linux-kernel - Re: [PATCH v4 1/5] rust: implement `kernel::sync::Refcount`

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Message-Id: <DATD8G28N47E.HWFYVQ7MRX8P@kernel.org>
Date: Sun, 22 Jun 2025 23:05:09 +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>
Cc: <rust-for-linux@...r.kernel.org>, "Fiona Behrens" <me@...enk.dev>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/5] rust: implement `kernel::sync::Refcount`

On Sun Jun 22, 2025 at 2:57 PM CEST, Gary Guo wrote:
> +impl Refcount {
> +    /// Construct a new [`Refcount`] from an initial value.
> +    #[inline]
> +    pub fn new(value: i32) -> Self {

Should we really allow users to set a negative value from the get-go?
Here a `u31` might come in real handy...

> +        // SAFETY: There are no safety requirements for this FFI call.
> +        Self(Opaque::new(unsafe { bindings::REFCOUNT_INIT(value) }))
> +    }
> +
> +    #[inline]
> +    fn as_ptr(&self) -> *mut bindings::refcount_t {
> +        self.0.get()
> +    }
> +
> +    /// Set a refcount's value.
> +    #[inline]
> +    pub fn set(&self, value: i32) {

Same here. We should of course provide a `saturate` function, but I
don't see a reason to set it to another negative value.

---
Cheers,
Benno

> +        // SAFETY: `self.as_ptr()` is valid.
> +        unsafe { bindings::refcount_set(self.as_ptr(), value) }
> +    }