OMPI/COMM: be more conservative about when a comm is ready

Only return a pointer to a ompi_commuicator_t struct now from

- ompi_comm_lookup
- ompi_comm_lookup_cid

when the communicator has a PML associated with it.

The ompi_comm_lookup function will continue to return NULL if the entry designated by the c_index
argument in the ompi_mpi_communicator table is OMPI_COMM_SENTINEL.

OLD COMMIT MESSAGE BEFORE REFACTOR

This patch addresses a race condition in OB1. One way this race condition is encountered is when using MPI_Comm_spawn under oversubscribed conditions.

The fundamental reason for this race condition existing is that the CID allocation procedure for intercommunicators does not have a barrier in the onpi_comm_activate_nb procedure. As a result, it is possible for a process to receive a fragement (message) from another process participating in the spawn procedure and still be in the cid allocation procedure (within ompi_comm_next_cid_nb). The process may have allocated a suitable slot in the ompi_mpi_communicators table but not yet associated it with a PML.

So in this code path it is necessary to check both

a valid cid for the incoming message headers' context is present in ompi_mpi_communicators and
a PML is associated with this communicator.
This problem is specific to inter communicators at the time of this PR as intracommunicators have a barrier like behavior in ompi_comm_activate_nb.

Signed-off-by: Howard Pritchard <[email protected]>

Author: hppritcha

ompi/communicator/communicator.h

@@ -22,7 +22,7 @@
  * Copyright (c) 2015      Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
  * Copyright (c) 2016-2017 IBM Corporation. All rights reserved.
- * Copyright (c) 2018-2022 Triad National Security, LLC. All rights
+ * Copyright (c) 2018-2024 Triad National Security, LLC. All rights
  *                         reserved.
  * Copyright (c) 2023      Advanced Micro Devices, Inc. All rights reserved.
  * $COPYRIGHT$
@@ -568,6 +568,13 @@ static inline ompi_communicator_t *ompi_comm_lookup (const uint32_t c_index)
         comm = NULL;
     }
 
+    /*
+     * return NULL if comm doesn't yet have an associated PML
+     */
+    if ((NULL != comm) && !OMPI_COMM_IS_PML_ADDED(comm)) {
+        comm = NULL;
+    }
+
     return comm;
 }
 
@@ -584,6 +591,13 @@ static inline ompi_communicator_t *ompi_comm_lookup_cid (const ompi_comm_extende
 {
     ompi_communicator_t *comm = NULL;
     (void) opal_hash_table_get_value_ptr (&ompi_comm_hash, &cid, sizeof (cid), (void **) &comm);
+    /*
+     * return NULL if the comm does not yet have an asociated PML
+     */
+    if ((NULL != comm) && !OMPI_COMM_IS_PML_ADDED(comm)) {
+        comm = NULL;
+    }
+
     return comm;
 }