Comment # 6 on bug 935086 from Takashi Iwai

Is this a hang in kernel level?  You can enable magic sysrq beforehand by
changing to kernel.sysrq=1 in /etc/sysctl.conf (and applying it), and see
whether you have a control via magic sysrq.
For example, alt-sysrq-s will trigger sync, alt-sysrq-t will show the stack
traces of all running tasks.

Also, did you see whether you get any better information with rd.debug?