6.ReentrantLock

Explicit reentrant mutual exclusion with interruptible/timed acquisition, optional fairness, Conditions, and try/finally release.

How fair ReentrantLock reduces barging by honoring queued predecessors, and the throughput/latency trade-off of fairness.

Default ReentrantLock barging behavior, why it can improve throughput, and how AQS queues still handle contended waiters.