diff --git a/src/sort.c b/src/sort.c
index 7410abc..9d156ac 100644
--- a/src/sort.c
+++ b/src/sort.c
@@ -3196,6 +3196,7 @@ merge_tree_init (size_t nthreads, size_t nlines, struct line *dest)
   root->parent = NULL;
   root->level = MERGE_END;
   root->queued = false;
+  fprintf (stderr, "merge_tree_init init_node %p\n", root);
   pthread_mutex_init (&root->lock, NULL);
 
   init_node (root, root + 1, dest, nthreads, nlines, false);
@@ -3238,6 +3239,7 @@ init_node (struct merge_node *restrict parent,
   node->parent = parent;
   node->level = parent->level + 1;
   node->queued = false;
+  fprintf (stderr, "init_node init_node %p\n", node);
   pthread_mutex_init (&node->lock, NULL);
 
   if (nthreads > 1)
@@ -3335,6 +3337,7 @@ queue_pop (struct merge_node_queue *queue)
   while (! (node = heap_remove_top (queue->priority_queue)))
     pthread_cond_wait (&queue->cond, &queue->mutex);
   pthread_mutex_unlock (&queue->mutex);
+  fprintf (stderr, "queue_pop lock_node %p\n", node);
   lock_node (node);
   node->queued = false;
   return node;
@@ -3455,8 +3458,10 @@ queue_check_insert_parent (struct merge_node_queue *queue,
 {
   if (node->level > MERGE_ROOT)
     {
+      fprintf (stderr, "queue_check_insert_parent lock_node %p\n", node->parent);
       lock_node (node->parent);
       queue_check_insert (queue, node->parent);
+      fprintf (stderr, "queue_check_insert_parent unlock_node %p\n", node->parent);
       unlock_node (node->parent);
     }
   else if (node->nlo + node->nhi == 0)
@@ -3483,6 +3488,7 @@ merge_loop (struct merge_node_queue *queue,
 
       if (node->level == MERGE_END)
         {
+          fprintf (stderr, "merge_loop_MERGE_END unlock_node %p\n", node);
           unlock_node (node);
           /* Reinsert so other threads can pop it. */
           queue_insert (queue, node);
@@ -3492,6 +3498,7 @@ merge_loop (struct merge_node_queue *queue,
       queue_check_insert (queue, node);
       queue_check_insert_parent (queue, node);
 
+      fprintf (stderr, "merge_loop unlock_node %p\n", node);
       unlock_node (node);
     }
 }
@@ -3608,6 +3615,7 @@ sortlines (struct line *restrict lines, size_t nthreads,
       merge_loop (queue, total_lines, tfp, temp_output);
     }
 
+  fprintf (stderr, "sortlines destroy_node %p\n", node);
   pthread_mutex_destroy (&node->lock);
 }
 
@@ -3917,6 +3925,7 @@ sort (char *const *files, size_t nfiles, char const *output_file,
               sortlines (line, nthreads, buf.nlines, root,
                          &queue, tfp, temp_output);
               queue_destroy (&queue);
+              fprintf (stderr, "sort destroy_node %p\n", root);
               pthread_mutex_destroy (&root->lock);
               merge_tree_destroy (merge_tree);
             }