I am sorry to post in a very old thread, but I figured that it might help others looking for an answer in a search engine...

I just had the very same problem, which might be due to this bug, at least according to my research.
According to that message, OpenMPI is catching the SIGCHLD signal, which is necessary for QProcess "finished" signal to work correctly (and thus for QProcess::waitForFinished()).

The message says that this was changed in OpenMPI series 1.6 and 1.7.
As of today, the latest Ubuntu 15.10 ships with OpenMPI 1.6.5, which apparently is still affected by this (mis-)behaviour.

Upgrading to a later OpenMPI version should therefore fix the problem.
I did this by installing OpenMPI 1.10.2 from source, which is very easily done by following these instructions, and I can confirm that the problem is solved.

Hope this helps...