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

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Message-Id: <BD4724FF-4AB7-4551-B71C-C22E6E709F19@collabora.com>
Date: Thu, 4 Sep 2025 12:15:43 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Benno Lossin <lossin@...nel.org>
Cc: 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>,
 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 4 Sep 2025, at 12:13, Benno Lossin <lossin@...nel.org> wrote:
> 
> 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 :)

I’ll fix this :)

— Daniel

> 
> ---
> 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> {