Deadlocks

A deadlock occurs when processes are waiting for each other in a circular chain. Four conditions must ALL be true (Coffman conditions): Mutual Exclusion (resource not sharable), Hold and Wait (holding one, waiting for another), No Preemption (can't force release), Circular Wait (A→B→C→A). Prevention: break any one condition. Detection: resource allocation graph. Recovery: kill a process, preempt resource.

Key Concepts

Deep Dive: The Four Conditions

Thread A holds Lock 1, wants Lock 2
Thread B holds Lock 2, wants Lock 1
→ Deadlock! (circular wait)

Condition	How to Prevent
Mutual Exclusion	Use lock-free algorithms
Hold and Wait	Acquire all locks at once
No Preemption	Allow lock timeout
Circular Wait	Lock ordering (always acquire in same order)

Deep Dive: Deadlock in Java

// DEADLOCK EXAMPLE
Object lock1 = new Object(), lock2 = new Object();

Thread t1 = new Thread(() -> {
    synchronized (lock1) {
        synchronized (lock2) { /* ... */ }
    }
});
Thread t2 = new Thread(() -> {
    synchronized (lock2) {    // Opposite order!
        synchronized (lock1) { /* ... */ }
    }
});

// FIX: Use consistent lock ordering
Thread t2Fixed = new Thread(() -> {
    synchronized (lock1) {    // Same order as t1
        synchronized (lock2) { /* ... */ }
    }
});

Deep Dive: Deadlock vs Livelock vs Starvation

Problem	Description
Deadlock	Processes blocked forever, no progress
Livelock	Processes keep changing state but no progress
Starvation	A process never gets resources (low priority)

Common Interview Questions

• What is a deadlock? What are the four conditions?
• How do you prevent deadlocks?
• How do you detect a deadlock?
• What is the difference between deadlock and livelock?
• Give an example of deadlock in Java.