Re: [RFC 2/2] rust: sync: Add atomic support

[Gary Guo <gary@garyguo.net>]

On Thu, 13 Jun 2024 09:30:26 -0700
Boqun Feng <boqun.feng@gmail.com> wrote:

> > > diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> > > new file mode 100644
> > > index 000000000000..b0f852cf1741
> > > --- /dev/null
> > > +++ b/rust/kernel/sync/atomic.rs
> > > @@ -0,0 +1,63 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +
> > > +//! Atomic primitives.
> > > +//!
> > > +//! These primitives have the same semantics as their C counterparts, for precise definitions of
> > > +//! the semantics, please refer to tools/memory-model. Note that Linux Kernel Memory (Consistency)
> > > +//! Model is the only model for Rust development in kernel right now, please avoid to use Rust's
> > > +//! own atomics.
> > > +
> > > +use crate::bindings::{atomic64_t, atomic_t};
> > > +use crate::types::Opaque;
> > > +
> > > +mod r#impl;
> > > +
> > > +/// Atomic 32bit signed integers.
> > > +pub struct AtomicI32(Opaque<atomic_t>);
> > > +
> > > +/// Atomic 64bit signed integers.
> > > +pub struct AtomicI64(Opaque<atomic64_t>);
> > 
> > 
> > Can we avoid two types and use a generic `Atomic<T>` and then implement
> > on `Atomic<i32>` and `Atomic<i64>` instead? Like the recent
> > generic NonZero in Rust standard library or the atomic crate
> > (https://docs.rs/atomic/).
> > 
> 
> We can always add a layer on top of what we have here to provide the
> generic `Atomic<T>`. However, I personally don't think generic
> `Atomic<T>` is a good idea, for a few reasons:
> 
> *	I'm not sure it will bring benefits to users, the current atomic
> 	users in kernel are pretty specific on the size of atomic they
> 	use, so they want to directly use AtomicI32 or AtomicI64 in
> 	their type definitions rather than use a `Atomic<T>` where their
> 	users can provide type later.

You can write `Atomic<i32>` and `Atomic<i64>`.

> 
> *	I can also see the future where we have different APIs on
> 	different types of atomics, for example, we could have a:
> 
> 		impl AtomicI64 {
> 		    pub fn split(&self) -> (&AtomicI32, &AtomicI32)
> 		}
> 
> 	which doesn't exist for AtomicI32. Note this is not a UB because
> 	we write our atomic implementation in asm, so it's perfectly
> 	fine for mix-sized atomics.

You can still have

	impl Atomic<i64> {
	    pub fn split(&self) -> (&Atomic<i32>, &Atomic<i32>)
	}

I see `Atomic<i32>/Atomic<i64>` being generally more flexible than
`AtomicI32/AtomicI64`, without very little downsides. We may not have
generic users but I think it doesn't do any harm to have it in a form
that makes future generics easy.

Best,
Gary