OpenMP program hangs with >256 threads

While trying to characterize some Phi performance over large numbers of OpenMP threads I've noticed strange behavior where programs hang with >256 threads.

I've pared things down to the following example:

#include <omp.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
  int threads = 257;
  // omp_set_dynamic(1);
#pragma omp parallel
#pragma omp single
  {
    printf("single %d of %d\n", omp_get_thread_num(), omp_get_num_threads());
  }
#pragma omp parallel num_threads(threads)
  {
    for (int j=0; j<10; j++) {
      printf("thread %d iteration: %d\n", omp_get_thread_num(), j);
    }
  }
}

If I compile/run this on my Phi host it works fine.  If I compile and run this on the Phi, it prints out the 10 iterations then hangs.  If you set threads <=256 then it works fine on the Phi.  If you omp_set_dynamic(1) and set the threads >256 then it works fine.

The key seems to be having the first default parallel region followed by a second one that uses more than 256 threads.  I haven't found a good description of the nuances of dynamic threads, but I can see how omp_set_dynamic might be required (though it didn't seem obvious to me that having different parallel regions with different numbers of threads was all that "dynamic" ;^)).  I'm definitely not sure why simple hanging for >256 threads is the appropriate behavior.