`UnsafePinned<T>` is useful for cases where a value might be shared with C
code but not directly used by it. In particular this is added for
additional data in the `MiscDeviceRegistration` which will be shared
between `fops->open` and the containing struct.

Similar to `Opaque` but guarantees that the value is always initialized
and that the inner value is dropped when `UnsafePinned` is dropped.

This was originally proposed for the IRQ abstractions [0] and is also
useful for other where the inner data may be aliased, but is always valid
and automatic `Drop` is desired.

Link: https://lore.kernel.org/rust-for-linux/CAH5fLgiOASgjoYKFz6kWwzLaH07DqP2ph+3YyCDh2+gYqGpABA@mail.gmail.com [0]
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
 rust/kernel/types.rs | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 2bbaab83b9d65da667a07e85b3c89c7fa881b53c..3c2f6ac62d161f1187b5e7ade86689eec667ff4d 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -253,6 +253,9 @@ fn drop(&mut self) {
 ///
 /// `Opaque<T>` is meant to be used with FFI objects that are never interpreted by Rust code.
 ///
+/// In cases where the contained data is only used by Rust, is not allowed to be
+/// uninitialized and automatic [`Drop`] is desired [`UnsafePinned`] should be used instead.
+///
 /// It is used to wrap structs from the C side, like for example `Opaque<bindings::mutex>`.
 /// It gets rid of all the usual assumptions that Rust has for a value:
 ///
@@ -573,3 +576,57 @@ pub enum Either<L, R> {
 /// [`NotThreadSafe`]: type@NotThreadSafe
 #[allow(non_upper_case_globals)]
 pub const NotThreadSafe: NotThreadSafe = PhantomData;
+
+/// Stores a value that may be used from multiple mutable pointers.
+///
+/// `UnsafePinned` gets rid of some of the usual assumptions that Rust has for a value:
+/// - The value is allowed to be mutated, when a `&UnsafePinned<T>` exists on the Rust side.
+/// - No uniqueness for mutable references: it is fine to have multiple `&mut UnsafePinned<T>`
+///   point to the same value.
+///
+/// To avoid the ability to use [`core::mem::swap`] this still needs to be used through a
+/// [`core::pin::Pin`] reference.
+///
+/// This is useful for cases where a value might be shared with C code
+/// but not interpreted by it or in cases where it can not always be guaranteed that the
+/// references are unique.
+///
+/// This is similar to [`Opaque<T>`] but is guaranteed to always contain valid data and will
+/// call the [`Drop`] implementation of `T` when dropped.
+#[repr(transparent)]
+pub struct UnsafePinned<T> {
+    value: UnsafeCell<T>,
+    _pin: PhantomPinned,
+}
+
+impl<T> UnsafePinned<T> {
+    /// Creates a new [`UnsafePinned`] value.
+    pub const fn new(value: T) -> Self {
+        Self {
+            value: UnsafeCell::new(value),
+            _pin: PhantomPinned,
+        }
+    }
+
+    /// Create an [`UnsafePinned`] pin-initializer from the given pin-initializer.
+    pub fn try_pin_init<E>(value: impl PinInit<T, E>) -> impl PinInit<Self, E> {
+        // SAFETY:
+        //   - In case of an error in `value` the error is returned, otherwise `slot` is fully
+        //     initialized, since `self.value` is initialized and `_pin` is a zero sized type.
+        //   - The `Pin` invariants of `self.value` are upheld, since no moving occurs.
+        unsafe { init::pin_init_from_closure(move |slot| value.__pinned_init(Self::raw_get(slot))) }
+    }
+
+    /// Returns a raw pointer to the contained data.
+    pub const fn get(&self) -> *mut T {
+        UnsafeCell::get(&self.value).cast::<T>()
+    }
+
+    /// Gets the value behind `this`.
+    ///
+    /// This function is useful to get access to the value without creating intermediate
+    /// references.
+    pub const fn raw_get(this: *const Self) -> *mut T {
+        UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
+    }
+}

-- 
2.48.1

On Fri Jan 31, 2025 at 4:08 PM CET, Christian Schrefl wrote:
> @@ -573,3 +576,57 @@ pub enum Either<L, R> {
>  /// [`NotThreadSafe`]: type@NotThreadSafe
>  #[allow(non_upper_case_globals)]
>  pub const NotThreadSafe: NotThreadSafe = PhantomData;
> +
> +/// Stores a value that may be used from multiple mutable pointers.
> +///
> +/// `UnsafePinned` gets rid of some of the usual assumptions that Rust has for a value:
> +/// - The value is allowed to be mutated, when a `&UnsafePinned<T>` exists on the Rust side.
> +/// - No uniqueness for mutable references: it is fine to have multiple `&mut UnsafePinned<T>`
> +///   point to the same value.

We have another patch series [1] in transit that changes the wording on
the `Opaque<T>` type. I think we should mirror the same wording here.

[1]: https://lore.kernel.org/rust-for-linux/20250305053438.1532397-2-dirk.behme@de.bosch.com/

> +///
> +/// To avoid the ability to use [`core::mem::swap`] this still needs to be used through a

IIRC typing out the whole path makes it also appear in the docs. I think
we should just use `mem::swap` and omit `core` of course you need to
add this to make it work:
    
    /// [`mem::swap`]: core::mem::swap

> +/// [`core::pin::Pin`] reference.

Here, I would link to `Pin`, but say "[pinned](core::pin::Pin) pointer."
instead, since eg `Pin<Box<T>>` also is okay to use.

> +///
> +/// This is useful for cases where a value might be shared with C code
> +/// but not interpreted by it or in cases where it can not always be guaranteed that the
> +/// references are unique.
> +///
> +/// This is similar to [`Opaque<T>`] but is guaranteed to always contain valid data and will
> +/// call the [`Drop`] implementation of `T` when dropped.
> +#[repr(transparent)]
> +pub struct UnsafePinned<T> {
> +    value: UnsafeCell<T>,
> +    _pin: PhantomPinned,
> +}
> +
> +impl<T> UnsafePinned<T> {
> +    /// Creates a new [`UnsafePinned`] value.
> +    pub const fn new(value: T) -> Self {
> +        Self {
> +            value: UnsafeCell::new(value),
> +            _pin: PhantomPinned,
> +        }
> +    }
> +
> +    /// Create an [`UnsafePinned`] pin-initializer from the given pin-initializer.
> +    pub fn try_pin_init<E>(value: impl PinInit<T, E>) -> impl PinInit<Self, E> {
> +        // SAFETY:
> +        //   - In case of an error in `value` the error is returned, otherwise `slot` is fully
> +        //     initialized, since `self.value` is initialized and `_pin` is a zero sized type.
> +        //   - The `Pin` invariants of `self.value` are upheld, since no moving occurs.
> +        unsafe { init::pin_init_from_closure(move |slot| value.__pinned_init(Self::raw_get(slot))) }

Ah this is a bit suboptimal, but I guess there currently isn't a better
way to do this. I'll add it to my list of things to improve with
pin-init.

> +    }
> +
> +    /// Returns a raw pointer to the contained data.
> +    pub const fn get(&self) -> *mut T {
> +        UnsafeCell::get(&self.value).cast::<T>()
> +    }
> +
> +    /// Gets the value behind `this`.
> +    ///
> +    /// This function is useful to get access to the value without creating intermediate
> +    /// references.
> +    pub const fn raw_get(this: *const Self) -> *mut T {
> +        UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()

Why the cast to `MaybeUninit<T>`? I think this can just be:

    UnsafeCell::raw_get(&raw const this.value)

---
Cheers,
Benno

> +    }
> +}