linux-kernel - Re: [PATCH v2 1/5] rust: retain pointer mut-ness in `container

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Message-ID: <D8ECSHQTFGM5.E4N1XJBDTJUY@proton.me>
Date: Wed, 12 Mar 2025 14:22:31 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Tamir Duberstein <tamird@...il.com>, Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...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>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Brendan Higgins <brendan.higgins@...ux.dev>, David Gow <davidgow@...gle.com>, Rae Moar <rmoar@...gle.com>, Bjorn Helgaas <bhelgaas@...gle.com>, Luis Chamberlain <mcgrof@...nel.org>, Russ Weight <russ.weight@...ux.dev>, Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, linux-pci@...r.kernel.org, linux-block@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 1/5] rust: retain pointer mut-ness in `container_of!`

On Sun Mar 9, 2025 at 5:00 PM CET, Tamir Duberstein wrote:
> Avoid casting the input pointer to `*const _`, allowing the output
> pointer to be `*mut` if the input is `*mut`. This allows a number of
> `*const` to `*mut` conversions to be removed at the cost of slightly
> worse ergonomics when the macro is used with a reference rather than a
> pointer; the only example of this was in the macro's own doctest.
>
> Signed-off-by: Tamir Duberstein <tamird@...il.com>

One tiny nit below, but even without that:

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>

> ---
>  rust/kernel/lib.rs      |  5 ++---
>  rust/kernel/pci.rs      |  2 +-
>  rust/kernel/platform.rs |  2 +-
>  rust/kernel/rbtree.rs   | 23 ++++++++++-------------
>  4 files changed, 14 insertions(+), 18 deletions(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 7697c60b2d1a..9cd6b6864739 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -187,7 +187,7 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
>  /// }
>  ///
>  /// let test = Test { a: 10, b: 20 };
> -/// let b_ptr = &test.b;
> +/// let b_ptr: *const _ = &test.b;

You could also use `&raw test.b` to get a pointer instead of relying on
the pointer coercion. That syntax is stable since 1.82.0, so older
compilers would need to enable the `raw_ref_op` feature.

I created an orthogonal good-first-issue for changing uses of
`addr_of[_mut]!` to `&raw [mut]`, so maybe that can also be done there:

    https://github.com/Rust-for-Linux/linux/issues/1148

---
Cheers,
Benno