[CentOS] unable to umount

Hi,

on an EL5 XEN DOM0 system I have following volume

$ df -h /srv 
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdc1             917G  858G   60G  94% /srv

that partition was used by virtual machines but they were all halted.

service xendomains stop

$ xm list
Name                                      ID Mem(MiB) VCPUs State   Time(s)
Domain-0                                   0     3000     2 r-----    695.1

$ service xend stop


nothing is using the partition
$ lsof |grep srv
<empty>

$ fuser -m /srv
<empty>

$ fuser -km /srv
<empty>


but i can not umount /srv

$ umount /srv
umount: /srv: device is busy
umount: /srv: device is busy


what could keeping the device "busy" ... ?

__
Thanks,

LF