How Maxscale deals with "Select for update transactions"

We are facing intermitent deadlock issues that doesnt got reflected in the maxscale log nor the mariadb log. Only in the applications triying to do sql transactions in the database. We use mostly readwritesplit with 2 nodes (yes, i know, bad design) and we dont know exactly how maxscale deals with the select for update queries. 1. to the slave, because is a read query (select) 2. to the master, because is a write query (update) Maxscale version 2.1.16, MariaDB 2.1.29 Thanks!