[PATCH v2 16/52] x86, cpuid: Fix CPU hotplug callback registration

["Srivatsa S. Bhat" <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>]

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

Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

 arch/x86/kernel/cpuid.c |   15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpuid.c b/arch/x86/kernel/cpuid.c
index 7d9481c..3225ae6c 100644
--- a/arch/x86/kernel/cpuid.c
+++ b/arch/x86/kernel/cpuid.c
@@ -198,14 +198,15 @@ static int __init cpuid_init(void)
 		goto out_chrdev;
 	}
 	cpuid_class->devnode = cpuid_devnode;
-	get_online_cpus();
+
+	cpu_notifier_register_begin();
 	for_each_online_cpu(i) {
 		err = cpuid_device_create(i);
 		if (err != 0)
 			goto out_class;
 	}
-	register_hotcpu_notifier(&cpuid_class_cpu_notifier);
-	put_online_cpus();
+	__register_hotcpu_notifier(&cpuid_class_cpu_notifier);
+	cpu_notifier_register_done();
 
 	err = 0;
 	goto out;
@@ -215,7 +216,7 @@ out_class:
 	for_each_online_cpu(i) {
 		cpuid_device_destroy(i);
 	}
-	put_online_cpus();
+	cpu_notifier_register_done();
 	class_destroy(cpuid_class);
 out_chrdev:
 	__unregister_chrdev(CPUID_MAJOR, 0, NR_CPUS, "cpu/cpuid");
@@ -227,13 +228,13 @@ static void __exit cpuid_exit(void)
 {
 	int cpu = 0;
 
-	get_online_cpus();
+	cpu_notifier_register_begin();
 	for_each_online_cpu(cpu)
 		cpuid_device_destroy(cpu);
 	class_destroy(cpuid_class);
 	__unregister_chrdev(CPUID_MAJOR, 0, NR_CPUS, "cpu/cpuid");
-	unregister_hotcpu_notifier(&cpuid_class_cpu_notifier);
-	put_online_cpus();
+	__unregister_hotcpu_notifier(&cpuid_class_cpu_notifier);
+	cpu_notifier_register_done();
 }
 
 module_init(cpuid_init);

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