There's a couple of methods like deferred counting, differential counting, or biased references. They're usually not completely atomic free but generally provide guaranteed constant overhead or tweak when or how the memory can be shared.
- Nim's ARC only allows one thread to manage a reference count at a time, but enables an isolated graph of memory to be moved to another thread. The concept is called `Isolate`, and is very similar to Rust's single owner of mutable references. There's still WIP to have the compiler automate the checks, but it's usable now (I used it with FreeRTOS's xQueue mechanism just fine). https://github.com/nim-lang/RFCs/issues/244
- Nim's ARC only allows one thread to manage a reference count at a time, but enables an isolated graph of memory to be moved to another thread. The concept is called `Isolate`, and is very similar to Rust's single owner of mutable references. There's still WIP to have the compiler automate the checks, but it's usable now (I used it with FreeRTOS's xQueue mechanism just fine). https://github.com/nim-lang/RFCs/issues/244
- Python's new non-GIL proposal that does this using biased references: https://hackaday.com/2021/11/03/python-ditches-the-gils-and-...
- The source of Python's biased references: https://iacoma.cs.uiuc.edu/iacoma-papers/pact18.pdf
- Defered Reference counting: https://dl.acm.org/doi/pdf/10.1145/3453483.3454060
It's pretty cool stuff!