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_notifier_register_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_notifier_register_done();


Fix the clocksource dummy-timer code by using this latter form of callback
registration.

Cc: Daniel Lezcano <daniel.lezc...@linaro.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.b...@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..ad35725 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_notifier_register_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_notifier_register_done();
+       return err;
 }
 early_initcall(dummy_timer_register);

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