Call to boundary() with 8 threads causes a crash when running Multigrid MPI

    The following code crashes for some levels of refinement when using 8 threads and 9 levels of refinement.

    #include "grid/multigrid3D.h"
    scalar m[];
    int maxlevel = 5;
    int main(int argc,char ** argv){
      if (argc>1)
      printf("thread #%d was here\n",pid());
      printf("thread #%d is done\n",pid());

    How to crash?

    Compile and run the code following like so;

    :~$ CC99='mpicc -std=c99' qcc -O2 -Wall -D_MPI=1 MGmpiboundary.c -o maybebug -lm
    :~$ mpirun -np 8 ./maybebug 

    There are no issues for this case. However when running with nine levels of refinement;

    :~$ mpirun -np 8 ./maybebug 9

    An error is raised: segmentation fault that seems to correspond with the call the boundary() function as one may get this:

    thread #1 was here
    thread #6 was here
    thread #3 was here
    thread #0 was here
    thread #4 was here
    thread #2 was here
    thread #5 was here
    thread #7 was here
    mpirun noticed that process rank 6 with PID 14254 on node antoon-XPS-15-9550 exited on signal 11 (Segmentation fault).

    The bug appears on a test laptop and also on a bigger system (a workstation) that allowed testing with 10 levels of refinement. Here some manupulation of the data associated with the m[] field worked fine, untill the call to boundary() was made.

    Note that running the executable using a single or 64 threads does not display this issue, i.e. for 9 levels of refinement.