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