Use atomics using atomic-polyfill

[psykora]

Replace use of core::sync::atomic by atomic_polyfill

Motivation: When playing with rust and esp-rs/esp-wifi, I noticed that the examples does not compile with the embassy master.
This is quick fix for the problem. I noticed that the polyfill is used as drop-in replacement on other places

• Target: riscv32imc-unknown-none-elf
• Device: esp32c3

[Dirbaio]

See discussion here #745 (comment)

and rust-lang/rust#99668

[psykora]

Hi Dirbaio,

Thanks for pointing out there is another workaround. I didn't know it. (To be honest my experience with rust on MCU is close to zero 😳).

However, I still don't understand why we can't use atomic-polyfill here. What is different from other cases?

Rationale:

• It is not newly introduced to embassy.
  It is used on numerous places (just quickly looked for occurrences of atomic_polyfill in this repository) for example.:
  embassy-executor/src/raw/run_queue.rs
• To my understanding atomic-polyfill should just reexport core::sync::atomic:: on targets where there is native support.

Thanks for understanding,

[Dirbaio]

It has overhead on targets without hardware CAS support, because it has to acquire a critical section for each operation. This is also needed in load/store operations to prevent them from racing with CAS operations. If you never do CAS on that atomic value, it's useless overhead.

That code you linked does do CAS operations.