On Tue, 22 Jan 2019, Andrea Parri wrote: > > @@ -131,7 +159,7 @@ let rec rcu-fence = rcu-gp | srcu-gp | > > (rcu-fence ; rcu-link ; rcu-fence) > > > > (* rb orders instructions just as pb does *) > > -let rb = prop ; po ; rcu-fence ; po? ; hb* ; pb* > > +let rb = prop ; po ; rcu-fence ; po? ; hb* ; pb* ; [marked] > > Testing has revealed some subtle semantics changes for some RCU tests > _without_ unmarked memory accesses; an example is reported at the end > of this email. I suspect that the improvements you mentioned in this > thread can restore the original semantics but I'm reporting this here > for further reference. > > With the above definition of 'rb', we're losing links which originate > or target RCU fences, so that this definition is in fact a relaxation > w.r.t. the current semantics (even when limiting to marked accesses). > The test below, for example, is currently forbidden by the LKMM, but > it becomes allowed with this patch. > > FWIW, I checked that including the RCU fences in 'marked' can restore > the original semantics of these tests; I'm still not sure whether this > change can make sense though.... > > Thoughts?
Ah, a very good discovery. I think changing marked to ~plain in a few places would be a better solution. Or maybe allowing plain accesses in those places will also be okay -- it's hard to judge at this point. > Oh, one last (and unrelated) nit before I forget: IIUC, we used to > upper-case set names, so I'd also suggest s/marked/Marked, s/plain/Plain > and similarly for the other sets to be introduced. Okay, I'll follow that convention. Alan
