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

Paul E. McKenney
2023-01-18T21:06:48Z

On Wed, Jan 18, 2023 at 08:57:22PM +0100, Jonas Oberhauser wrote:
>
>
> 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 :) :( :)).

Well, Alan's insistance on rigor has keep LKMM out of trouble more times
than I can count. ;-)

> 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.

It looks to be "different domain values", but maybe I should just run
some experiments. ;-)

Thanx, Paul