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

[Peter Zijlstra]

On Thu, Jun 13, 2024 at 09:30:26AM -0700, Boqun Feng wrote:

> 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.
> 
> *	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.
> 
> So let's start with some basic and simple until we really have a need
> for generic `Atomic<T>`. Thoughts?

Not on the generic thing, but on the lack of long. atomic_long_t is
often used when we have pointers with extra bits on. Then you want a
number type in order to be able to manipulate the low bits.