linux-kernel - Re: [PATCH] rust: sync: fix safety comment for `static_lock

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Message-Id: <DBIJLR7XNI6U.21PMPODHE83DZ@kernel.org>
Date: Tue, 22 Jul 2025 13:21:17 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "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>, "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Danilo
 Krummrich" <dakr@...nel.org>, "Lyude Paul" <lyude@...hat.com>, "Mitchell
 Levy" <levymitchell0@...il.com>, "Wedson Almeida Filho"
 <wedsonaf@...il.com>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rust: sync: fix safety comment for `static_lock_class`

On Wed May 21, 2025 at 1:17 AM CEST, Benno Lossin wrote:
> The safety comment mentions lockdep -- which from a Rust perspective
> isn't important -- and doesn't mention the real reason for why it's
> sound to create `LockClassKey` as uninitialized memory.
>
> Signed-off-by: Benno Lossin <lossin@...nel.org>
> ---
>
> I don't think we need to backport this.
>
> ---
>  rust/kernel/sync.rs | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 36a719015583..a10c812d8777 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -93,8 +93,11 @@ fn drop(self: Pin<&mut Self>) {
>  macro_rules! static_lock_class {
>      () => {{
>          static CLASS: $crate::sync::LockClassKey =
> -            // SAFETY: lockdep expects uninitialized memory when it's handed a statically allocated
> -            // lock_class_key
> +            // Lockdep expects uninitialized memory when it's handed a statically allocated `struct
> +            // lock_class_key`.
> +            //
> +            // SAFETY: `LockClassKey` transparently wraps `Opaque` which permits uninitialized
> +            // memory.
>              unsafe { ::core::mem::MaybeUninit::uninit().assume_init() };

Looking at this patch with fresh eyes (thanks for the bump, Alice :) I
think we should rather have a public unsafe function on `LockClassKey`
that creates an uninitialized lock class key. I'd like to avoid the
`MaybeUninit::uninit().assume_init()` pattern, as it might confuse
people & it looks very wrong.

We can take this patch, as it definitely is an improvement, but I think
we should also just fix this properly. Any thoughts?

---
Cheers,
Benno

>          $crate::prelude::Pin::static_ref(&CLASS)
>      }};
>
> base-commit: a5806cd506af5a7c19bcd596e4708b5c464bfd21