linux-kernel - Re: [PATCH v2 3/3] rust: lock: add a Pin<&mut T> accessor

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Message-Id: <DCK43W485VCY.3KE72NNMDP32D@kernel.org>
Date: Thu, 04 Sep 2025 17:13:59 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Daniel Almeida" <daniel.almeida@...labora.com>, "Peter Zijlstra"
 <peterz@...radead.org>, "Ingo Molnar" <mingo@...hat.com>, "Will Deacon"
 <will@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Waiman Long"
 <longman@...hat.com>, "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>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] rust: lock: add a Pin<&mut T> accessor

On Thu Aug 28, 2025 at 10:52 PM CEST, Daniel Almeida wrote:
> In order for callers to be able to access the inner T safely if T: !Unpin,
> there needs to be a way to get a Pin<&mut T>. Add this accessor and a
> corresponding example to tell users how it works.
>
> This is not useful on its own for now, because we do not support pin
> projections yet. This means that the following is not going to compile:
>
>     let mut data: MutexGuard<'_, Data> = mutex.lock();
>     let mut data: Pin<&mut Data> = data.as_mut();
>     let foo = &mut data.foo;
>
> A future patch can enable the behavior above by implementing support for
> pin projections. Said patch is in the works already and will possibly
> land on 6.18.
>
> Link: https://github.com/Rust-for-Linux/linux/issues/1181
> Suggested-by: Benno Lossin <lossin@...nel.org>
> Suggested-by: Boqun Feng <boqun.feng@...il.com>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Daniel Almeida <daniel.almeida@...labora.com>

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

> ---
>  rust/kernel/sync/lock.rs | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index 9242790d15dbf65d66518d060a8a777aac558cfc..7191804a244da05db74294fdec598f1a4732682c 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -245,6 +245,31 @@ pub(crate) fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
>  
>          cb()
>      }
> +
> +    /// Returns a pinned mutable reference to the protected data.
> +    ///
> +    /// The guard implements [`DerefMut`] when `T: Unpin`, so for [`Unpin`]
> +    /// types [`DerefMut`] should be used instead of this function.
> +    ///
> +    /// [`DerefMut`]: core::ops::DerefMut
> +    /// [`Unpin`]: core::marker::Unpin
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// # use kernel::sync::{Mutex, MutexGuard};
> +    /// # use core::pin::Pin;
> +    /// struct Data;
> +    ///
> +    /// fn example(mutex: &Mutex<Data>) {
> +    ///   let mut data: MutexGuard<'_, Data> = mutex.lock();
> +    ///   let mut data: Pin<&mut Data> = data.as_mut();
> +    ///  }

The formatting looks off in this one, there should be 4 spaces of
indentation here; there are also 2 spaces in front of the `}`.

Also `Data` implements `Unpin`, so you're not following your own
recommendation from above :)

---
Cheers,
Benno

> +    /// ```
> +    pub fn as_mut(&mut self) -> Pin<&mut T> {
> +        // SAFETY: `self.lock.data` is structurally pinned.
> +        unsafe { Pin::new_unchecked(&mut *self.lock.data.get()) }
> +    }
>  }
>  
>  impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {