An attempt to fix MDEV-33189

storage/innobase/buf/buf0flu.cc

@@ -2511,6 +2511,8 @@ static void buf_flush_page_cleaner()
     }
     else if (buf_flush_async_lsn <= oldest_lsn)
       goto check_oldest_and_set_idle;
+    else
+      mysql_mutex_lock(&buf_pool.mutex);
 
     n= n >= n_flushed ? n - n_flushed : 0;
     goto LRU_flush;