Re: [PATCH 1/3] rust: add Aliased type

[Boqun Feng]

Hi Christian,

[Cc Daniel and Danilo]

Thanks for the patch!

On Sun, Jan 19, 2025 at 11:11:13PM +0100, Christian Schrefl wrote:
> This type is useful for cases where a value might be shared with C code
> but not interpreted by it.
> In partiquarly this is added to for data that is shared between a Driver
> and a MiscDevice implementation.
> 
> Similar to Opaque but guarantees that the value is initialized and the
> inner value is dropped when Aliased is dropped.
> 
> This was origianally proposed for the IRQ abstractions [0], but also
> useful for other cases where Data may be aliased, but is always valid
> and automatic drop is desired.
> 
> Link: https://lore.kernel.org/rust-for-linux/CAH5fLgiOASgjoYKFz6kWwzLaH07DqP2ph+3YyCDh2+gYqGpABA@xxxxxxxxxxxxxx [0]
> Suggested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Signed-off-by: Christian Schrefl <chrisi.schrefl@xxxxxxxxx>
> ---
>  rust/kernel/types.rs | 40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 3aea6af9a0bca70ee42b4bad2fe31a99750cbf11..5640128c9a9055476a0040033946ba6caa6e7076 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -528,3 +528,43 @@ pub enum Either<L, R> {
>  /// [`NotThreadSafe`]: type@NotThreadSafe
>  #[allow(non_upper_case_globals)]
>  pub const NotThreadSafe: NotThreadSafe = PhantomData;
> +
> +/// Stores a value that may be aliased.
> +///
> +/// This is similar to `Opaque<T>` but is guaranteed to contain valid data and will
> +/// Call the Drop implementation of T when dropped.
> +#[repr(transparent)]
> +pub struct Aliased<T> {

As I already mentioned [1], the name `Aliased` is more reflecting the
fact that this wrapper will avoid generating the "noalias" attribute(?)
on the reference/pointer to the type rather than an intuitive idea about
"why or when do I need this". Moreover, I think the argument about the
naming of the counterpart in unstable Rust (UnsafePinned) makes sense to
me [2]: this type alone won't prevent `&mut Aliased` getting `swap`, and
it has to be used with `Pin`.

Therefore, I think we should use a different name, perhaps
`(Always)Shared`, or just use `UnsafePinned`, or, as always, looking
fowards to a better name from anybody ;-)

[1]: https://lore.kernel.org/rust-for-linux/Z407egxOy7oNLpq8@boqun-archlinux/
[2]: https://rust-lang.github.io/rfcs/3467-unsafe-pinned.html#naming

Regards,
Boqun

> +    value: UnsafeCell<T>,
> +    _pin: PhantomPinned,
> +}
> +
> +impl<T> Aliased<T> {
> +    /// Creates a new `Aliased` value.
> +    pub const fn new(value: T) -> Self {
> +        Self {
> +            value: UnsafeCell::new(value),
> +            _pin: PhantomPinned,
> +        }
> +    }
> +    /// Create an `Aliased` 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 the slot is fully initialized,
> +        // since value is initialized and _pin is a Zero sized type.
> +        // The pin invariants of 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 opaque 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
>