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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 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.
    J

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