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

[Boqun Feng]

On Fri, Jun 14, 2024 at 09:22:24PM +0000, Benno Lossin wrote:
> On 14.06.24 16:33, Boqun Feng wrote:
> > On Fri, Jun 14, 2024 at 11:59:58AM +0200, Miguel Ojeda wrote:
> >> On Thu, Jun 13, 2024 at 9:05 PM Boqun Feng <boqun.feng at gmail.com> wrote:
> >>>
> >>> Does this make sense?
> >>
> >> Implementation-wise, if you think it is simpler or more clear/elegant
> >> to have the extra lower level layer, then that sounds fine.
> >>
> >> However, I was mainly talking about what we would eventually expose to
> >> users, i.e. do we want to provide `Atomic<T>` to begin with? If yes,
> > 
> > The truth is I don't know ;-) I don't have much data on which one is
> > better. Personally, I think AtomicI32 and AtomicI64 make the users have
> > to think about size, alignment, etc, and I think that's important for
> > atomic users and people who review their code, because before one uses
> > atomics, one should ask themselves: why don't I use a lock? Atomics
> > provide the ablities to do low level stuffs and when doing low level
> > stuffs, you want to be more explicit than ergonomic.
> 
> How would this be different with `Atomic<i32>` and `Atomic<i64>`? Just

The difference is that with Atomic{I32,I64} APIs, one has to choose (and
think about) the size when using atomics, and cannot leave that option
open. It's somewhere unconvenient, but as I said, atomics variables are
different. For example, if someone is going to implement a reference
counter struct, they can define as follow:

	struct Refcount<T> {
	    refcount: AtomicI32,
	    data: UnsafeCell<T>
	}

but with atomic generic, people can leave that option open and do:

	struct Refcount<R, T> {
	    refcount: Atomic<R>,
	    data: UnsafeCell<T>
	}

while it provides configurable options for experienced users, but it
also provides opportunities for sub-optimal types, e.g. Refcount<u8, T>:
on ll/sc architectures, because `data` and `refcount` can be in the same
machine-word, the accesses of `refcount` are affected by the accesses of
`data`.

The point I'm trying to make here is: when you are using atomics, you
care about performance a lot (otherwise, why don't you use a lock?), and
because of that, you should care about the size of the atomics, because
it may affect the performance significantly.

Regards,
Boqun

> because the underlying `Atomic<I>` type is generic shouldn't change
> this, right?
> 
> ---
> Cheers,
> Benno
>