linux-kernel - Re: [tip:x86/urgent] x86: Add memory modify constraints to xchg() and cmpxchg()

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Message-ID: <4C50B6A7.3060401@zytor.com>
Date:	Wed, 28 Jul 2010 16:00:55 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	mingo@...hat.com, linux-kernel@...r.kernel.org, glommer@...hat.com,
	alan@...rguk.ukuu.org.uk, zamsden@...hat.com, stable@...nel.org,
	mtosatti@...hat.com, gregkh@...e.de, peter@...frader.org,
	tglx@...utronix.de, avi@...hat.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86: Add memory modify constraints to xchg()
 and 	cmpxchg()

On 07/28/2010 08:45 AM, Linus Torvalds wrote:
> On Tue, Jul 27, 2010 at 10:33 PM, tip-bot for H. Peter Anvin
> <hpa@...or.com> wrote:
>>
>> x86: Add memory modify constraints to xchg() and cmpxchg()
> 
> Ack. I assume this doesn't really change the code generated? At least
> not with a gcc that honors the whole memory clobber thing properly?
> 
> I also suspect that we can/should get rid of the __xg() thing - it was
> there just to make sure gcc didn't see the memory read as a single
> word and tried to optimize it. With the "+m" it probably doesn't
> matter any more (don't know if it ever did)
> 

For what it's worth, it fairly heavily preturbs code around
__set_64bit(), which implies it actually does something useful in that
case.  The rest of the code looks similar enough.

	-hpa

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