Re: XDP socket rings, and LKMM litmus tests

Alan Stern
2021-03-04T21:29:33Z

On Thu, Mar 04, 2021 at 11:05:15AM -0800, Paul E. McKenney wrote:
> On Thu, Mar 04, 2021 at 10:35:24AM -0500, Alan Stern wrote:
> > On Wed, Mar 03, 2021 at 09:04:07PM -0800, Paul E. McKenney wrote:
> > > On Wed, Mar 03, 2021 at 10:21:01PM -0500, Alan Stern wrote:
> > > > On Wed, Mar 03, 2021 at 02:03:48PM -0800, Paul E. McKenney wrote:
> > > > > On Wed, Mar 03, 2021 at 03:22:46PM -0500, Alan Stern wrote:
> >
> > > > > > > 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?
> > > > > >
> > > > > > By definition, an access to a local variable cannot participate in a
> > > > > > data race because all such accesses are confined to a single thread.
> > > > >
> > > > > True, but its value might have come from a load from a shared variable.
> > > >
> > > > Then that load could have participated in a data race. But the store to
> > > > the local variable cannot.
> > >
> > > Agreed. My thought was that if the ordering from the initial (non-local)
> > > load mattered, then that initial load must have participated in a
> > > data race. Is that true, or am I failing to perceive some corner case?
> >
> > Ordering can matter even when no data race is involved. Just think
> > about how much of the memory model is concerned with ordering of
> > marked accesses, which don't participate in data races unless there is
> > a conflicting plain access somewhere.
>
> Fair point. Should I have instead said "then that initial load must
> have run concurrently with a store to that same variable"?

I'm losing track of the point you were originally trying to make.

Does ordering matter when there are no conflicting accesses? Sure.
Consider this:

A: r1 = READ_ONCE(x);
B: WRITE_ONCE(y, r1);
smp_wmb();
C: WRITE_ONCE(z, 1);

Even if there are no other accesses to y at all (let alone any
conflicting ones), the mere existence of B forces A to be ordered before
C, and this is easily detectable by a litmus test.

Alan