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SubjectRe: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C
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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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