Messages in this thread | | | Date | Mon, 13 Jul 2015 17:56:06 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() |
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On Mon, Jul 13, 2015 at 03:24:18PM +0100, Will Deacon wrote: > On Mon, Jul 13, 2015 at 02:09:50PM +0100, Peter Hurley wrote: > > On 07/13/2015 08:15 AM, Will Deacon wrote: > > > smp_mb__after_unlock_lock is used to promote an UNLOCK + LOCK sequence > > > into a full memory barrier. > > > > > > However: > > > > > > - This ordering guarantee is already provided without the barrier on > > > all architectures apart from PowerPC > > > > > > - The barrier only applies to UNLOCK + LOCK, not general > > > RELEASE + ACQUIRE operations > > > > I'm unclear what you mean here: do you mean > > A) a memory barrier is not required between RELEASE M + ACQUIRE N when you > > want to maintain distinct order between those operations on all arches > > (with the possible exception of PowerPC), or, > > B) no one is using smp_mb__after_unlock_lock() in that way right now. > > My understanding is (B), but Peter and I don't seem to agree yet! > I'll tighten up the text once we reach a conclusion.
I'm fairly sure (but I've not looked) that nobody does in fact rely on this.
So I'm in agreement with B, and I'm quibbling on what exactly A means ;-)
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