Hi,

It seems that ioctl_by_bdev() in fs/block_dev.c has a problem.
When initrd is unmounted it can cause OOPS. 
This problem occurs in recent ac patches.
May be vanilla too.

change_root() in fs/super.c calls ioctl_by_bdev() in
fs/block_dev.c which does not set inode_fake.i_bdev.

But ioctl of ramdisk (rd_ioctl() in rd.c) accesses to
i_bdev->bd_openers of the inode and which causes OOPS.

I attach the patch.

- GO!