Practice · Concurrency and Synchronization

Q1

Two threads each execute counter++ once on a shared counter that starts at 0, with no synchronization. Which final values are possible?

A Any value B Only 2 C 1 or 2 D 0, 1 or 2

Q2

Of the 20 possible interleavings of two threads each doing load–add–store once, how many give the correct result 2?

A 20 B 2 C 1 D 10

Q3

What is a race condition?

A A situation where the result depends on the relative timing/order of operations of concurrent threads B A deadlock between two threads C A thread with a high priority D A program that runs faster than expected

Q4

Which requirement of a critical-section solution prevents a thread from waiting forever while others repeatedly enter?

A Atomicity B Progress C Mutual exclusion D Bounded waiting

Q5

In Peterson's algorithm, both threads set their flags to true at nearly the same time. Which one enters the critical section first?

A The one that wrote turn first — because the other one's later write gives the turn back B Always thread 0 C Both enter together D The one that set turn first

Q6

What does compare-and-swap (CAS) do?

A Atomically replaces a memory value with a new one only if it currently equals an expected value, and reports success B Sorts an array C Compares two strings D Swaps two registers

Q7

When is a spinlock preferable to a blocking mutex?

A On a single-core machine B When critical sections are very short and threads run on different cores, so spinning briefly is cheaper than a context switch C When the lock is held for seconds D Never

Q8

A semaphore has value 0. Process A executes wait(S). With the definition used in this chapter, what happens?

A S becomes 1. B S becomes −1 and A blocks. C A is terminated. D S stays 0 and A continues.

Q9

A mutex semaphore starts at 1. Operations: A wait, B wait, C wait, A signal. What is the semaphore value, and who is running in the critical section?

A 1; nobody B −1; B (C still waiting) C 0; C D −2; A

Q10

A semaphore with initial value 4 protects the 4 GPU slots of the EdgeCampus server. What kind of semaphore is it?

A Spinlock B Condition variable C Binary semaphore D Counting semaphore

Q11

Thread B must not start processing until thread A has loaded a model. Which initial semaphore value implements this signal (A does signal(S), B does wait(S))?

A 0 B The number of threads C 1 D −1

Q12

In the bounded-buffer solution, what are the initial values of mutex, empty and full for a buffer of size N?

A N, 1, 0 B 0, 0, N C 1, 0, N D 1, N, 0

Q13

A producer executes wait(mutex) before wait(empty). The buffer is full. What can happen?

A Nothing; the order does not matter. B The buffer grows automatically. C The producer blocks on empty while holding the mutex, so consumers can never enter to free a slot: deadlock. D The consumer produces instead.

Q14

In the "readers-first" solution to the readers–writers problem, what is the main risk?

A Readers can never read concurrently. B Writers can starve if readers keep arriving. C Two writers write at once. D Deadlock among readers.

Q15

Five dining philosophers each pick up their left chopstick, then their right one. What can go wrong?

A All five hold their left chopstick and wait forever for the right one: deadlock B Only starvation of one philosopher, never deadlock C Nothing D Two philosophers eat with the same chopstick

Q16

Why must a thread call wait() inside a while loop that re-checks the condition, rather than an if?

A Because Java forbids if with wait. B Because while releases the lock. C After waking up, the condition may no longer hold (another thread acted first, or a spurious wake-up occurred), so it must be checked again. D Because if is slower.

Q17

What does Java's volatile keyword guarantee for a shared variable?

A That it is stored on disk B That x++ is atomic C That only one thread can access it D That writes by one thread become visible to other threads (no stale cached value)

Q18

Which Java class gives a thread-safe counter without using a lock?

A AtomicInteger B Integer C ArrayList D String

Q19

Which Java class is a ready-made bounded buffer for producer–consumer designs?

A ArrayBlockingQueue B Thread C HashMap D StringBuilder

Q20

What is a key advantage of message passing over shared memory?

A It needs no operating system. B It is always faster. C There is no shared data to protect with locks, and it works between machines. D It never loses messages.

Q21

Two EdgeCampus web servers check "is GPU slot 3 free?" and both see "yes", then both book it. What kind of bug is this, and a proper fix?

A A priority inversion; change priorities B A race condition (check-then-act); make check and booking one atomic operation, e.g., a database transaction with a unique constraint or a distributed lock C A deadlock; restart the servers D A memory leak; add RAM

Q22

Each of two threads increments an unsynchronized shared counter k = 3 times. What is the smallest possible final value?

A 6 B 1 C 2 D 3

Q23

Linux kernels often spin briefly, then block when a lock is busy. Why?

A To increase context switches B Because blocking is impossible in the kernel C To confuse attackers D Many locks are released within microseconds (spinning avoids an expensive context switch); if not, blocking stops wasting CPU

Q24

Why does the bounded-buffer solution need both the counting semaphores (empty, full) and the mutex?

A The mutex is optional; it only improves speed. B Because semaphores cannot be negative. C The counting semaphores are optional when there is one producer. D empty/full count slots and make producers/consumers wait, but two producers could still write the same slot at the same time; the mutex protects the buffer indices and contents.

Q25

A thread holds a ReentrantLock and an exception is thrown inside the critical section. Where must unlock() be called to avoid other threads waiting forever?

A In the constructor B It is released automatically when the method returns C In a finally block D At the end of the try block

Q26

Kubernetes runs three replicas of its controller manager but only one must act at a time. Which mechanism does it use?

A Disabling interrupts B Leader election with a lease stored in the cluster's consistent key-value store (etcd) C A semaphore in each pod D A Java synchronized block

Q27 Short answer

Show an interleaving of two threads, each executing x = x + 1 (load, add, store) once with x initially 5, that ends with x = 6, and one that ends with x = 7. Then fix the code in Java in two different ways.

Q28 Short answer

Trace a counting semaphore slots = 2 (two GPU slots) for this sequence: A wait, B wait, C wait, D wait, A signal, C signal, B signal, D signal. After each operation give the value, who is waiting (FIFO) and who holds a slot.

Q29 Short answer

Explain why the dining-philosophers solution "always pick up the lower-numbered chopstick first" cannot deadlock.

Q30 Short answer

EdgeCampus receives AR frames from phones (producers) and processes them on 4 GPU workers (consumers) through a buffer of 64 frames. Design the synchronization (Java or semaphores) and explain what happens when the buffer is full or empty. What policy would you choose for frames when the buffer is full, given that AR needs fresh frames?