[petsc-users] Bug report VecNorm

[Stephan Köhler]

Dear PETSc/Tao team,

there is a bug in the voector interface:  In the function

VecNorm, see, eg. https://petsc.org/release/src/vec/vec/interface/rvector.c.html#VecNorm line 197 the check for consistency in line 214 is done on the wrong communicator.  The communicator should be PETSC_COMM_SELF.

Otherwise the program may hang when PetscCheck is executed.

Please find a minimal example attached.


Kind regards,
Stephan Köhler

--
Stephan Köhler
TU Bergakademie Freiberg
Institut für numerische Mathematik und Optimierung

Akademiestraße 6
09599 Freiberg
Gebäudeteil Mittelbau, Zimmer 2.07

Telefon: +49 (0)3731 39-3188 (Büro)


#include "petscvec.h"

int main(int argc, char **args)
{
  PetscMPIInt size, rank;
  Vec         vec;
  PetscReal   norm;
  PetscBool   flg = PETSC_FALSE, minflg = PETSC_FALSE;
  MPI_Comm    comm;
  PetscScalar *xx;

  PetscCall(PetscInitialize(&argc, &args, PETSC_NULLPTR, PETSC_NULLPTR));

  comm = PETSC_COMM_WORLD;
  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  PetscCheck(size > 1, comm, PETSC_ERR_ARG_WRONG, "example should be called with more than 1 MPI rank.");

  PetscCall(VecCreateMPI(comm, (rank+1)*10, PETSC_DETERMINE, &vec));
  PetscCall(VecSet(vec, 1.0));
  PetscCall(VecNorm(vec, NORM_INFINITY, &norm));

  PetscSynchronizedPrintf(PETSC_COMM_WORLD, "rank = %d, size = %d, norm = %lf\n", rank, size, norm);
  PetscSynchronizedFlush(comm, PETSC_STDOUT);

  if(rank == 0)
  {
    PetscCall(VecGetArrayWrite(vec, &xx));
    PetscCall(VecRestoreArrayWrite(vec, &xx));
  }

  PetscCall(VecNormAvailable(vec, NORM_INFINITY, &flg, &norm));

  PetscSynchronizedPrintf(comm, "rank = %d, size = %d, flg = %d, norm = %lf\n", rank, size, flg, norm);
  PetscSynchronizedFlush(comm, PETSC_STDOUT);

  PetscCall(MPIU_Allreduce(&flg, &minflg, 1, MPIU_BOOL, MPI_LAND, PetscObjectComm((PetscObject)vec)));
  /* wrong */
  PetscCheck(flg == minflg, PetscObjectComm((PetscObject)vec), PETSC_ERR_ARG_WRONGSTATE, "Some MPI processes have cached norm, others do not. This may happen when some MPI processes call VecGetArray() and some others do not.");
  /* this is correct */
  // PetscCheck(flg == minflg, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "Some MPI processes have cached norm, others do not. This may happen when some MPI processes call VecGetArray() and some others do not.");

  PetscCall(VecDestroy(&vec));

  PetscFinalize();

  return 0;

}

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