Apa iku deadlock? Apa papat kahanané, lan kepiye carane nyegah?

Question

IT Interview Practice · Accepted Answer

**Deadlock iku kahanan ing endi loro utawa luwih thread saben-saben ngenteni selawasé kanggo sumber daya sing dicekel liyané, dadi ora ana sing bisa maju.** Iki mbutuhaké papat kahanan sing kudu ana bebarengan (kahanan Coffman).

## Papat kahanan

1. **Mutual exclusion** — sumber daya dicekel kanthi eksklusif.
2. **Hold and wait** — thread nyekel siji sumber daya nalika ngenteni liyané.
3. **No preemption** — sumber daya ora bisa dijupuk kanthi peksa.
4. **Circular wait** — siklus thread sing saben-saben ngenteni sabanjuré.

Pecahna salah siji lan deadlock dadi mokal.

```text
Thread 1 holds A, wants B ─┐
                           ├─ circular wait → deadlock
Thread 2 holds B, wants A ─┘
```

## Tuladha lan solusi

```python
# DEADLOCK: threads lock in opposite orders
def t1():
    with lockA:
        with lockB: ...     # T1: A then B
def t2():
    with lockB:
        with lockA: ...     # T2: B then A  ← cycle!

# FIX: impose a global lock ordering (always A before B)
def safe():
    with lockA:
        with lockB: ...     # everyone acquires in the same order
```

## Pencegahan & deteksi

- **Urutan lock**: tansah jupuk lock ing urutan global sing konsisten — mecah circular wait.
- **Timeout / try-lock**: `tryAcquire(timeout)`, mundur lan coba manèh — mecah hold-and-wait.
- **Siji lock kasar** utawa struktur lock-free ngindhari masalahé blas.
- **Deteksi**: watchdog mbangun grafik wait-for lan golèk siklus, banjur matèni/nganyari korban.

## Kenapa iki penting

Deadlock mbekukake sistem kanthi sepi — ora crash, mung panjaluk sing mandheg. Nyebut papat kahanan nuduhaké sampeyan bisa nalar babagan *kenapa* iku kedadéan lan milih pencegahan sing pas (biasané urutan lock sing konsisten), tinimbang nyebar lock lan ngarep-arep.

Papat kahanan

Tuladha lan solusi

Pencegahan & deteksi

Kenapa iki penting

Pitakon sing gegandhengan