The Problem

Server fails to boot with latest kernel with the below errors from console logs:

Initializing network drop monitor service
md: Waiting for all devices to be available before autodetect
md: If you don't use raid, use raid=noautodetect
md: Autodetecting RAID arrays.
md: Scanned 0 and added 0 devices.
md: autorun ...
md: ... autorun DONE.
VFS: Cannot open root device "UUID=[UUID]" or unknown-block(0,0)
Please append a correct "root=" boot option; here are the available partitions:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
Pid: 1, comm: swapper Not tainted 2.6.39-400.325.1.el6uek.x86_64 #1
Call Trace:
 [] panic+0xa6/0x1bd
 [] ? printk+0x41/0x47
 [] mount_block_root+0x1ea/0x29e
 [] mount_root+0x56/0x5a
 [] prepare_namespace+0x170/0x1a9
 [] kernel_init+0x2aa/0x2c0
 [] kernel_thread_helper+0x4/0x10
 [] ? parse_early_options+0x20/0x20
 [] ? gs_change+0x13/0x13

Server is sucessfully booting with with the older kernels.

The Solution

The installed latest kernel version did not update the initrd line in the /etc/grub.conf file

# grep -v "^#" /boot/grub/grub.conf | egrep 'Oracle Linux Server|initrd'
title Oracle Linux Server Unbreakable Enterprise Kernel (2.6.39-400.325.1.el6uek.x86_64)
<..initrd line="" missing="" ..="">
title Oracle Linux Server Red Hat Compatible Kernel (2.6.32-754.35.1.el6.x86_64)
initrd /initramfs-2.6.32-754.35.1.el6.x86_64.img
title Oracle Linux Server Unbreakable Enterprise Kernel (2.6.39-400.298.2.el6uek.x86_64)
initrd /initramfs-2.6.39-400.298.2.el6uek.x86_64.img

The initrd file is created for the latest kernel

# ls -l /boot/initramfs-2.6.39-400.325.1.el6uek.x86_64.img
-rw-r--r-- 1 root root 23940723 Dec 2 06:07 /boot/initramfs-2.6.39-400.325.1.el6uek.x86_64.img
# du -hs /boot/initramfs-2.6.39-400.325.1.el6uek.x86_64.img
23M /boot/initramfs-2.6.39-400.325.1.el6uek.x86_64.img 

1. Take a back up of the current configuration /etc/grub.conf file.

2. Edit the /etc/grub.conf file and append the missing line. For example:

initrd /initramfs-2.6.39-400.325.1.el6uek.x86_64.img

3. Reboot with latest kernel.

4. Incase the above action fails, then boot in to older kernel and reinstall the latest kernel, to update the grub.conf properly. For example:

# yum reinstall kernel-uek-2.6.39-400.325.1.el6uek.x86_6