Martin Sulzmann

Bas van den Heuvel, Martin Sulzmann, Peter Thiemann

Deadlocks are a major source of bugs in concurrent programs. They are hard to predict, because they may only occur under specific scheduling conditions. Dynamic analysis attempts to identify potential deadlocks by examining a single execution trace of the program. A standard approach involves monitoring sequences of lock acquisitions in each thread, with the goal of identifying deadlock patterns. A deadlock pattern is characterized by a cyclic chain of lock acquisitions, where each lock is held by one thread while being requested by the next. However, it is well known that not all deadlock patterns identified in this way correspond to true deadlocks, as they may be impossible to manifest under any schedule. We tackle this deficiency by proposing a new method based on partial orders to eliminate false positives: lock acquisitions must be unordered under a given partial order, and not preceded by other deadlock patterns. We prove soundness (no falsely predicted deadlocks) for the novel TRW partial order, and completeness (no deadlocks missed) for a slightly weakened variant of TRW. Both partial orders can be computed efficiently and report the same deadlocks for an extensive benchmark suite.

Martin Sulzmann

Locks are a standard mechanism for synchronizing concurrent threads. The standard lock set construction assumes that critical sections are confined to a single thread, and therefore only accounts for locks acquired within that thread. The commonly used notion of a critical section implicitly assumes that protected events belong to the same thread. We show that this assumption is not valid for general C/Pthread executions. Using a trace model that captures the essence of C/Pthread programs, we give a trace-based characterization of critical sections that does not impose a per-thread restriction. As a result, critical sections may span multiple threads. Such \emph{multi-thread} critical sections arise naturally in real programs and close a semantic gap in the standard lock set construction.

Daniel Kraus, Jeremias Rößler, Martin Sulzmann

Ensuring the correct visual appearance of graphical user interfaces (GUIs) is important because visual bugs can cause substantial losses for businesses. An application might behave functionally correct in an automated test, but visual bugs can make the GUI effectively unusable for the user. Most of today's approaches for visual testing are pixel-based and tend to have flaws that are characteristic for image differencing. For instance, minor and unimportant visual changes often cause false positives, which confuse the user with unnecessary error reports. Our idea is to introduce an abstract GUI state (AGS), where we define structural relations to identify relevant GUI changes and ignore those that are unimportant from the user's point of view. In addition, we explore several strategies to address the GUI element identification problem in terms of AGS. This allows us to provide rich diagnostic information that help the user to better interpret changes. Based on the principles of golden master testing, we can support a fully-automated approach to visual testing by using the AGS. We have implemented our approach to visually test web pages and our experiments show that we are able to reliably detect GUI changes.