[PATCH v3 20/52] x86, therm_throt.c: Fix CPU hotplug callback registration

[Srivatsa S. Bhat]

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 thermal throttle code in x86 by using this latter form of callback
registration.

Cc: Tony Luck <tony.luck at intel.com>
Cc: Borislav Petkov <bp at alien8.de>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Ingo Molnar <mingo at kernel.org>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: x86 at kernel.org
Cc: linux-edac at vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
---

 arch/x86/kernel/cpu/mcheck/therm_throt.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mcheck/therm_throt.c b/arch/x86/kernel/cpu/mcheck/therm_throt.c
index 3eec7de..e05dfa3 100644
--- a/arch/x86/kernel/cpu/mcheck/therm_throt.c
+++ b/arch/x86/kernel/cpu/mcheck/therm_throt.c
@@ -319,7 +319,7 @@ static __init int thermal_throttle_init_device(void)
 	if (!atomic_read(&therm_throt_en))
 		return 0;
 
-	register_hotcpu_notifier(&thermal_throttle_cpu_notifier);
+	cpu_notifier_register_begin();
 
 #ifdef CONFIG_HOTPLUG_CPU
 	mutex_lock(&therm_cpu_lock);
@@ -333,6 +333,9 @@ static __init int thermal_throttle_init_device(void)
 	mutex_unlock(&therm_cpu_lock);
 #endif
 
+	__register_hotcpu_notifier(&thermal_throttle_cpu_notifier);
+	cpu_notifier_register_done();
+
 	return 0;
 }
 device_initcall(thermal_throttle_init_device);