Is it safe to use mysqldump to backup and restore a master-master cluster
I would strongly urge you to use Percona's XtraBackup. You're already using their storage engine - take the time to get to know this tool! Good reasons not to use mysqldump can be found here. If you really have to get text file SQL dumps of your data, check out mydumper, see here and here for why (disclaimer - haven't used).