This is what I was trying to draw your attention to. The POSIX standard does
not contain any requirement on how pre-emptable threads are. At no time was I
making any reference to any implementation, past, present, or future.
Further, this isn't a simple "quality of implementation" issue; there are
important performance tradeoffs involved here.
I have yet to hear an argument that makes it clear why MPI should endorse
POSIX threads (with this critical and deliberate fuzziness) over other threads
- including any subsequent threads standards that might emerge in the future.