[PATCH v3 29/52] x86, kvm: 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 kvm code in x86 by using this latter form of callback registration.

Cc: Gleb Natapov <gleb at kernel.org>
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: kvm at vger.kernel.org
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
---

 arch/x86/kvm/x86.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2b85784..4db6f56 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5365,7 +5365,8 @@ static void kvm_timer_init(void)
 	int cpu;
 
 	max_tsc_khz = tsc_khz;
-	register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
+
+	cpu_notifier_register_begin();
 	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
 #ifdef CONFIG_CPU_FREQ
 		struct cpufreq_policy policy;
@@ -5382,6 +5383,10 @@ static void kvm_timer_init(void)
 	pr_debug("kvm: max_tsc_khz = %ld\n", max_tsc_khz);
 	for_each_online_cpu(cpu)
 		smp_call_function_single(cpu, tsc_khz_changed, NULL, 1);
+
+	__register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
+	cpu_notifier_register_done();
+
 }
 
 static DEFINE_PER_CPU(struct kvm_vcpu *, current_vcpu);