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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 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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