Concurrency Control: Locks, Semaphores, and Monitors

Learning Objectives Define concurrency, race conditions, and the critical section problem. Implement mutual exclusion using locks (mutexes) to prevent data corruption. Differentiate between binary and counting semaphores and apply them to resource allocation problems. Describe how monitors encapsulate shared data and synchronization logic to simplify concurrent programming. Analyze code snippets to identify potential race conditions and propose a correct synchronization solution. Design a solution to the classic Producer-Consumer problem using semaphores or monitors. Compare and contrast the use cases, advantages, and disadvantages of locks, semaphores, and monitors. Ever wondered how you and a friend can edit the same Google Doc simultaneously without overwriting each oth...

TermDefinitionExample Race ConditionAn error condition where the outcome of a program depends on the unpredictable sequence or timing of operations from multiple threads accessing shared data.Two threads try to increment a shared counter `count` (initially 5). Thread A reads `count` (5), Thread B reads `count` (5). A computes `5+1=6` and writes `6` to `count`. B then computes `5+1=6` and also writes `6`. The final result is 6, when it should have been 7. Critical SectionA segment of code that accesses a shared resource and must not be executed by more than one thread at a time to prevent race conditions.In the code `count++`, the sequence of reading the value of `count`, incrementing it, and writing it back is the critical section. Mutual Exclusion (Mutex)A property that ensures only one...

Lock/Mutex Pattern for Mutual Exclusion lock.acquire(); // --- Critical Section Start --- // Access shared resource // --- Critical Section End --- lock.release(); This is the fundamental pattern for protecting a block of code. A thread must acquire the lock before entering the critical section. Crucially, it must release the lock when it leaves, allowing other waiting threads to proceed. Semaphore P/V (Wait/Signal) Operations wait(S): Decrements semaphore S. If S becomes negative, the thread blocks. signal(S): Increments semaphore S. If S is not positive, a blocked thread is unblocked. Used to control access to a pool of N resources. `wait()` is called to request a resource, and `signal()` is called to release one. This pattern is the foundation for solving many complex syn...