linux-kernel - Re: [PATCH v8 4/9] rust: sync: atomic: Add generic atomics

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Message-Id: <DC0AGT8F3JH1.2PZIXKX9AQBWD@kernel.org>
Date: Tue, 12 Aug 2025 09:59:39 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <lkmm@...ts.linux.dev>,
 <linux-arch@...r.kernel.org>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Gary Guo" <gary@...yguo.net>,
 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>,
 "Alan Stern" <stern@...land.harvard.edu>
Subject: Re: [PATCH v8 4/9] rust: sync: atomic: Add generic atomics

On Sat Jul 19, 2025 at 5:08 AM CEST, Boqun Feng wrote:
> To provide using LKMM atomics for Rust code, a generic `Atomic<T>` is
> added, currently `T` needs to be Send + Copy because these are the
> straightforward usages and all basic types support this.
>
> Implement `AtomicType` for `i32` and `i64`, and so far only basic
> operations load() and store() are introduced.
>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

Reviewed-by: Benno Lossin <lossin@...nel.org>

> ---
>  rust/kernel/sync/atomic.rs           | 274 +++++++++++++++++++++++++++
>  rust/kernel/sync/atomic/predefine.rs |  15 ++
>  2 files changed, 289 insertions(+)
>  create mode 100644 rust/kernel/sync/atomic/predefine.rs

> +/// # Limitations
> +///
> +/// Because C primitives are used to implement the atomic operations, and a C function requires a
> +/// valid object of a type to operate on (i.e. no `MaybeUninit<_>`), hence at the Rust <-> C
> +/// surface, only types with all the bits initialized can be passed. As a result, types like `(u8,
> +/// u16)` (padding bytes are uninitialized) are currently not supported. Note that technically
> +/// these types can be supported if some APIs are removed for them and the inner implementation is
> +/// tweaked, but the justification of support such a type is not strong enough at the moment. This
> +/// should be resolved if there is an implementation for `MaybeUninit<i32>` as `AtomicImpl`.

I'd remove the last sentence, as it makes it sound like one only would
have to add that impl and be finished.

---
Cheers,
Benno