Re: [PATCH v1 1/3] rust: revocable: Switch to kernel::sync atomic primitives

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Dec 31, 2025 at 07:29:17AM +0800, Boqun Feng wrote:
> On Tue, Dec 30, 2025 at 06:37:16PM +0900, FUJITA Tomonori wrote:
> > Convert uses of `AtomicBool` to `Atomic<bool>`.
> > 
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> > ---
> >  rust/kernel/revocable.rs | 24 ++++++++++++++++--------
> >  1 file changed, 16 insertions(+), 8 deletions(-)
> > 
> > diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
> > index 0f4ae673256d..47cd2b45eada 100644
> > --- a/rust/kernel/revocable.rs
> > +++ b/rust/kernel/revocable.rs
> > @@ -7,12 +7,20 @@
> >  
> >  use pin_init::Wrapper;
> >  
> > -use crate::{bindings, prelude::*, sync::rcu, types::Opaque};
> > +use crate::{
> > +    bindings,
> > +    prelude::*,
> > +    sync::atomic::{
> > +        ordering::Relaxed,
> > +        Atomic, //
> > +    },
> > +    sync::rcu,
> > +    types::Opaque, //
> > +};
> >  use core::{
> >      marker::PhantomData,
> >      ops::Deref,
> > -    ptr::drop_in_place,
> > -    sync::atomic::{AtomicBool, Ordering},
> > +    ptr::drop_in_place, //
> >  };
> >  
> >  /// An object that can become inaccessible at runtime.
> > @@ -65,7 +73,7 @@
> >  /// ```
> >  #[pin_data(PinnedDrop)]
> >  pub struct Revocable<T> {
> > -    is_available: AtomicBool,
> > +    is_available: Atomic<bool>,
> >      #[pin]
> >      data: Opaque<T>,
> 
> I actually think we should use Atomic<i32> instead of Atomic<bool> here,
> because in most of the cases `T` is a aligned to at least 4-bytes (e.g.
> Devres uses a Revocable<IoMem>) And in that case, using Atomic<bool>
> doesn't save extra memory but makes the xchg() operation slower on
> architectures that don't support byte-wise RmW instructions (e.g.
> riscv).
> 
> My rule of thumb for using Atomic<bool> is: when you really care about
> the memory usage, and you can guarantee using Atomic<bool> is saving
> memory. For a general "flag" usage, Atomic<bool> is not strictly better
> than Atomic<i32> in most cases imo.

In Binder I believe we only use load/store with Atomic<bool>. Does it
matter in that case?

Alice