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

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Date:	Mon, 22 Aug 2011 16:43:59 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 04:29 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.

> There are a few additional xadd users which should be converted unless
> I'm mistaken:
>
> rwsem_atomic_update() in asm/rwsem.h.
>
> atomic_add_return() in asm/atomic.h.
>
> atomic64_add_return() in asm/atomic64_64.h.
>
> atom_asr() in asm/uv/uv_bau.h (*VOMIT* - the UV people have created a
> whole different type in private code...)
>

Yeah, I hadn't done a full xadd audit.  I'll take a look.

    J
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