Re: [PATCH v1] rust: sync: atomic: Add i32-backed Flag for atomic booleans

[Gary Guo <gary@garyguo.net>]

On Sat, 03 Jan 2026 19:44:48 +0900 (JST)
FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:

> On Thu, 1 Jan 2026 21:04:30 +0000
> Gary Guo <gary@garyguo.net> wrote:
> 
> > On Thu,  1 Jan 2026 19:27:18 +0900
> > FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:
> >   
> >> Add a new Flag enum (Clear/Set) with #[repr(i32)] and implement
> >> AtomicType for it, so users can use Atomic<Flag> for boolean flags.
> >> 
> >> 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>
> >> ---
> >>  rust/kernel/sync/atomic.rs | 35 +++++++++++++++++++++++++++++++++++
> >>  1 file changed, 35 insertions(+)
> >> 
> >> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> >> index 4aebeacb961a..d98ab51ae4fc 100644
> >> --- a/rust/kernel/sync/atomic.rs
> >> +++ b/rust/kernel/sync/atomic.rs
> >> @@ -560,3 +560,38 @@ pub fn fetch_add<Rhs, Ordering: ordering::Ordering>(&self, v: Rhs, _: Ordering)
> >>          unsafe { from_repr(ret) }
> >>      }
> >>  }
> >> +
> >> +/// An atomic flag type backed by `i32`.  
> > 
> > I would recommend that we document that the backing type is the
> > (perf-)optimal type on the target architecure, so arch can decide to use
> > i8 as backing type if they prefer.  
> 
> I'm not sure I fully understand the intent yet.
> 
> Do you mean we should document Flag as being backed by the
> (perf-)optimal integer type for the target architecture, so that the
> backing type can remain an implementation detail and potentially be
> selected per-arch (e.g. i8 on x86) via cfg?

Yes, I don't want anyone to rely on it being i32 (at least for now, before
a concrete use case of doing so appears).

> 
> Yeah, that sounds like a good addition.
> 
> +
> +impl From<Flag> for bool {
> +    fn from(f: Flag) -> Self {
> +        f == Flag::Set
> +    }
> +}

A `#[inline]` is warranted here. Also, it'll be good to have the
conversion for the other direction too.

Best,
Gary