Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Alan Stern
2023-01-15T16:23:38Z

On Sat, Jan 14, 2023 at 09:15:10PM -0800, Paul E. McKenney wrote:
> On Sat, Jan 14, 2023 at 03:19:06PM -0500, Alan Stern wrote:
> > On Sat, Jan 14, 2023 at 10:15:37AM -0800, Paul E. McKenney wrote:
> > > Nevertheless, here is the resulting .bell fragment:
> > >
> > > ------------------------------------------------------------------------
> > >
> > > (* Compute matching pairs of Srcu-lock and Srcu-unlock *)
> > > let srcu-rscs = ([Srcu-lock] ; data ; [Srcu-unlock]) & loc
> > >
> > > (* Validate nesting *)
> > > flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
> > > flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking
> > >
> > > (* Check for use of synchronize_srcu() inside an RCU critical section *)
> > > flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep
> > >
> > > (* Validate SRCU dynamic match *)
> > > flag ~empty different-values(srcu-rscs) as srcu-bad-nesting
> >
> > I forgot to mention... An appropriate check for one srcu_read_lock()
> > matched to more than one srcu_read_unlock() would be something like
> > this:
> >
> > flag ~empty (srcu-rscs^-1 ; srcu-rscs) \ id as multiple-unlocks
>
> I have added this, thank you!
>
> > Alan
> >
> > PS: Do you agree that we should change the names of the first two flags
> > above to unbalanced-srcu-lock and unbalanced-srcu-unlock, respectively
> > (and similarly for the rcu checks)? It might help to be a little more
> > specific about how the locking is wrong when we detect an error.
>
> I have made this change, again, thank you!
>
> But I also added this:
>
> flag empty srcu-rscs as no-srcu-readers
>
> And it is always flagged. So far, I have not found any sort of relation
> that connects Srcu-lock to Srcu-unlock other than po. I tried data,
> ctrl, addr, rf, rfi, and combinations thereof.
>
> What am I missing here?

I don't think you're missing anything. This is a matter for Boqun or
Luc; it must have something to do with the way herd treats the
srcu_read_lock() and srcu_read_unlock() primitives.

Alan