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

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Date:	Mon, 02 Aug 2010 17:07:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.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 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.

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

> +       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() ?
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