x86, kvm: 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 kvm code in x86 by using this latter form of callback registration.

Cc: Gleb Natapov <gleb@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2b8578432d5bccd296fa6d5859e3575c0fe4aa02..4db6f56e7b30b2cc79a6bdcf68b7b29ff04b5527 100644 (file)
--- 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);