linux-kernel - Re: [PATCH v3 1/4] rust: implement `kernel::sync::Refcount`

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Message-ID: <20250221160217.2a6772c3@eugeo>
Date: Fri, 21 Feb 2025 16:02:17 +0000
From: Gary Guo <gary@...yguo.net>
To: Tamir Duberstein <tamird@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 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>, Lyude Paul <lyude@...hat.com>, Wedson Almeida Filho
 <wedsonaf@...il.com>, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] rust: implement `kernel::sync::Refcount`

On Wed, 19 Feb 2025 17:12:19 -0500
Tamir Duberstein <tamird@...il.com> wrote:

> On Wed, Feb 19, 2025 at 3:17 PM Gary Guo <gary@...yguo.net> wrote:
> >
> > 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>
> > Signed-off-by: Gary Guo <gary@...yguo.net>
> > ---
> >  rust/helpers/refcount.c      | 10 +++++
> >  rust/kernel/sync.rs          |  2 +
> >  rust/kernel/sync/refcount.rs | 86 ++++++++++++++++++++++++++++++++++++
> >  3 files changed, 98 insertions(+)
> >  create mode 100644 rust/kernel/sync/refcount.rs
> >
>
> <snip>
>
> > +    /// Decrement a refcount.
> > +    ///
> > +    /// It will `WARN` on underflow and fail to decrement when saturated.
> > +    ///
> > +    /// Provides release memory ordering, such that prior loads and stores are done
> > +    /// before.
> > +    #[inline]
> > +    pub fn dec(&self) {
> > +        // SAFETY: `self.as_ptr()` is valid.
> > +        unsafe { bindings::refcount_dec(self.as_ptr()) }
> > +    }
> > +
> > +    /// 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.
> > +    #[inline]
> > +    #[must_use = "use `dec` instead you do not need to test if it is 0"]  
> 
> The word "if" seems to be missing?

Ack

> 
> The C function comment includes this bit:
> 
>  * Use of this function is not recommended for the normal reference counting
>  * use case in which references are taken and released one at a time.  In these
>  * cases, refcount_dec(), or one of its variants, should instead be used to
>  * decrement a reference count.
> 
> Do we need to include this warning?

I think you're referring to refcount_sub_and_test? This is
refcount_dec_and_test.

Best,
Gary

> 
> 
>  [...]