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