Re: XDP socket rings, and LKMM litmus tests

Paul E. McKenney
2021-03-04T19:07:14Z

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"?

Thanx, Paul