[Mpi-forum] user-defined reductions and re-entrancy

[Jeff Hammond]

Is there anything in the MPI standard preventing this sort of disgusting
behavior?

/* user code increments evil once between every call to this function so
that it should never fail in single-threaded execution */
static int evil = 0;
MPI_User_function(void* invec, void* inoutvec, int *len, MPI_Datatype
*datatype)
{
  evil++;
  double * in = (double*)invec;
  double * out = (double*)inoutvec;
  if (*datatype == MPI_DOUBLE && (evil%2)==0) {
    for (int i=0; i<*len; i++) out[i] = evil*in[i];
  } else {
    MPI_Abort(1,MPI_COMM_WORLD);
  }
}

My goal is to illustrate a user-defined reduction that is not reentrant.
If you dislike my example, then please mentally replace it with a better
one of your own creation :-)

The more general question is, are there any constraints on whether
implementations may internally use multiple threads to call user-defined
reductions?  I do not see any, but perhaps this is an oversight on my part.

Thanks,

Jeff

--
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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