Hi Andreas,
Cloud9 is a project that has an extensive POSIX environment model
which also lets you symbolically execute multithreaded programs. It is
based on KLEE.
http://dslab.epfl.ch/pubs/cloud9.pdf
Portend builds on top of Cloud9 to perform race detection/classification:
http://dslab.epfl.ch/pubs/portend.pdf
Cloud9 code base is also public, it might be of interest to you:
Best,