[uml-user] udev hangs on boot when using a cow file

[Dan B. <dba...@or...>]

I'm having issues when using cow files with CentOS6 system images.  It 
is specific to CentOS6.  When I tried with a debian image built with 
debootstrap, the system booted just fine.  I am experiencing this issue 
both with a custom built UML kernel as well as the kernel I obtained 
from the debian repository (2.6.32-1um-4+34squeeze1)

The issue I am experiencing is that during the boot process, udev hangs 
forever and the boot process does not complete.  It only occurs when I 
use a cow file by specifying just the cow file on the command line like 
"ubd0=cow_fs" rather than "ubd0=cow_fs,root_fs".  When I specify both 
the cow file and the backing file, the problem doesn't happen.  I am 
able to specify ubd0 either way with a debian image and the system boots 
as expected.  I have tried editing the appropriate rc script in the 
CentOS6 image to take udev out of the boot process, and the system 
boots, but has problems related to udev not running.

I have also tried specifying the "udevtimeout" parameter on the kernel 
command line to see if I could force it to time out quickly and carry on 
with booting, but that made no difference.  This leads me to believe 
that udev isn't just spinning it's wheels waiting for something, but 
it's actually hung or crashed.

I'm kind of at a loss here.  I don't know what interaction between the 
cow fs and udev would cause udev to hang, or if it's something that's 
only tangentially related to using the cow fs that's causing the 
problem.  It may even be a bug in udev.  I'm assuming that the kernel 
presents the block device ubd0/a to the system the same way, regardless 
if the device is specified to the kernel as "cow_file,backing_file" or 
just "cow_file", so I don't know why that would cause an issue.  
However, the manner that I use to specify ubd0 on the command line is 
literally the only thing that is different between a system that boots 
successfully, and one that hangs at starting udev.

The environment in which I run the UML kernel doesn't seem to make a 
difference; I have run the kernel on both my Debian Squeeze box and one 
of our CentOS6 servers (which is where these machines will be running in 
production).

Dan