Chapter 9: Threads and Synchronization — Doppelgangers and Collaboration

Vol 3: Computer Core Expedition · Chapter 9

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Metadata Card

Attribute	Value
Keywords	Thread Models, Mutex, Semaphore, Condition Variable, Deadlock

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Your Progress

"You now know that a process is an isolated world. But what if you want multiple execution streams within the same world, sharing the same memory? That's where threads come in."

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Encounter 1: Threads vs Processes

	Process	Thread
Address space	Separate	Shared
Context switch cost	High (TLB flush)	Low
Communication	IPC (pipes, sockets)	Shared memory
Isolation	Strong	Weak

Encounter 2: Race Conditions

When multiple threads access shared data without synchronization, the result depends on the unpredictable timing of thread interleaving.

// Race condition: two threads both increment counter
counter++;  // read, add, write — three operations that can interleave

Encounter 3: Mutex (Mutual Exclusion)

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void thread_safe_increment() {
    pthread_mutex_lock(&mutex);
    counter++;  // Protected section
    pthread_mutex_unlock(&mutex);
}

Encounter 4: Semaphore

A counter used to control access to a pool of resources:

• sem_wait(sem): Decrement, block if zero
• sem_post(sem): Increment, wake a waiting thread

Encounter 5: Deadlock

Four necessary conditions:

1. Mutual exclusion: Resource cannot be shared
2. Hold and wait: Thread holds one resource while waiting for another
3. No preemption: Resource can't be forcibly taken
4. Circular wait: Threads form a circular dependency chain

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Verification Checklist

• [ ] Can explain the difference between processes and threads
• [ ] Can identify and fix a race condition with a mutex
• [ ] Can explain the difference between mutex and semaphore
• [ ] Can identify the four deadlock conditions

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Chapter 10: CPU Scheduling