On Wed, Mar 03, 2021 at 06:37:36PM +0100, maranget wrote: > > > > On 3 Mar 2021, at 18:12, Alan Stern <st...@rowland.harvard.edu> 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.
Shouldn't similar reasoning apply to data and address dependencies? For example, suppose there is a control dependency from a load to a register variable, and then a data dependency from the register variable to a store. This should be treated as an overall data dependency from the load to the store. Does your change to herd7 do this? I couldn't tell from the description in the PR. Also, do you think it's reasonable to add a restriction to herd7 against taking the address of a local variable? > > (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?” Aren't they already the same thing? It's not possible to have an rfe from a register, is it? Alan > > 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
