mysqldump on slave hangs

[Thomas Kettenbach]

We have a master / slave configuration with a mixed MyISAM / InnoDB Database. Once a day we make a backup of the databases on the slave. mysqldump does the
'FLUSH TABLES WITH READLOCK'

Another process makes a 'FLUSH TABLES', while the master keeps on getting updated the slave sql replication issues a 'UPDATE table SET ... WHERE ...' -- this one hangs on 'WAIT FOR RELEASE OF READ LOCK'. So far, so good.

However the other two processes with 'FLUSH TABLES WITH READLOCK' and 'FLUSH TABLES' seem to hang and can't continue.

My questions in this issue are:
- What are the 'flush table' processes doing? How can I get more info on that?
- Why is the 'read lock' not released?
- How can i see active locks held on the mysqld?

Thanks for working during the eoy period and hopefully answering this issue.