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

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Date:	Thu, 19 Jun 2008 09:30:20 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Jeremy Fitzhardinge <jeremy@...p.org>, 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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction
 abstraction

On Thu, 19 Jun 2008, Ingo Molnar wrote:
> 
> Below is the commit, it needed a small amount of massaging to apply the 
> void * -> unsigned long * change in the x86/bitops topic.

Well, that's your bug right there.

The macros very much depended on the pointers being "void *", due to the 
pointer arithmetic (which is a gcc extension that we use extensively - 
"void *" arithmetic works as if it was a byte pointer).

When you changed "addr" to "volatile unsigned long *", now the arithmetic 
ended up multiplying the offset by the size of "long" before adding it.

That said, the _original_ patch wasn't tested either, but quite frankly, 
looking over the patch one more time, I can't really see how it could 
break.

		Linus
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