tune2fs's man page includes a warning:
Never use tune2fs to change parameters of a read/write
mounted filesystem! Use this utility at your own risk.
You're modifying a filesystem!
*I* understand this warning, since many of the things tune2fs can do to
a file system will likely cause kernel confusion if done to a
read/write mounted file system. *One* of the things tune2fs is set the
file system's label, which can also be done with e2label. e2label does
not include the WARNING. I have a batch of file systems on two SCSI
disk that I want to set labels on and want to know if I need to boot up
a rescue disk or if I can just do this on a running system. I would
think that e2label would NOT be dangerous on live mounted file systems,
but it is not clear.
(It would be easier & simplier to do it on a live system, since I would
have a set of 'nice' tools to automate the process.)