Re: [PATCH v1] rust: list: Use AtomicFlag in AtomicTracker

["Gary Guo" <gary@garyguo.net>]

On Mon Jan 19, 2026 at 11:17 PM GMT, FUJITA Tomonori wrote:
> Make AtomicTracker use AtomicFlag instead of Atomic<bool> to avoid
> slow byte-sized RMWs on architectures that don't support them.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/kernel/list/arc.rs | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/rust/kernel/list/arc.rs b/rust/kernel/list/arc.rs
> index 2282f33913ee..3296726e07d9 100644
> --- a/rust/kernel/list/arc.rs
> +++ b/rust/kernel/list/arc.rs
> @@ -6,7 +6,7 @@
>  
>  use crate::alloc::{AllocError, Flags};
>  use crate::prelude::*;
> -use crate::sync::atomic::{ordering, Atomic};
> +use crate::sync::atomic::{ordering, AtomicFlag};
>  use crate::sync::{Arc, ArcBorrow, UniqueArc};
>  use core::marker::PhantomPinned;
>  use core::ops::Deref;
> @@ -469,7 +469,7 @@ impl<T, U, const ID: u64> core::ops::DispatchFromDyn<ListArc<U, ID>> for ListArc
>  /// If the boolean is `false`, then there is no [`ListArc`] for this value.
>  #[repr(transparent)]
>  pub struct AtomicTracker<const ID: u64 = 0> {
> -    inner: Atomic<bool>,
> +    inner: AtomicFlag,
>      // This value needs to be pinned to justify the INVARIANT: comment in `AtomicTracker::new`.
>      _pin: PhantomPinned,
>  }
> @@ -480,12 +480,12 @@ pub fn new() -> impl PinInit<Self> {
>          // INVARIANT: Pin-init initializers can't be used on an existing `Arc`, so this value will
>          // not be constructed in an `Arc` that already has a `ListArc`.
>          Self {
> -            inner: Atomic::new(false),
> +            inner: AtomicFlag::new(false),
>              _pin: PhantomPinned,
>          }
>      }
>  
> -    fn project_inner(self: Pin<&mut Self>) -> &mut Atomic<bool> {
> +    fn project_inner(self: Pin<&mut Self>) -> &mut AtomicFlag {
>          // SAFETY: The `inner` field is not structurally pinned, so we may obtain a mutable
>          // reference to it even if we only have a pinned reference to `self`.
>          unsafe { &mut Pin::into_inner_unchecked(self).inner }
> @@ -495,7 +495,7 @@ fn project_inner(self: Pin<&mut Self>) -> &mut Atomic<bool> {
>  impl<const ID: u64> ListArcSafe<ID> for AtomicTracker<ID> {
>      unsafe fn on_create_list_arc_from_unique(self: Pin<&mut Self>) {
>          // INVARIANT: We just created a ListArc, so the boolean should be true.
> -        *self.project_inner().get_mut() = true;
> +        self.project_inner().store(true, ordering::Relaxed);

I would if we can still provide `get_mut()`. You can just hand out a reference
to the LSB of the flag (of course, this is a different byte depending on
endianness..).

Best,
Gary

>      }
>  
>      unsafe fn on_drop_list_arc(&self) {
>
> base-commit: 47f079b9ac97c4b0c673bbbf57245952e9edf8a2