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    SubjectRe: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency
    On Thu, Nov 30, 2006 at 09:58:07AM +0530, Gautham R Shenoy wrote:
    > So can we ignore this circular-dep warning as a false positive?
    > Or is there a way to exploit this circular dependency ?
    > At the moment, I cannot think of way to exploit this circular dependency
    > unless we do something like try destroying the created workqueue when the
    > cpu is dead, i.e make the cpufreq governors cpu-hotplug-aware.
    > (eeks! that doesn't look good)

    Ok, I see that we are already doing it :(. So we can end up in a

    Here's the culprit callpath:

    !-> raw_notifier_call_chain(CPU_LOCK_ACQUIRE)
    ! !
    ! !-> workqueue_cpu_mutex(CPU_LOCK_ACQUIRE) [*]
    !-> raw_notifier_call_chain(CPU_DEAD)
    !-> cpufreq_cpu_callback (CPU_DEAD)
    !-> cpufreq_remove_dev
    !-> __cpufreq_governor(data, GOVERNOR_STOP)
    !-> policy->governor->governor()
    !-> cpufreq_governor_dbs(GOVERNOR_STOP)
    !-> destroy_workqueue() [*]

    [*] indicates function takes workqueue_mutex.

    So a deadlock!

    I wasn't able to observe this because I'm running Xeon SMP box on which
    you cannot offline cpu0. And cpufreq data is created only for cpu0,
    while all other cpus cpufreq_data just point to cpu0's cpufreq_data.

    So the mentioned callpath within cpufreq_remove_dev is never reached
    during the normal cpu offline cycle.

    However, if there are architectures which allow the first-booted-cpu
    (or to be precise, the cpu for which cpufreq_data is *actually* created)
    to be offlined and we are running Ondemand governor during the offline,
    we will see this deadlock.

    Gautham R Shenoy
    Linux Technology Center
    IBM India.
    "Freedom comes with a price tag of responsibility, which is still a bargain,
    because Freedom is priceless!"
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      Last update: 2006-11-30 07:39    [W:0.022 / U:40.400 seconds]
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