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

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Date:	Fri, 06 Aug 2010 13:43:12 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Peter Zijlstra" <peterz@...radead.org>
Cc:	"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 02.08.10 at 17:17, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> On 08/02/2010 08:07 AM, Peter Zijlstra wrote:
>> On Fri, 2010-07-16 at 18:03 -0700, Jeremy Fitzhardinge wrote:
>>> +       asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
>>> +                     : "+Q" (inc), "+m" (lock->slock) : : "memory", "cc");
>>   "+Q" (inc->slock)
>>
>>> +       for (;;) {
>>> +               if (inc.tickets.head == inc.tickets.tail)
>>> +                       return;
>>> +               cpu_relax();
>>> +               inc.tickets.head = ACCESS_ONCE(lock->tickets.head);
>>> +       }
>>> +       barrier();              /* make sure nothing creeps before the lock 
> is taken */
>>>   }
>> How will it ever get to that barrier() ?
> 
> The compiler treats this as being:

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.

Jan

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