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    SubjectRe: [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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