On Thu, Jul 05, 2018 at 10:57:28AM -0400, Alan Stern wrote: > On Thu, 5 Jul 2018, Will Deacon wrote: > > On Thu, Jul 05, 2018 at 10:21:36AM -0400, Alan Stern wrote: > > > On Wed, 4 Jul 2018, Will Deacon wrote: > > > > On Tue, Jul 03, 2018 at 01:28:17PM -0400, Alan Stern wrote: > > > > > Would this be allowed if smp_load_acquire() was implemented with > > > > > LDAPR? > > > > > If the answer is yes then we will have to remove the rfi-rel-acq and > > > > > rel-rf-acq-po relations from the memory model entirely. > > > > > > > > I don't understand what you mean by "rfi-rel-acq-po", and I assume you > > > > mean > > > > rel-rfi-acq-po for the other? Sounds like I'm confused here. > > > > > > "rfi-rel-acq" is the relation which was removed by the first of my two > > > patches (it is now back in business since Paul reverted the commits), > > > and "rel-rf-acq-po" is the relation that was introduced to replace it. > > > > Sorry, yes, I realised this after I'd replied. Curious: but why do you name > > the relations this way around, as opposed to e.g. rel-rfi-acq? It's > > obviously up to you, but I just couldn't figure out what inspired the > > ordering. > > I no longer remember the reason for naming "rfi-rel-acq" the way I did. > As you say, it doesn't make a lot of sense.
Fair enough! > The reason for "rel-rf-acq-po" instead of "rel-rfi-acq-po" was because > the second of the two patches uses that relation in a context where the > release and the acquire might very well run on different CPUs. Ok, that makes sense. I realised that I've only been thinking about RCpc making a difference in the rfi case, because Armv8 is multi-copy atomic so we don't allow early forwarding from a release to an RCpc acquire if they are from different CPUs. Again, I'd be interested in what other architectures have to say here (i.e. whether RCpc acquire/release instructions are likely to exist in a non multi-copy atomic architecture). Will
