/* Requires cpu_add_remove_lock to be held */
static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
{
	int ret, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;

	if (cpu_online(cpu) || !cpu_present(cpu))
		return -EINVAL;

	cpu_hotplug_begin();
	ret = __cpu_notify(CPU_UP_PREPARE | mod, hcpu, -1, &nr_calls);
	if (ret) {
		nr_calls--;
		printk("%s: attempt to bring up CPU %u failed\n",
				__func__, cpu);
		goto out_notify;
	}

	/* Arch-specific enabling code. */
	ret = __cpu_up(cpu);
	if (ret != 0)
		goto out_notify;
	BUG_ON(!cpu_online(cpu));

	/* Now call notifier in preparation. */
	cpu_notify(CPU_ONLINE | mod, hcpu);

out_notify:
	if (ret != 0)
		__cpu_notify(CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
	cpu_hotplug_done();

	return ret;
}

/* Requires cpu_add_remove_lock to be held */
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
	int err, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
	struct take_cpu_down_param tcd_param = {
		.mod = mod,
		.hcpu = hcpu,
	};

	if (num_online_cpus() == 1)
		return -EBUSY;

	if (!cpu_online(cpu))
		return -EINVAL;

	cpu_hotplug_begin();

	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
	if (err) {
		nr_calls--;
		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
		printk("%s: attempt to take down CPU %u failed\n",
				__func__, cpu);
		goto out_release;
	}

	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
	if (err) {
		/* CPU didn't die: tell everyone.  Can't complain. */
		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);

		goto out_release;
	}
	BUG_ON(cpu_online(cpu));

	/*
	 * The migration_call() CPU_DYING callback will have removed all
	 * runnable tasks from the cpu, there's only the idle task left now
	 * that the migration thread is done doing the stop_machine thing.
	 *
	 * Wait for the stop thread to go away.
	 */
	while (!idle_cpu(cpu))
		cpu_relax();

	/* This actually kills the CPU. */
	__cpu_die(cpu);

	/* CPU is completely dead: tell everyone.  Too late to complain. */
	cpu_notify_nofail(CPU_DEAD | mod, hcpu);

	check_for_tasks(cpu);

out_release:
	cpu_hotplug_done();
	if (!err)
		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
	return err;
}

static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
	int err, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
	struct take_cpu_down_param tcd_param = {
		.mod = mod,
		.hcpu = hcpu,
	};

	if (num_online_cpus() == 1)
		return -EBUSY;

	if (!cpu_online(cpu))
		return -EINVAL;

	cpu_hotplug_begin();

	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
	if (err) {
		nr_calls--;
		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
		printk("%s: attempt to take down CPU %u failed\n",
				__func__, cpu);
		goto out_release;
	}

	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
	if (err) {
		
		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);

		goto out_release;
	}
	BUG_ON(cpu_online(cpu));

	while (!idle_cpu(cpu))
		cpu_relax();

	
	__cpu_die(cpu);

	
	cpu_notify_nofail(CPU_DEAD | mod, hcpu);

	check_for_tasks(cpu);

	#ifdef CONFIG_HTC_ACPU_DEBUG
	{
		unsigned int status = 0;
		msm_proc_comm(PCOM_BACKUP_CPU_STATUS, (unsigned*)&cpu, (unsigned*) &status);
	}
	#endif
out_release:
	cpu_hotplug_done();
	if (!err)
		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
	return err;
}

/* Requires cpu_add_remove_lock to be held */
static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
{
	int ret, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
	struct task_struct *idle;

	cpu_hotplug_begin();

	if (cpu_online(cpu)) {
		ret = 0;
		goto out;
	}

	if (!cpu_present(cpu)) {
		ret = -EINVAL;
		goto out;
	}

	idle = idle_thread_get(cpu);
	if (IS_ERR(idle)) {
		ret = PTR_ERR(idle);
		goto out;
	}

	ret = smpboot_create_threads(cpu);
	if (ret)
		goto out;

	ret = __cpu_notify(CPU_UP_PREPARE | mod, hcpu, -1, &nr_calls);
	if (ret) {
		nr_calls--;
		printk(KERN_WARNING "%s: attempt to bring up CPU %u failed\n",
				__func__, cpu);
		goto out_notify;
	}

	/* Arch-specific enabling code. */
	ret = __cpu_up(cpu, idle);
	if (ret != 0)
		goto out_notify;
	BUG_ON(!cpu_online(cpu));

	/* Wake the per cpu threads */
	smpboot_unpark_threads(cpu);

	/* Now call notifier in preparation. */
	cpu_notify(CPU_ONLINE | mod, hcpu);

out_notify:
	if (ret != 0)
		__cpu_notify(CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
out:
	cpu_hotplug_done();
	trace_sched_cpu_hotplug(cpu, ret, 1);

	return ret;
}

/* Requires cpu_add_remove_lock to be held */
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
			   enum cpuhp_state target)
{
	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
	int prev_state, ret = 0;
	bool hasdied = false;

	if (num_online_cpus() == 1)
		return -EBUSY;

	if (!cpu_present(cpu))
		return -EINVAL;

	cpu_hotplug_begin();

	cpuhp_tasks_frozen = tasks_frozen;

	prev_state = st->state;
	st->target = target;
	/*
	 * If the current CPU state is in the range of the AP hotplug thread,
	 * then we need to kick the thread.
	 */
	if (st->state > CPUHP_TEARDOWN_CPU) {
		ret = cpuhp_kick_ap_work(cpu);
		/*
		 * The AP side has done the error rollback already. Just
		 * return the error code..
		 */
		if (ret)
			goto out;

		/*
		 * We might have stopped still in the range of the AP hotplug
		 * thread. Nothing to do anymore.
		 */
		if (st->state > CPUHP_TEARDOWN_CPU)
			goto out;
	}
	/*
	 * The AP brought itself down to CPUHP_TEARDOWN_CPU. So we need
	 * to do the further cleanups.
	 */
	ret = cpuhp_down_callbacks(cpu, st, cpuhp_bp_states, target);
	if (ret && st->state > CPUHP_TEARDOWN_CPU && st->state < prev_state) {
		st->target = prev_state;
		st->rollback = true;
		cpuhp_kick_ap_work(cpu);
	}

	hasdied = prev_state != st->state && st->state == CPUHP_OFFLINE;
out:
	cpu_hotplug_done();
	/* This post dead nonsense must die */
	if (!ret && hasdied)
		cpu_notify_nofail(CPU_POST_DEAD, cpu);
	return ret;
}

/* Requires cpu_add_remove_lock to be held */
static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
{
	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
	struct task_struct *idle;
	int ret = 0;

	cpu_hotplug_begin();

	if (!cpu_present(cpu)) {
		ret = -EINVAL;
		goto out;
	}

	/*
	 * The caller of do_cpu_up might have raced with another
	 * caller. Ignore it for now.
	 */
	if (st->state >= target)
		goto out;

	if (st->state == CPUHP_OFFLINE) {
		/* Let it fail before we try to bring the cpu up */
		idle = idle_thread_get(cpu);
		if (IS_ERR(idle)) {
			ret = PTR_ERR(idle);
			goto out;
		}
	}

	cpuhp_tasks_frozen = tasks_frozen;

	st->target = target;
	/*
	 * If the current CPU state is in the range of the AP hotplug thread,
	 * then we need to kick the thread once more.
	 */
	if (st->state > CPUHP_BRINGUP_CPU) {
		ret = cpuhp_kick_ap_work(cpu);
		/*
		 * The AP side has done the error rollback already. Just
		 * return the error code..
		 */
		if (ret)
			goto out;
	}

	/*
	 * Try to reach the target state. We max out on the BP at
	 * CPUHP_BRINGUP_CPU. After that the AP hotplug thread is
	 * responsible for bringing it up to the target state.
	 */
	target = min((int)target, CPUHP_BRINGUP_CPU);
	ret = cpuhp_up_callbacks(cpu, st, cpuhp_bp_states, target);
out:
	cpu_hotplug_done();
	return ret;
}

int cpu_down(unsigned int cpu)
{
    int err, notifier_rc;
    void *hcpu = (void *)(long)cpu;
    struct notifier_block *nb = NULL;

    if ( !cpu_hotplug_begin() )
        return -EBUSY;

    if ( (cpu >= NR_CPUS) || (cpu == 0) || !cpu_online(cpu) )
    {
        cpu_hotplug_done();
        return -EINVAL;
    }

    notifier_rc = notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE, hcpu, &nb);
    if ( notifier_rc != NOTIFY_DONE )
    {
        err = notifier_to_errno(notifier_rc);
        goto fail;
    }

    if ( (err = stop_machine_run(take_cpu_down, NULL, cpu)) < 0 )
        goto fail;

    __cpu_die(cpu);
    BUG_ON(cpu_online(cpu));

    notifier_rc = notifier_call_chain(&cpu_chain, CPU_DEAD, hcpu, NULL);
    BUG_ON(notifier_rc != NOTIFY_DONE);

    send_guest_global_virq(dom0, VIRQ_PCPU_STATE);
    cpu_hotplug_done();
    return 0;

 fail:
    notifier_rc = notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED, hcpu, &nb);
    BUG_ON(notifier_rc != NOTIFY_DONE);
    cpu_hotplug_done();
    return err;
}

int cpu_up(unsigned int cpu)
{
    int notifier_rc, err = 0;
    void *hcpu = (void *)(long)cpu;
    struct notifier_block *nb = NULL;

    if ( !cpu_hotplug_begin() )
        return -EBUSY;

    if ( (cpu >= NR_CPUS) || cpu_online(cpu) || !cpu_present(cpu) )
    {
        cpu_hotplug_done();
        return -EINVAL;
    }

    notifier_rc = notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu, &nb);
    if ( notifier_rc != NOTIFY_DONE )
    {
        err = notifier_to_errno(notifier_rc);
        goto fail;
    }

    err = __cpu_up(cpu);
    if ( err < 0 )
        goto fail;

    notifier_rc = notifier_call_chain(&cpu_chain, CPU_ONLINE, hcpu, NULL);
    BUG_ON(notifier_rc != NOTIFY_DONE);

    send_guest_global_virq(dom0, VIRQ_PCPU_STATE);

    cpu_hotplug_done();
    return 0;

 fail:
    notifier_rc = notifier_call_chain(&cpu_chain, CPU_UP_CANCELED, hcpu, &nb);
    BUG_ON(notifier_rc != NOTIFY_DONE);
    cpu_hotplug_done();
    return err;
}

static void acpi_processor_remove(struct acpi_device *device)
{
	struct acpi_processor *pr;

	if (!device || !acpi_driver_data(device))
		return;

	pr = acpi_driver_data(device);
	if (pr->id >= nr_cpu_ids)
		goto out;

	/*
	 * The only reason why we ever get here is CPU hot-removal.  The CPU is
	 * already offline and the ACPI device removal locking prevents it from
	 * being put back online at this point.
	 *
	 * Unbind the driver from the processor device and detach it from the
	 * ACPI companion object.
	 */
	device_release_driver(pr->dev);
	acpi_unbind_one(pr->dev);

	/* Clean up. */
	per_cpu(processor_device_array, pr->id) = NULL;
	per_cpu(processors, pr->id) = NULL;

	cpu_maps_update_begin();
	cpu_hotplug_begin();

	/* Remove the CPU. */
	arch_unregister_cpu(pr->id);
	acpi_unmap_lsapic(pr->id);

	cpu_hotplug_done();
	cpu_maps_update_done();

	try_offline_node(cpu_to_node(pr->id));

 out:
	free_cpumask_var(pr->throttling.shared_cpu_map);
	kfree(pr);
}

static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
{
	int ret, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;

	if (cpu_online(cpu) || !cpu_present(cpu))
		return -EINVAL;

	cpu_hotplug_begin();
	ret = __cpu_notify(CPU_UP_PREPARE | mod, hcpu, -1, &nr_calls);
	if (ret) {
		nr_calls--;
		printk(KERN_WARNING "%s: attempt to bring up CPU %u failed\n",
				__func__, cpu);
		goto out_notify;
	}

	
	ret = __cpu_up(cpu);
	if (ret != 0)
		goto out_notify;
	BUG_ON(!cpu_online(cpu));

	
	cpu_notify(CPU_ONLINE | mod, hcpu);

	#ifdef CONFIG_HTC_ACPU_DEBUG
	{
		unsigned int status = 1;
		msm_proc_comm(PCOM_BACKUP_CPU_STATUS, (unsigned*)&cpu, (unsigned*) &status);
	}
	#endif
out_notify:
	if (ret != 0)
		__cpu_notify(CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
	cpu_hotplug_done();

	return ret;
}

/* Requires cpu_add_remove_lock to be held */
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
	int err, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
	struct take_cpu_down_param tcd_param = {
		.mod = mod,
		.hcpu = hcpu,
	};

	if (num_online_cpus() == 1)
		return -EBUSY;

	if (!cpu_online(cpu))
		return -EINVAL;

	cpu_hotplug_begin();

	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
	if (err) {
		nr_calls--;
		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
		pr_warn("%s: attempt to take down CPU %u failed\n",
			__func__, cpu);
		goto out_release;
	}

	/*
	 * By now we've cleared cpu_active_mask, wait for all preempt-disabled
	 * and RCU users of this state to go away such that all new such users
	 * will observe it.
	 *
	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
	 * not imply sync_sched(), so explicitly call both.
	 *
	 * Do sync before park smpboot threads to take care the rcu boost case.
	 */
#ifdef CONFIG_PREEMPT
	synchronize_sched();
#endif
	synchronize_rcu();

	smpboot_park_threads(cpu);

	/*
	 * So now all preempt/rcu users must observe !cpu_active().
	 */

	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
	if (err) {
		/* CPU didn't die: tell everyone.  Can't complain. */
		smpboot_unpark_threads(cpu);
		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
		goto out_release;
	}
	BUG_ON(cpu_online(cpu));

	/*
	 * The migration_call() CPU_DYING callback will have removed all
	 * runnable tasks from the cpu, there's only the idle task left now
	 * that the migration thread is done doing the stop_machine thing.
	 *
	 * Wait for the stop thread to go away.
	 */
	while (!idle_cpu(cpu))
		cpu_relax();

	/* This actually kills the CPU. */
	__cpu_die(cpu);

	/* CPU is completely dead: tell everyone.  Too late to complain. */
	cpu_notify_nofail(CPU_DEAD | mod, hcpu);

	check_for_tasks(cpu);

out_release:
	cpu_hotplug_done();
	if (!err)
		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
	return err;
}

/* Requires cpu_add_remove_lock to be held */
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
	int mycpu, err, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
	struct take_cpu_down_param tcd_param = {
		.mod = mod,
		.hcpu = hcpu,
	};
	cpumask_var_t cpumask;
	cpumask_var_t cpumask_org;

	if (num_online_cpus() == 1)
		return -EBUSY;

	if (!cpu_online(cpu))
		return -EINVAL;

	/* Move the downtaker off the unplug cpu */
	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
		return -ENOMEM;
	if (!alloc_cpumask_var(&cpumask_org, GFP_KERNEL))  {
		free_cpumask_var(cpumask);
		return -ENOMEM;
	}

	cpumask_copy(cpumask_org, tsk_cpus_allowed(current));
	cpumask_andnot(cpumask, cpu_online_mask, cpumask_of(cpu));
	set_cpus_allowed_ptr(current, cpumask);
	free_cpumask_var(cpumask);
	migrate_disable();
	mycpu = smp_processor_id();
	if (mycpu == cpu) {
		printk(KERN_ERR "Yuck! Still on unplug CPU\n!");
		migrate_enable();
		err = -EBUSY;
		goto restore_cpus;
	}

	cpu_hotplug_begin();
	err = cpu_unplug_begin(cpu);
	if (err) {
		printk("cpu_unplug_begin(%d) failed\n", cpu);
		goto out_cancel;
	}

	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
	if (err) {
		nr_calls--;
		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
		printk("%s: attempt to take down CPU %u failed\n",
				__func__, cpu);
		goto out_release;
	}

	__cpu_unplug_wait(cpu);
	smpboot_park_threads(cpu);

	/* Notifiers are done. Don't let any more tasks pin this CPU. */
	cpu_unplug_sync(cpu);

	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
	if (err) {
		/* CPU didn't die: tell everyone.  Can't complain. */
		smpboot_unpark_threads(cpu);
		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
		goto out_release;
	}
	BUG_ON(cpu_online(cpu));

	/*
	 * The migration_call() CPU_DYING callback will have removed all
	 * runnable tasks from the cpu, there's only the idle task left now
	 * that the migration thread is done doing the stop_machine thing.
	 *
	 * Wait for the stop thread to go away.
	 */
	while (!idle_cpu(cpu))
		cpu_relax();

	/* This actually kills the CPU. */
	__cpu_die(cpu);

	/* CPU is completely dead: tell everyone.  Too late to complain. */
	cpu_notify_nofail(CPU_DEAD | mod, hcpu);

	check_for_tasks(cpu);

out_release:
	cpu_unplug_done(cpu);
out_cancel:
	migrate_enable();
	cpu_hotplug_done();
	if (!err)
		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
restore_cpus:
	set_cpus_allowed_ptr(current, cpumask_org);
	free_cpumask_var(cpumask_org);
	return err;
//.........这里部分代码省略.........

/* Requires cpu_add_remove_lock to be held */
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
	int err, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
	struct take_cpu_down_param tcd_param = {
		.caller = current,
		.mod = mod,
		.hcpu = hcpu,
	};
	unsigned long timeout;
	unsigned long flags;
	struct task_struct *g, *p;

	if (num_online_cpus() == 1)
		return -EBUSY;

	if (!cpu_online(cpu))
		return -EINVAL;

	cpu_hotplug_begin();
	set_cpu_active(cpu, false);
	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
	if (err) {
		set_cpu_active(cpu, true);

		nr_calls--;
		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
		printk("%s: attempt to take down CPU %u failed\n",
				__func__, cpu);
		goto out_release;
	}

	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
	if (err) {
		set_cpu_active(cpu, true);
		/* CPU didn't die: tell everyone.  Can't complain. */
		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);

		goto out_release;
	}
	BUG_ON(cpu_online(cpu));

	timeout = jiffies + HZ;
	/* Wait for it to sleep (leaving idle task). */
	while (!idle_cpu(cpu)) {
		msleep(1);
		if (time_after(jiffies, timeout)) {
			printk("%s: CPU%d not idle after offline. Running tasks:\n", __func__, cpu);
			read_lock_irqsave(&tasklist_lock, flags);
			do_each_thread(g, p) {
				if (!p->se.on_rq || task_cpu(p) != cpu)
					continue;
				sched_show_task(p);
			} while_each_thread(g, p);
			read_unlock_irqrestore(&tasklist_lock, flags);
			timeout = jiffies + HZ;
		}
	}

	/* This actually kills the CPU. */
	__cpu_die(cpu);

	/* CPU is completely dead: tell everyone.  Too late to complain. */
	cpu_notify_nofail(CPU_DEAD | mod, hcpu);

	check_for_tasks(cpu);

out_release:
	cpu_hotplug_done();
	if (!err)
		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
	return err;
}

/* Requires cpu_add_remove_lock to be held */
static int _cpu_down(unsigned int cpu, int tasks_frozen)
{
	int err, nr_calls = 0;
	void *hcpu = (void *)(long)cpu;
	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
	struct take_cpu_down_param tcd_param = {
		.mod = mod,
		.hcpu = hcpu,
	};

	if (num_online_cpus() == 1)
		return -EBUSY;

	if (!cpu_online(cpu))
		return -EINVAL;

	cpu_hotplug_begin();

	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
	if (err) {
		nr_calls--;
		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
		pr_warn("%s: attempt to take down CPU %u failed\n",
			__func__, cpu);
		goto out_release;
	}

	/*
	 * By now we've cleared cpu_active_mask, wait for all preempt-disabled
	 * and RCU users of this state to go away such that all new such users
	 * will observe it.
	 *
	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
	 * not imply sync_sched(), so wait for both.
	 *
	 * Do sync before park smpboot threads to take care the rcu boost case.
	 */
	if (IS_ENABLED(CONFIG_PREEMPT))
		synchronize_rcu_mult(call_rcu, call_rcu_sched);
	else
		synchronize_rcu();

	smpboot_park_threads(cpu);

	/*
	 * Prevent irq alloc/free while the dying cpu reorganizes the
	 * interrupt affinities.
	 */
	irq_lock_sparse();

	/*
	 * So now all preempt/rcu users must observe !cpu_active().
	 */
	err = stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
	if (err) {
		/* CPU didn't die: tell everyone.  Can't complain. */
		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
		irq_unlock_sparse();
		goto out_release;
	}
	BUG_ON(cpu_online(cpu));

	/*
	 * The migration_call() CPU_DYING callback will have removed all
	 * runnable tasks from the cpu, there's only the idle task left now
	 * that the migration thread is done doing the stop_machine thing.
	 *
	 * Wait for the stop thread to go away.
	 */
	while (!per_cpu(cpu_dead_idle, cpu))
		cpu_relax();
	smp_mb(); /* Read from cpu_dead_idle before __cpu_die(). */
	per_cpu(cpu_dead_idle, cpu) = false;

	/* Interrupts are moved away from the dying cpu, reenable alloc/free */
	irq_unlock_sparse();

	hotplug_cpu__broadcast_tick_pull(cpu);
	/* This actually kills the CPU. */
	__cpu_die(cpu);

	/* CPU is completely dead: tell everyone.  Too late to complain. */
	tick_cleanup_dead_cpu(cpu);
	cpu_notify_nofail(CPU_DEAD | mod, hcpu);

	check_for_tasks(cpu);

out_release:
	cpu_hotplug_done();
	if (!err)
		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
	return err;
}