Re: [PATCH v2] rust: add `ARef::into_raw`

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Aug 21, 2024 at 9:59 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> From: Kartik Prajapati <kartikprajapati987@gmail.com>
>
> Add a method for `ARef` that is analogous to `Arc::into_raw`. It is the
> inverse operation of `ARef::from_raw`, and allows you to convert the
> `ARef` back into a raw pointer while retaining ownership of the
> refcount.
>
> This new function will be used by [1] for converting the type in an
> `ARef` using `ARef::from_raw(ARef::into_raw(me).cast())`. The author has

Oh, this should refer to me, oops.

> also needed the same function for other use-cases in the past, but [1]
> is the first to go upstream.
>
> This was implemented independently by Kartik and Alice. The two versions
> were merged by Alice, so all mistakes are Alice's.
>
> Link: https://lore.kernel.org/r/20240801-vma-v3-1-db6c1c0afda9@google.com [1]
> Closes: https://github.com/Rust-for-Linux/linux/issues/1044
> Signed-off-by: Kartik Prajapati <kartikprajapati987@gmail.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> Sending a v2 that is merged with [1]. I didn't realize that I was
> duplicating work, sorry! I'll update the author information to list
> Kartik as the primary author. I kept my function body so that it returns
> NonNull to mirror the type used by `ARef::from_raw`.
>
> This change was previously included in
> https://lore.kernel.org/all/20240801-vma-v3-1-db6c1c0afda9@google.com/
> but is being split out in its own commit at Danilo's suggestion.
>
> Link: https://github.com/Rust-for-Linux/linux/pull/1056 [1]
> ---
> Changes in v2:
> - Add example from [1], and fix the imports in the example so it compiles.
> - Update author information to reflect merge with [1].
> - Link to v1: https://lore.kernel.org/r/20240801-aref-into-raw-v1-1-33401e2fbac8@google.com
> ---
>  rust/kernel/types.rs | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index ee7dd1f963ef..9e7ca066355c 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -7,7 +7,7 @@
>  use core::{
>      cell::UnsafeCell,
>      marker::{PhantomData, PhantomPinned},
> -    mem::MaybeUninit,
> +    mem::{ManuallyDrop, MaybeUninit},
>      ops::{Deref, DerefMut},
>      pin::Pin,
>      ptr::NonNull,
> @@ -396,6 +396,35 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
>              _p: PhantomData,
>          }
>      }
> +
> +    /// Consumes the `ARef`, returning a raw pointer.
> +    ///
> +    /// This function does not change the refcount. After calling this function, the caller is
> +    /// responsible for the refcount previously managed by the `ARef`.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use core::ptr::NonNull;
> +    /// use kernel::types::{ARef, AlwaysRefCounted};
> +    ///
> +    /// struct Empty {}
> +    ///
> +    /// unsafe impl AlwaysRefCounted for Empty {
> +    ///     fn inc_ref(&self) {}
> +    ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
> +    /// }
> +    ///
> +    /// let mut data = Empty {};
> +    /// let ptr = NonNull::<Empty>::new(&mut data as *mut _).unwrap();
> +    /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
> +    /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
> +    ///
> +    /// assert_eq!(ptr, raw_ptr);
> +    /// ```
> +    pub fn into_raw(me: Self) -> NonNull<T> {
> +        ManuallyDrop::new(me).ptr
> +    }
>  }
>
>  impl<T: AlwaysRefCounted> Clone for ARef<T> {
>
> ---
> base-commit: e26fa546042add70944d018b930530d16b3cf626
> change-id: 20240801-aref-into-raw-e0518131442f
>
> Best regards,
> --
> Alice Ryhl <aliceryhl@google.com>
>