Common Concurrency Problems

Researchers have spent a great deal of time and effort looking into con-currency bugs over many years. Much of the early work focused on deadlock, a topic which we’ve touched on in the past chapters but will now dive into deeply [C+71]. More recent work focuses on studying other types of common concurrency bugs (i.e., non-deadlock bugs). In this chapter, we take a brief look at some example concurrency problems found in real code bases, to better understand what problems to look out for. And thus our central issue for this chapter: CRUX: HOW TO HANDLE COMMON CONCURRENCY BUGS Concurrency bugs tend to come in a variety of common patterns. Knowing which ones to look out for is the first step to writing more ro-bust, correct concurrent code. 32.1 What Types Of Bugs Exist? The first, and most obvious, question is this: what types of concur-rency bugs manifest in complex, concurrent programs? This question is difficult to answer in general, but fortunately, some others have done the work for us. Specifically, we rely upon a study by Lu et al. [L+08], which analyzes a number of popular concurrent applications in great detail to understand what types of bugs arise in practice. The study focuses on four major and important open-source applica-tions: MySQL (a popular database management system), Apache (a well-known web server), Mozilla (the famous web browser), and OpenOffice (a free version of the MS Office suite, which some people actually use). In the study, the authors examine concurrency bugs that have been found and fixed in each of these code bases, turning the developers ’ work into a quantitative bug analysis; understanding these results can help you un-derstand what types of problems actually occur in mature code bases.