MPI: почему процессы ведут себя не всегда так, как задумано логикой программы?

#include <mpi.h>
#include <stdio.h>

void ring( int m, int argc, char** argv );

int main ( int argc, char** argv ) {
	ring( 3, argc, argv );

	return 0;
}

void ring( int m, int argc, char **argv ) {
	int i = 0, ProcNum, ProcRank, done = 0, buf, process;
	MPI_Status status;

	MPI_Init( &argc, &argv );

		//for ( i = 0; i < m; i++ ) {
		while( i != m ) {
				MPI_Comm_rank( MPI_COMM_WORLD, &ProcRank );
				MPI_Comm_size( MPI_COMM_WORLD, &ProcNum  );

				if ( ProcRank == 0 ) {
					MPI_Send( &ProcRank, 1, MPI_INT, 1, 0, MPI_COMM_WORLD );
				}
				else {
					MPI_Recv( &buf, 1, MPI_INT, ProcRank - 1, 0, MPI_COMM_WORLD, &status );
					printf( "bom-bom from %d to %d on itteration: %d\n", buf, ProcRank, i );

					process = ProcRank + 1;
					if ( process >= ProcNum )
						process = 0;

					MPI_Send( &ProcRank, 1, MPI_INT, process, 0, MPI_COMM_WORLD );
				}

				MPI_Barrier( MPI_COMM_WORLD );
				if ( ProcRank == 0 ) {
					MPI_Recv( &buf, 1, MPI_INT, ProcNum - 1, 0, MPI_COMM_WORLD, &status );
					printf( "bom-bom from %d to %d on itteration: %d\n", buf, ProcRank, i );
					printf( "DONE\n\n" );
				}

				i += 1;
				MPI_Barrier( MPI_COMM_WORLD );
		}

	MPI_Finalize();
}

$ mpirun -np 4 ./prog
bom-bom from 0 to 1 on itteration: 0
bom-bom from 1 to 2 on itteration: 0
bom-bom from 2 to 3 on itteration: 0
bom-bom from 2 to 3 on itteration: 1
bom-bom from 3 to 0 on itteration: 0
DONE

bom-bom from 3 to 0 on itteration: 1
DONE

bom-bom from 0 to 1 on itteration: 1
bom-bom from 0 to 1 on itteration: 2
bom-bom from 1 to 2 on itteration: 1
bom-bom from 1 to 2 on itteration: 2
bom-bom from 2 to 3 on itteration: 2
bom-bom from 3 to 0 on itteration: 2
DONE

$ mpirun -np 4 ./prog
bom-bom from 0 to 1 on itteration: 0
bom-bom from 1 to 2 on itteration: 0
bom-bom from 2 to 3 on itteration: 0
bom-bom from 3 to 0 on itteration: 0
DONE

bom-bom from 0 to 1 on itteration: 1
bom-bom from 1 to 2 on itteration: 1
bom-bom from 2 to 3 on itteration: 1
bom-bom from 3 to 0 on itteration: 1
DONE

bom-bom from 1 to 2 on itteration: 2
bom-bom from 2 to 3 on itteration: 2
bom-bom from 0 to 1 on itteration: 2
bom-bom from 3 to 0 on itteration: 2
DONE