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Date: Wed, 05 Jan 2011 07:01:57 +0100 From: Mike Galbraith <efault@....de> To: Nelson Elhage <nelhage@...lice.com> Cc: Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-kernel@...r.kernel.org Subject: Re: cgroup scheduling: Adding kthreadd to a non-RT cgroup can deadlock the kernel 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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