  Distributed software is very tricky to implement correctly as some errors
only occur in peculiar situations. For such errors testing is not effective.
Mathematically proving correctness is hard and time consuming, and therefore,
it is rarely done. Fortunately, there is a technique in between, namely model
checking, that, if applied with skill, is both efficient and able to find rare
errors.
  In this tutorial we show how to create behavioural models of parallel
software, how to specify requirements using modal formulas, and how to verify
these. For that we use the mCRL2 language and toolset (https://www.mcrl2.org/).
We discuss the design of an evolution of well-known mutual exclusion protocols,
and how model checking not only provides insight in their behaviour and
correctness, but also guides their design.
