CST 334 Week 6

A Bug's Life: Concurrency

During week 6 of CST 334, we continued the discussion on the topic of concurrency, focusing on semaphores and their role in handling synchronization in concurrent programming. A semaphore is an object with an integer value which can be manipulated with routines like sem_wait() and sem_post(). Introduced by Edsger Dijkstra, semaphores come in two main types: binary semaphores, which function as locks, and counting semaphores, which are useful for managing resources.

The core operations of semaphores are sem_wait() and sem_post(), which decrement and increment the semaphore value. These operations are used to control access to the critical sections of code, ensuring that only a limited number of threads can execute a particular section at any given time. This helps in preventing issues like race conditions where multiple threads might try to modify shared data simultaneously.

The week also covered common concurrency problems, including atomicity-violation bugs and order-violation bugs. Atomicity-violation bugs occur when a sequence of operations that should be executed as a single, indivisible step is interrupted, leading to inconsistent states. The fix typically involves using locks to ensure that the critical section of code are executed atomically.

Order-violation bugs happen when the expected order of operations is disrupted, causing incorrect program behavior. For instance, if one thread expects a variable to be initialized by another thread before it uses it, any deviation from this order can cause errors. Using condition variables to enforce the proper sequence of operations is a common solution to these bugs.

Lastly, we delved into the concept of deadlock, where two or more threads are stuck waiting for each other to release resources, and none of them can proceed. Preventing deadlock involves strategies such as lock ordering, where all threads acquire locks in a predetermined order to avoid circular dependencies. Understanding and applying these techniques is crucial for writing concurrent programs.

The most significant insight from this week's material came from a study by Lu et al., which analyzed concurrency bugs in popular open-source applications like MySQL, Apache, Mozilla, and OpenOffice. The study identified 105 concurrency bugs, the majority of which (~70%) were non-deadlock bugs such as atomicity and order violations, while the remainder were deadlock bugs. Open-source code is subject to extensive scrutiny, but it is likely that concurrency issues are prevalent in many, if not all, major codebases. This study underscored the importance of the topics being covered here.