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

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Message-ID: <4C5C6DCE.4020206@zytor.com>
Date:	Fri, 06 Aug 2010 13:17:18 -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 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.

	-hpa
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