Re: Plain accesses and data races in the Linux Kernel Memory Model

[Alan Stern]

On Tue, 22 Jan 2019, Andrea Parri wrote:

> > @@ -131,7 +159,7 @@ let rec rcu-fence = rcu-gp | srcu-gp |
> >  	(rcu-fence ; rcu-link ; rcu-fence)
> >  
> >  (* rb orders instructions just as pb does *)
> > -let rb = prop ; po ; rcu-fence ; po? ; hb* ; pb*
> > +let rb = prop ; po ; rcu-fence ; po? ; hb* ; pb* ; [marked]
> 
> Testing has revealed some subtle semantics changes for some RCU tests
> _without_ unmarked memory accesses; an example is reported at the end
> of this email.  I suspect that the improvements you mentioned in this
> thread can restore the original semantics but I'm reporting this here
> for further reference.
> 
> With the above definition of 'rb', we're losing links which originate
> or target RCU fences, so that this definition is in fact a relaxation
> w.r.t. the current semantics (even when limiting to marked accesses).
> The test below, for example, is currently forbidden by the LKMM, but
> it becomes allowed with this patch.
> 
> FWIW, I checked that including the RCU fences in 'marked' can restore
> the original semantics of these tests; I'm still not sure whether this
> change can make sense though....
> 
> Thoughts?

Ah, a very good discovery.  I think changing marked to ~plain in a few
places would be a better solution.  Or maybe allowing plain accesses in
those places will also be okay -- it's hard to judge at this point.

> Oh, one last (and unrelated) nit before I forget: IIUC, we used to
> upper-case set names, so I'd also suggest s/marked/Marked, s/plain/Plain
> and similarly for the other sets to be introduced.

Okay, I'll follow that convention.

Alan