RDMA osc: perform CAS in shared memory if possible

Signed-off-by: Joseph Schuchart <schuchart@hlrs.de>

ompi/mca/osc/rdma/osc_rdma_accumulate.c

@@ -812,7 +812,16 @@ int ompi_osc_rdma_compare_and_swap (const void *origin_addr, const void *compare
         lock_acquired = true;
     }
 
-    /* either we have and exclusive lock (via MPI_Win_lock() or the accumulate lock) or the
+    /* operate in (shared) memory if there is only a single node
+     * OR if we have an exclusive lock
+     * OR if other processes won't try to use the network either */
+    bool use_shared_mem = module->single_node ||
+                          (ompi_osc_rdma_peer_local_base (peer) &&
+                              (ompi_osc_rdma_peer_is_exclusive (peer) ||
+                                  !module->acc_single_intrinsic));
+
+    if (!use_shared_mem) {
+        /* either we have an exclusive lock (via MPI_Win_lock() or the accumulate lock) or the
          * user has indicated that they will only use the same op (or same op and no op) for
          * operations on overlapping memory ranges. that indicates it is safe to go ahead and
          * use network atomic operations. */
@@ -821,6 +830,7 @@ int ompi_osc_rdma_compare_and_swap (const void *origin_addr, const void *compare
         if (OMPI_SUCCESS == ret) {
             return OMPI_SUCCESS;
         }
+    }
 
     if (!(lock_acquired || ompi_osc_rdma_peer_is_exclusive (peer))) {
         (void) ompi_osc_rdma_lock_acquire_exclusive (module, peer, offsetof (ompi_osc_rdma_state_t, accumulate_lock));