unable to resize root partition on EC2 centos

i created my EC2 Machine using Community Image of Centos 6.3 x64. i have added a 35 GB disk. Now when i do #df -h

Filesystem            Size  Used Avail Use% Mounted on
/dev/xvda1            7.9G  1.2G  6.4G  16% /
tmpfs                 7.3G     0  7.3G   0% /dev/shm

my disk is 35GB but its showing 8 GB in root and 7 as tmpfs.

i tried to use resize2fs but it didnt work on centos. disk has ext4 partation..

# resize2fs /dev/xvda
resize2fs 1.41.12 (17-May-2010)
resize2fs: Device or resource busy while trying to open /dev/xvda
Couldn't find valid filesystem superblock.

or even if i tried resize2fs /dev/xvda1 it says device has nothing to do.

any idea or other way, its my root disk(/). so cant unmount it.