Jeff and I encountered a problem when cross-compiling OMPI for Linux. Turned out that we had an old test in the code that looked for threads to have different pids. Since it couldn't be tested when cross-compiling, the test simply assumed this was the case for Linux under those conditions - which broke the build for current Linux kernels.
Different pids for threads was last seen in the old RH 4 series (kernel 2.6.9 or so). Some code (e.g., waitpid) was also provided to support this unusual situation - this code was in fact broken when we updated the event library. So even if we were in an old kernel, the code base would neither compile nor run.
Rather than trying to continue to support these old kernels, we have removed all the stale code that was covered by OPAL_THREADS_HAVE_DIFFERENT_PIDS. This removed some complexity from a few PLM modules and removed the broken code.
Jeff modified the corresponding .m4 test so we now detect an older kernel, print out a nice "we don't support this" message (along with noting that earlier versions of OMPI do), and then abort the build.
If you know of some reason to restore support for old Linux kernels, and someone willing to do the work to "refresh" that support, please let us know.
Ralph & Jeff