[PATCH 35/51] acpi-cpufreq: Fix CPU hotplug callback registration

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

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 acpi-cpufreq code by using this latter form of callback registration.

Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: cpufreq@vger.kernel.org
Cc: linux-pm@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 drivers/cpufreq/acpi-cpufreq.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
index 18448a7..e2eb471 100644
--- a/drivers/cpufreq/acpi-cpufreq.c
+++ b/drivers/cpufreq/acpi-cpufreq.c
@@ -907,15 +907,16 @@ static void __init acpi_cpufreq_boost_init(void)
 
 		acpi_cpufreq_driver.boost_supported = true;
 		acpi_cpufreq_driver.boost_enabled = boost_state(0);
-		get_online_cpus();
+
+		cpu_maps_update_begin();
 
 		/* Force all MSRs to the same value */
 		boost_set_msrs(acpi_cpufreq_driver.boost_enabled,
 			       cpu_online_mask);
 
-		register_cpu_notifier(&boost_nb);
+		__register_cpu_notifier(&boost_nb);
 
-		put_online_cpus();
+		cpu_maps_update_done();
 	}
 }