When you mount an ext4 you get a kernel thread performing the journaling. This has been so since around 2.6.22 I think. You mount /dev/sda3 and see this process.
$ ps -aefl | grep jbd
1 S root 297 2 0 80 0 - 0 kjourn 05:27 ? 00:00:00 [jbd2/sda5-8]
1 S root 510 2 0 80 0 - 0 kjourn 05:27 ? 00:00:00 [jbd2/sda3-8]
I recently wanted to fsck this same so as root i did a 'umount /dev/sda3' and tried the fsck which promptly complained the /dev/sda3 was in use. Using lsof I discovered the kernel thread 'kjourn above was still running, tho' the device was clearly umounted.
Remounting/umounting, sync'ing etc mode no difference. The kjourn thread appears to ignore all signals.
I finally rebooted w/ the mount disabled in fstab, but that's not really acceptable generally.
A few googles showed little
Anyone know how to deal with this ?