Re: [RFC] tools/memory-model: Rule out OOTA

On Fri, Jul 25, 2025 at 07:23:23AM +0200, Hernan Ponce de Leon wrote:
> On 7/24/2025 4:14 PM, Paul E. McKenney wrote:
> > On Wed, Jul 23, 2025 at 09:39:05AM -0700, Paul E. McKenney wrote:
> > > On Wed, Jul 23, 2025 at 09:26:32AM +0200, Hernan Ponce de Leon wrote:
> > > > On 7/23/2025 2:43 AM, Paul E. McKenney wrote:
> > > > > On Mon, Jan 06, 2025 at 10:40:03PM +0100, Jonas Oberhauser wrote:
> > > > > > The current LKMM allows out-of-thin-air (OOTA), as evidenced in the following
> > > > > > example shared on this list a few years ago:
> > > > > 
> > > > > Apologies for being slow, but I have finally added the litmus tests in
> > > > > this email thread to the https://github.com/paulmckrcu/litmus repo.
> > > > 
> > > > I do not understand some of the comments in the preamble of the tests:
> > > > 
> > > > (*
> > > >   * Result: Never
> > > >   *
> > > >   * But Sometimes in LKMM as of early 2025, given that 42 is a possible
> > > >   * value for things like S19..
> > > >   *
> > > >   * https://lore.kernel.org/all/[email protected]/
> > > >   *)
> > > > 
> > > > I see that herd7 reports one of the states to be [b]=S16. Is this supposed
> > > > to be some kind of symbolic state (i.e., any value is possible)?
> > > 
> > > Exactly!
> > > 
> > > > The value in the "Result" is what we would like the model to say if we would
> > > > have a perfect version of dependencies, right?
> > > 
> > > In this case, yes.
> > 
> > I should hasten to add that, compiler optimizations being what they are,
> > "perfect" may or may not be attainable, and even if attainable, might
> > not be maintainable.
> 
> Yes, I just wanted to clarify if this is what herd7 + the current model are
> saying or what developers should expect.

Good point, and I added explicit words to this effect in the comments
of those aspirational OOTA litmus tests, so thank you!

							Thanx, Paul

> Hernan
> 
> > 
> > I am pretty sure that you all already understood that, but I felt the
> > need to make it explicit.  ;-)
> > 
> > 							Thanx, Paul
>