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

Boqun Feng
2024-06-15T02:46:18Z

On Fri, Jun 14, 2024 at 06:28:00PM -0700, John Hubbard wrote:
> On 6/14/24 6:24 PM, Boqun Feng wrote:
> > On Fri, Jun 14, 2024 at 06:03:37PM -0700, John Hubbard wrote:
> > > On 6/14/24 2:59 AM, Miguel Ojeda wrote:
> > > > On Thu, Jun 13, 2024 at 9:05 PM Boqun Feng <boqun.feng@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,
> > > > then we could make the lower layer private already.
> > > >
> > > > We can defer that extra layer/work if needed even if we go for
> > > > `Atomic<T>`, but it would be nice to understand if we have consensus
> > > > for an eventual user-facing API, or if someone has any other opinion
> > > > or concerns on one vs. the other.
> > >
> > > Well, here's one:
> > >
> > > The reason that we have things like atomic64_read() in the C code is
> > > because C doesn't have generics.
> > >
> > > In Rust, we should simply move directly to Atomic<T>, as there are,
> > > after all, associated benefits. And it's very easy to see the connection
> >
> > What are the associated benefits you are referring to? Rust std doesn't
> > use Atomic<T>, that somewhat proves that we don't need it.
> Just the stock things that a generic provides: less duplicated code,

It's still a bit handwavy, sorry.

Admittedly, I haven't looked into too much Rust concurrent code, maybe
it's even true for C code ;-) So I took a look at the crate that Gary
mentioned (the one provides generic atomic APIs):

https://crates.io/crates/atomic

there's a "Dependent" tab where you can see the other crates that
depends on it. With a quick look, I haven't found any Rust concurrent
project I'm aware of (no crossbeam, no tokio, no futures). On the other
hand, there is a non-generic based atomic library:

https://crates.io/crates/portable-atomic

which has more projects depend on it, and there are some Rust concurrent
projects that I'm aware of: futures, async-task etc. Note that people
can get the non-generic based atomic API from Rust std library, and
the "portable-atomic" crate is only 2-year old, while "atomic" crate is
8-year old.

More interestingly, the same author of "atomic" crate, who is an expert
in concurrent areas, has another project (there are a lot projects from
the author, but this is the one I'm mostly aware of) "parking_lot",
which "provides implementations of Mutex, RwLock, Condvar and Once that
are smaller, faster and more flexible than those in the Rust standard
library, as well as a ReentrantMutex type which supports recursive
locking.", and it doesn't use the "atomic" crate either.

These data could mean nothing, there are multiple reasons affecting the
popularity of a library. But all the above seems to suggests that you
don't really need generic on atomic, at least for a lot of meaningful
concurent code.

So if we were to make a decision right now, I don't see that generic
atomics are winning. Of course, as I said previously, we can always add
them if we have learned more and have the consensus.

(Don't make me wrong, I love generic in general, I just want to avoid
the "I have a generic hammer and everything looks like generic nails"
situation.)

Regards,
Boqun

> automatic support for future types (although here it's really just
> integer types we care about of course).
>
>
> thanks,
> --
> John Hubbard
> NVIDIA
>