The implementation uses atomics and futexes (yay!) and is heavily based on the
implementation I did for my learning project named "Let's write synchronization
primitives" [0].
That project, in fact, started when I tried to implement pthread_once() for
Serenity (because it was needed for another project of mine, stay tuned ;) ) and
was not very sure I got every case right. So now, after learning some more about
code patterns around atomics and futexes, I am reasonably sure, and it's time to
contribute the implementation of pthread_once() to Serenity :^)
[0] To be published at https://github.com/bugaevc/lets-write-sync-primitives
