Menene deadlock, kuma yaya kake hanawa, kaucewa, ko gano shi?

Question

IT Interview Practice · Accepted Answer

**Deadlock** yanayi ne inda threads biyu ko fiye kowanne ya **toshe har abada**, kowanne yana jiran wani albarkatu da wani ke riƙe. Babu abin da ke ci gaba.

## Sharuɗɗa huɗu da suka wajaba (Coffman)

Dukkan huɗu dole su kasance lokaci guda — cire ko ɗaya kuma deadlock ba zai yiwu ba:

```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
```

## Sarrafa deadlock

- **Prevention (hanawa)** — ƙira ta yadda sharaɗi ɗaya ba zai taɓa kasancewa ba. Mafi amfani: kashe **circular wait** ta hanyar sanya **global lock ordering** (kullum karɓi A kafin B). Ko kai hari kan **hold-and-wait** ta hanyar kama dukkan locks da ake bukata lokaci guda.
- **Avoidance (kaucewa)** — ba da buƙata ne kawai idan tsarin ya ci gaba da kasancewa a yanayi "mai aminci" inda kowane thread zai iya kammalawa (**Banker's algorithm**). Yana bukatar sanin mafi girman buƙatun albarkatu na kowane thread tun farko, don haka ba safai ake amfani da shi a aikace ba.
- **Detection & recovery (ganowa da farfaɗowa)** — bar deadlocks su faru, lokaci-lokaci a bincika **wait-for graph** don neman zagaye, sa'an nan a farfaɗo ta hanyar kashewa ko mayar da wani wanda aka zaɓa (victim). Databases suna yin haka daidai: suna gano deadlocks kuma suna soke transaction ɗaya da kuskuren "deadlock victim".
- **Ignore ("ostrich algorithm")** — idan deadlocks ba kasafai ba ne, kawai a sake farawa; OSes da yawa suna karɓar wannan ga wasu albarkatu.

## Me ya sa yake da muhimmanci

Deadlock batu ne na gargajiya na hira domin tsarin sharuɗɗa huɗu kayan gano matsala ne madaidaici: idan aka samu tsayawa (hang), za ka iya tambaya "wanne sharaɗi zan iya karyawa?" A tsarin gaske amsar da aka fi amfani da ita ita ce **consistent lock ordering** (wanda ke hana circular wait) — ƙa'ida da kake amfani da ita duk sa'ad da ka ɗauki lock fiye da ɗaya. Fahimtar cewa database ɗinka yana jefa kurakuran deadlock-victim, da sanin cewa kawai ka sake gwada wannan transaction, aikace-aikace ne kai tsaye mai amfani.

Sharuɗɗa huɗu da suka wajaba (Coffman)

Sarrafa deadlock

Me ya sa yake da muhimmanci

Tambayoyi masu alaƙa