Messages in this thread | | | Date | Fri, 06 Aug 2010 13:33:41 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C |
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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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