Classic Problems in Concurrency

Every student of distributed computing should know quite a bit about the fundamentals of concurrency. A good chunk of that is knowing why the following problems (not necessarily their solutions) are worthy of study.

The classic problems mentioned below are well defined; so do a web search.

1 Dining Philosophers Problem

Illustrates deadlock
Illustrates livelock
Illustrates malicious cooperation
Assume or not: Communication among them

2 Mutual Exclusion Problem

Consider two or more processes. Each P_i has an area of code C_i "sensitive enough" that we call it a "critical section" CS.
Assume CS always terminates.
"Sensitive enough" == shared variable, usually

2.1 Mutual Exclusion Problem

Find a solution that satisfies four requirements:
1. R1 Mutual Exclusion: Number of processes in the CS, at any time == 0 or 1.
2. R2: Deadlock-Free + Live-lock-free
3. R3: No Unnecessary Delay
4. R4: Eventual Entry or Bounded Waiting: A process wishing to enter its CS, must be able to enter it in a finite amount of wait.

2.2 Mutual Exclusion Problem

Typically solved with semaphores.
1. m : semaphore := 1
2. Entry to CS: P(m);
3. Exit from CS: V(m);

2.3 Mutual Exclusion Problem

Also, read about solutions such as Dekker's and Peterson's, using ordinary variables.

2.4 Mutual Exclusion Problem

Two processes can illustrate the essence of the issue, but we are also interested in "starvation-free" semaphore based solutions, especially in distributed systems.
1. The above solution is not starvation free.
Starvation-free Mutex Solutions Using Split Binary Semaphores
1. ../Semaphores/udding-morris-algs.html

3 Readers-Writers Problem

Read-Sharing a resource
Exclusive update of the resource
Andrews' Notes on Passing The Baton Technique
Parnas readers-writers

4 Producers-Consumers Problem

aka Bounded Buffer Problem
Producers-Consumers Problem

5 Cigarette Smokers Problem

What problems of concurrency can semaphores solve?
Introduces an array of semaphores.
Cigarette Smokers Problem cannot be solved without arrays.
Cigarette Smokers Problem