What is a deadlock, and how do you prevent, avoid, or detect it?

Question

IT Interview Practice · Accepted Answer

A **deadlock** is a state where two or more threads are each **blocked forever**, each waiting for a resource that another holds. Nothing makes progress.

## The four necessary (Coffman) conditions

All four must hold at once — remove any one and deadlock is impossible:

```text
1. Mutual exclusion   → a resource is held exclusively (only one holder at a time)
2. Hold and wait      → a thread holds one resource while waiting to acquire another
3. No preemption      → resources can't be forcibly taken; only released voluntarily
4. Circular wait      → a cycle: T1 waits for R2 (held by T2), T2 waits for R1 (held by T1)
```

```text
   T1 ──holds──▶ R1        T1 wants R2 ──▶ (held by T2)
   T2 ──holds──▶ R2        T2 wants R1 ──▶ (held by T1)   ← cycle = deadlock
```

## Handling deadlock

- **Prevention** — design so one condition can never hold. The most practical: kill **circular wait** by imposing a **global lock ordering** (always acquire A before B). Or attack **hold-and-wait** by grabbing all needed locks at once.
- **Avoidance** — grant a request only if the system stays in a "safe" state where every thread can still finish (the **Banker's algorithm**). It needs each thread's maximum resource needs up front, so it's rarely used in practice.
- **Detection & recovery** — let deadlocks happen, periodically scan the **wait-for graph** for a cycle, then recover by killing or rolling back a victim. Databases do exactly this: they detect deadlocks and abort one transaction with a "deadlock victim" error.
- **Ignore (the "ostrich algorithm")** — if deadlocks are rare, just restart; many OSes accept this for some resources.

## Why it matters

Deadlock is a classic interview topic because the four-condition framework is a precise diagnostic tool: given a hang, you can ask "which condition can I break?" In real systems the go-to answer is **consistent lock ordering** (which prevents circular wait) — a rule you apply every time you take more than one lock. Recognizing that your database throws deadlock-victim errors, and knowing to simply retry that transaction, is a direct practical application.

The four necessary (Coffman) conditions

Handling deadlock

Why it matters

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