Re: [PATCH v2] rust: sync: atomic: Add i32-backed Flag for atomic booleans

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Jan 09, 2026 at 09:36:41AM +0900, FUJITA Tomonori wrote:
> On Thu, 8 Jan 2026 13:54:58 +0100
> Alice Ryhl <aliceryhl@google.com> wrote:
> 
> > On Thu, Jan 8, 2026 at 1:50 PM FUJITA Tomonori
> > <fujita.tomonori@gmail.com> wrote:
> >>
> >> Add a new Flag enum (Clear/Set) and implement AtomicType for it, so
> >> users can use Atomic<Flag> for boolean flags.
> >>
> >> The backing integer type is an implementation detail; it is currently
> >> i32 but may change in the future.
> >>
> >> Document when Atomic<Flag> is generally preferable to Atomic<bool>: in
> >> particular, when RMW operations such as xchg()/cmpxchg() may be used
> >> and minimizing memory usage is not the top priority. On some
> >> architectures without byte-sized RMW instructions, Atomic<bool> can be
> >> slower for RMW operations.
> >>
> >> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> > 
> > Can we have a `struct AtomicFlag(Atomic<Flag>)` here whose load() /
> > store() methods use real booleans? Otherwise this seems inconvenient
> > to use.
> 
> Do you mean something along these lines?
> 
> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> index 473d3c07e234..19750be1a239 100644
> --- a/rust/kernel/sync/atomic.rs
> +++ b/rust/kernel/sync/atomic.rs
> @@ -622,3 +622,29 @@ fn from(b: bool) -> Self {
>          }
>      }
>  }
> +
> +/// A convenience wrapper around [`Atomic<Flag>`] that exposes a `bool` API.
> +pub struct AtomicFlag(Atomic<Flag>);
> +
> +impl AtomicFlag {
> +    /// Creates a new atomic flag.
> +    pub const fn new(b: bool) -> Self {
> +        if b {
> +            AtomicFlag(Atomic::new(Flag::Set))
> +        } else {
> +            AtomicFlag(Atomic::new(Flag::Clear))
> +        }
> +    }
> +
> +    /// Loads the value from the atomic flag.
> +    #[inline(always)]
> +    pub fn load<Ordering: ordering::AcquireOrRelaxed>(&self, o: Ordering) -> bool {
> +        self.0.load(o).into()
> +    }
> +
> +    /// Stores a value to the atomic flag.
> +    #[inline(always)]
> +    pub fn store<Ordering: ordering::ReleaseOrRelaxed>(&self, b: bool, o: Ordering) {
> +        self.0.store(b.into(), o)
> +    }
> +}

Yes, that's what I meant.

Alice