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

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Message-Id: <DATW5HJ259PB.1289VEJEUBT2Z@kernel.org>
Date: Mon, 23 Jun 2025 13:54:38 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <lkmm@...ts.linux.dev>, <linux-arch@...r.kernel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
 "Will Deacon" <will@...nel.org>, "Peter Zijlstra" <peterz@...radead.org>,
 "Mark Rutland" <mark.rutland@....com>, "Wedson Almeida Filho"
 <wedsonaf@...il.com>, "Viresh Kumar" <viresh.kumar@...aro.org>, "Lyude
 Paul" <lyude@...hat.com>, "Ingo Molnar" <mingo@...nel.org>, "Mitchell Levy"
 <levymitchell0@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Linus Torvalds"
 <torvalds@...ux-foundation.org>, "Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

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