C11 Fails to Define Data Races Involving Interrupts

Users of our data-race detector for C/C++ programs, RV-Predict/C, frequently tell us at Runtime Verification, Inc., (RV) that they would like to see a variant that detects data races between threads and interrupts in embedded systems. A solitary application thread may share data locations with one or more interrupt service routines (ISRs). Data accesses by the ISRs and the application thread may conflict. The conflicting accesses can lead to unexpected behavior in safety-critical software. Users hope to detect conflicting accesses using runtime verification, and fix their software to avoid the conflicts.

What does it mean to have a “data race” between a thread and an interrupt? We quickly realized at RV that we resorted to a vague analogy between threads and interrupts to define a data race. It was important to produce a formal definition. We are concerned with interrupts only in C programs, so we sought an authoritative definition in the C11 standard. We were surprised when we could not find one!
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Testing K Framework using RV-Predict

This blog post is the beginning of a series of blog posts recording our experience of hunting data races in complex real-world applications using RV-Predict. To start with, we will choose the K framework as our first test subject.

Why the K framework? Well, there are a number of reasons. First of all, it’s a very cool project! Check out its website for a quick introduction. It is also the foundation of our RV-Match product, which aims to provide strong correctness guarantees through formal verification. Therefore, it’s very important that the underlying K Framework is free from bugs including data races. Finally, the K framework is complex. It is written in a combination of Java and Scala and makes use of the Java 8 features extensively. It also takes advantage of many 3rd-party libraries such as Guice, Guava, Nailgun, etc. As a practical race detector, RV-Predict must be able to handle all these aspects gracefully.
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Detecting popular data races in Java using RV-Predict

Data races are a common kind of concurrency bug in multithreaded applications. A data race can be defined as two threads accessing a shared memory location concurrently and at least one of the accesses is a write. Data races are notoriously difficult to find and reproduce because they often happen under very specific circumstances. Therefore, you could have a successful pass of the tests most of the time but some test fails once in a while with some mysterious error message far from the root cause of the data race.

Despite all the effort on solving this problem, it remains a challenge in practice to detect data races effectively and efficiently. RV-Predict aims to change this undesired situation. In this blog post, we will summarize some of the most popular kinds of data races in Java and show you how to catch them using RV-Predict. The examples in this post are included in the RV-Predict distribution under the examples/ directory.

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Using RV-Predict to track down race conditions

We’ve all had it happen: you write some multithreaded code, but when you run it, it crashes with some kind of error every second or third or hundredth run. The traditional way of trying to debug this problem is very tedious and involves attempting to reproduce the error (difficult) and then tracing the inconsistent state backwards to find the source of the race condition (even more difficult).

Enter RV-Predict. Today I was developing code for RV-Match when I ran into an intermittently occurring NullPointerException in its parser. Instead of trying to reproduce the bug and tracking its behavior backwards, I assumed going in that it was caused by a race condition, and ran RV-Predict on the program. After it crashed, it spat out the following error report: Continue reading