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SubjectRe: [PATCH] cgroup/cpuset: remove circular dependency deadlock
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On 09/07/2017 02:26 PM, Boqun Feng wrote:
> 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

Hi Boqun,

cpu_down() in normal process can also trigger this.



Regards
Prateek
>
>>>
>>> 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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