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    Subject[PATCH 30/51] powercap, intel-rapl: Fix CPU hotplug callback registration
    Date
    Subsystems that want to register CPU hotplug callbacks, as well as perform
    initialization for the CPUs that are already online, often do it as shown
    below:

    get_online_cpus();

    for_each_online_cpu(cpu)
    init_cpu(cpu);

    register_cpu_notifier(&foobar_cpu_notifier);

    put_online_cpus();

    This is wrong, since it is prone to ABBA deadlocks involving the
    cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
    with CPU hotplug operations).

    Instead, the correct and race-free way of performing the callback
    registration is:

    cpu_maps_update_begin();

    for_each_online_cpu(cpu)
    init_cpu(cpu);

    /* Note the use of the double underscored version of the API */
    __register_cpu_notifier(&foobar_cpu_notifier);

    cpu_maps_update_done();


    Fix the intel-rapl code in the powercap driver by using this latter form
    of callback registration. But retain the calls to get/put_online_cpus(),
    since they also protect the function rapl_cleanup_data(). By nesting
    get/put_online_cpus() *inside* cpu_maps_update_begin/done(), we avoid
    the ABBA deadlock possibility mentioned above.

    Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
    Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
    Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    drivers/powercap/intel_rapl.c | 10 ++++++++--
    1 file changed, 8 insertions(+), 2 deletions(-)

    diff --git a/drivers/powercap/intel_rapl.c b/drivers/powercap/intel_rapl.c
    index 3c67683..b460d46 100644
    --- a/drivers/powercap/intel_rapl.c
    +++ b/drivers/powercap/intel_rapl.c
    @@ -1369,6 +1369,9 @@ static int __init rapl_init(void)

    return -ENODEV;
    }
    +
    + cpu_maps_update_begin();
    +
    /* prevent CPU hotplug during detection */
    get_online_cpus();
    ret = rapl_detect_topology();
    @@ -1380,20 +1383,23 @@ static int __init rapl_init(void)
    ret = -ENODEV;
    goto done;
    }
    - register_hotcpu_notifier(&rapl_cpu_notifier);
    + __register_hotcpu_notifier(&rapl_cpu_notifier);
    done:
    put_online_cpus();
    + cpu_maps_update_done();

    return ret;
    }

    static void __exit rapl_exit(void)
    {
    + cpu_maps_update_begin();
    get_online_cpus();
    - unregister_hotcpu_notifier(&rapl_cpu_notifier);
    + __unregister_hotcpu_notifier(&rapl_cpu_notifier);
    rapl_unregister_powercap();
    rapl_cleanup_data();
    put_online_cpus();
    + cpu_maps_update_done();
    }

    module_init(rapl_init);


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