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SubjectRe: [PATCH] cgroup/cpuset: remove circular dependency deadlock
On Thu, Sep 07, 2017 at 09:28:48AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 07, 2017 at 11:34:12AM +0530, Prateek Sood wrote:
> > Remove circular dependency deadlock in a scenario where hotplug of CPU is
> > being done while there is updation in cgroup and cpuset triggered from
> > userspace.
> >
> > Example scenario:
> > kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
> >
> > kworker/0:0 - percpu_down_write(&cpu_hotplug_lock) [held]
> > flush(work) [no high prio workqueue available on CPU]
> > wait_for_completion()

Hi Prateek,

so this is:

_cpu_down():
cpus_write_lock(); // percpu_down_write(&cpu_hotlug_lock)
cpuhp_invoke_callbacks():
workqueue_offine_cpu():
wq_update_unbound_numa():
alloc_unbound_pool():
get_unbound_pool():
create_worker():
kthread_create_on_node():
wake_up_process(kthreadd_task);
wait_for_completion(); // create->done

, right?

Wonder running in a kworker is necessary to trigger this, I mean running
a cpu_down() in a normal process context could also trigger this, no?
Just ask out of curiosity.

Regards,
Boqun

> >
> > kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting]
> >
> > init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held]
> > lock(cpuset_mutex) [waiting]
> >
> > init:1 - lock(cpuset_mutex) [held]
> > percpu_down_read(&cpu_hotplug_lock) [waiting]
>
> That's both unreadable and useless :/ You want to tell what code paths
> that were, not which random tasks happened to run them.
>
>
[...]
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