freeze_bdev - - lock a filesystem and force it into a consistent state
struct super_block * freeze_bdev (struct block_device * bdev);
bdev blockdevice to lock
This takes the block device bd_mount_sem to make sure no new mounts happen on bdev until thaw_bdev is called. If a superblock is found on this device, we take the s_umount semaphore on it to make sure nobody unmounts until the snapshot creation is done.