cgroup scheduling: Adding kthreadd to a non-RT cgroup can deadlock the kernel

Hi,

I've found a bug where, on CONFIG_RT_GROUP_SCHED systems, adding the kthreadd task to a cgroup with cpu.rt_runtime_us = 0 (as some cgroup configuration scripts do, when they move all processes into a default cgroup), can result in deadlocks in the kernel.

On 2.6.37, the problem can be triggered via CPU hotplug. The following sequence of events will deadlock on an SMP system:

1. Add kthreadd to a cpu cgroup with rt_runtime_us = 0
2. echo 0 > /sys/devices/system/cpu/cpu1/online
3. echo 1 > /sys/devices/system/cpu/cpu1/online
4. echo 0 > /sys/devices/system/cpu/cpu1/online
5. echo 1 > /sys/devices/system/cpu/cpu1/online

In line (3), the CPU hotplug will cause us to create a new ksoftirqd/1 thread. Since that thread is forked from kthreadd, it will end up in the same cgroup, also without any realtime access.

In step (4), cpu_callback in softirq.c will attempt to kill ksoftirqd by setting it to SCHED_FIFO and using kthread_stop(). It does this with 'sched_setscheduler_nocheck', which bypasses the usual checks that prevent setting a process to an SCHED_FIFO if it is in a cgroup that would prevent it from running.

Thus, ksoftirqd ends up at SCHED_FIFO but with a zero rt_runtime_us, and is never scheduled again, and kthread_stop blocks waiting on it.

In (5), we try to call the CPU notifier chain again, but it is still locked from (4), and we deadlock.

For reasons I don't fully understand, just adding ksoftirqd/1 to a cgroup and then taking CPU 1 offline doesn't result in a hang, so I think there may be some detail of this situation I don't fully understand, but I'm pretty confident in the general analysis.

Before 2.6.34, we can trigger a similar problem just by adding kthreadd to a cgroup and then calling stop_machine (e.g. by removing a module), since stop_machine created a new RT workqueue on each invocation. This is how I first found this problem: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/693594

- Nelson

Greetings,

Hm. Seems to me this is just another of the myriad ways a privileged user can shoot himself in the foot.

-Mike

IMHO its a bug to move kthreadd into a cgroup. Simply don't do that.

Ok. I bothered reporting this here since the kernel /does/ try to prevent you from shooting yourself in the foot in related ways, by moving existing SCHED_FIFO tasks into a cgroup without any RT runtime, so I figured it might make sense to add a check here. libcgroup, for example, will try to move all processes into a default cgroup, ignoring errors, and relies on the kernel to prevent it from shooting itself in the foot.

It sounds like you consider that behavior a bug, though, so I'll go report this bug there.

- Nelson

Right, so we try to catch obvious cases, but its near impossible to catch all cases.

And its not only the scheduler controller, I bet you can get into trouble with some of the other controllers as well.