MPI_DOORBELL
#define mpi_read_db(s) mpi_read((s), MPI_DOORBELL)
#define mpi_write_db(s, v) mpi_write((s), MPI_DOORBELL, (v))
if (mpi_wait_ne(sc, MPI_DOORBELL, MPI_DOORBELL_STATE,
if (mpi_wait_ne(sc, MPI_DOORBELL, MPI_DOORBELL_STATE,
if (mpi_wait_eq(sc, MPI_DOORBELL, MPI_DOORBELL_STATE,
if (mpi_wait_eq(sc, MPI_DOORBELL, MPI_DOORBELL_STATE,