linux-kernel - Re: [PATCH 00/18] x86: Ticket lock + cmpxchg cleanup

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Date:	Wed, 24 Aug 2011 16:10:21 -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 00/18] x86: Ticket lock + cmpxchg cleanup

On 08/24/2011 04:05 PM, H. Peter Anvin wrote:
> On 08/24/2011 03:59 PM, Jeremy Fitzhardinge wrote:
>> On 08/24/2011 03:53 PM, Linus Torvalds wrote:
>>> On Wed, Aug 24, 2011 at 1:10 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>>> Could we just kill SMP support for OOSTORE machines now?  That would be
>>>> the cleanest possible fix...
>>> No it wouldn't. The asm version would *still* be cleaner than the "C
>>> plus random barriers".
>>>
>>> It's not like the C version is "portable" in any case.
>> If there's no need to have a locked instruction, then it could simply be:
>>
>>     barrier();
>>     lock->head++;
>>     barrier();
>>
>> with no need for asm at all.
>>
> That's not guaranteed in any way to generate a locally atomic instruction.

Doesn't need to be.  The final write needs to be locally atomic, but we
assume that a lot.

    J
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