shared vs exclusive lock

flock Contention Lab: Exclusive, Shared, and LOCK_NB Fail Rates

Hands-on flock lab: uncontended LOCK_EX p50 0.33 us; waiter ~50 ms hold; w8 p50 ~36 ms; LOCK_NB 99% fail; LOCK_SH ~6.7x wall. Real numbers, no Docker.