linux-kernel - Re: [PATCH v5 1/5] rust: implement `kernel::sync::Refcount`

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Message-Id: <DC0AOTWSN7X8.2ZCPAFGHB9XJ@kernel.org>
Date: Tue, 12 Aug 2025 10:10:08 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Gary Guo" <gary@...nel.org>, "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>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>, "Will Deacon" <will@...nel.org>,
 "Peter Zijlstra" <peterz@...radead.org>, "Mark Rutland"
 <mark.rutland@....com>, "Tamir Duberstein" <tamird@...il.com>, "Ingo
 Molnar" <mingo@...nel.org>, "Mitchell Levy" <levymitchell0@...il.com>,
 "Lyude Paul" <lyude@...hat.com>, "Wedson Almeida Filho"
 <wedsonaf@...il.com>, "Viresh Kumar" <viresh.kumar@...aro.org>
Cc: <rust-for-linux@...r.kernel.org>, "Fiona Behrens" <me@...enk.dev>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/5] rust: implement `kernel::sync::Refcount`

On Thu Jul 24, 2025 at 1:32 AM CEST, Gary Guo wrote:
> From: Gary Guo <gary@...yguo.net>
>
> This is a wrapping layer of `include/linux/refcount.h`. Currently the
> kernel refcount has already been used in `Arc`, however it calls into
> FFI directly.
>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> Reviewed-by: Fiona Behrens <me@...enk.dev>
> Signed-off-by: Gary Guo <gary@...yguo.net>

Reviewed-by: Benno Lossin <lossin@...nel.org>

> ---
>  rust/helpers/refcount.c      | 10 ++++
>  rust/kernel/sync.rs          |  2 +
>  rust/kernel/sync/refcount.rs | 98 ++++++++++++++++++++++++++++++++++++
>  3 files changed, 110 insertions(+)
>  create mode 100644 rust/kernel/sync/refcount.rs

> +    #[inline]
> +    fn as_ptr(&self) -> *mut bindings::refcount_t {

I think we should make this `pub(crate)` from the get-go.

> +        self.0.get()
> +    }
> +

> +    /// Decrement a refcount and test if it is 0.
> +    ///
> +    /// It will `WARN` on underflow and fail to decrement when saturated.
> +    ///
> +    /// Provides release memory ordering, such that prior loads and stores are done
> +    /// before, and provides an acquire ordering on success such that memory deallocation
> +    /// must come after.
> +    ///
> +    /// Returns true if the resulting refcount is 0, false otherwise.
> +    ///
> +    /// # Notes
> +    ///
> +    /// A common pattern of using `Refcount` is to free memory when the reference count reaches
> +    /// zero. This means that the reference to `Refcount` could become invalid after calling this
> +    /// function. This is fine as long as the reference to `Refcount` is no longer used when this
> +    /// function returns `false`. It is not necessary to use raw pointers in this scenario, see
> +    /// https://github.com/rust-lang/rust/issues/55005.

Missing `<>` around link.

---
Cheers,
Benno