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

[Gary Guo <gary@garyguo.net>]

On Wed, 31 Dec 2025 18:35:02 +0900 (JST)
FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:

> On Wed, 31 Dec 2025 07:29:17 +0800
> Boqun Feng <boqun.feng@gmail.com> wrote:
> 
> >>  /// 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).  
> 
> I agree. Recent RISC-V support byte-wise swap instructions so this may
> not appliy to them. Among the architectures currently supporting Rust,
> perhaps only loongarch could have this issue.

The narrower atomic instructions are extensions and not part of the base
ISA. RISC-V arch use alternative mechanisms for this, but using the
mechanism has an overhead, let alone the hardwares that do not have the
extensions.

Best,
Gary