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.
>
>> 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". ;-)
Thanks, Akira
>
> --Elad
