> On 3 Mar 2021, at 18:12, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, Mar 02, 2021 at 03:50:19PM -0800, Paul E. McKenney wrote: >> On Tue, Mar 02, 2021 at 04:14:46PM -0500, Alan Stern wrote: > >>> This result is wrong, apparently because of a bug in herd7. There >>> should be control dependencies from each of the two loads in P0 to each >>> of the two stores, but herd7 doesn't detect them. >>> >>> Maybe Luc can find some time to check whether this really is a bug and >>> if it is, fix it. >> >> I agree that herd7's control dependency tracking could be improved. >> >> But sadly, it is currently doing exactly what I asked Luc to make it do, >> which is to confine the control dependency to its "if" statement. But as >> usual I wasn't thinking globally enough. And I am not exactly sure what >> to ask for. Here a store to a local was control-dependency ordered after >> a read, and so that should propagate to a read from that local variable. >> Maybe treat local variables as if they were registers, so that from >> herd7's viewpoint the READ_ONCE()s are able to head control-dependency >> chains in multiple "if" statements? >> >> Thoughts? > > Local variables absolutely should be treated just like CPU registers, if > possible. In fact, the compiler has the option of keeping local > variables stored in registers. > And indeed local variables are treated as registers by herd7. > (Of course, things may get complicated if anyone writes a litmus test > that uses a pointer to a local variable, Especially if the pointer > could hold the address of a local variable in one execution and a > shared variable in another! Or if the pointer is itself a shared > variable and is dereferenced in another thread!) > > But even if local variables are treated as non-shared storage locations, > we should still handle this correctly. Part of the problem seems to lie > in the definition of the to-r dependency relation; the relevant portion > is: In fact, I’d rather change the computation of “dep” here control-dependency “ctrl”. Notice that “ctrl” is computed by herd7 and present in the initial environment of the Cat interpreter. I have made a PR to herd7 that performs the change. The commit message states the new definition. > > (dep ; [Marked] ; rfi) > > Here dep is the control dependency from the READ_ONCE to the > local-variable store, and the rfi refers to the following load of the > local variable. The problem is that the store to the local variable > doesn't go in the Marked class, because it is notated as a plain C > assignment. (And likewise for the following load.) > This is a related issue, I am not sure, but perhaps it can be formulated as "should rfi and rf on registers behave the same?” > Should we change the model to make loads from and stores to local > variables always count as Marked? > > What should have happened if the local variable were instead a shared > variable which the other thread didn't access at all? It seems like a > weak point of the memory model that it treats these two things > differently. > > Alan
