Re: [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg operations

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jun 30, 2025 at 10:51:00AM -0400, Alan Stern wrote:
> On Mon, Jun 30, 2025 at 12:16:27PM +0200, Andreas Hindborg wrote:
> > "Boqun Feng" <boqun.feng@gmail.com> writes:
> > > in atomic/ordering.rs, I think I can extend it to:
> > >
> > > //! - [`Acquire`] provides ordering between the load part of the annotated operation and all the
> > > //!   following memory accesses, and if there is a store part, it has Relaxed ordering.
> > > //! - [`Release`] provides ordering between all the preceding memory accesses and the store part of
> > > //!   the annotated operation, and if there is load part, it has Relaxed ordering
> > >
> > > This aligns with what we usually describe things in tool/memory-model/.
> > 
> > Cool. When you start to go into details of ordering concepts, I feel

Well, avoiding going too much into the details is what I wanted for
those documentation comments ;-)

> > like something is missing though. For example for this sentence:
> > 
> >   [`Release`] provides ordering between all the preceding memory
> >   accesses and the store part of the annotated operation.
> > 
> > I guess this provided ordering is only guaranteed to be observable for
> > threads that read the same location with `Acquire` or stronger ordering?
> > 
> > If we start expanding on the orderings, rather than deferring to LKMM,
> > we should include this info.

I'm not sure I follow you on this one. I'm not trying to expand on
orderings, instead I'm trying to resolve your feedback that my previous
version didn't mention what ordering the unspecific part of a
read-modify-write has (like the store part of an Acquire and load part
of the Release). And to me, they are naturally just Relaxed, but I feel
that given the feedback I got from you, maybe I should explicitly
mention they are Relaxed.

It's simply just making things more explicit and I'm still deferring to
LKMM about the exact meaning of the ordering.

> 
> The problem with the word "ordering" is that it is too general, not 
> specific enough.  You need more context to know exactly what the 
> ordering means.
> 
> For example, ordering store A against store B (which comes later in the 
> code) could mean that the CPU executes A before it executes B.  Or it 
> could mean that a different CPU will see the data from A before it sees 
> the data from B.
> 
> A more explicit description would be helpful.
> 

Except that the explicit description should be in tools/memory-model/
instead of the comments of a function or the Rust atomic module. Like
Documentation/atomic_t.txt vs
tools/memory-model/Documentation/explanation.txt. Because one is for
people to get a quick idea about what these annotations/suffixes mean,
and the other is the more elaborate on their precise meaning. Most of
the cases, people won't need to get the subtilly of the memory model to
write correct code.

Regards,
Boqun

> Alan Stern