On 08/06/2010 01:33 PM, Jeremy Fitzhardinge wrote:
quoted text
> 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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