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

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Date:	Mon, 02 Aug 2010 08:17:53 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C

  On 08/02/2010 08:07 AM, Peter Zijlstra wrote:
> On Fri, 2010-07-16 at 18:03 -0700, Jeremy Fitzhardinge wrote:
>> +       register union {
>> +               struct __raw_tickets tickets;
>> +               unsigned short slock;
>> +       } inc = { .slock = 1<<  TICKET_SHIFT };
>    register arch_spinlock_t inc = { .tickets = { .head = 1, .tail = 0 } };
>
> > From a quick look you can basically replace all TICKET_SHIFT usage (1<<
> TICKET_SHIFT) with such a constant.

Mostly.  In the later patch to convert trylock in to C, you need it to 
construct an argument for cmpxchg (which can only take a scalar, even if 
it does have a struct packed into it).

> [ Also, does gcc really listen to the register hint these days? ]

It doesn't make much different in this case.  I think the only real 
effect is that its illegal to take the address of a register variable.

>> +       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:

	for (;;) {
		if (inc.tickets.head == inc.tickets.tail)
			goto out;
		...
	}
out:	barrier();
}

(Which would probably be a reasonable way to clarify the code.)

Without the barrier there's a risk of locked-region code being scheduled 
before the for(;;) loop.

     J
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