Upstream kernel says this isn't a kernel bug:
https://lkml.org/lkml/2011/1/5/53
** Package changed: linux (Ubuntu) = libcgroup (Ubuntu)
** Summary changed:
- rmmod hangs if cgroup-bin is installed
+ cgroup-bin should not move kthreadd into a default cgroup
** Description changed:
Steps to
** Description changed:
Steps to reproduce:
1. Install cgroup-bin from universe on a stock Lucid machine (I've only
tested amd64, but I suspect it shouldn't matter)
2. Load an arbitrary module (e.g. modprobe rds)
3. Unload the module loaded in (2) (e.g. rmmod rds)
The 'rmmod'
The bug is somehow related to how cgroup puts every process into a
default cgroup. You can un-wedge the hung 'rmmod' via
pgrep kstop /mnt/cgroup/cpu/tasks
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The problem appears to be that the 'sysdefault' cgroup has a
rt_runtime_us value of 0, and the stop_machine kthreads are getting
created inside that cgroup (since the init script moves everything into
the sysdefault cgroup), and so never get run.
The 'cpu' cgroup subsys will prevent moving RT
