Concuerror

By Stavros Aronis

Concuerror is a stateless model checking tool for Erlang programs. It can be used to detect and debug concurrency errors, such as deadlocks and errors due to race conditions. Moreover it can verify the absence of such errors, because it tests programs systematically, unlike techniques based on randomness.

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How to use Concuerror?

Read the basic tutorial here.

What are race conditions?

Concuerror’s operation is based on detecting pairs of operations that are racing, i.e. could have a different result if scheduled in the opposite order. You can see some of these pairs by using the --show_races option.

Remember, however, that not all race conditions are necessarily bad! An example of benign racing is when worker processes report their progress by sending messages to a work managing process. These messages are racing, as they may reach (and be received) by the managing process in different orders.

Race conditions become can lead to errors when the program expects certain events to occur in a particular order, but that order is not always guaranteed. Such errors are often hard to reproduce, when they require rare, particular schedulings.

Concuerror systematically explores all “meaningfully different” schedulings, detecting all such errors or verifying their absence.

How does Concuerror work?

Concuerror schedules the Erlang processes spawned in the test as if only a single scheduler was available. During execution, the tool records a trace of any calls to built-in operations that can behave differently depending on the scheduling (e.g., receive statements, registry operations, ETS operations). It then analyzes the trace, detecting pairs of operations that are really racing. Based on this analysis, it explores more schedulings, reversing the order of execution of such pairs. This is a technique known as stateless model checking with dynamic partial order reduction.

Read more details about how Concuerror works here.