Learning Objectives

At the end of this lecture, you should be able to:

• Use mutex locks and condition variables to solve more concurrency problems.

Topics

In this lecture, we will cover the following topics:

• Mutex locks.
• Condition variables.
• The sleeping barber’s problem.

The Problem

• The sleeping barber is a famous concurrency problem that illustrates some of the issues when writing multi-threaded code, especially when dealing with multithreaded operating system.

• So here is the setup of the problem:

  1. We have one barber that owns a barber shop.

  2. There is one work chair where the barber can actively cut a customer’s hair.

  3. There are NUM_WAITING_CHAIRS waiting chairs in the waiting area.

  4. If there are no customers in the shop, the barber will fall asleep on the work chair.

  5. When a customer arrives, if the barber is asleep, the customer will wake the barber up and get a haircut.

  6. Alternative, if the barber is busy, the customer will enter the waiting area. Two cases might arise as well:

     • If the waiting area is full, then the customer gets pissed and leaves.
     • If there is an empty chair in the waiting area, the customer will join the waiting area.