From: Timo Heister <timo.heister@gmail.com>
Date: Mon, 21 Oct 2019 11:19:43 +0000 (+0200)
Subject: add a barrier to avoid MPI race in ConsensusAlgorithm
X-Git-Tag: v9.2.0-rc1~960^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5bcf04365db318c3dc143b16d79cc28c2d27e6f1;p=dealii.git

add a barrier to avoid MPI race in ConsensusAlgorithm

As discussed in #8929, messages exchanged in this function can be mixed
up if called twice in succesion. Add a Barrier to make sure this won't
happen.
---

diff --git a/include/deal.II/base/mpi_compute_index_owner_internal.h b/include/deal.II/base/mpi_compute_index_owner_internal.h
index dea27428e4..00642c81b5 100644
--- a/include/deal.II/base/mpi_compute_index_owner_internal.h
+++ b/include/deal.II/base/mpi_compute_index_owner_internal.h
@@ -759,6 +759,11 @@ namespace Utilities
               }
 #  endif
 
+            // This barrier is important to make sure that two successive calls
+            // to this functions do not overlap and we confuse messages. See the
+            // discussion in https://github.com/dealii/dealii/issues/8929
+            MPI_Barrier(comm);
+
 #endif // DEAL_II_WITH_MPI
 
             return requested_indices;