Task Synchronization

Computer Organization & Embedded System — Real-Time Operating and Control System, NEC licence examination syllabus (Nepal Engineering Council).

Task Synchronization

The classic problems that show up in every OS exam, worldwide.

A race condition happens when the result of concurrent execution depends on timing/order — fixed by ensuring mutual exclusion over the critical section.

🔒 Mutex vs Semaphore

MutexBinary lock, ownership-based — only the thread that locked it can unlock it.

Binary semaphore0 or 1, similar to mutex but no ownership concept.

Counting semaphoreAllows N concurrent users of a resource.

Deadlock — the four necessary conditions

All 4 must hold simultaneously for deadlock: 1. Mutual Exclusion — resources can't be shared 2. Hold and Wait — a process holds one resource while waiting for another 3. No Preemption — resources can't be forcibly taken back 4. Circular Wait — P1 waits for P2's resource, P2 waits for P1's resource

Priority inversion: a high-priority task gets blocked waiting on a resource held by a low-priority task — fixed by priority inheritance (temporarily boost the low-priority task's priority to that of the waiting high-priority task, so it finishes and releases the resource faster).

🍜 Classical Synchronization Problems

Producer-ConsumerProducer fills a buffer, consumer empties it — needs semaphores to prevent producer overfilling or consumer reading an empty buffer.

Readers-WritersMany readers can read simultaneously, but a writer needs exclusive access.

Dining Philosophers5 philosophers, 5 forks, each needs 2 forks to eat — classic deadlock-risk scenario if everyone grabs their left fork simultaneously.

💡 Be ready to explain how the Dining Philosophers deadlock can be avoided (e.g., only let a philosopher pick up both forks if both are free — an ordering rule breaks the circular-wait condition).

Syllabus points

Race condition, critical section
Mutex vs semaphore (binary/counting)
Deadlock (conditions + handling)
IPC: shared memory, message queue, mailbox, pipe, signal

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