linux-kernel - Re: [PATCH 07/15] x86: add xadd helper macro

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Message-ID: <4E532F65.7010609@zytor.com>
Date:	Mon, 22 Aug 2011 21:41:09 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 07/15] x86: add xadd helper macro

On 08/22/2011 04:43 PM, Jeremy Fitzhardinge wrote:
> On 08/22/2011 04:29 PM, H. Peter Anvin wrote:
>> On 08/22/2011 04:15 PM, Jeremy Fitzhardinge wrote:
>>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>>>
>>> Add a common xadd implementation.
>>>
>>> This has the side effect of generating a bad instruction if you try to
>>> use it on a 64-bit value on a 32-bit system - but don't do that.
>>>
>> It would be better to barf at that point, so we get the error with a C
>> line... also, there needs to be a default clause with
>> __compiletime_error() in it.
> 
> OK, but the "standard of care" here is the old "calling undefined
> function" link error; getting a bad asm instruction is a little more
> helpful, in a sense.  But agreed on the default: case.  I'll see if I
> can fix up xadd and cmpxchg to fail better as well.
> 

__compiletime_error() is better than either, obviously...

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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