diff --git a/ompi/mca/coll/libnbc/coll_libnbc.h b/ompi/mca/coll/libnbc/coll_libnbc.h
index b66ce60fa5e..c8034c53165 100644
--- a/ompi/mca/coll/libnbc/coll_libnbc.h
+++ b/ompi/mca/coll/libnbc/coll_libnbc.h
@@ -75,8 +75,8 @@ struct ompi_coll_libnbc_component_t {
     opal_free_list_t requests;
     opal_list_t active_requests;
     int32_t active_comms;
-    opal_atomic_lock_t progress_lock;
-    opal_mutex_t lock;
+    opal_atomic_lock_t progress_lock; /* protect from recursive calls */
+    opal_mutex_t lock;                /* protect access to the active_requests list */
 };
 typedef struct ompi_coll_libnbc_component_t ompi_coll_libnbc_component_t;
 
diff --git a/ompi/mca/coll/libnbc/coll_libnbc_component.c b/ompi/mca/coll/libnbc/coll_libnbc_component.c
index fe8bdcc902a..57454d1a003 100644
--- a/ompi/mca/coll/libnbc/coll_libnbc_component.c
+++ b/ompi/mca/coll/libnbc/coll_libnbc_component.c
@@ -263,8 +263,11 @@ ompi_coll_libnbc_progress(void)
     ompi_coll_libnbc_request_t* request, *next;
     int res;
 
+    /* return if invoked recursively */
     if (opal_atomic_trylock(&mca_coll_libnbc_component.progress_lock)) return 0;
 
+    /* process active requests, and use mca_coll_libnbc_component.lock to access the
+     * mca_coll_libnbc_component.active_requests list */
     OPAL_THREAD_LOCK(&mca_coll_libnbc_component.lock);
     OPAL_LIST_FOREACH_SAFE(request, next, &mca_coll_libnbc_component.active_requests,
                            ompi_coll_libnbc_request_t) {