Unit testing concurrent software - what do you do?

Question

As software gets more and more concurrent, how do you handle testing the core behaviour of the type with your unit tests (not the parallel behaviour, just the core

Answer 1

Given that your TPL will have its own separate unit test you don't need to verify that.

Given that I write two tests for each module:
1) A single threaded unit test that uses some environment variable or #define to turn of the TPL so that I can test my module for functional correctness.
2) A stress test that runs the module in its threaded deployable mode. This test attempts to find concurrency issues and should use lots of random data.

The second test often includes many modules and so is probably more of an integration/system test.