All the following commands did not make kernel reread partition :

partprobe /dev/sda (warning : kernel failed to reread ....)
hdparm -z /dev/sda (BLKRRPART failed : device or resource busy)
blockdev -rereadpt /dev/sda (BLKRRPART failed : device or resource busy)
sfdisk -R /dev/sda (BLKRRPART failed : device or resource busy)

i still need a reboot to make it work

http://serverfault.com/questions/36038/reread-partition-table-without-rebooting