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

Alan Stern
2023-01-11T17:05:16Z

On Wed, Jan 11, 2023 at 04:45:46PM +0000, Jonas Oberhauser wrote:
>
>
> -----Original Message-----
> From: Alan Stern [mailto:stern@rowland.harvard.edu]
> > On Wed, Jan 11, 2023 at 11:33:33AM +0000, Jonas Oberhauser wrote:
> > > Considering how much effort it is to keep the documentation up-to-date
> > > even for small changes, I'm extremely oscillation-averse.
> > > Interestingly as I go through the documentation while preparing each
> > > patch I often find some remarks hinting at the content of the patch,
> > > e.g. "fences don't link events on different CPUs" and "rcu-fence is
> > > able to link events on different CPUs. (Perhaps this fact should lead
> > > us to say that rcu-fence isn't really a fence at all!)" in the current
> > > explanation.txt.
> >
> > > [...] that I'll use strong-order right away, and then rename the handful of
> > > other fences-but-not-really-at-all to '-order' as well.
> >
> > Minor snag: There already is an rcu-order relation in the memory model.
> > Maybe we need a different word from "order". Or maybe rcu-order should be renamed.
>
> Yeah, I noticed (it's in the same section I'm quoting from above). There are
> some other minor things that might need editing in that section, e.g.,
> "Written symbolically, X ->rcu-fence Y means
> there are fence events E and F such that:
>
> X ->po E ->rcu-order F ->po Y."
> But in fact the definition is
> let rcu-fence = po ; rcu-order ; po?
> which allows for F = Y and not F ->po Y.

Yeah, that should be fixed.

> I'll need to get a better understanding of rcu-order before I can form an
> opinion of how things could be organized. The only thing I'm certain of is that
> strong-order and rcu-fence should end up with the same suffix :D
>
> Just looking at it from afar, it almost looks like there's a simpler,
> non-recursive definition of rcu-order trying to come out. I assume you've tried
> various things and they don't work xP ?

What is there to try? As far as I know, the only construct in the cat
language that can be used to get the effect of counting is a recursive
definition.

> Is it because you use the recursion to "count" the grace periods and read-side
> critical sections, in the sense of maintaining the inequality invariant between
> them? I wonder if there's a "pumping lemma" that can show this can't be done
> with a non-recursive definition.

Such a lemma would have to be based on the other constructs available in
the language. The only things I can think of which even come close are
the * and + operators, and they are insufficient (because they are no
stronger than regular expressions, which are well known to be too weak
-- there isn't even a regular expression for strings in which the
parentheses are balanced).

Alan