Menene deadlock? Menene sharuɗɗa huɗu, kuma ta yaya kake hana shi?

Question

IT Interview Practice · Accepted Answer

**Deadlock hali ne inda threads biyu ko fiye kowanne ke jira har abada ga albarkatun da sauran ke riƙewa, don haka babu wanda zai iya ci gaba.** Yana buƙatar sharuɗɗa huɗu su kasance a lokaci guda (Coffman conditions).

## Sharuɗɗa huɗu

1. **Mutual exclusion** — ana riƙe albarkatu na keɓe.
2. **Hold and wait** — thread yana riƙe albarkatu ɗaya yayin da yake jiran wani.
3. **No preemption** — ba za a iya ƙwace albarkatu da ƙarfi ba.
4. **Circular wait** — zagayen threads kowanne yana jiran na gaba.

Karya ɗaya kuma deadlock ba zai yiwu ba.

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

## Misali da gyara

```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
```

## Rigakafi & ganowa

- **Lock ordering**: ko da yaushe acquire locks a tsari na global mai daidaici — yana karya circular wait.
- **Timeouts / try-lock**: `tryAcquire(timeout)`, back off ka retry — yana karya hold-and-wait.
- **Coarse lock ɗaya** ko lock-free structures suna kauce wa matsalar gaba ɗaya.
- **Ganowa**: watchdog yana gina wait-for graph kuma yana neman zagaye, sannan yana kashe/sāke fara victim.

## Me ya sa yake da muhimmanci

Deadlocks suna daskare tsarin a shiru — babu crash, kawai requests da suka rataye. Kiran sharuɗɗa huɗu yana nuna kana iya tunani game da *dalilin* da yake faruwa kuma ka zaɓi rigakafi da aka nufa (yawanci lock ordering mai daidaici), maimakon yaɗa locks kana fata.

Sharuɗɗa huɗu

Misali da gyara

Rigakafi & ganowa

Me ya sa yake da muhimmanci

Tambayoyi masu alaƙa