What Is Deadlock? — 10 Minutes of OS, Day 8

GPT drew the thumbnail as a deadlock: Processes holding resources while waiting on one another in a circle. The center looks like the resource, doesn't it? It has drawn a surprisingly three-dimensional circuit! A computer that suddenly stops responding may be experiencing . Several processes wait for resources held by one another, . This post explains the conditions that create deadlock and ways to resolve it. Because it is a common interview topic, understanding concrete examples matters.

Deadlock occurs when two or more processes each hold resources and wait indefinitely for resources held by the others. For example, P1 holds a printer and P2 holds a scanner; each requests the other's resource and waits, so both stop.

P1 has the printer and waits for P2's scanner. P2 has the scanner and waits for P1's printer. This mutual blocking is deadlock.

Deadlock arises when all four conditions hold.

Breaking any one condition prevents deadlock. Strategies therefore remove at least one of these conditions.

.

Before granting a resource request, check whether the resulting system state will remain safe. The is a representative example. It resembles a bank deciding whether a loan would put its overall available funds at risk.

Allow deadlock to occur but inspect the system periodically to detect it. Use a or . Detection algorithms add overhead, so use them carefully.

After detecting deadlock, restore normal operation. Main approaches include:

Deadlock also occurs frequently with threads. For example, two Java threads may each hold one object's lock while waiting for the other object's lock.

// Thread 1
synchronized(objA) {
  synchronized(objB) {
    // 작업
  }
}
// Thread 2
synchronized(objB) {
  synchronized(objA) {
    // 작업
  }
}

Use a consistent lock-acquisition order or lock timeouts to address this.

We examined deadlock, its conditions, and prevention, avoidance, detection, and recovery through examples. Interviews often check , not just definitions, so practice connecting each condition and strategy to a real situation. This concludes the long sequence of posts about processes. Next, we will explore , the computer's basic working-memory structure.