Use a mutex for 64 bit atomics on xtensa targets

[ellenhp]

Motivation

I'm using a library that includes tokio as a transitive dependency on an xtensa architecture (ESP32). The ESP32 doesn't have 64 bit atomics, so tokio fails to build.

Solution

Adding xtensa to the list of target architectures that use mutexes to simulate atomic 64 bit integers. :)

[Darksonn]

Thanks.

[ellenhp]

Spoke a bit with some Rust ESP-32 people and they indicated this was the right way to go about this change. Their only warning was that most of tokio still won't build which I knew already. Ready for review. Thank you!

[Darksonn]

I don't really understand why CI is stuck. Are you able to push an empty commit?

[ellenhp]

I just amended the commit without changing anything and then force-pushed, hopefully that's fine. I know it says somewhere not to rebase and force-push during review but I assume that's just because it can make review hard for larger PRs. Hopefully for 6 lines of diff it was fine to do that.

[Darksonn]

Yes, it's ok in this case. Another thing you could have done is push an empty commit with git commit --allow-empty

[ellenhp]

I spoke a bit more with some Rust ESP32 people and we're considering maybe changing our max atomic width to 64 and implementing the 64 bit atomic types with a mutex. Tokio handles these platforms explicitly but much of the rust ecosystem assumes 64 atomics will always exist, so I suggested doing that as a potential option. I have no idea if that's the right thing to do or the wrong thing to do, but I wanted to put this into a draft state while people with more experience weigh in. Would hate to have to revert something if it got merged in the meantime.

[Darksonn]

Let me know what you find out.

[Darksonn]

Any status on this?

[ellenhp]

Hey! I'm not sure because I hit another blocker (Brian Smith choosing not to review briansmith/ring#1459) for my project so it ended up not really being possible to use the ESP32 for what I wanted. I haven't been in touch with the rest of the community since then.

[Darksonn]

Okay. I will close this for now. If anyone needs this in the future, they can open a new PR.

[MabezDev]

@ellenhp FYI we recently changed the STD targets to support 64bit atomics so in theory this PR is no longer needed anyway :).