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

[Jonas Oberhauser]

On 1/18/2023 4:50 AM, Paul E. McKenney wrote:
> On Tue, Jan 17, 2023 at 03:15:06PM -0500, Alan Stern wrote:
> > On Tue, Jan 17, 2023 at 09:43:08AM -0800, Paul E. McKenney wrote:
> > > On Tue, Jan 17, 2023 at 10:56:34AM -0500, Alan Stern wrote:
> > > > Isn't it true that the current code will flag srcu-bad-nesting if a
> > > > litmus test has non-nested overlapping SRCU read-side critical sections?
> > > Now that you mention it, it does indeed, flagging srcu-bad-nesting.
> > >
> > > Just to see if I understand, different-values yields true if the set
> > > contains multiple elements with the same value mapping to different
> > > values.  Or, to put it another way, if the relation does not correspond
> > > to a function.
> > As I understand it, given a relation r (i.e., a set of pairs of events),
> > different-values(r) returns the sub-relation consisting of those pairs
> > in r for which the value associated with the first event of the pair is
> > different from the value associated with the second event of the pair.
> OK, so different-values(r) is different than (r \ id) because the
> former operates on values and the latter on events?

I think you can say that (if you allow yourself to be a little bit loose 
with words, as I allow myself to be, much to the chagrin of Alan :) :( :)).

If you had a .value functional relation that relates every event to the 
value of that event, then
   different-values(r) = r \ .value ; .value^-1
i.e., it relates events x and y iff: 1) r relates x and y, and 2) the 
value of x is not equal to the value of y.

You could write this as
   different-values(r) = r \ .value ; value-id ; .value^-1
where value-id is like id but for values, i.e., relates every value v to 
itself.

You could say that this difference operates on the values of the events, 
rather than on the events itself.
In contrast,
    r \ id
works directly on the events and relates x and y iff: 1) r relates x and 
y, and 2) the event x is not equal to the event y.

In this sense I think your characterization is appropriate.