Document what "memory model" guarantees each Atomic method provides

[alexdowad]

From reading the source, it appears that #compare_and_set, #update, and #try_update are all equivalent to a full memory barrier. It also appears that #get, #set, and #swap provide no concurrency guarantees whatsoever.

This is crucial information when using Atomic to write correct, high-performance, concurrent code. It should be included right in the RDoc for each method, and also mentioned in the README.

[jdantonio]

Closing because this gem has been deprecated.

[headius]

I'll try to answer the question even though future discussion should move to concurrent-ruby.

In the JRuby code, Atomic always uses a volatile field. That will represent a memory barrier for both set and get, as in the JDK's AtomicReference class.

I'm not as familiar with the C code.

[pitr-ch]

followup in ruby-concurrency/concurrent-ruby#232