Shared state and the futex
workq builds a bounded MPMC job queue three ways — std::mutex + condvars, a buffered channel, Mutex + Condvar — and we look under all of them at the one primitive doing the work: the futex, whose uncontended fast path is a userspace CAS with no syscall and whose slow path is FUTEX_WAIT/WAKE, proven by strace scaling from 5,186 to 488,818 calls, then broken on purpose under ThreadSanitizer and go -race while Rust refuses to compile the race at all.
This chapter has moved. The C++ concurrency material has its own dedicated, runnable book: C++ Concurrency and Parallelism.
It maps to Shared State and the Futex there.
The full, runnable C++ treatment of concurrency and parallelism — threads and jthread, mutexes and the memory model, atomics and lock-free structures, futures and coroutines, fibers, Asio, NUMA and mechanical sympathy, and the low-latency patterns — now lives in that book, each chapter with a Docker or KVM-lab demo and a Lua verify.lua behind it. This book keeps its Linux systems-programming material; the C++ concurrency compendium was extracted so it could grow on its own.