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    Subject[PATCH 37/51] clocksource, dummy-timer: 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 clocksource dummy-timer code by using this latter form of callback
    registration.

    Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    drivers/clocksource/dummy_timer.c | 11 ++++++++---
    1 file changed, 8 insertions(+), 3 deletions(-)

    diff --git a/drivers/clocksource/dummy_timer.c b/drivers/clocksource/dummy_timer.c
    index b3eb582..995d446 100644
    --- a/drivers/clocksource/dummy_timer.c
    +++ b/drivers/clocksource/dummy_timer.c
    @@ -56,14 +56,19 @@ static struct notifier_block dummy_timer_cpu_nb = {

    static int __init dummy_timer_register(void)
    {
    - int err = register_cpu_notifier(&dummy_timer_cpu_nb);
    + int err = 0;
    +
    + cpu_maps_update_begin();
    + err = __register_cpu_notifier(&dummy_timer_cpu_nb);
    if (err)
    - return err;
    + goto out;

    /* We won't get a call on the boot CPU, so register immediately */
    if (num_possible_cpus() > 1)
    dummy_timer_setup();

    - return 0;
    +out:
    + cpu_maps_update_done();
    + return err;
    }
    early_initcall(dummy_timer_register);


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