The idea of Reader-Writer Lock is quite simple. In normal lock, we don't differentiate the threads. That said, each thread who wants to enter the critical section must first acquire the lock. But on a second thought, you may find that threads actually have different behavior inside the critical section: some threads just want to see the values of shared variable, while others really want to update those variables.

Condition variable is used for a thread to wait for some condition to be true before continuing. The implementation is quite simple compared to lock, yet the difficult part is to understand how a CV is supposed to used.

Lock is basically just a semaphore whose initial counter is 1. lock_acquire is like P, while lock_release is like V. You probably want to go over my previous post about semaphore

Semaphore denotes a certain number of shared resources. Basically, it's one counter and two operations on this counter, namely P and V. P is used to acquire one resource (thus decrementing the counter) while V is used to release one resource (thus incrementing the counter).