[CS Fundamentals] Managing Deadlock in Operating Systems: Prevention and Detection Techniques

---

Definition & When it Happens

Deadlock is a situation in which two or more processes are unable to proceed because each is waiting for the other(s) to release resources, creating a cycle of dependency. In real-life, deadlock kinda seems like this situatoin:

Multiple processes might need the same resource. Let’s say there are Process A, Process B, Resource A and Resource B.

• Process A holds Resource B, and needs Resource A

• Process B holds Resource A, and needs Resource B

Neither can proceed because Process A is holding the resource that process B needs, and Process B is also holding the resource that process A needs.

Each is waiting on the other to release the resource it needs, resulting in a deadlock.

---

4 Key Conditions That Cause Deadlock (Coffman Conditions)

We call these four conditions “Coffman Conditions” in computer science.

1. Mutual Exclusion

   • Only one process can use the resource at a time.

2. Hold and Wait

   • There must be a process that is holding at least one resource and waiting to acquire additional resources held by other processes.

3. No Preemption

   • Resources that are assigned to a process cannot be forcibly taken away.

4. Circular Wait

   • A set of processes are waiting for resources in such a way that there is a circular chain.

   • This diagram explains the circular wait situation.

If at least one of these four conditions is not satisifed, the deadlock won’t happen.

---

How to Handle Deadlock

1. Prevention

   • It is important to design a system in a such way that those conditions do not occur.

   • Eliminate one of Coffman conditions (High cost).

     • For example, we can have multiple processes sharing the same resource, which solves mutual exclusion.

2. Avoidance

   1. Banker’s Alogorithm

      • It is inspired by the way a bank might manage loan requests to avoid bankruptcy.

      • When a process requests a resource, check if the system remains in a safe state after allocating the resource.

        • A system is in a safe state if it does not cause a deadlock and if there is a sequence of proccesses that can eventually finish, and is able to allow the next sequence to proceed.

   2. Resource-Allocation Graph (RAG) Algorithm

      • A graphical method for modeling resource allocation and detecting deadlock in a system.

3. Detection

   1. You can detect a deadlock situation using resource-allocation graph (RAG) algorithm.

   2. When resources are requested, use a detection algorithm and see if an overhead occurs.

---

Reference

https://github.com/gyoogle/tech-interview-for-developer/blob/master/Computer%20Science/Operating%20System/DeadLock.md