linux-kernel - [PATCH 4/4] rust: lock: Add a Pin<&mut T> accessor

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Message-ID: <20250919091241.32138-5-boqun.feng@gmail.com>
Date: Fri, 19 Sep 2025 11:12:41 +0200
From: Boqun Feng <boqun.feng@...il.com>
To: "Peter Zijlstra" <peterz@...adead.org>,
	"Ingo Molnar" <mingo@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>
Cc: rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	lkmm@...ts.linux.dev,
	"Will Deacon" <will@...nel.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	Waiman Long <longman@...hat.com>,
	"Miguel Ojeda" <ojeda@...nel.org>,
	alex.gaynor@...il.com,
	"Gary Guo" <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	"Benno Lossin" <lossin@...nel.org>,
	"Alice Ryhl" <aliceryhl@...gle.com>,
	"Trevor Gross" <tmgross@...ch.edu>,
	"Danilo Krummrich" <dakr@...nel.org>,
	"Andreas Hindborg" <a.hindborg@...nel.org>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: [PATCH 4/4] rust: lock: Add a Pin<&mut T> accessor

From: Daniel Almeida <daniel.almeida@...labora.com>

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 requires the pin projection functionality [1] for better ergonomic.

Link: https://github.com/Rust-for-Linux/linux/issues/1181
Link: https://lore.kernel.org/rust-for-linux/20250912174148.373530-1-lossin@kernel.org/ [1]
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>
[boqun: Apply Daniel's fix to the code example, add the reference to pin
projection patch and remove out-of-date part in the commit log]
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---

Notes:
    SubmissionLink: https://lore.kernel.org/all/20250828-lock-t-when-t-is-pinned-v2-3-b067c4b93fd6@collabora.com/

 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 9242790d15db..cb00fdb94ffd 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, marker::PhantomPinned};
+    /// struct Data(PhantomPinned);
+    ///
+    /// fn example(mutex: &Mutex<Data>) {
+    ///     let mut data: MutexGuard<'_, Data> = mutex.lock();
+    ///     let mut data: Pin<&mut Data> = data.as_mut();
+    /// }
+    /// ```
+    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> {
-- 
2.51.0