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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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