Deadlock Handling

Process Synchronization

Deadlock Handling

Round Robin

Process Scheduling and Synchronization

Deadlock handling refers to the techniques used by an operating system to deal with deadlock situations where processes are stuck waiting for resources.

What is Deadlock Handling?

It is the set of methods used to prevent, avoid, detect, and recover from deadlocks in a system.

Deadlock Prevention

Avoidance (Banker’s Algorithm)

Detection

Recovery

Deadlock Prevention (Operating System)

What is Deadlock Prevention?

Deadlock prevention is a technique used to ensure that deadlock never occurs by eliminating at least one of the four necessary conditions required for deadlock.

Four Necessary Conditions for Deadlock

Deadlock can occur only if all these conditions are present:

Prevent deadlock by eliminating at least one of the four necessary conditions:

Mutual Exclusion → Make resources sharable (if possible)

Hold and Wait → Request all resources at once

No Pre-emption → Allow resource pre-emption

Circular Wait → Impose ordering of resources

Prevention = break any one of these conditions

Methods of Deadlock Prevention

1. Eliminate Mutual Exclusion

Make resources sharable (if possible)

Example: Shared printers (spooling)

2. Eliminate Hold and Wait

Process must request all resources at once

Or release held resources before requesting new ones

3. Eliminate No Preemption

Allow system to take resources back

If a process cannot get required resource, it releases current ones

4. Eliminate Circular Wait

Assign a fixed order (resource hierarchy) to resources

Processes must request resources in increasing order

Example

Suppose:

P1 holds R1 and waits for R2

P2 holds R2 and waits for R1

Apply resource ordering → no circular wait → no deadlock

Advantages

Completely avoids deadlock

Simple concept

Disadvantages

Low resource utilization

Reduced system performance

Processes may wait longer

Short Answer (Exam Ready)

Deadlock prevention is a method that avoids deadlock by ensuring at least one of the necessary conditions (mutual exclusion, hold and wait, no pre-emption, circular wait) does not occur.

Methods of Deadlock Handling

1. Deadlock Prevention

Prevent deadlock by eliminating at least one of the four necessary conditions:

Mutual Exclusion → Make resources sharable (if possible)

Hold and Wait → Request all resources at once

No Preemption → Allow resource preemption

Circular Wait → Impose ordering of resources

2. Deadlock Avoidance

Deadlock avoidance ensures that the system never enters an unsafe state by carefully allocating resources.

Avoid deadlock by making safe resource allocation decisions

System checks if allocation keeps it in a safe state

What is Banker’s Algorithm?

It is a deadlock avoidance algorithm that checks whether granting a resource request will keep the system in a safe state.

Named “Banker’s” because it works like a bank:

A banker gives loans only if he is sure all customers can repay

Key Concepts

1. Safe State

A state where all processes can complete without deadlock

2. Unsafe State

A state that may lead to deadlock

3. Data Structures Used

Available → Available resources

Max → Maximum demand of each process

Allocation → Resources currently allocated

Need = Max − Allocation

Working Steps

Check if request ≤ Need

Check if request ≤ Available

Temporarily allocate resources

Check for safe sequence

If safe → grant request

If unsafe → deny request

Example

Process	Allocation	Max	Need
P1	1	3	2
P2	1	2	1

Available = 1

Check Safe Sequence

Available = 1

P2 can complete (Need = 1)
→ Available becomes 2

Now P1 can complete (Need = 2)

Safe Sequence = P2 → P1

Advantages

Avoids deadlock completely

Ensures safe execution

Disadvantages

Requires prior knowledge of max resources

Complex for large systems

May reduce resource utilization

Short Answer (Exam Ready)

Banker’s Algorithm is a deadlock avoidance technique that allocates resources only if the

3. Deadlock Detection

Allow deadlock to occur, and then detect it

Use Resource Allocation Graph (RAG)

Check for cycles in the system

4. Deadlock Recovery

After detection, recover using:

Process Termination

Kill one or more processes

Resource Preemption

Take resources from some processes and give to others

Comparison Table

Method	Idea	Advantage	Disadvantage
Prevention	Eliminate conditions	No deadlock	Low resource utilization
Avoidance	Safe state check	Efficient	Complex
Detection	Find after occurrence	Flexible	Overhead
Recovery	Fix after detection	Simple	Data loss possible

Key Concepts

Mutual Exclusion

Circular Wait

Deadlock Detection

Resource Allocation

Short Answer (Exam Ready)

Deadlock handling consists of prevention, avoidance, detection, and recovery techniques used to manage deadlocks in operating systems.