/* CPU control.
   * (C) 2001, 2002, 2003, 2004 Rusty Russell
   *
   * This code is licenced under the GPL.
   */

  #include <linux/sched/mm.h>

  #include <linux/proc_fs.h>
  #include <linux/smp.h>
  #include <linux/init.h>
  #include <linux/notifier.h>

  #include <linux/sched/signal.h>

  #include <linux/sched/hotplug.h>

  #include <linux/sched/isolation.h>

  #include <linux/sched/task.h>

  #include <linux/sched/smt.h>

  #include <linux/unistd.h>
  #include <linux/cpu.h>

  #include <linux/oom.h>
  #include <linux/rcupdate.h>

  #include <linux/export.h>

  #include <linux/bug.h>

  #include <linux/kthread.h>
  #include <linux/stop_machine.h>

  #include <linux/mutex.h>

  #include <linux/gfp.h>

  #include <linux/suspend.h>

  #include <linux/lockdep.h>

  #include <linux/tick.h>

  #include <linux/irq.h>

  #include <linux/nmi.h>

  #include <linux/smpboot.h>

  #include <linux/relay.h>

  #include <linux/slab.h>

  #include <linux/percpu-rwsem.h>

  #include <trace/events/power.h>

  #define CREATE_TRACE_POINTS
  #include <trace/events/cpuhp.h>

  #include "smpboot.h"

  /**
   * cpuhp_cpu_state - Per cpu hotplug state storage
   * @state:	The current cpu state
   * @target:	The target state

   * @thread:	Pointer to the hotplug thread
   * @should_run:	Thread should execute

   * @rollback:	Perform a rollback

   * @single:	Single callback invocation
   * @bringup:	Single callback bringup or teardown selector
   * @cb_state:	The state for a single callback (install/uninstall)

   * @result:	Result of the operation

   * @done_up:	Signal completion to the issuer of the task for cpu-up
   * @done_down:	Signal completion to the issuer of the task for cpu-down

   */
  struct cpuhp_cpu_state {
  	enum cpuhp_state	state;
  	enum cpuhp_state	target;

  	enum cpuhp_state	fail;

  #ifdef CONFIG_SMP
  	struct task_struct	*thread;
  	bool			should_run;

  	bool			rollback;

  	bool			single;
  	bool			bringup;

  	struct hlist_node	*node;

  	struct hlist_node	*last;

  	enum cpuhp_state	cb_state;

  	int			result;

  	struct completion	done_up;
  	struct completion	done_down;

  #endif

  };

  static DEFINE_PER_CPU(struct cpuhp_cpu_state, cpuhp_state) = {
  	.fail = CPUHP_INVALID,
  };

  #ifdef CONFIG_SMP
  cpumask_t cpus_booted_once_mask;
  #endif

  #if defined(CONFIG_LOCKDEP) && defined(CONFIG_SMP)

  static struct lockdep_map cpuhp_state_up_map =
  	STATIC_LOCKDEP_MAP_INIT("cpuhp_state-up", &cpuhp_state_up_map);
  static struct lockdep_map cpuhp_state_down_map =
  	STATIC_LOCKDEP_MAP_INIT("cpuhp_state-down", &cpuhp_state_down_map);

  static inline void cpuhp_lock_acquire(bool bringup)

  {
  	lock_map_acquire(bringup ? &cpuhp_state_up_map : &cpuhp_state_down_map);
  }

  static inline void cpuhp_lock_release(bool bringup)

  {
  	lock_map_release(bringup ? &cpuhp_state_up_map : &cpuhp_state_down_map);
  }
  #else

  static inline void cpuhp_lock_acquire(bool bringup) { }
  static inline void cpuhp_lock_release(bool bringup) { }

  #endif

  /**
   * cpuhp_step - Hotplug state machine step
   * @name:	Name of the step
   * @startup:	Startup function of the step
   * @teardown:	Teardown function of the step

   * @cant_stop:	Bringup/teardown can't be stopped at this step

   */
  struct cpuhp_step {

  	const char		*name;
  	union {

  		int		(*single)(unsigned int cpu);
  		int		(*multi)(unsigned int cpu,
  					 struct hlist_node *node);
  	} startup;

  	union {

  		int		(*single)(unsigned int cpu);
  		int		(*multi)(unsigned int cpu,
  					 struct hlist_node *node);
  	} teardown;

  	struct hlist_head	list;

  	bool			cant_stop;
  	bool			multi_instance;

  };

  static DEFINE_MUTEX(cpuhp_state_mutex);

  static struct cpuhp_step cpuhp_hp_states[];

  static struct cpuhp_step *cpuhp_get_step(enum cpuhp_state state)
  {

  	return cpuhp_hp_states + state;

  }

  /**
   * cpuhp_invoke_callback _ Invoke the callbacks for a given state
   * @cpu:	The cpu for which the callback should be invoked

   * @state:	The state to do callbacks for

   * @bringup:	True if the bringup callback should be invoked

   * @node:	For multi-instance, do a single entry callback for install/remove
   * @lastp:	For multi-instance rollback, remember how far we got

   *

   * Called from cpu hotplug and from the state register machinery.

   */

  static int cpuhp_invoke_callback(unsigned int cpu, enum cpuhp_state state,

  				 bool bringup, struct hlist_node *node,
  				 struct hlist_node **lastp)

  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);

  	struct cpuhp_step *step = cpuhp_get_step(state);

  	int (*cbm)(unsigned int cpu, struct hlist_node *node);
  	int (*cb)(unsigned int cpu);
  	int ret, cnt;

  	if (st->fail == state) {
  		st->fail = CPUHP_INVALID;
  
  		if (!(bringup ? step->startup.single : step->teardown.single))
  			return 0;
  
  		return -EAGAIN;
  	}

  	if (!step->multi_instance) {

  		WARN_ON_ONCE(lastp && *lastp);

  		cb = bringup ? step->startup.single : step->teardown.single;

  		if (!cb)
  			return 0;

  		trace_cpuhp_enter(cpu, st->target, state, cb);

  		ret = cb(cpu);

  		trace_cpuhp_exit(cpu, st->state, state, ret);

  		return ret;
  	}

  	cbm = bringup ? step->startup.multi : step->teardown.multi;

  	if (!cbm)
  		return 0;
  
  	/* Single invocation for instance add/remove */
  	if (node) {

  		WARN_ON_ONCE(lastp && *lastp);

  		trace_cpuhp_multi_enter(cpu, st->target, state, cbm, node);
  		ret = cbm(cpu, node);
  		trace_cpuhp_exit(cpu, st->state, state, ret);
  		return ret;
  	}
  
  	/* State transition. Invoke on all instances */
  	cnt = 0;
  	hlist_for_each(node, &step->list) {

  		if (lastp && node == *lastp)
  			break;

  		trace_cpuhp_multi_enter(cpu, st->target, state, cbm, node);
  		ret = cbm(cpu, node);
  		trace_cpuhp_exit(cpu, st->state, state, ret);

  		if (ret) {
  			if (!lastp)
  				goto err;
  
  			*lastp = node;
  			return ret;
  		}

  		cnt++;
  	}

  	if (lastp)
  		*lastp = NULL;

  	return 0;
  err:
  	/* Rollback the instances if one failed */

  	cbm = !bringup ? step->startup.multi : step->teardown.multi;

  	if (!cbm)
  		return ret;
  
  	hlist_for_each(node, &step->list) {
  		if (!cnt--)
  			break;

  
  		trace_cpuhp_multi_enter(cpu, st->target, state, cbm, node);
  		ret = cbm(cpu, node);
  		trace_cpuhp_exit(cpu, st->state, state, ret);
  		/*
  		 * Rollback must not fail,
  		 */
  		WARN_ON_ONCE(ret);

  	}
  	return ret;
  }

  #ifdef CONFIG_SMP

  static bool cpuhp_is_ap_state(enum cpuhp_state state)
  {
  	/*
  	 * The extra check for CPUHP_TEARDOWN_CPU is only for documentation
  	 * purposes as that state is handled explicitly in cpu_down.
  	 */
  	return state > CPUHP_BRINGUP_CPU && state != CPUHP_TEARDOWN_CPU;
  }

  static inline void wait_for_ap_thread(struct cpuhp_cpu_state *st, bool bringup)
  {
  	struct completion *done = bringup ? &st->done_up : &st->done_down;
  	wait_for_completion(done);
  }
  
  static inline void complete_ap_thread(struct cpuhp_cpu_state *st, bool bringup)
  {
  	struct completion *done = bringup ? &st->done_up : &st->done_down;
  	complete(done);
  }
  
  /*
   * The former STARTING/DYING states, ran with IRQs disabled and must not fail.
   */
  static bool cpuhp_is_atomic_state(enum cpuhp_state state)
  {
  	return CPUHP_AP_IDLE_DEAD <= state && state < CPUHP_AP_ONLINE;
  }

  /* Serializes the updates to cpu_online_mask, cpu_present_mask */

  static DEFINE_MUTEX(cpu_add_remove_lock);

  bool cpuhp_tasks_frozen;
  EXPORT_SYMBOL_GPL(cpuhp_tasks_frozen);

  /*

   * The following two APIs (cpu_maps_update_begin/done) must be used when
   * attempting to serialize the updates to cpu_online_mask & cpu_present_mask.

   */
  void cpu_maps_update_begin(void)
  {
  	mutex_lock(&cpu_add_remove_lock);
  }
  
  void cpu_maps_update_done(void)
  {
  	mutex_unlock(&cpu_add_remove_lock);
  }

  /*
   * If set, cpu_up and cpu_down will return -EBUSY and do nothing.

   * Should always be manipulated under cpu_add_remove_lock
   */
  static int cpu_hotplug_disabled;

  #ifdef CONFIG_HOTPLUG_CPU

  DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);

  void cpus_read_lock(void)

  {

  	percpu_down_read(&cpu_hotplug_lock);

  }

  EXPORT_SYMBOL_GPL(cpus_read_lock);

  int cpus_read_trylock(void)
  {
  	return percpu_down_read_trylock(&cpu_hotplug_lock);
  }
  EXPORT_SYMBOL_GPL(cpus_read_trylock);

  void cpus_read_unlock(void)

  {

  	percpu_up_read(&cpu_hotplug_lock);

  }

  EXPORT_SYMBOL_GPL(cpus_read_unlock);

  void cpus_write_lock(void)

  {

  	percpu_down_write(&cpu_hotplug_lock);

  }

  void cpus_write_unlock(void)

  {

  	percpu_up_write(&cpu_hotplug_lock);

  }

  void lockdep_assert_cpus_held(void)

  {

  	/*
  	 * We can't have hotplug operations before userspace starts running,
  	 * and some init codepaths will knowingly not take the hotplug lock.
  	 * This is all valid, so mute lockdep until it makes sense to report
  	 * unheld locks.
  	 */
  	if (system_state < SYSTEM_RUNNING)
  		return;

  	percpu_rwsem_assert_held(&cpu_hotplug_lock);

  }

  static void lockdep_acquire_cpus_lock(void)
  {

  	rwsem_acquire(&cpu_hotplug_lock.dep_map, 0, 0, _THIS_IP_);

  }
  
  static void lockdep_release_cpus_lock(void)
  {

  	rwsem_release(&cpu_hotplug_lock.dep_map, _THIS_IP_);

  }

  /*
   * Wait for currently running CPU hotplug operations to complete (if any) and
   * disable future CPU hotplug (from sysfs). The 'cpu_add_remove_lock' protects
   * the 'cpu_hotplug_disabled' flag. The same lock is also acquired by the
   * hotplug path before performing hotplug operations. So acquiring that lock
   * guarantees mutual exclusion from any currently running hotplug operations.
   */
  void cpu_hotplug_disable(void)
  {
  	cpu_maps_update_begin();

  	cpu_hotplug_disabled++;

  	cpu_maps_update_done();
  }

  EXPORT_SYMBOL_GPL(cpu_hotplug_disable);

  static void __cpu_hotplug_enable(void)
  {
  	if (WARN_ONCE(!cpu_hotplug_disabled, "Unbalanced cpu hotplug enable
  "))
  		return;
  	cpu_hotplug_disabled--;
  }

  void cpu_hotplug_enable(void)
  {
  	cpu_maps_update_begin();

  	__cpu_hotplug_enable();

  	cpu_maps_update_done();
  }

  EXPORT_SYMBOL_GPL(cpu_hotplug_enable);

  
  #else
  
  static void lockdep_acquire_cpus_lock(void)
  {
  }
  
  static void lockdep_release_cpus_lock(void)
  {
  }

  #endif	/* CONFIG_HOTPLUG_CPU */

  /*
   * Architectures that need SMT-specific errata handling during SMT hotplug
   * should override this.
   */
  void __weak arch_smt_update(void) { }

  #ifdef CONFIG_HOTPLUG_SMT
  enum cpuhp_smt_control cpu_smt_control __read_mostly = CPU_SMT_ENABLED;

  void __init cpu_smt_disable(bool force)

  {

  	if (!cpu_smt_possible())

  		return;
  
  	if (force) {

  		pr_info("SMT: Force disabled
  ");
  		cpu_smt_control = CPU_SMT_FORCE_DISABLED;

  	} else {

  		pr_info("SMT: disabled
  ");

  		cpu_smt_control = CPU_SMT_DISABLED;

  	}

  }

  /*
   * The decision whether SMT is supported can only be done after the full

   * CPU identification. Called from architecture code.

   */
  void __init cpu_smt_check_topology(void)
  {

  	if (!topology_smt_supported())

  		cpu_smt_control = CPU_SMT_NOT_SUPPORTED;
  }

  static int __init smt_cmdline_disable(char *str)
  {
  	cpu_smt_disable(str && !strcmp(str, "force"));

  	return 0;
  }
  early_param("nosmt", smt_cmdline_disable);
  
  static inline bool cpu_smt_allowed(unsigned int cpu)
  {

  	if (cpu_smt_control == CPU_SMT_ENABLED)

  		return true;

  	if (topology_is_primary_thread(cpu))

  		return true;
  
  	/*
  	 * On x86 it's required to boot all logical CPUs at least once so
  	 * that the init code can get a chance to set CR4.MCE on each

  	 * CPU. Otherwise, a broadcasted MCE observing CR4.MCE=0b on any

  	 * core will shutdown the machine.
  	 */

  	return !cpumask_test_cpu(cpu, &cpus_booted_once_mask);

  }

  
  /* Returns true if SMT is not supported of forcefully (irreversibly) disabled */
  bool cpu_smt_possible(void)
  {
  	return cpu_smt_control != CPU_SMT_FORCE_DISABLED &&
  		cpu_smt_control != CPU_SMT_NOT_SUPPORTED;
  }
  EXPORT_SYMBOL_GPL(cpu_smt_possible);

  #else
  static inline bool cpu_smt_allowed(unsigned int cpu) { return true; }
  #endif

  static inline enum cpuhp_state
  cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
  {
  	enum cpuhp_state prev_state = st->state;
  
  	st->rollback = false;
  	st->last = NULL;
  
  	st->target = target;
  	st->single = false;
  	st->bringup = st->state < target;
  
  	return prev_state;
  }
  
  static inline void
  cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
  {
  	st->rollback = true;
  
  	/*
  	 * If we have st->last we need to undo partial multi_instance of this
  	 * state first. Otherwise start undo at the previous state.
  	 */
  	if (!st->last) {
  		if (st->bringup)
  			st->state--;
  		else
  			st->state++;
  	}
  
  	st->target = prev_state;
  	st->bringup = !st->bringup;
  }
  
  /* Regular hotplug invocation of the AP hotplug thread */
  static void __cpuhp_kick_ap(struct cpuhp_cpu_state *st)
  {
  	if (!st->single && st->state == st->target)
  		return;
  
  	st->result = 0;
  	/*
  	 * Make sure the above stores are visible before should_run becomes
  	 * true. Paired with the mb() above in cpuhp_thread_fun()
  	 */
  	smp_mb();
  	st->should_run = true;
  	wake_up_process(st->thread);

  	wait_for_ap_thread(st, st->bringup);

  }
  
  static int cpuhp_kick_ap(struct cpuhp_cpu_state *st, enum cpuhp_state target)
  {
  	enum cpuhp_state prev_state;
  	int ret;
  
  	prev_state = cpuhp_set_state(st, target);
  	__cpuhp_kick_ap(st);
  	if ((ret = st->result)) {
  		cpuhp_reset_state(st, prev_state);
  		__cpuhp_kick_ap(st);
  	}
  
  	return ret;
  }

  static int bringup_wait_for_ap(unsigned int cpu)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);

  	/* Wait for the CPU to reach CPUHP_AP_ONLINE_IDLE */

  	wait_for_ap_thread(st, true);

  	if (WARN_ON_ONCE((!cpu_online(cpu))))
  		return -ECANCELED;

  	/* Unpark the hotplug thread of the target cpu */

  	kthread_unpark(st->thread);

  	/*
  	 * SMT soft disabling on X86 requires to bring the CPU out of the
  	 * BIOS 'wait for SIPI' state in order to set the CR4.MCE bit.  The

  	 * CPU marked itself as booted_once in notify_cpu_starting() so the

  	 * cpu_smt_allowed() check will now return false if this is not the
  	 * primary sibling.
  	 */
  	if (!cpu_smt_allowed(cpu))
  		return -ECANCELED;

  	if (st->target <= CPUHP_AP_ONLINE_IDLE)
  		return 0;
  
  	return cpuhp_kick_ap(st, st->target);

  }

  static int bringup_cpu(unsigned int cpu)
  {
  	struct task_struct *idle = idle_thread_get(cpu);
  	int ret;

  	/*
  	 * Some architectures have to walk the irq descriptors to
  	 * setup the vector space for the cpu which comes online.
  	 * Prevent irq alloc/free across the bringup.
  	 */
  	irq_lock_sparse();

  	/* Arch-specific enabling code. */
  	ret = __cpu_up(cpu, idle);

  	irq_unlock_sparse();

  	if (ret)

  		return ret;

  	return bringup_wait_for_ap(cpu);

  }

  static int finish_cpu(unsigned int cpu)
  {
  	struct task_struct *idle = idle_thread_get(cpu);
  	struct mm_struct *mm = idle->active_mm;
  
  	/*
  	 * idle_task_exit() will have switched to &init_mm, now
  	 * clean up any remaining active_mm state.
  	 */
  	if (mm != &init_mm)
  		idle->active_mm = &init_mm;
  	mmdrop(mm);
  	return 0;
  }

  /*
   * Hotplug state machine related functions
   */

  static void undo_cpu_up(unsigned int cpu, struct cpuhp_cpu_state *st)

  {

  	for (st->state--; st->state > st->target; st->state--)
  		cpuhp_invoke_callback(cpu, st->state, false, NULL, NULL);

  }

  static inline bool can_rollback_cpu(struct cpuhp_cpu_state *st)
  {
  	if (IS_ENABLED(CONFIG_HOTPLUG_CPU))
  		return true;
  	/*
  	 * When CPU hotplug is disabled, then taking the CPU down is not
  	 * possible because takedown_cpu() and the architecture and
  	 * subsystem specific mechanisms are not available. So the CPU
  	 * which would be completely unplugged again needs to stay around
  	 * in the current state.
  	 */
  	return st->state <= CPUHP_BRINGUP_CPU;
  }

  static int cpuhp_up_callbacks(unsigned int cpu, struct cpuhp_cpu_state *st,

  			      enum cpuhp_state target)

  {
  	enum cpuhp_state prev_state = st->state;
  	int ret = 0;
  
  	while (st->state < target) {

  		st->state++;

  		ret = cpuhp_invoke_callback(cpu, st->state, true, NULL, NULL);

  		if (ret) {

  			if (can_rollback_cpu(st)) {
  				st->target = prev_state;
  				undo_cpu_up(cpu, st);
  			}

  			break;
  		}
  	}
  	return ret;
  }

  /*
   * The cpu hotplug threads manage the bringup and teardown of the cpus
   */
  static void cpuhp_create(unsigned int cpu)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);

  	init_completion(&st->done_up);
  	init_completion(&st->done_down);

  }
  
  static int cpuhp_should_run(unsigned int cpu)
  {
  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
  
  	return st->should_run;
  }

  /*
   * Execute teardown/startup callbacks on the plugged cpu. Also used to invoke
   * callbacks when a state gets [un]installed at runtime.

   *
   * Each invocation of this function by the smpboot thread does a single AP
   * state callback.
   *
   * It has 3 modes of operation:
   *  - single: runs st->cb_state
   *  - up:     runs ++st->state, while st->state < st->target
   *  - down:   runs st->state--, while st->state > st->target
   *
   * When complete or on error, should_run is cleared and the completion is fired.

   */
  static void cpuhp_thread_fun(unsigned int cpu)
  {
  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);

  	bool bringup = st->bringup;
  	enum cpuhp_state state;

  	if (WARN_ON_ONCE(!st->should_run))
  		return;

  	/*

  	 * ACQUIRE for the cpuhp_should_run() load of ->should_run. Ensures
  	 * that if we see ->should_run we also see the rest of the state.

  	 */
  	smp_mb();

  	/*
  	 * The BP holds the hotplug lock, but we're now running on the AP,
  	 * ensure that anybody asserting the lock is held, will actually find
  	 * it so.
  	 */
  	lockdep_acquire_cpus_lock();

  	cpuhp_lock_acquire(bringup);

  	if (st->single) {

  		state = st->cb_state;
  		st->should_run = false;
  	} else {
  		if (bringup) {
  			st->state++;
  			state = st->state;
  			st->should_run = (st->state < st->target);
  			WARN_ON_ONCE(st->state > st->target);

  		} else {

  			state = st->state;
  			st->state--;
  			st->should_run = (st->state > st->target);
  			WARN_ON_ONCE(st->state < st->target);

  		}

  	}
  
  	WARN_ON_ONCE(!cpuhp_is_ap_state(state));

  	if (cpuhp_is_atomic_state(state)) {
  		local_irq_disable();
  		st->result = cpuhp_invoke_callback(cpu, state, bringup, st->node, &st->last);
  		local_irq_enable();

  		/*
  		 * STARTING/DYING must not fail!
  		 */
  		WARN_ON_ONCE(st->result);

  	} else {

  		st->result = cpuhp_invoke_callback(cpu, state, bringup, st->node, &st->last);
  	}
  
  	if (st->result) {
  		/*
  		 * If we fail on a rollback, we're up a creek without no
  		 * paddle, no way forward, no way back. We loose, thanks for
  		 * playing.
  		 */
  		WARN_ON_ONCE(st->rollback);
  		st->should_run = false;

  	}

  	cpuhp_lock_release(bringup);

  	lockdep_release_cpus_lock();

  
  	if (!st->should_run)

  		complete_ap_thread(st, bringup);

  }
  
  /* Invoke a single callback on a remote cpu */

  static int

  cpuhp_invoke_ap_callback(int cpu, enum cpuhp_state state, bool bringup,
  			 struct hlist_node *node)

  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);

  	int ret;

  
  	if (!cpu_online(cpu))
  		return 0;

  	cpuhp_lock_acquire(false);
  	cpuhp_lock_release(false);
  
  	cpuhp_lock_acquire(true);
  	cpuhp_lock_release(true);

  	/*
  	 * If we are up and running, use the hotplug thread. For early calls
  	 * we invoke the thread function directly.
  	 */
  	if (!st->thread)

  		return cpuhp_invoke_callback(cpu, state, bringup, node, NULL);

  	st->rollback = false;
  	st->last = NULL;
  
  	st->node = node;
  	st->bringup = bringup;

  	st->cb_state = state;

  	st->single = true;

  	__cpuhp_kick_ap(st);

  	/*

  	 * If we failed and did a partial, do a rollback.

  	 */

  	if ((ret = st->result) && st->last) {
  		st->rollback = true;
  		st->bringup = !bringup;
  
  		__cpuhp_kick_ap(st);
  	}

  	/*
  	 * Clean up the leftovers so the next hotplug operation wont use stale
  	 * data.
  	 */
  	st->node = st->last = NULL;

  	return ret;

  }
  
  static int cpuhp_kick_ap_work(unsigned int cpu)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);

  	enum cpuhp_state prev_state = st->state;
  	int ret;

  	cpuhp_lock_acquire(false);
  	cpuhp_lock_release(false);
  
  	cpuhp_lock_acquire(true);
  	cpuhp_lock_release(true);

  
  	trace_cpuhp_enter(cpu, st->target, prev_state, cpuhp_kick_ap_work);
  	ret = cpuhp_kick_ap(st, st->target);
  	trace_cpuhp_exit(cpu, st->state, prev_state, ret);
  
  	return ret;

  }
  
  static struct smp_hotplug_thread cpuhp_threads = {
  	.store			= &cpuhp_state.thread,
  	.create			= &cpuhp_create,
  	.thread_should_run	= cpuhp_should_run,
  	.thread_fn		= cpuhp_thread_fun,
  	.thread_comm		= "cpuhp/%u",
  	.selfparking		= true,
  };
  
  void __init cpuhp_threads_init(void)
  {
  	BUG_ON(smpboot_register_percpu_thread(&cpuhp_threads));
  	kthread_unpark(this_cpu_read(cpuhp_state.thread));
  }

  #ifdef CONFIG_HOTPLUG_CPU

  #ifndef arch_clear_mm_cpumask_cpu
  #define arch_clear_mm_cpumask_cpu(cpu, mm) cpumask_clear_cpu(cpu, mm_cpumask(mm))
  #endif

  /**
   * clear_tasks_mm_cpumask - Safely clear tasks' mm_cpumask for a CPU
   * @cpu: a CPU id
   *
   * This function walks all processes, finds a valid mm struct for each one and
   * then clears a corresponding bit in mm's cpumask.  While this all sounds
   * trivial, there are various non-obvious corner cases, which this function
   * tries to solve in a safe manner.
   *
   * Also note that the function uses a somewhat relaxed locking scheme, so it may
   * be called only for an already offlined CPU.
   */

  void clear_tasks_mm_cpumask(int cpu)
  {
  	struct task_struct *p;
  
  	/*
  	 * This function is called after the cpu is taken down and marked
  	 * offline, so its not like new tasks will ever get this cpu set in
  	 * their mm mask. -- Peter Zijlstra
  	 * Thus, we may use rcu_read_lock() here, instead of grabbing
  	 * full-fledged tasklist_lock.
  	 */

  	WARN_ON(cpu_online(cpu));

  	rcu_read_lock();
  	for_each_process(p) {
  		struct task_struct *t;

  		/*
  		 * Main thread might exit, but other threads may still have
  		 * a valid mm. Find one.
  		 */

  		t = find_lock_task_mm(p);
  		if (!t)
  			continue;

  		arch_clear_mm_cpumask_cpu(cpu, t->mm);

  		task_unlock(t);
  	}
  	rcu_read_unlock();
  }

  /* Take this CPU down. */

  static int take_cpu_down(void *_param)

  {

  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
  	enum cpuhp_state target = max((int)st->target, CPUHP_AP_OFFLINE);

  	int err, cpu = smp_processor_id();

  	int ret;

  	/* Ensure this CPU doesn't handle any more interrupts. */
  	err = __cpu_disable();
  	if (err < 0)

  		return err;

  	/*
  	 * We get here while we are in CPUHP_TEARDOWN_CPU state and we must not
  	 * do this step again.
  	 */
  	WARN_ON(st->state != CPUHP_TEARDOWN_CPU);
  	st->state--;

  	/* Invoke the former CPU_DYING callbacks */

  	for (; st->state > target; st->state--) {
  		ret = cpuhp_invoke_callback(cpu, st->state, false, NULL, NULL);
  		/*
  		 * DYING must not fail!
  		 */
  		WARN_ON_ONCE(ret);
  	}

  	/* Give up timekeeping duties */
  	tick_handover_do_timer();

  	/* Remove CPU from timer broadcasting */
  	tick_offline_cpu(cpu);

  	/* Park the stopper thread */

  	stop_machine_park(cpu);

  	return 0;

  }

  static int takedown_cpu(unsigned int cpu)

  {

  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);

  	int err;

  	/* Park the smpboot threads */

  	kthread_park(per_cpu_ptr(&cpuhp_state, cpu)->thread);

  	/*

  	 * 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_cpuslocked(take_cpu_down, NULL, cpumask_of(cpu));

  	if (err) {

  		/* CPU refused to die */

  		irq_unlock_sparse();

  		/* Unpark the hotplug thread so we can rollback there */
  		kthread_unpark(per_cpu_ptr(&cpuhp_state, cpu)->thread);

  		return err;

  	}

  	BUG_ON(cpu_online(cpu));

  	/*

  	 * The teardown callback for CPUHP_AP_SCHED_STARTING 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.

  	 */

  	wait_for_ap_thread(st, false);

  	BUG_ON(st->state != CPUHP_AP_IDLE_DEAD);

  	/* 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);

  	tick_cleanup_dead_cpu(cpu);

  	rcutree_migrate_callbacks(cpu);

  	return 0;
  }

  static void cpuhp_complete_idle_dead(void *arg)
  {
  	struct cpuhp_cpu_state *st = arg;

  	complete_ap_thread(st, false);

  }

  void cpuhp_report_idle_dead(void)
  {
  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
  
  	BUG_ON(st->state != CPUHP_AP_OFFLINE);

  	rcu_report_dead(smp_processor_id());

  	st->state = CPUHP_AP_IDLE_DEAD;
  	/*
  	 * We cannot call complete after rcu_report_dead() so we delegate it
  	 * to an online cpu.
  	 */
  	smp_call_function_single(cpumask_first(cpu_online_mask),
  				 cpuhp_complete_idle_dead, st, 0);

  }

  static void undo_cpu_down(unsigned int cpu, struct cpuhp_cpu_state *st)
  {

  	for (st->state++; st->state < st->target; st->state++)
  		cpuhp_invoke_callback(cpu, st->state, true, NULL, NULL);

  }
  
  static int cpuhp_down_callbacks(unsigned int cpu, struct cpuhp_cpu_state *st,
  				enum cpuhp_state target)
  {
  	enum cpuhp_state prev_state = st->state;
  	int ret = 0;
  
  	for (; st->state > target; st->state--) {
  		ret = cpuhp_invoke_callback(cpu, st->state, false, NULL, NULL);
  		if (ret) {
  			st->target = prev_state;

  			if (st->state < prev_state)
  				undo_cpu_down(cpu, st);

  			break;
  		}
  	}
  	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;

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

  	if (!cpu_present(cpu))

  		return -EINVAL;

  	cpus_write_lock();

  
  	cpuhp_tasks_frozen = tasks_frozen;

  	prev_state = cpuhp_set_state(st, 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) {

  		st->target = max((int)target, 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;

  
  		st->target = target;

  	}
  	/*

  	 * The AP brought itself down to CPUHP_TEARDOWN_CPU. So we need

  	 * to do the further cleanups.
  	 */

  	ret = cpuhp_down_callbacks(cpu, st, target);

  	if (ret && st->state == CPUHP_TEARDOWN_CPU && st->state < prev_state) {

  		cpuhp_reset_state(st, prev_state);
  		__cpuhp_kick_ap(st);

  	}

  out:

  	cpus_write_unlock();

  	/*
  	 * Do post unplug cleanup. This is still protected against
  	 * concurrent CPU hotplug via cpu_add_remove_lock.
  	 */
  	lockup_detector_cleanup();

  	arch_smt_update();

  	return ret;

  }

  static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
  {
  	if (cpu_hotplug_disabled)
  		return -EBUSY;
  	return _cpu_down(cpu, 0, target);
  }

  static int cpu_down(unsigned int cpu, enum cpuhp_state target)

  {

  	int err;

  	cpu_maps_update_begin();

  	err = cpu_down_maps_locked(cpu, target);

  	cpu_maps_update_done();

  	return err;
  }

  /**
   * cpu_device_down - Bring down a cpu device
   * @dev: Pointer to the cpu device to offline
   *
   * This function is meant to be used by device core cpu subsystem only.
   *
   * Other subsystems should use remove_cpu() instead.
   */
  int cpu_device_down(struct device *dev)

  {

  	return cpu_down(dev->id, CPUHP_OFFLINE);

  }

  int remove_cpu(unsigned int cpu)
  {
  	int ret;
  
  	lock_device_hotplug();
  	ret = device_offline(get_cpu_device(cpu));
  	unlock_device_hotplug();
  
  	return ret;
  }
  EXPORT_SYMBOL_GPL(remove_cpu);

  void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
  {
  	unsigned int cpu;
  	int error;
  
  	cpu_maps_update_begin();
  
  	/*
  	 * Make certain the cpu I'm about to reboot on is online.
  	 *
  	 * This is inline to what migrate_to_reboot_cpu() already do.
  	 */
  	if (!cpu_online(primary_cpu))
  		primary_cpu = cpumask_first(cpu_online_mask);
  
  	for_each_online_cpu(cpu) {
  		if (cpu == primary_cpu)
  			continue;
  
  		error = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
  		if (error) {
  			pr_err("Failed to offline CPU%d - error=%d",
  				cpu, error);
  			break;
  		}
  	}
  
  	/*
  	 * Ensure all but the reboot CPU are offline.
  	 */
  	BUG_ON(num_online_cpus() > 1);
  
  	/*
  	 * Make sure the CPUs won't be enabled by someone else after this
  	 * point. Kexec will reboot to a new kernel shortly resetting
  	 * everything along the way.
  	 */
  	cpu_hotplug_disabled++;
  
  	cpu_maps_update_done();

  }

  
  #else
  #define takedown_cpu		NULL

  #endif /*CONFIG_HOTPLUG_CPU*/

  /**

   * notify_cpu_starting(cpu) - Invoke the callbacks on the starting CPU

   * @cpu: cpu that just started
   *

   * It must be called by the arch code on the new cpu, before the new cpu
   * enables interrupts and before the "boot" cpu returns from __cpu_up().
   */
  void notify_cpu_starting(unsigned int cpu)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
  	enum cpuhp_state target = min((int)st->target, CPUHP_AP_ONLINE);

  	int ret;

  	rcu_cpu_starting(cpu);	/* Enables RCU usage on this CPU. */

  	cpumask_set_cpu(cpu, &cpus_booted_once_mask);

  	while (st->state < target) {

  		st->state++;

  		ret = cpuhp_invoke_callback(cpu, st->state, true, NULL, NULL);
  		/*
  		 * STARTING must not fail!
  		 */
  		WARN_ON_ONCE(ret);

  	}
  }

  /*

   * Called from the idle task. Wake up the controlling task which brings the

   * hotplug thread of the upcoming CPU up and then delegates the rest of the
   * online bringup to the hotplug thread.

   */

  void cpuhp_online_idle(enum cpuhp_state state)

  {

  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);

  
  	/* Happens for the boot cpu */
  	if (state != CPUHP_AP_ONLINE_IDLE)
  		return;

  	/*
  	 * Unpart the stopper thread before we start the idle loop (and start
  	 * scheduling); this ensures the stopper task is always available.
  	 */
  	stop_machine_unpark(smp_processor_id());

  	st->state = CPUHP_AP_ONLINE_IDLE;

  	complete_ap_thread(st, true);

  }

  /* 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;

  	cpus_write_lock();

  	if (!cpu_present(cpu)) {

  		ret = -EINVAL;
  		goto out;
  	}

  	/*

  	 * The caller of cpu_up() might have raced with another
  	 * caller. Nothing to do.

  	 */
  	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;

  	cpuhp_set_state(st, 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, target);

  out:

  	cpus_write_unlock();

  	arch_smt_update();

  	return ret;
  }

  static int cpu_up(unsigned int cpu, enum cpuhp_state target)

  {
  	int err = 0;

  	if (!cpu_possible(cpu)) {

  		pr_err("can't online cpu %d because it is not configured as may-hotadd at boot time
  ",
  		       cpu);

  #if defined(CONFIG_IA64)

  		pr_err("please check additional_cpus= boot parameter
  ");

  #endif
  		return -EINVAL;
  	}

  	err = try_online_node(cpu_to_node(cpu));
  	if (err)
  		return err;

  	cpu_maps_update_begin();

  
  	if (cpu_hotplug_disabled) {

  		err = -EBUSY;

  		goto out;
  	}

  	if (!cpu_smt_allowed(cpu)) {
  		err = -EPERM;
  		goto out;
  	}

  	err = _cpu_up(cpu, 0, target);

  out:

  	cpu_maps_update_done();

  	return err;
  }

  /**
   * cpu_device_up - Bring up a cpu device
   * @dev: Pointer to the cpu device to online
   *
   * This function is meant to be used by device core cpu subsystem only.
   *
   * Other subsystems should use add_cpu() instead.
   */
  int cpu_device_up(struct device *dev)

  {

  	return cpu_up(dev->id, CPUHP_ONLINE);

  }

  int add_cpu(unsigned int cpu)
  {
  	int ret;
  
  	lock_device_hotplug();
  	ret = device_online(get_cpu_device(cpu));
  	unlock_device_hotplug();
  
  	return ret;
  }
  EXPORT_SYMBOL_GPL(add_cpu);

  /**
   * bringup_hibernate_cpu - Bring up the CPU that we hibernated on
   * @sleep_cpu: The cpu we hibernated on and should be brought up.
   *
   * On some architectures like arm64, we can hibernate on any CPU, but on
   * wake up the CPU we hibernated on might be offline as a side effect of
   * using maxcpus= for example.
   */
  int bringup_hibernate_cpu(unsigned int sleep_cpu)

  {

  	int ret;
  
  	if (!cpu_online(sleep_cpu)) {
  		pr_info("Hibernated on a CPU that is offline! Bringing CPU up.
  ");

  		ret = cpu_up(sleep_cpu, CPUHP_ONLINE);

  		if (ret) {
  			pr_err("Failed to bring hibernate-CPU up!
  ");
  			return ret;
  		}
  	}
  	return 0;
  }

  void bringup_nonboot_cpus(unsigned int setup_max_cpus)
  {
  	unsigned int cpu;
  
  	for_each_present_cpu(cpu) {
  		if (num_online_cpus() >= setup_max_cpus)
  			break;
  		if (!cpu_online(cpu))

  			cpu_up(cpu, CPUHP_ONLINE);

  	}

  }

  #ifdef CONFIG_PM_SLEEP_SMP

  static cpumask_var_t frozen_cpus;

  int freeze_secondary_cpus(int primary)

  {

  	int cpu, error = 0;

  	cpu_maps_update_begin();

  	if (primary == -1) {

  		primary = cpumask_first(cpu_online_mask);

  		if (!housekeeping_cpu(primary, HK_FLAG_TIMER))
  			primary = housekeeping_any_cpu(HK_FLAG_TIMER);
  	} else {
  		if (!cpu_online(primary))
  			primary = cpumask_first(cpu_online_mask);
  	}

  	/*
  	 * We take down all of the non-boot CPUs in one shot to avoid races

  	 * with the userspace trying to use the CPU hotplug at the same time
  	 */

  	cpumask_clear(frozen_cpus);

  	pr_info("Disabling non-boot CPUs ...
  ");

  	for_each_online_cpu(cpu) {

  		if (cpu == primary)

  			continue;

  		if (pm_wakeup_pending()) {

  			pr_info("Wakeup pending. Abort CPU freeze
  ");
  			error = -EBUSY;
  			break;
  		}

  		trace_suspend_resume(TPS("CPU_OFF"), cpu, true);

  		error = _cpu_down(cpu, 1, CPUHP_OFFLINE);

  		trace_suspend_resume(TPS("CPU_OFF"), cpu, false);

  		if (!error)

  			cpumask_set_cpu(cpu, frozen_cpus);

  		else {

  			pr_err("Error taking CPU%d down: %d
  ", cpu, error);

  			break;
  		}
  	}

  	if (!error)

  		BUG_ON(num_online_cpus() > 1);

  	else

  		pr_err("Non-boot CPUs are not disabled
  ");

  
  	/*
  	 * Make sure the CPUs won't be enabled by someone else. We need to do

  	 * this even in case of failure as all freeze_secondary_cpus() users are
  	 * supposed to do thaw_secondary_cpus() on the failure path.

  	 */
  	cpu_hotplug_disabled++;

  	cpu_maps_update_done();

  	return error;
  }

  void __weak arch_thaw_secondary_cpus_begin(void)

  {
  }

  void __weak arch_thaw_secondary_cpus_end(void)

  {
  }

  void thaw_secondary_cpus(void)

  {
  	int cpu, error;
  
  	/* Allow everyone to use the CPU hotplug again */

  	cpu_maps_update_begin();

  	__cpu_hotplug_enable();

  	if (cpumask_empty(frozen_cpus))

  		goto out;

  	pr_info("Enabling non-boot CPUs ...
  ");

  	arch_thaw_secondary_cpus_begin();

  	for_each_cpu(cpu, frozen_cpus) {

  		trace_suspend_resume(TPS("CPU_ON"), cpu, true);

  		error = _cpu_up(cpu, 1, CPUHP_ONLINE);

  		trace_suspend_resume(TPS("CPU_ON"), cpu, false);

  		if (!error) {

  			pr_info("CPU%d is up
  ", cpu);

  			continue;
  		}

  		pr_warn("Error taking CPU%d up: %d
  ", cpu, error);

  	}

  	arch_thaw_secondary_cpus_end();

  	cpumask_clear(frozen_cpus);

  out:

  	cpu_maps_update_done();

  }

  static int __init alloc_frozen_cpus(void)

  {
  	if (!alloc_cpumask_var(&frozen_cpus, GFP_KERNEL|__GFP_ZERO))
  		return -ENOMEM;
  	return 0;
  }
  core_initcall(alloc_frozen_cpus);

  
  /*

   * When callbacks for CPU hotplug notifications are being executed, we must
   * ensure that the state of the system with respect to the tasks being frozen
   * or not, as reported by the notification, remains unchanged *throughout the
   * duration* of the execution of the callbacks.
   * Hence we need to prevent the freezer from racing with regular CPU hotplug.
   *
   * This synchronization is implemented by mutually excluding regular CPU
   * hotplug and Suspend/Hibernate call paths by hooking onto the Suspend/
   * Hibernate notifications.
   */
  static int
  cpu_hotplug_pm_callback(struct notifier_block *nb,
  			unsigned long action, void *ptr)
  {
  	switch (action) {
  
  	case PM_SUSPEND_PREPARE:
  	case PM_HIBERNATION_PREPARE:

  		cpu_hotplug_disable();

  		break;
  
  	case PM_POST_SUSPEND:
  	case PM_POST_HIBERNATION:

  		cpu_hotplug_enable();

  		break;
  
  	default:
  		return NOTIFY_DONE;
  	}
  
  	return NOTIFY_OK;
  }

  static int __init cpu_hotplug_pm_sync_init(void)

  {

  	/*
  	 * cpu_hotplug_pm_callback has higher priority than x86
  	 * bsp_pm_callback which depends on cpu_hotplug_pm_callback
  	 * to disable cpu hotplug to avoid cpu hotplug race.
  	 */

  	pm_notifier(cpu_hotplug_pm_callback, 0);
  	return 0;
  }
  core_initcall(cpu_hotplug_pm_sync_init);

  #endif /* CONFIG_PM_SLEEP_SMP */

  int __boot_cpu_id;

  #endif /* CONFIG_SMP */

  /* Boot processor state steps */

  static struct cpuhp_step cpuhp_hp_states[] = {

  	[CPUHP_OFFLINE] = {
  		.name			= "offline",

  		.startup.single		= NULL,
  		.teardown.single	= NULL,

  	},
  #ifdef CONFIG_SMP
  	[CPUHP_CREATE_THREADS]= {

  		.name			= "threads:prepare",

  		.startup.single		= smpboot_create_threads,
  		.teardown.single	= NULL,

  		.cant_stop		= true,

  	},

  	[CPUHP_PERF_PREPARE] = {

  		.name			= "perf:prepare",
  		.startup.single		= perf_event_init_cpu,
  		.teardown.single	= perf_event_exit_cpu,

  	},

  	[CPUHP_WORKQUEUE_PREP] = {

  		.name			= "workqueue:prepare",
  		.startup.single		= workqueue_prepare_cpu,
  		.teardown.single	= NULL,

  	},

  	[CPUHP_HRTIMERS_PREPARE] = {

  		.name			= "hrtimers:prepare",
  		.startup.single		= hrtimers_prepare_cpu,
  		.teardown.single	= hrtimers_dead_cpu,

  	},

  	[CPUHP_SMPCFD_PREPARE] = {

  		.name			= "smpcfd:prepare",

  		.startup.single		= smpcfd_prepare_cpu,
  		.teardown.single	= smpcfd_dead_cpu,

  	},

  	[CPUHP_RELAY_PREPARE] = {
  		.name			= "relay:prepare",
  		.startup.single		= relay_prepare_cpu,
  		.teardown.single	= NULL,
  	},

  	[CPUHP_SLAB_PREPARE] = {
  		.name			= "slab:prepare",
  		.startup.single		= slab_prepare_cpu,
  		.teardown.single	= slab_dead_cpu,

  	},

  	[CPUHP_RCUTREE_PREP] = {

  		.name			= "RCU/tree:prepare",

  		.startup.single		= rcutree_prepare_cpu,
  		.teardown.single	= rcutree_dead_cpu,

  	},

  	/*

  	 * On the tear-down path, timers_dead_cpu() must be invoked
  	 * before blk_mq_queue_reinit_notify() from notify_dead(),
  	 * otherwise a RCU stall occurs.
  	 */

  	[CPUHP_TIMERS_PREPARE] = {

  		.name			= "timers:prepare",

  		.startup.single		= timers_prepare_cpu,

  		.teardown.single	= timers_dead_cpu,

  	},

  	/* Kicks the plugged cpu into life */

  	[CPUHP_BRINGUP_CPU] = {
  		.name			= "cpu:bringup",

  		.startup.single		= bringup_cpu,

  		.teardown.single	= finish_cpu,

  		.cant_stop		= true,

  	},

  	/* Final state before CPU kills itself */
  	[CPUHP_AP_IDLE_DEAD] = {
  		.name			= "idle:dead",
  	},
  	/*
  	 * Last state before CPU enters the idle loop to die. Transient state
  	 * for synchronization.
  	 */
  	[CPUHP_AP_OFFLINE] = {
  		.name			= "ap:offline",
  		.cant_stop		= true,
  	},

  	/* First state is scheduler control. Interrupts are disabled */
  	[CPUHP_AP_SCHED_STARTING] = {
  		.name			= "sched:starting",

  		.startup.single		= sched_cpu_starting,
  		.teardown.single	= sched_cpu_dying,

  	},