Updates to the RHEL kernel change the location of and/or delete any custom kernel modules that have been compiled (this applies most notably right now to graphics drivers). In the past this has been OK as I usually just recompile any modules against the new kernel after updating. Unfortunately, it seems that, starting in RHEL 5.3, the kernel will panic whenever it is asked to load a module that does not exist, instead of just throwing a polite error.
I noticed this with graphics drivers – the machine screen will go blank AND ssh will not function if X11 asks the kernel to load a graphics driver module that no longer exists. So, it is important when updating kernels now to remove any references to custom modules (i.e., modules not managed by yum) prior to rebooting after a kernel update. (Yet another reason that we don’t pull down kernel updates automatically!)
In the case of the graphics driver — which is the only custom module as of Feb 2009 — this is painless because the driver installer backs up the vanilla conf file. This means that you have to delete /etc/X11/xorg.conf and rename /etc/X11/xorg.conf.original-x (the vanilla backup, where “x” is a small integer or zero) to /etc/X11/xorg.conf prior to reboot. If you forget, you must boot into knoppix or something similar off a CD, mount the hard disk and make this same change.
-James
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