On Wed, Dec 22, 2021 at 11:35:02PM +0900, Akira Yokosawa wrote:
> Hi,
>
> On Wed, 22 Dec 2021 07:15:54 -0500, Elad Lahav wrote:
> > Hi Akira,
> >
> >> I think this is mostly covered in Section 9.5.2.1 "Publish-Subscribe
> >> Mechanism" and Figure 9.10 "Publication/Subscription Constraints".
> >
> > I don't believe it is. I think you can infer that from reading sections 9.5
> > and chapter 15, but it is never made explicit.
>
> I agree it is not explicit.
>
> >
> > Sub-section 9.5.2.1 talks about the use of rcu_dereference() to obtain a
> > pointer, but it doesn't go very deep into how that's implemented. You would
> > think that the writer's use of store-release semantics would necessitate
> > the reader's use of load-acquire semantics, but the discussion here, the
> > previous examples, and a quick inspection of how rcu_dereference() is
> > implemented, suggest that a READ_ONCE() is sufficient (or some less
> > Linux-kernel-specific equivalent).
> >
> > If I am a naive reader (and a lazy one, who haven't read chapter 15 and made
> > the necessary connection), I could write something like:
> >
> > struct foo_s {
> > int a;
> > } foo;
> > int b;
> > struct foo_s *g_foop = &foo;
> >
> > // Reader
> > rcu_read_lock();
> > struct foo *l_foop = rcu_dereference(g_foop);
> > use(l_foop->a);
> > use(b);
> > rcu_read_unlock();
> >
> > // Writer
> > struct foo *l_foop = malloc(sizeof(struct foo));
> > l_foop->a = 1;
> > b = 2;
> > // Release semantics ensure previous stores are observed
> > rcu_assign_pointer(g_foop, l_foop);
> >
> > But since b has no address dependency to g_foop and since neither
> > rcu_read_lock() nor rcu_dereference() impose acquire semantics, then
> > the reader may not observe the new value of b.
>
> No, it may not.
> So what you want is the mention of "address dependency" somewhere in
> Section 9.5.2.1?
>
> >
> > Again, I may be missing something, but this seems to be a major point
> > that needs to be explained and emphasized.
>
> I guess Paul is intentionally avoiding discussions on memory ordering
> here in Section 9.5.
>
> Such a discussion would easily frighten naive readers...
>
> Anyway, I guess Paul would have some good compromise to satisfy you.
Actually, I am going to ask Elad to propose the location and wording of an
addition to Section 9.5 covering this, so that we can discuss and refine
it. This addition might be a new paragraph, a footnote, a quick quiz,
a citation, or what have you. And the refining might switch back and
forth among these options a few times. But we do have to start somewhere.
I am thinking in terms of this going in after the release that I was
naively planning to do yesterday. (The objective universe had other
ideas.)
> >> BTW, I couldn't figure out what you meant by "the Spanish
> >> Acquisition"...
> > It's a reference to an old Monty Python skit. Apologies for the silly pun...
>
> Ah, now I guess I see the pun of "acquire" and "acquisition". ;-)
And here I was hoping that Elad was purchasing a house in Barcelona
or some such. ;-)
(Sorry, couldn't resist!)
Thanx, Paul
