Backupping my root filesystem w/o restarting

[Sfynx]

I want to make regular backups, preferably with tar, of the root filesystem of my little Gentoo server. This is the drive config:

/dev/hda1: boot, 7.8MB
/dev/hda2: swap, 36MB
/dev/hda3: root, 790MB

I have a fully working smbfs mountpoint on /mnt/backup to a Windows share where the backup is to be placed.

The root filesystem (/dev/hda3) is active and mounted and I want to get it remounted as read-only before safely performing the backup procedure (to prevent that it is being changed while backupping). After the backup, it should be remounted read-write again, thereby resuming normal server operation.

The problem is that I cannot get it properly remounted. / is in use, also after unmounting all the stuff in fstab... and devfs.

I know it is possible to make another root partition that contains a backup procedure to have the bigger root partition unmounted during the backup, but I do not want to restart my server to make a backup if that is possible (I love uptimes! ) Does anyone have a suggestion?
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[delta407]

man mount.

"mount -o remount,ro /"
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