In essence, a data race is when two more threads attempt to read or write to the same resource without any synchronization mechanism in place. Here, only a concurrent read and write, or two simultaneous writes, are actually harmful; therefore, only these two types of access get reported.

In an earlier section on basic Helgrind usage, we saw some examples of this type of error in the log. There it concerned the simultaneous writing and reading of a variable. As we also covered in that section, Helgrind does not concern itself with whether a write or read was atomic, but merely reports a potential issue.

Much like with lock order problems, this again means that one has to judge each data race report on its merit, as many will likely be false-positives.