In this chapter, we discussed the atomic primitives available on modern CPU architectures and their reflection in Rust. We built higher-level synchronization primitives of the sort discussed in Chapter 4, Sync and Send – the Foundation of Rust Concurrency, and Chapter 5, Locks – Mutex, Condvar, Barriers, and RWLock, as well as our own semaphore, which does not exist in Rust. The semaphore implementation could be improved, depending on your needs, and I warmly encourage the readers to give that a shot. We also ran into a common problem of atomic programming, memory reclamation, which we discussed in terms of a Michael Scott queue. We'll discuss approaches to this problem in-depth in the next chapter.