Topic in Operating Systems
Process synchronization is the coordination of concurrent processes or threads so that shared data stays consistent when several of them access it at once. Questions start with race conditions and the critical-section problem, whose solutions must satisfy mutual exclusion, progress and bounded waiting. You will be asked to trace Peterson's solution, explain hardware support such as test-and-set and compare-and-swap, and compare mutex locks, spinlocks and counting or binary semaphores with their wait() and signal() operations. Classic problems (bounded buffer, readers-writers, dining philosophers) are heavily used, along with monitors and condition variables, priority inversion, and the priority inheritance and ceiling protocols.
Below are 30 practice questions from a pool of 210 Concurrency & Synchronization MCQs, one of 12 topics in Operating Systems. Each shows the correct answer with an explanation; when you are ready, take a timed quiz to test recall under exam conditions.
Each question below shows the correct answer with a full explanation. Use these to build conceptual understanding before attempting a timed quiz.
Q1. A race condition occurs when:
A race condition occurs when multiple processes or threads access shared data concurrently and the final result depends on the particular order of execution, leading to unpredictable outcomes.
Q2. The critical section is:
The critical section is the portion of code where a process accesses shared resources (variables, files, etc.). Only one process should be in its critical section at a time to prevent race conditions.
Q3. Mutual exclusion means:
Mutual exclusion ensures that when one process is executing in its critical section, no other process is allowed to execute in its critical section, preventing concurrent access to shared resources.
Q4. A mutex lock is:
A mutex (mutual exclusion) lock is a synchronization tool with two operations: acquire (lock) and release (unlock). A process must acquire the lock before entering the critical section and release it when done.
Q5. A semaphore is:
A semaphore is an integer variable accessed through two atomic operations: wait (P/down) decrements it, and signal (V/up) increments it. It is used for synchronization and mutual exclusion.
Q6. A binary semaphore can have values:
A binary semaphore can only have values 0 or 1, functioning similarly to a mutex lock. It is used to provide mutual exclusion for accessing a single shared resource.
Q7. A counting semaphore can have values:
A counting semaphore can range over an unrestricted integer domain. It is used to control access to a resource with a finite number of instances, tracking available resources.
Q8. Busy waiting (spinlock) means:
Busy waiting (spinning) occurs when a process repeatedly checks a condition in a loop while waiting, consuming CPU cycles. A spinlock is a lock that uses busy waiting.
Q9. What are the three requirements for a solution to the critical section problem?
A correct solution must satisfy: (1) Mutual exclusion - only one process in CS at a time, (2) Progress - selection of next process cannot be postponed indefinitely, (3) Bounded waiting - a limit on how long a process waits.
Q10. The wait() operation on a semaphore is also known as:
The wait() operation is also known as P (from Dutch 'proberen' meaning 'to test'). It decrements the semaphore value and blocks the process if the value becomes negative.
Q11. What is Peterson's solution for the critical section problem?
Peterson's solution is a classic software-based solution for two processes. It uses a 'turn' variable and a 'flag' array to ensure mutual exclusion, progress, and bounded waiting without hardware support.
Q12. The test-and-set instruction is:
Test-and-set is an atomic hardware instruction that reads a value, sets it to true, and returns the old value - all in a single uninterruptible operation, useful for implementing locks.
Q13. The compare-and-swap (CAS) instruction:
CAS atomically compares the content of a memory location with an expected value and, only if they match, modifies it to a new value. It is the foundation for many lock-free data structures.
Q14. A monitor is:
A monitor is a high-level synchronization construct where shared data variables are encapsulated with the procedures that operate on them. Only one process can be active within a monitor at a time.
Q15. Condition variables in monitors are used with which operations?
Condition variables in monitors use wait() (process suspends itself) and signal() (resumes one suspended process). Unlike semaphores, signal() on a condition variable has no effect if no process is waiting.
Q16. In the bounded buffer problem, what are the semaphores typically used?
The bounded buffer uses three semaphores: mutex (binary, for mutual exclusion of buffer access), empty (counting, initialized to buffer size for empty slots), and full (counting, initialized to 0 for filled slots).
Q17. In the readers-writers problem, what is the key synchronization challenge?
Multiple readers can read simultaneously without issues, but a writer needs exclusive access to the shared data. The challenge is allowing concurrent reads while ensuring exclusive write access.
Q18. What is a spinlock most useful for?
Spinlocks are most useful for short critical sections on multiprocessors because busy waiting on one CPU while another CPU executes the critical section avoids the overhead of a context switch.
Q19. What is priority inversion?
Priority inversion occurs when a high-priority process is blocked waiting for a resource held by a low-priority process, and a medium-priority process preempts the low-priority process, effectively blocking the high-priority process.
Q20. Priority inheritance protocol solves priority inversion by:
Priority inheritance temporarily elevates the priority of the process holding a resource to the priority of the highest-priority process waiting for that resource, preventing medium-priority processes from causing inversion.
Q21. What is the dining philosophers problem and why is it significant?
The dining philosophers problem has 5 philosophers sharing 5 chopsticks. It illustrates how concurrent processes competing for limited resources can lead to deadlock and starvation if not properly synchronized.
Q22. How can the dining philosophers problem be solved to prevent deadlock?
Solutions include: allowing at most n-1 philosophers to sit, picking up both chopsticks atomically, using asymmetric ordering (odd picks left first, even picks right first), or using monitor-based solutions.
Q23. What is a read-write lock and when is it preferred over a mutex?
A read-write lock allows multiple threads to hold it for reading simultaneously but requires exclusive access for writing. It is preferred when reads are much more frequent than writes.
Q24. What is a memory barrier (memory fence)?
A memory barrier is an instruction that forces all memory operations before it to complete before any memory operations after it begin, preventing CPU and compiler reorderings that could cause synchronization bugs.
Q25. What are lock-free algorithms and why are they important?
Lock-free algorithms use atomic operations (CAS, fetch-and-add) instead of traditional locks. They guarantee system-wide progress, avoid deadlock, and often provide better performance and scalability under contention.
Q26. In the readers-writers problem, what is the difference between the first and second variants?
In the first variant, readers have priority (may cause writer starvation). In the second variant, writers have priority (once a writer is waiting, no new readers are admitted, which may cause reader starvation).
Q27. What is the difference between Hoare-style and Mesa-style monitors?
In Hoare-style monitors, signal() immediately switches to the waiting process. In Mesa-style (used by Java, Pthreads), signal() is a hint, so the awakened process must recheck the condition (use while, not if).
Q28. What is transactional memory?
Transactional memory allows a sequence of read/write operations to execute atomically. If a conflict is detected, the transaction is aborted and retried. It can be implemented in hardware (HTM) or software (STM).
Q29. What is the ABA problem in lock-free programming?
The ABA problem occurs when a memory location is read as A, another thread changes it to B then back to A, and the original thread's CAS succeeds thinking nothing changed. Solutions include version counters or hazard pointers.
Q30. What is a futex (fast userspace mutex) in Linux?
A futex combines user-space atomic operations for the fast (uncontended) path with kernel-based waiting for the slow (contended) path, providing efficient synchronization by avoiding system calls in the common case.
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