On white, 8 point threads and 64 vertical threads hangs

It's not clear what the cause is, but HOMMEXX appears to hang in repro_sum_int when running with 8 point threads and 64 vertical threads. gdb says it's actually called poll from the parallel library (IIRC), so it's potentially another issue with MPI.

Reproduce the hang with 8 point threads and 64 vertical threads, then inspect the stack around repro_sum_int and poll in the parallel library. Determine whether the wait is caused by MPI or another parallel-runtime issue; done means the configuration no longer hangs or the responsible external component is documented.

Written by the indexing model from the issue text.