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

[Benno Lossin]

On 15.06.24 03:33, Boqun Feng wrote:
> 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`.

I think this is a non-issue. We have two options of counteracting this:
1. We can just point this out in reviews and force people to use
   `Atomic<T>` with a concrete type. In cases where there really is the
   need to be generic, we can have it.
2. We can add a private trait in the bounds for the generic, nobody
   outside of the module can access it and thus they need to use a
   concrete type:

        // needs a better name
        trait Integer {}
        impl Integer for i32 {}
        impl Integer for i64 {}

        pub struct Atomic<T: Integer> {
            /* ... */
        }

And then in the other module, you can't do this (with compiler error):

        pub struct Refcount<R: Integer, T> {
                            // ^^^^^^^ not found in this scope
                            // note: trait `crate::atomic::Integer` exists but is inaccessible
            refcount: Atomic<R>,
            data: UnsafeCell<T>,
        }

I think that we can start with approach 2 and if we find a use-case
where generics are really unavoidable, we can either put it in the same
module as `Atomic<T>`, or change the access of `Integer`.

---
Cheers,
Benno

> 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.