Re: XDP socket rings, and LKMM litmus tests

Alan Stern
2021-03-04T00:07:27Z

On Wed, Mar 03, 2021 at 06:37:36PM +0100, maranget wrote:
>
>
> > On 3 Mar 2021, at 18:12, Alan Stern <stern@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