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Chapter 3

Deadlock, Prevention, Detection, and Banker's Algorithm

Master deadlock conditions and strategy families, then apply safe-state reasoning.

Estimated time: 32 min

Deadlock is when each process waits for another in a cycle and nobody can move.

Banker's and deadlock conditions are frequent long-answer and numerical exam topics.

Deadlock Core

From notes

Clear explanation

Deadlock occurs when processes hold resources and form a circular wait, each depending on another to release.

What it really means

Imagine four cars blocking each other at a one-lane crossing.

Key takeaway

Breaking any one required condition can prevent deadlock.

Banker's Safe-State Check

From notes

Clear explanation

Banker's simulates whether all processes can finish under current allocation + request using Available, Allocation, Max, Need matrices.

What it really means

A cautious banker lends only when all customers can still be satisfied in some order.

Key takeaway

Banker's is predictive avoidance, not post-failure recovery.

How to Solve Banker's Numerical

Added clarity

Clear explanation

Compute Need matrix first, then repeatedly find a process with Need <= Available, mark it finished, and release its Allocation to Available.

What it really means

You check who can finish with current resources and let that completion unlock more resources.

Example

If Available=(1,5,2,0) and a process has Need=(0,4,2,0), it can proceed and later add its allocation back.

Key takeaway

Safe-sequence construction is the core of every Banker's answer.

Request Grant Check (Exam Order)

Added clarity

Clear explanation

For incoming Request_i, check Request_i <= Need_i, then Request_i <= Available, then simulate temporary allocation and re-run safety algorithm.

What it really means

You test whether lending now still allows everyone to eventually finish.

Example

Even if request fits Available, it must be denied if simulated state has no safe sequence.

Key takeaway

Always write all three checks explicitly in request questions.

  • - Listing deadlock conditions without explaining why each is required
  • - Confusing safe state with deadlock state
  • - Skipping Need matrix computation in Banker's questions
  • - Four conditions: mutual exclusion, hold-wait, no preemption, circular wait
  • - Safe state means some completion sequence exists
  • - Banker's grants request only if resulting state remains safe
Need = Max - Allocation

Exam lens for this topic

What evaluators usually expect in structured exam answers.

Must-use keywords

  • - Mutual Exclusion
  • - Hold and Wait
  • - No Preemption
  • - Circular Wait
  • - Safe Sequence

Answer flow

  • - Define deadlock and list four necessary conditions
  • - Compute Need matrix using Need = Max - Allocation
  • - Derive safe sequence using Need <= Available
  • - For request problems, validate Request<=Need and Request<=Available before safety check

Diagram expectations

  • - Resource allocation graph or matrix tables

Repeated pattern: Allocation/Max/Available matrix format is a standard recurring pattern.

Mini quiz

Quick self-check from this topic before moving ahead.

1. Banker's algorithm approves a request only when:

Practice Questions

  • What is deadlock? Explain four conditions of deadlock.

    Source: Summer 2025 Q3(A)

    Answer focus: Definition + clear condition reasoning.

  • Discuss Banker's algorithm for deadlock avoidance.

    Source: Summer 2023 Q3(B) OR

    Answer focus: Safe sequence logic with matrix operations.

Practice from papers (end-of-topic set)

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