Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

["Benno Lossin" <lossin@kernel.org>]

On Mon Jun 23, 2025 at 7:19 AM CEST, Boqun Feng wrote:
> On Sat, Jun 21, 2025 at 12:32:12PM +0100, Gary Guo wrote:
>> Note tha the transparent new types restriction on `AllowAtomic` is not
>> sufficient for this, as I can define
>> 
>
> Nice catch! I do agree we should disallow `MyWeirdI32`, and I also agree
> that we should put transmutability as safety requirement for
> `AllowAtomic`. However, I would suggest we still keep
> `into_repr`/`from_repr`, and require the implementation to make them
> provide the same results as transmute(), as a correctness precondition
> (instead of a safety precondition), in other words, you can still write
> a `MyWeirdI32`, and it won't cause safety issues, but it'll be
> incorrect.

Hmm I don't like keeping the function when we add the transmute
requirement.

> The reason why I think we should keep `into_repr`/`from_repr` but add
> a correctness precondition is that they are easily to implement as safe
> code for basic types, so it'll be better than a transmute() call. Also
> considering `Atomic<*mut T>`, would transmuting between integers and
> pointers act the same as expose_provenance() and
> from_exposed_provenance()?

Hmmm, this is indeed a problem for pointers. I guess we do need the
functions...

But this also prevents us from adding the transmute requirement, as it
doesn't hold for pointers. Maybe we need to add the requirement that
`into_repr`/`from_repr` preserve the binary representation?

---
Cheers,
Benno