From: Luca Heltai <luca.heltai@sissa.it>
Date: Mon, 2 Dec 2019 17:32:50 +0000 (+0100)
Subject: Fixed deadlock.
X-Git-Tag: v9.2.0-rc1~828^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fpull%2F9121%2Fhead;p=dealii.git

Fixed deadlock.
---

diff --git a/include/deal.II/base/mpi.h b/include/deal.II/base/mpi.h
index 9af1832740..5b3dbc87f6 100644
--- a/include/deal.II/base/mpi.h
+++ b/include/deal.II/base/mpi.h
@@ -1500,13 +1500,16 @@ namespace Utilities
         else
           send_to.emplace_back(m.first);
 
-      // Shortcut, when running in serial
-      if (send_to.size() == 0)
-        return received_objects;
-
       const auto n_point_point_communications =
         Utilities::MPI::compute_n_point_to_point_communications(comm, send_to);
 
+      // If we have something to send, or we expect something from other
+      // processors, we need to visit one of the two scopes below. Otherwise,
+      // no other action is required by this mpi process, and we can safely
+      // return.
+      if (send_to.size() == 0 && n_point_point_communications == 0)
+        return received_objects;
+
       // Protect the following communication:
       static CollectiveMutex      mutex;
       CollectiveMutex::ScopedLock lock(mutex, comm);