Add documentation for HDF5 and exceptions

diff --git a/include/deal.II/base/hdf5.h b/include/deal.II/base/hdf5.h
index c24dc47780e0c8ba847c095458c4280042a58506..0263e1f408e47a80657c18ad9d83741fd83483d5 100644 (file)
--- a/include/deal.II/base/hdf5.h
+++ b/include/deal.II/base/hdf5.h
@@ -326,6 +326,38 @@ DEAL_II_NAMESPACE_OPEN
  * thread-safe HDF5 version serializes the API but does not provide any level of
  * concurrency. To achieve high parallel performance with HDF5, we advice to use
  * HDF5 with MPI.
+ *
+ * # Exceptions and parallel HDF5
+ * In certain corner cases, exceptions combined with parallel HDF5 can freeze
+ * deal.II. If one MPI process throws an exception and not the other ones, then
+ * deal.II freezes. The reason is that the following happens:
+ * 1. On the processor that calls the exception we try to close the parallel
+ *   MPI file (H5F_try_close) which ultimately results in a barrier
+ *   (MPI_barrier). This is triggered by the destructor of an HDF5Object.
+ * 2. On the processor that does not call the exception we call
+ *   H5VL_dataset_write() and end up in MPI_Allreduce().
+ *
+ * This could happen if an error is found in all the processes except one. For
+ * example, the following code could potentially freeze deal.II:
+ * @code
+ * HDF5::File data_file(filename,
+ *                            HDF5::File::FileAccessMode::create,
+ *                            mpi_communicator);
+ * {
+ *  auto  dataset_a =
+ *        data_file.create_dataset<double>(dataset_name_a, dataset_dimensions_a);
+ *  dataset_a.write_selection(data_a, coordinates_a);
+ * }
+ *
+ * AssertThrow(Utilities::MPI::this_mpi_process(mpi_communicator) == 0,
+ *                   ExcInternalError());
+ *
+ * {
+ *  auto  dataset_b =
+ *        data_file.create_dataset<double>(dataset_name_b, dataset_dimensions_b);
+ *  dataset_b.write_selection(data_b, coordinates_b);
+ * }
+ * @endcode
  */
 // clang-format on
 namespace HDF5