not cmpxchg, just xchg. 

In other words, is:

	lock btr $_PAGE_BIT_RW, (%rbx)

much cheaper than

	mov	$0, %rax
	xchg	%rax, (%rbx)
	and	$~_PAGE_RW, %rax
	mov	%rax, (%rbx)

?

It's the same number of locked RMW operations, so aside from being a few 
instructions longer, I think it would be much the same.

I guess the correct answer is "lmbench".

    J
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