linux-kernel - Re: [PATCH v2] rust: adding UniqueRefCounted and UniqueRef types

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Message-ID: <87zfi2jjd4.fsf@kernel.org>
Date: Mon, 03 Mar 2025 15:09:59 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Oliver Mangold" <oliver.mangold@...me>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Alex Gaynor"
 <alex.gaynor@...il.com>,  "Boqun Feng" <boqun.feng@...il.com>,  "Gary Guo"
 <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Benno
 Lossin" <benno.lossin@...ton.me>,  "Alice Ryhl" <aliceryhl@...gle.com>,
  "Trevor Gross" <tmgross@...ch.edu>,  <linux-kernel@...r.kernel.org>,
  <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v2] rust: adding UniqueRefCounted and UniqueRef types

"Oliver Mangold" <oliver.mangold@...me> writes:

[...]

> +/// Types that are [`AlwaysRefCounted`] and can be safely converted to an [`UniqueRef`]
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that the methods of the trait
> +/// change the reference count of the underlying object such that:
> +/// - the uniqueness invariant is upheld, i.e. it is not possible
> +///   to obtain another reference by any means (other than through the [`UniqueRef`])
> +///   until the [`UniqueRef`] is dropped or converted to an [`ARef`].
> +/// - [`UniqueRefCounted::dec_ref`] correctly frees the underlying object.
> +/// - [`UniqueRefCounted::unique_to_shared`] set the reference count to the value
> +/// - that the returned [`ARef`] expects for an object with a single reference
> +///   in existence.
> +pub unsafe trait UniqueRefCounted: AlwaysRefCounted + Sized {
> +    /// Checks if the [`ARef`] is unique and convert it
> +    /// to an [`UniqueRef`] it that is that case.
> +    /// Otherwise it returns again an [`ARef`] to the same
> +    /// underlying object.
> +    fn try_shared_to_unique(this: ARef<Self>) -> Result<UniqueRef<Self>, ARef<Self>>;

This could just be `try_into_unique`, since the type of `this` gives the
rest of the context.

> +    /// Converts the [`UniqueRef`] into an [`ARef`].
> +    fn unique_to_shared(this: UniqueRef<Self>) -> ARef<Self>;

Similarly, this could be `into_shared`.


Best regards,
Andreas Hindborg