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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 On 08/06/2010 02:09 PM, H. Peter Anvin wrote:
> On 08/06/2010 01:33 PM, Jeremy Fitzhardinge wrote:
> > On 08/06/2010 01:17 PM, H. Peter Anvin wrote:
> > > On 08/06/2010 07:53 AM, Jeremy Fitzhardinge wrote:
> > > > On 08/06/2010 05:43 AM, Jan Beulich wrote:
> > > > > You certainly mean "the compiler currently treats this as being:" - I
> > > > > don't think there's a guarantee it'll always be doing so.
> > > > >
> > > > > > for (;;) {
> > > > > > if (inc.tickets.head == inc.tickets.tail)
> > > > > > goto out;
> > > > > > ...
> > > > > > }
> > > > > > out: barrier();
> > > > > > }
> > > > > >
> > > > > > (Which would probably be a reasonable way to clarify the code.)
> > > > > I therefore think it needs to be written this way.
> > > > Agreed.
> > > A call/return to an actual out-of-line function is a barrier (and will
> > > always be a barrier, as it is the fundamental ABI sequence points),
> > > but to an inline function it is not.
> > Yes. So the goto explicitly puts the barrier into the control flow which
> > should stop the compiler from doing anything unexpected.
> In this particular case, though, I would somewhat expect the more 
> conventional:
> while (inc.tickets.head != inc.tickets.tail) {
>     cpu_relax();
>     inc.tickets.head = ACCESS_ONCE(lock->tickets_head);
> }
Yes, that makes sense for the plain spinlock version.  But the full 
code, including the pv-ticketlock spin timeout, ends up being:
static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
{
        register struct __raw_tickets inc;

        inc = __ticket_spin_claim(lock);

        for (;;) {
                unsigned count = SPIN_THRESHOLD;

                do {
                        if (inc.head == inc.tail)
                                goto out;
                        cpu_relax();
                        inc.head = ACCESS_ONCE(lock->tickets.head);
                } while (--count);
                __ticket_lock_spinning(lock, inc.tail);
        }
out:    barrier();              /* make sure nothing creeps before the lock is 
taken */
}

So the goto form is closer to the final form.  If it weren't for this, 
I'd also prefer the while() form.
(If you config PARAVIRT_SPINLOCKS=n, then __ticket_lock_spinning() 
becomes an empty inline, which causes gcc to collapse the whole thing 
into a simple infinite loop (ie, it eliminates "count" and the inner loop).)

    J

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