linux-kernel - Re: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C

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Message-ID: <4C5C7A0A.9080107@zytor.com>
Date:	Fri, 06 Aug 2010 14:09:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Jan Beulich <JBeulich@...ell.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Avi Kivity <avi@...hat.com>, Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C

On 08/06/2010 01:33 PM, Jeremy Fitzhardinge wrote:
> On 08/06/2010 01:17 PM, H. Peter Anvin wrote:
>> On 08/06/2010 07:53 AM, Jeremy Fitzhardinge wrote:
>>> On 08/06/2010 05:43 AM, Jan Beulich wrote:
>>>> You certainly mean "the compiler currently treats this as being:" - I
>>>> don't think there's a guarantee it'll always be doing so.
>>>>
>>>>> for (;;) {
>>>>> if (inc.tickets.head == inc.tickets.tail)
>>>>> goto out;
>>>>> ...
>>>>> }
>>>>> out: barrier();
>>>>> }
>>>>>
>>>>> (Which would probably be a reasonable way to clarify the code.)
>>>> I therefore think it needs to be written this way.
>>>
>>> Agreed.
>>>
>>
>> A call/return to an actual out-of-line function is a barrier (and will
>> always be a barrier, as it is the fundamental ABI sequence points),
>> but to an inline function it is not.
>
> Yes. So the goto explicitly puts the barrier into the control flow which
> should stop the compiler from doing anything unexpected.
>

In this particular case, though, I would somewhat expect the more 
conventional:

while (inc.tickets.head != inc.tickets.tail) {
	cpu_relax();
	inc.tickets.head = ACCESS_ONCE(lock->tickets_head);
}

	-hpa
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