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

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Message-ID: <20250827204901.75a47868.gary@garyguo.net>
Date: Wed, 27 Aug 2025 20:49:01 +0100
From: Gary Guo <gary@...yguo.net>
To: "Benno Lossin" <lossin@...nel.org>
Cc: "Gary Guo" <gary@...nel.org>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex
 Gaynor" <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>,
 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>,
 <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 Tue, 12 Aug 2025 10:10:08 +0200
"Benno Lossin" <lossin@...nel.org> wrote:

> 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.

I think it's probably fine as is. I think this method won't be useful
outside this module unless people add a `from_ptr` later. But I'm also
fine changing this.

Best,
Gary

> 
> > +        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