Bug#11766591 59733: POSSIBLE DEADLOCK WHEN BUFFERED CHANGES ARE DISCARDED

Tweak the faulty UNIV_SYNC_DEBUG diagnostics a little bit more.

ibuf_add_free_page(): Lower the latching order of the newly allocated page
only after acquiring the ibuf_mutex.

storage/innobase/ibuf/ibuf0ibuf.c

@@ -1683,14 +1683,14 @@ ibuf_add_free_page(
 
 	page = buf_page_get(space, page_no, RW_X_LATCH, &mtr);
 
-#ifdef UNIV_SYNC_DEBUG
-	buf_page_dbg_add_level(page, SYNC_TREE_NODE_NEW);
-#endif /* UNIV_SYNC_DEBUG */
-
 	ibuf_enter();
 
 	mutex_enter(&ibuf_mutex);
 
+#ifdef UNIV_SYNC_DEBUG
+	buf_page_dbg_add_level(page, SYNC_TREE_NODE_NEW);
+#endif /* UNIV_SYNC_DEBUG */
+
 	root = ibuf_tree_root_get(ibuf_data, space, &mtr);
 
 	/* Add the page to the free list and update the ibuf size data */

storage/innodb_plugin/ibuf/ibuf0ibuf.c

@@ -1766,16 +1766,15 @@ ibuf_add_free_page(void)
 		block = buf_page_get(
 			IBUF_SPACE_ID, 0, page_no, RW_X_LATCH, &mtr);
 
-		buf_block_dbg_add_level(block, SYNC_IBUF_TREE_NODE_NEW);
+		ibuf_enter();
 
+		mutex_enter(&ibuf_mutex);
+
+		buf_block_dbg_add_level(block, SYNC_IBUF_TREE_NODE_NEW);
 
 		page = buf_block_get_frame(block);
 	}
 
-	ibuf_enter();
-
-	mutex_enter(&ibuf_mutex);
-
 	root = ibuf_tree_root_get(&mtr);
 
 	/* Add the page to the free list and update the ibuf size data */