Re: cgroup scheduling: Adding kthreadd to a non-RT cgroup candeadlock the kernel

[Mike Galbraith]

On Tue, 2011-01-04 at 23:54 -0500, Nelson Elhage wrote:
> Hi,

Greetings,

> 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.

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

	-Mike

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