FLUSH TABLES WITH READ LOCK on one node appears to lock other nodes as well.
In a 3 nodes Galera cluster if I do FTWRL on one node and apply simple DDL or DML on other nodes I don't have any locking.
To reproduce the issue I used a trivial mysqlslap test:
on my node 1
$ mysqlslap --concurrency=10 --iterations=200 --number-int-cols=2 --number-char-cols=3 --auto-generate-sql -uroot -h127.0.0.1 -P10021
With this as soon as FTWRL is executed on another node, say 3, all threads of mysqlslap on node 1 will hang with something like:
At this point, writing to any other node is also impossible, statements will hang, and so the whole cluster is locked.
FTWRL is used for backups, for example, so potentially a backup can lock the whole cluster.