Re: [PATCH v4 1/2] rust: sync: atomic: Add performance-optimal-integer-backed Flag for atomic booleans

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Jan 20, 2026 at 08:08:51AM +0900, FUJITA Tomonori wrote:
> On Mon, 19 Jan 2026 12:10:25 +0900 (JST)
> FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:
> 
> > On Mon, 19 Jan 2026 09:22:10 +0800
> > Boqun Feng <boqun.feng@gmail.com> wrote:
> > 
> >> On Thu, Jan 15, 2026 at 11:12:29AM +0900, FUJITA Tomonori 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 may vary by
> >>> architecture and 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>
> >>> ---
> >>>  rust/kernel/sync/atomic.rs | 71 ++++++++++++++++++++++++++++++++++++++
> >>>  1 file changed, 71 insertions(+)
> >>> 
> >>> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> >>> index ca9cab77abf0..58f57903460f 100644
> >>> --- a/rust/kernel/sync/atomic.rs
> >>> +++ b/rust/kernel/sync/atomic.rs
> >> 
> >> I prefer we can move `Flag` into atomic/predefine.rs, I queued in
> >> rust-sync with the following version, please let me know whether it
> >> works, thank you all!
> > 
> > Works for me, thanks!
> 
> If we like to keep the intra-doc-links, a workaround like the
> followings seems to be necessary.
> 
> diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
> index f6c7538cb47b..92e312c06595 100644
> --- a/rust/kernel/sync/atomic/predefine.rs
> +++ b/rust/kernel/sync/atomic/predefine.rs
> @@ -156,6 +156,13 @@ fn rhs_into_delta(rhs: usize) -> isize_atomic_repr {
>  /// flag.store(Flag::Set, Relaxed);
>  /// assert_eq!(Flag::Set, flag.load(Relaxed));
>  /// ```
> +///
> +/// [`Atomic<Flag>`]: kernel::sync::atomic::Atomic
> +/// [`Atomic<bool>`]: kernel::sync::atomic::Atomic
> +/// [`Atomic::xchg()`]: kernel::sync::atomic::Atomic::xchg
> +/// [`Atomic::cmpxchg()`]: kernel::sync::atomic::Atomic::cmpxchg
> +/// [`Atomic::load()`]: kernel::sync::atomic::Atomic::load
> +/// [`Atomic::store()`]: kernel::sync::atomic::Atomic::store
>  #[derive(Clone, Copy, PartialEq, Eq)]
>  #[cfg_attr(any(CONFIG_X86_64, CONFIG_UML, CONFIG_ARM, CONFIG_ARM64), repr(i8))]
>  #[cfg_attr(
> 


Good point! I was missing that, however I think we could use `super`
instead of `kernel::sync::atomic` in this case. Already pushed to
rust-sync with the following changes, thanks!

Regards,
Boqun

------------->8
diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
index f6c7538cb47b..5faa2fe2f4b6 100644
--- a/rust/kernel/sync/atomic/predefine.rs
+++ b/rust/kernel/sync/atomic/predefine.rs
@@ -156,6 +156,13 @@ fn rhs_into_delta(rhs: usize) -> isize_atomic_repr {
 /// flag.store(Flag::Set, Relaxed);
 /// assert_eq!(Flag::Set, flag.load(Relaxed));
 /// ```
+///
+/// [`Atomic<Flag>`]: super::Atomic
+/// [`Atomic<bool>`]: super::Atomic
+/// [`Atomic::xchg()`]: super::Atomic::xchg
+/// [`Atomic::cmpxchg()`]: super::Atomic::cmpxchg
+/// [`Atomic::load()`]: super::Atomic::load
+/// [`Atomic::store()`]: super::Atomic::store
 #[derive(Clone, Copy, PartialEq, Eq)]
 #[cfg_attr(any(CONFIG_X86_64, CONFIG_UML, CONFIG_ARM, CONFIG_ARM64), repr(i8))]
 #[cfg_attr(