I read your last posts and I have a similar problem when I test modules by modprobe

modprobe cloop is ok but not modprobe unionfs

# modprobe unionfs
unionfs: Unknown symbol __rcuref_hash
FATAL: Error inserting unionfs (/lib/modules/2.6.15/kernel/fs/unionfs/unionfs.ko): Unknown symbol in module, or unknown parameter (see dmesg)

# dmesg
unionfs: Unknown symbol __rcuref_hash