resize rootfs

[hagayg]

I noticed i running low on disk space :

xbian@xbian ~ $ df -Bm
Filesystem 1M-blocks Used Available Use% Mounted on
rootfs 658M 623M 36M 95% /
/dev/mmcblk0p2 658M 623M 36M 95% /
tmpfs 38M 1M 38M 1% /run
tmpfs 5M 0M 5M 0% /run/lock
tmpfs 10M 0M 10M 0% /dev
tmpfs 75M 0M 75M 0% /run/shm
/dev/mmcblk0p1 34M 11M 24M 32% /boot

So i used xbian config menu to resize my partition to full sd card size (i got 4gb one).
After rebooting i got the same reulsts as shown before.

After little digging i found that my partition is indeed 4gb but the rootfs isn't :

Disk /dev/mmcblk0: 3973 MB, 3973054464 bytes
4 heads, 16 sectors/track, 121248 cylinders, total 7759872 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000f06a6

Device Boot Start End Blocks Id System
/dev/mmcblk0p1 2048 71679 34816 c W95 FAT32 (LBA)
/dev/mmcblk0p2 71680 7759871 3844096 83 Linux


So tried to resize it using :

sudo resize2fs /dev/mmcblk0p2

and got :

resize2fs: Permission denied to resize filesystem

Other parameter told me the its in use and can be done.

Am i missing something ?

Thanks in advance.