On Wed, Mar 03, 2021 at 12:12:21PM -0500, Alan Stern 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. > > (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!) Good point! I did miss this complication. ;-) As you say, when its address is taken, the "local" variable needs to be treated as is it were shared. There are exceptions where the pointed-to local is still used only by its process. Are any of these exceptions problematic? > 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: > > (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.) > > Should we change the model to make loads from and stores to local > variables always count as Marked? As long as the initial (possibly unmarked) load would be properly complained about. And I cannot immediately think of a situation where this approach would break that would not result in a data race being flagged. Or is this yet another failure of my imagination? > 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. But is this really any different than the situation where a global variable is only accessed by a single thread? Thanx, Paul
