Doc states re. BACKUP DB command:

TABLESPACE tablespace-name
A list of names used to specify the table spaces to be backed up.
ONLINE
Specifies online backup. The default is offline backup. Online backups are only available for databases configured with logretain or userexit enabled. During an online backup, DB2 obtains IN (Intent None) locks on all tables existing in SMS table spaces as they are processed and S (Share) locks on LOB data in SMS table spaces

So what ablout DMS tablespaces (and the db is configured with LOGRETAIN=RECOVERY and USEREXIT=ON) and online backup?

Ruby