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

[Gary Guo <gary@garyguo.net>]

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

> On Wed, Feb 19, 2025 at 3:17 PM Gary Guo <gary@garyguo.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@google.com>
> > Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> > Signed-off-by: Gary Guo <gary@garyguo.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

> 
> 
>  [...]