Re: XDP socket rings, and LKMM litmus tests

Paul E. McKenney
2021-03-03T20:58:13Z

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