Deadlock in Operating System

Deadlock is one of the most important Operating System topics because it explains why processes can stop making progress even when resources exist in the system.

What is Deadlock?

Deadlock is a state where two or more processes wait forever because each process is holding a resource and waiting for another resource held by another process.

Simple Example

Imagine:

• process P1 holds resource R1 and waits for R2
• process P2 holds resource R2 and waits for R1

Now both processes keep waiting. This is deadlock.

Coffman Conditions

Deadlock can occur only if these four conditions hold together:

1. mutual exclusion
2. hold and wait
3. no preemption
4. circular wait

Mutual Exclusion

A resource can be used by only one process at a time.

Hold and Wait

A process holds one resource and waits for another.

No Preemption

Resources cannot be forcefully taken away; they must be released voluntarily.

Circular Wait

Processes wait in a circular chain.

Deadlock Prevention

Deadlock prevention means breaking at least one Coffman condition.

Examples:

• avoid hold and wait
• allow preemption in some cases
• impose resource ordering to avoid circular wait

Deadlock Avoidance

Deadlock avoidance means the system checks resource allocation carefully so it never enters an unsafe state.

Common concept:

• Banker's Algorithm

Deadlock Detection

In detection-based approach, deadlock is allowed to happen and then detected later by the system.

Deadlock Recovery

Recovery methods:

• terminate one or more processes
• preempt some resources
• roll back a process if possible

Interview-style answer

Deadlock is a condition where processes wait indefinitely for resources held by each other. It occurs when mutual exclusion, hold and wait, no preemption, and circular wait exist together. Deadlock can be handled using prevention, avoidance, detection, and recovery techniques.

Quick Revision Points

• deadlock blocks progress
• four conditions are required together
• prevention breaks one condition
• avoidance checks safe state
• recovery may require killing processes

Viva Questions

1. What is deadlock?
2. What are the four deadlock conditions?
3. What is deadlock prevention?
4. What is deadlock avoidance?
5. What is the difference between prevention and avoidance?