Merge pull request #12434 from gkatev/han_root_node_rbuf_fix

coll/han: Fix issue in reduce leading to invalid rbuf pointer

Author: bosilca

ompi/mca/coll/han/coll_han_reduce.c

@@ -4,6 +4,8 @@
  *                         reserved.
  * Copyright (c) 2020      Bull S.A.S. All rights reserved.
  * Copyright (c) 2022      IBM Corporation. All rights reserved
+ * Copyright (c) 2024      Computer Architecture and VLSI Systems (CARV)
+ *                         Laboratory, ICS Forth. All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -139,11 +141,18 @@ mca_coll_han_reduce_intra(const void *sbuf,
                          "[%d]: root_low_rank %d root_up_rank %d\n", w_rank, root_low_rank,
                          root_up_rank));
 
-    void *tmp_rbuf = rbuf;
+    /* node leaders require a buffer to store intermediate results */
+    void *tmp_rbuf = NULL;
     void *tmp_rbuf_to_free = NULL;
-    if (low_rank == root_low_rank && root_up_rank != up_rank) {
-        /* allocate 2 segments on node leaders that are not the global root */
+    if (w_rank == root) {
+        /* the global root already has one */
+        tmp_rbuf = rbuf;
+    } else if (low_rank == root_low_rank) {
+        /* allocate 2 temporary segments on node leaders that are not the global root */
         tmp_rbuf = malloc(2*extent*seg_count);
+        if (NULL == tmp_rbuf) {
+            return OMPI_ERR_OUT_OF_RESOURCE;
+        }
         tmp_rbuf_to_free = tmp_rbuf;
     }
 
@@ -176,7 +185,7 @@ mca_coll_han_reduce_intra(const void *sbuf,
             t->sbuf = (char *) t->sbuf + extent * t->seg_count;
         }
 
-        if (up_rank == root_up_rank) {
+        if (w_rank == root) {
             t->rbuf = (char *) t->rbuf + extent * t->seg_count;
         }
         t->cur_seg = t->cur_seg + 1;