clocksource, dummy-timer: Fix CPU hotplug callback registration

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.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/clocksource/dummy_timer.c b/drivers/clocksource/dummy_timer.c
index b3eb582d6a6f1956bc870067188bb03a090fd30b..ad357254172890ff4170c2a32634f57ad831b26e 100644 (file)
--- 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);