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

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Message-ID: <aL47zBoNpvjYxec5@tardis-2.local>
Date: Sun, 7 Sep 2025 19:13:32 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Daniel Almeida <daniel.almeida@...labora.com>
Cc: Benno Lossin <lossin@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	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 Thu, Sep 04, 2025 at 12:15:43PM -0300, Daniel Almeida wrote:
> 
> 
> > 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 :)
> 

If the fix is small, feel free to send a diff and I can fold it when
queueing (i.e. no need to resend the whole series). I'm trying to send
it to tip before -rc6 so there will be some more tests. Thanks!

Regards,
Boqun

> - 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> {
> 
>