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

["H. Peter Anvin" <hpa@xxxxxxxxx>]

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);
}

        -hpa

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel