Hello,

Our library's autobuild (CTF, which uses MPI extensively and in relatively sophisticated ways) started failing on multiple architectures after github workflows moved to later OS versions (and so later MPI versions). I believe I have narrowed the issue to an MPI bug associated with very basic usage of MPI Reduce. The following test code runs into a segmentation fault inside MPI when running with 2 MPI processes with the latest Ubuntu MPI build and MPI 4.0. It works for smaller values of message size (n) or if the root is rank 0. The usage of MPI_IN_PLACE adheres with the MPI standard.

Best,
Edgar Solomonik

#include <mpi.h>
#include <iostream>

int main(int argc, char ** argv){
  int64_t n = 257;

  MPI_Init(&argc, &argv);
  int rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  double * A = (double*)malloc(sizeof(double)*n);
  for (int i=0; i<n; i++){
    A[i] = (double)i;
  }

  if (rank == 1){
    MPI_Reduce(MPI_IN_PLACE, A, n, MPI_DOUBLE, MPI_SUM, 1, MPI_COMM_WORLD);
  } else {
    MPI_Reduce(A, NULL, n, MPI_DOUBLE, MPI_SUM, 1, MPI_COMM_WORLD);
  }

  free(A);

  MPI_Finalize();

  return 0;
}