text: /* CPU control.
   * (C) 2001, 2002, 2003, 2004 Rusty Russell
   *
   * This code is licenced under the GPL.
   */
  #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/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;
  	bool			booted_once;
  	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,
  };
  
  #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)
  {
  	percpu_rwsem_assert_held(&cpu_hotplug_lock);
  }
  
  static void lockdep_acquire_cpus_lock(void)
  {
  	rwsem_acquire(&cpu_hotplug_lock.rw_sem.dep_map, 0, 0, _THIS_IP_);
  }
  
  static void lockdep_release_cpus_lock(void)
  {
  	rwsem_release(&cpu_hotplug_lock.rw_sem.dep_map, 1, _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\n"))
  		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_control == CPU_SMT_FORCE_DISABLED ||
  		cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
  		return;
  
  	if (force) {
  		pr_info("SMT: Force disabled\n");
  		cpu_smt_control = CPU_SMT_FORCE_DISABLED;
  	} else {
  		pr_info("SMT: disabled\n");
  		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 broadacasted MCE observing CR4.MCE=0b on any
  	 * core will shutdown the machine.
  	 */
  	return !per_cpu(cpuhp_state, cpu).booted_once;
  }
  #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 stopper thread and the hotplug thread of the target cpu */
  	stop_machine_unpark(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 cpu_notify_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);
  }
  
  /*
   * 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 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) {
  			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
  /**
   * 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;
  		cpumask_clear_cpu(cpu, mm_cpumask(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();
  	/* 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 do_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;
  }
  
  int cpu_down(unsigned int cpu)
  {
  	return do_cpu_down(cpu, CPUHP_OFFLINE);
  }
  EXPORT_SYMBOL(cpu_down);
  
  #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. */
  	st->booted_once = true;
  	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
   * stopper and 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;
  
  	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 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;
  
  	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 do_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\n",
  		       cpu);
  #if defined(CONFIG_IA64)
  		pr_err("please check additional_cpus= boot parameter\n");
  #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;
  }
  
  int cpu_up(unsigned int cpu)
  {
  	return do_cpu_up(cpu, CPUHP_ONLINE);
  }
  EXPORT_SYMBOL_GPL(cpu_up);
  
  #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 (!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 ...\n");
  	for_each_online_cpu(cpu) {
  		if (cpu == primary)
  			continue;
  		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\n", cpu, error);
  			break;
  		}
  	}
  
  	if (!error)
  		BUG_ON(num_online_cpus() > 1);
  	else
  		pr_err("Non-boot CPUs are not disabled\n");
  
  	/*
  	 * Make sure the CPUs won't be enabled by someone else. We need to do
  	 * this even in case of failure as all disable_nonboot_cpus() users are
  	 * supposed to do enable_nonboot_cpus() on the failure path.
  	 */
  	cpu_hotplug_disabled++;
  
  	cpu_maps_update_done();
  	return error;
  }
  
  void __weak arch_enable_nonboot_cpus_begin(void)
  {
  }
  
  void __weak arch_enable_nonboot_cpus_end(void)
  {
  }
  
  void enable_nonboot_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 ...\n");
  
  	arch_enable_nonboot_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\n", cpu);
  			continue;
  		}
  		pr_warn("Error taking CPU%d up: %d\n", cpu, error);
  	}
  
  	arch_enable_nonboot_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	= NULL,
  		.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,
  	},
  	[CPUHP_AP_RCUTREE_DYING] = {
  		.name			= "RCU/tree:dying",
  		.startup.single		= NULL,
  		.teardown.single	= rcutree_dying_cpu,
  	},
  	[CPUHP_AP_SMPCFD_DYING] = {
  		.name			= "smpcfd:dying",
  		.startup.single		= NULL,
  		.teardown.single	= smpcfd_dying_cpu,
  	},
  	/* Entry state on starting. Interrupts enabled from here on. Transient
  	 * state for synchronsization */
  	[CPUHP_AP_ONLINE] = {
  		.name			= "ap:online",
  	},
  	/*
  	 * Handled on controll processor until the plugged processor manages
  	 * this itself.
  	 */
  	[CPUHP_TEARDOWN_CPU] = {
  		.name			= "cpu:teardown",
  		.startup.single		= NULL,
  		.teardown.single	= takedown_cpu,
  		.cant_stop		= true,
  	},
  	/* Handle smpboot threads park/unpark */
  	[CPUHP_AP_SMPBOOT_THREADS] = {
  		.name			= "smpboot/threads:online",
  		.startup.single		= smpboot_unpark_threads,
  		.teardown.single	= smpboot_park_threads,
  	},
  	[CPUHP_AP_IRQ_AFFINITY_ONLINE] = {
  		.name			= "irq/affinity:online",
  		.startup.single		= irq_affinity_online_cpu,
  		.teardown.single	= NULL,
  	},
  	[CPUHP_AP_PERF_ONLINE] = {
  		.name			= "perf:online",
  		.startup.single		= perf_event_init_cpu,
  		.teardown.single	= perf_event_exit_cpu,
  	},
  	[CPUHP_AP_WATCHDOG_ONLINE] = {
  		.name			= "lockup_detector:online",
  		.startup.single		= lockup_detector_online_cpu,
  		.teardown.single	= lockup_detector_offline_cpu,
  	},
  	[CPUHP_AP_WORKQUEUE_ONLINE] = {
  		.name			= "workqueue:online",
  		.startup.single		= workqueue_online_cpu,
  		.teardown.single	= workqueue_offline_cpu,
  	},
  	[CPUHP_AP_RCUTREE_ONLINE] = {
  		.name			= "RCU/tree:online",
  		.startup.single		= rcutree_online_cpu,
  		.teardown.single	= rcutree_offline_cpu,
  	},
  #endif
  	/*
  	 * The dynamically registered state space is here
  	 */
  
  #ifdef CONFIG_SMP
  	/* Last state is scheduler control setting the cpu active */
  	[CPUHP_AP_ACTIVE] = {
  		.name			= "sched:active",
  		.startup.single		= sched_cpu_activate,
  		.teardown.single	= sched_cpu_deactivate,
  	},
  #endif
  
  	/* CPU is fully up and running. */
  	[CPUHP_ONLINE] = {
  		.name			= "online",
  		.startup.single		= NULL,
  		.teardown.single	= NULL,
  	},
  };
  
  /* Sanity check for callbacks */
  static int cpuhp_cb_check(enum cpuhp_state state)
  {
  	if (state <= CPUHP_OFFLINE || state >= CPUHP_ONLINE)
  		return -EINVAL;
  	return 0;
  }
  
  /*
   * Returns a free for dynamic slot assignment of the Online state. The states
   * are protected by the cpuhp_slot_states mutex and an empty slot is identified
   * by having no name assigned.
   */
  static int cpuhp_reserve_state(enum cpuhp_state state)
  {
  	enum cpuhp_state i, end;
  	struct cpuhp_step *step;
  
  	switch (state) {
  	case CPUHP_AP_ONLINE_DYN:
  		step = cpuhp_hp_states + CPUHP_AP_ONLINE_DYN;
  		end = CPUHP_AP_ONLINE_DYN_END;
  		break;
  	case CPUHP_BP_PREPARE_DYN:
  		step = cpuhp_hp_states + CPUHP_BP_PREPARE_DYN;
  		end = CPUHP_BP_PREPARE_DYN_END;
  		break;
  	default:
  		return -EINVAL;
  	}
  
  	for (i = state; i <= end; i++, step++) {
  		if (!step->name)
  			return i;
  	}
  	WARN(1, "No more dynamic states available for CPU hotplug\n");
  	return -ENOSPC;
  }
  
  static int cpuhp_store_callbacks(enum cpuhp_state state, const char *name,
  				 int (*startup)(unsigned int cpu),
  				 int (*teardown)(unsigned int cpu),
  				 bool multi_instance)
  {
  	/* (Un)Install the callbacks for further cpu hotplug operations */
  	struct cpuhp_step *sp;
  	int ret = 0;
  
  	/*
  	 * If name is NULL, then the state gets removed.
  	 *
  	 * CPUHP_AP_ONLINE_DYN and CPUHP_BP_PREPARE_DYN are handed out on
  	 * the first allocation from these dynamic ranges, so the removal
  	 * would trigger a new allocation and clear the wrong (already
  	 * empty) state, leaving the callbacks of the to be cleared state
  	 * dangling, which causes wreckage on the next hotplug operation.
  	 */
  	if (name && (state == CPUHP_AP_ONLINE_DYN ||
  		     state == CPUHP_BP_PREPARE_DYN)) {
  		ret = cpuhp_reserve_state(state);
  		if (ret < 0)
  			return ret;
  		state = ret;
  	}
  	sp = cpuhp_get_step(state);
  	if (name && sp->name)
  		return -EBUSY;
  
  	sp->startup.single = startup;
  	sp->teardown.single = teardown;
  	sp->name = name;
  	sp->multi_instance = multi_instance;
  	INIT_HLIST_HEAD(&sp->list);
  	return ret;
  }
  
  static void *cpuhp_get_teardown_cb(enum cpuhp_state state)
  {
  	return cpuhp_get_step(state)->teardown.single;
  }
  
  /*
   * Call the startup/teardown function for a step either on the AP or
   * on the current CPU.
   */
  static int cpuhp_issue_call(int cpu, enum cpuhp_state state, bool bringup,
  			    struct hlist_node *node)
  {
  	struct cpuhp_step *sp = cpuhp_get_step(state);
  	int ret;
  
  	/*
  	 * If there's nothing to do, we done.
  	 * Relies on the union for multi_instance.
  	 */
  	if ((bringup && !sp->startup.single) ||
  	    (!bringup && !sp->teardown.single))
  		return 0;
  	/*
  	 * The non AP bound callbacks can fail on bringup. On teardown
  	 * e.g. module removal we crash for now.
  	 */
  #ifdef CONFIG_SMP
  	if (cpuhp_is_ap_state(state))
  		ret = cpuhp_invoke_ap_callback(cpu, state, bringup, node);
  	else
  		ret = cpuhp_invoke_callback(cpu, state, bringup, node, NULL);
  #else
  	ret = cpuhp_invoke_callback(cpu, state, bringup, node, NULL);
  #endif
  	BUG_ON(ret && !bringup);
  	return ret;
  }
  
  /*
   * Called from __cpuhp_setup_state on a recoverable failure.
   *
   * Note: The teardown callbacks for rollback are not allowed to fail!
   */
  static void cpuhp_rollback_install(int failedcpu, enum cpuhp_state state,
  				   struct hlist_node *node)
  {
  	int cpu;
  
  	/* Roll back the already executed steps on the other cpus */
  	for_each_present_cpu(cpu) {
  		struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
  		int cpustate = st->state;
  
  		if (cpu >= failedcpu)
  			break;
  
  		/* Did we invoke the startup call on that cpu ? */
  		if (cpustate >= state)
  			cpuhp_issue_call(cpu, state, false, node);
  	}
  }
  
  int __cpuhp_state_add_instance_cpuslocked(enum cpuhp_state state,
  					  struct hlist_node *node,
  					  bool invoke)
  {
  	struct cpuhp_step *sp;
  	int cpu;
  	int ret;
  
  	lockdep_assert_cpus_held();
  
  	sp = cpuhp_get_step(state);
  	if (sp->multi_instance == false)
  		return -EINVAL;
  
  	mutex_lock(&cpuhp_state_mutex);
  
  	if (!invoke || !sp->startup.multi)
  		goto add_node;
  
  	/*
  	 * Try to call the startup callback for each present cpu
  	 * depending on the hotplug state of the cpu.
  	 */
  	for_each_present_cpu(cpu) {
  		struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
  		int cpustate = st->state;
  
  		if (cpustate < state)
  			continue;
  
  		ret = cpuhp_issue_call(cpu, state, true, node);
  		if (ret) {
  			if (sp->teardown.multi)
  				cpuhp_rollback_install(cpu, state, node);
  			goto unlock;
  		}
  	}
  add_node:
  	ret = 0;
  	hlist_add_head(node, &sp->list);
  unlock:
  	mutex_unlock(&cpuhp_state_mutex);
  	return ret;
  }
  
  int __cpuhp_state_add_instance(enum cpuhp_state state, struct hlist_node *node,
  			       bool invoke)
  {
  	int ret;
  
  	cpus_read_lock();
  	ret = __cpuhp_state_add_instance_cpuslocked(state, node, invoke);
  	cpus_read_unlock();
  	return ret;
  }
  EXPORT_SYMBOL_GPL(__cpuhp_state_add_instance);
  
  /**
   * __cpuhp_setup_state_cpuslocked - Setup the callbacks for an hotplug machine state
   * @state:		The state to setup
   * @invoke:		If true, the startup function is invoked for cpus where
   *			cpu state >= @state
   * @startup:		startup callback function
   * @teardown:		teardown callback function
   * @multi_instance:	State is set up for multiple instances which get
   *			added afterwards.
   *
   * The caller needs to hold cpus read locked while calling this function.
   * Returns:
   *   On success:
   *      Positive state number if @state is CPUHP_AP_ONLINE_DYN
   *      0 for all other states
   *   On failure: proper (negative) error code
   */
  int __cpuhp_setup_state_cpuslocked(enum cpuhp_state state,
  				   const char *name, bool invoke,
  				   int (*startup)(unsigned int cpu),
  				   int (*teardown)(unsigned int cpu),
  				   bool multi_instance)
  {
  	int cpu, ret = 0;
  	bool dynstate;
  
  	lockdep_assert_cpus_held();
  
  	if (cpuhp_cb_check(state) || !name)
  		return -EINVAL;
  
  	mutex_lock(&cpuhp_state_mutex);
  
  	ret = cpuhp_store_callbacks(state, name, startup, teardown,
  				    multi_instance);
  
  	dynstate = state == CPUHP_AP_ONLINE_DYN;
  	if (ret > 0 && dynstate) {
  		state = ret;
  		ret = 0;
  	}
  
  	if (ret || !invoke || !startup)
  		goto out;
  
  	/*
  	 * Try to call the startup callback for each present cpu
  	 * depending on the hotplug state of the cpu.
  	 */
  	for_each_present_cpu(cpu) {
  		struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
  		int cpustate = st->state;
  
  		if (cpustate < state)
  			continue;
  
  		ret = cpuhp_issue_call(cpu, state, true, NULL);
  		if (ret) {
  			if (teardown)
  				cpuhp_rollback_install(cpu, state, NULL);
  			cpuhp_store_callbacks(state, NULL, NULL, NULL, false);
  			goto out;
  		}
  	}
  out:
  	mutex_unlock(&cpuhp_state_mutex);
  	/*
  	 * If the requested state is CPUHP_AP_ONLINE_DYN, return the
  	 * dynamically allocated state in case of success.
  	 */
  	if (!ret && dynstate)
  		return state;
  	return ret;
  }
  EXPORT_SYMBOL(__cpuhp_setup_state_cpuslocked);
  
  int __cpuhp_setup_state(enum cpuhp_state state,
  			const char *name, bool invoke,
  			int (*startup)(unsigned int cpu),
  			int (*teardown)(unsigned int cpu),
  			bool multi_instance)
  {
  	int ret;
  
  	cpus_read_lock();
  	ret = __cpuhp_setup_state_cpuslocked(state, name, invoke, startup,
  					     teardown, multi_instance);
  	cpus_read_unlock();
  	return ret;
  }
  EXPORT_SYMBOL(__cpuhp_setup_state);
  
  int __cpuhp_state_remove_instance(enum cpuhp_state state,
  				  struct hlist_node *node, bool invoke)
  {
  	struct cpuhp_step *sp = cpuhp_get_step(state);
  	int cpu;
  
  	BUG_ON(cpuhp_cb_check(state));
  
  	if (!sp->multi_instance)
  		return -EINVAL;
  
  	cpus_read_lock();
  	mutex_lock(&cpuhp_state_mutex);
  
  	if (!invoke || !cpuhp_get_teardown_cb(state))
  		goto remove;
  	/*
  	 * Call the teardown callback for each present cpu depending
  	 * on the hotplug state of the cpu. This function is not
  	 * allowed to fail currently!
  	 */
  	for_each_present_cpu(cpu) {
  		struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
  		int cpustate = st->state;
  
  		if (cpustate >= state)
  			cpuhp_issue_call(cpu, state, false, node);
  	}
  
  remove:
  	hlist_del(node);
  	mutex_unlock(&cpuhp_state_mutex);
  	cpus_read_unlock();
  
  	return 0;
  }
  EXPORT_SYMBOL_GPL(__cpuhp_state_remove_instance);
  
  /**
   * __cpuhp_remove_state_cpuslocked - Remove the callbacks for an hotplug machine state
   * @state:	The state to remove
   * @invoke:	If true, the teardown function is invoked for cpus where
   *		cpu state >= @state
   *
   * The caller needs to hold cpus read locked while calling this function.
   * The teardown callback is currently not allowed to fail. Think
   * about module removal!
   */
  void __cpuhp_remove_state_cpuslocked(enum cpuhp_state state, bool invoke)
  {
  	struct cpuhp_step *sp = cpuhp_get_step(state);
  	int cpu;
  
  	BUG_ON(cpuhp_cb_check(state));
  
  	lockdep_assert_cpus_held();
  
  	mutex_lock(&cpuhp_state_mutex);
  	if (sp->multi_instance) {
  		WARN(!hlist_empty(&sp->list),
  		     "Error: Removing state %d which has instances left.\n",
  		     state);
  		goto remove;
  	}
  
  	if (!invoke || !cpuhp_get_teardown_cb(state))
  		goto remove;
  
  	/*
  	 * Call the teardown callback for each present cpu depending
  	 * on the hotplug state of the cpu. This function is not
  	 * allowed to fail currently!
  	 */
  	for_each_present_cpu(cpu) {
  		struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
  		int cpustate = st->state;
  
  		if (cpustate >= state)
  			cpuhp_issue_call(cpu, state, false, NULL);
  	}
  remove:
  	cpuhp_store_callbacks(state, NULL, NULL, NULL, false);
  	mutex_unlock(&cpuhp_state_mutex);
  }
  EXPORT_SYMBOL(__cpuhp_remove_state_cpuslocked);
  
  void __cpuhp_remove_state(enum cpuhp_state state, bool invoke)
  {
  	cpus_read_lock();
  	__cpuhp_remove_state_cpuslocked(state, invoke);
  	cpus_read_unlock();
  }
  EXPORT_SYMBOL(__cpuhp_remove_state);
  
  #if defined(CONFIG_SYSFS) && defined(CONFIG_HOTPLUG_CPU)
  static ssize_t show_cpuhp_state(struct device *dev,
  				struct device_attribute *attr, char *buf)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, dev->id);
  
  	return sprintf(buf, "%d\n", st->state);
  }
  static DEVICE_ATTR(state, 0444, show_cpuhp_state, NULL);
  
  static ssize_t write_cpuhp_target(struct device *dev,
  				  struct device_attribute *attr,
  				  const char *buf, size_t count)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, dev->id);
  	struct cpuhp_step *sp;
  	int target, ret;
  
  	ret = kstrtoint(buf, 10, &target);
  	if (ret)
  		return ret;
  
  #ifdef CONFIG_CPU_HOTPLUG_STATE_CONTROL
  	if (target < CPUHP_OFFLINE || target > CPUHP_ONLINE)
  		return -EINVAL;
  #else
  	if (target != CPUHP_OFFLINE && target != CPUHP_ONLINE)
  		return -EINVAL;
  #endif
  
  	ret = lock_device_hotplug_sysfs();
  	if (ret)
  		return ret;
  
  	mutex_lock(&cpuhp_state_mutex);
  	sp = cpuhp_get_step(target);
  	ret = !sp->name || sp->cant_stop ? -EINVAL : 0;
  	mutex_unlock(&cpuhp_state_mutex);
  	if (ret)
  		goto out;
  
  	if (st->state < target)
  		ret = do_cpu_up(dev->id, target);
  	else
  		ret = do_cpu_down(dev->id, target);
  out:
  	unlock_device_hotplug();
  	return ret ? ret : count;
  }
  
  static ssize_t show_cpuhp_target(struct device *dev,
  				 struct device_attribute *attr, char *buf)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, dev->id);
  
  	return sprintf(buf, "%d\n", st->target);
  }
  static DEVICE_ATTR(target, 0644, show_cpuhp_target, write_cpuhp_target);
  
  
  static ssize_t write_cpuhp_fail(struct device *dev,
  				struct device_attribute *attr,
  				const char *buf, size_t count)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, dev->id);
  	struct cpuhp_step *sp;
  	int fail, ret;
  
  	ret = kstrtoint(buf, 10, &fail);
  	if (ret)
  		return ret;
  
  	/*
  	 * Cannot fail STARTING/DYING callbacks.
  	 */
  	if (cpuhp_is_atomic_state(fail))
  		return -EINVAL;
  
  	/*
  	 * Cannot fail anything that doesn't have callbacks.
  	 */
  	mutex_lock(&cpuhp_state_mutex);
  	sp = cpuhp_get_step(fail);
  	if (!sp->startup.single && !sp->teardown.single)
  		ret = -EINVAL;
  	mutex_unlock(&cpuhp_state_mutex);
  	if (ret)
  		return ret;
  
  	st->fail = fail;
  
  	return count;
  }
  
  static ssize_t show_cpuhp_fail(struct device *dev,
  			       struct device_attribute *attr, char *buf)
  {
  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, dev->id);
  
  	return sprintf(buf, "%d\n", st->fail);
  }
  
  static DEVICE_ATTR(fail, 0644, show_cpuhp_fail, write_cpuhp_fail);
  
  static struct attribute *cpuhp_cpu_attrs[] = {
  	&dev_attr_state.attr,
  	&dev_attr_target.attr,
  	&dev_attr_fail.attr,
  	NULL
  };
  
  static const struct attribute_group cpuhp_cpu_attr_group = {
  	.attrs = cpuhp_cpu_attrs,
  	.name = "hotplug",
  	NULL
  };
  
  static ssize_t show_cpuhp_states(struct device *dev,
  				 struct device_attribute *attr, char *buf)
  {
  	ssize_t cur, res = 0;
  	int i;
  
  	mutex_lock(&cpuhp_state_mutex);
  	for (i = CPUHP_OFFLINE; i <= CPUHP_ONLINE; i++) {
  		struct cpuhp_step *sp = cpuhp_get_step(i);
  
  		if (sp->name) {
  			cur = sprintf(buf, "%3d: %s\n", i, sp->name);
  			buf += cur;
  			res += cur;
  		}
  	}
  	mutex_unlock(&cpuhp_state_mutex);
  	return res;
  }
  static DEVICE_ATTR(states, 0444, show_cpuhp_states, NULL);
  
  static struct attribute *cpuhp_cpu_root_attrs[] = {
  	&dev_attr_states.attr,
  	NULL
  };
  
  static const struct attribute_group cpuhp_cpu_root_attr_group = {
  	.attrs = cpuhp_cpu_root_attrs,
  	.name = "hotplug",
  	NULL
  };
  
  #ifdef CONFIG_HOTPLUG_SMT
  
  static const char *smt_states[] = {
  	[CPU_SMT_ENABLED]		= "on",
  	[CPU_SMT_DISABLED]		= "off",
  	[CPU_SMT_FORCE_DISABLED]	= "forceoff",
  	[CPU_SMT_NOT_SUPPORTED]		= "notsupported",
  };
  
  static ssize_t
  show_smt_control(struct device *dev, struct device_attribute *attr, char *buf)
  {
  	return snprintf(buf, PAGE_SIZE - 2, "%s\n", smt_states[cpu_smt_control]);
  }
  
  static void cpuhp_offline_cpu_device(unsigned int cpu)
  {
  	struct device *dev = get_cpu_device(cpu);
  
  	dev->offline = true;
  	/* Tell user space about the state change */
  	kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
  }
  
  static void cpuhp_online_cpu_device(unsigned int cpu)
  {
  	struct device *dev = get_cpu_device(cpu);
  
  	dev->offline = false;
  	/* Tell user space about the state change */
  	kobject_uevent(&dev->kobj, KOBJ_ONLINE);
  }
  
  static int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
  {
  	int cpu, ret = 0;
  
  	cpu_maps_update_begin();
  	for_each_online_cpu(cpu) {
  		if (topology_is_primary_thread(cpu))
  			continue;
  		ret = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
  		if (ret)
  			break;
  		/*
  		 * As this needs to hold the cpu maps lock it's impossible
  		 * to call device_offline() because that ends up calling
  		 * cpu_down() which takes cpu maps lock. cpu maps lock
  		 * needs to be held as this might race against in kernel
  		 * abusers of the hotplug machinery (thermal management).
  		 *
  		 * So nothing would update device:offline state. That would
  		 * leave the sysfs entry stale and prevent onlining after
  		 * smt control has been changed to 'off' again. This is
  		 * called under the sysfs hotplug lock, so it is properly
  		 * serialized against the regular offline usage.
  		 */
  		cpuhp_offline_cpu_device(cpu);
  	}
  	if (!ret)
  		cpu_smt_control = ctrlval;
  	cpu_maps_update_done();
  	return ret;
  }
  
  static int cpuhp_smt_enable(void)
  {
  	int cpu, ret = 0;
  
  	cpu_maps_update_begin();
  	cpu_smt_control = CPU_SMT_ENABLED;
  	for_each_present_cpu(cpu) {
  		/* Skip online CPUs and CPUs on offline nodes */
  		if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
  			continue;
  		ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
  		if (ret)
  			break;
  		/* See comment in cpuhp_smt_disable() */
  		cpuhp_online_cpu_device(cpu);
  	}
  	cpu_maps_update_done();
  	return ret;
  }
  
  static ssize_t
  store_smt_control(struct device *dev, struct device_attribute *attr,
  		  const char *buf, size_t count)
  {
  	int ctrlval, ret;
  
  	if (sysfs_streq(buf, "on"))
  		ctrlval = CPU_SMT_ENABLED;
  	else if (sysfs_streq(buf, "off"))
  		ctrlval = CPU_SMT_DISABLED;
  	else if (sysfs_streq(buf, "forceoff"))
  		ctrlval = CPU_SMT_FORCE_DISABLED;
  	else
  		return -EINVAL;
  
  	if (cpu_smt_control == CPU_SMT_FORCE_DISABLED)
  		return -EPERM;
  
  	if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
  		return -ENODEV;
  
  	ret = lock_device_hotplug_sysfs();
  	if (ret)
  		return ret;
  
  	if (ctrlval != cpu_smt_control) {
  		switch (ctrlval) {
  		case CPU_SMT_ENABLED:
  			ret = cpuhp_smt_enable();
  			break;
  		case CPU_SMT_DISABLED:
  		case CPU_SMT_FORCE_DISABLED:
  			ret = cpuhp_smt_disable(ctrlval);
  			break;
  		}
  	}
  
  	unlock_device_hotplug();
  	return ret ? ret : count;
  }
  static DEVICE_ATTR(control, 0644, show_smt_control, store_smt_control);
  
  static ssize_t
  show_smt_active(struct device *dev, struct device_attribute *attr, char *buf)
  {
  	bool active = topology_max_smt_threads() > 1;
  
  	return snprintf(buf, PAGE_SIZE - 2, "%d\n", active);
  }
  static DEVICE_ATTR(active, 0444, show_smt_active, NULL);
  
  static struct attribute *cpuhp_smt_attrs[] = {
  	&dev_attr_control.attr,
  	&dev_attr_active.attr,
  	NULL
  };
  
  static const struct attribute_group cpuhp_smt_attr_group = {
  	.attrs = cpuhp_smt_attrs,
  	.name = "smt",
  	NULL
  };
  
  static int __init cpu_smt_state_init(void)
  {
  	return sysfs_create_group(&cpu_subsys.dev_root->kobj,
  				  &cpuhp_smt_attr_group);
  }
  
  #else
  static inline int cpu_smt_state_init(void) { return 0; }
  #endif
  
  static int __init cpuhp_sysfs_init(void)
  {
  	int cpu, ret;
  
  	ret = cpu_smt_state_init();
  	if (ret)
  		return ret;
  
  	ret = sysfs_create_group(&cpu_subsys.dev_root->kobj,
  				 &cpuhp_cpu_root_attr_group);
  	if (ret)
  		return ret;
  
  	for_each_possible_cpu(cpu) {
  		struct device *dev = get_cpu_device(cpu);
  
  		if (!dev)
  			continue;
  		ret = sysfs_create_group(&dev->kobj, &cpuhp_cpu_attr_group);
  		if (ret)
  			return ret;
  	}
  	return 0;
  }
  device_initcall(cpuhp_sysfs_init);
  #endif
  
  /*
   * cpu_bit_bitmap[] is a special, "compressed" data structure that
   * represents all NR_CPUS bits binary values of 1<<nr.
   *
   * It is used by cpumask_of() to get a constant address to a CPU
   * mask value that has a single bit set only.
   */
  
  /* cpu_bit_bitmap[0] is empty - so we can back into it */
  #define MASK_DECLARE_1(x)	[x+1][0] = (1UL << (x))
  #define MASK_DECLARE_2(x)	MASK_DECLARE_1(x), MASK_DECLARE_1(x+1)
  #define MASK_DECLARE_4(x)	MASK_DECLARE_2(x), MASK_DECLARE_2(x+2)
  #define MASK_DECLARE_8(x)	MASK_DECLARE_4(x), MASK_DECLARE_4(x+4)
  
  const unsigned long cpu_bit_bitmap[BITS_PER_LONG+1][BITS_TO_LONGS(NR_CPUS)] = {
  
  	MASK_DECLARE_8(0),	MASK_DECLARE_8(8),
  	MASK_DECLARE_8(16),	MASK_DECLARE_8(24),
  #if BITS_PER_LONG > 32
  	MASK_DECLARE_8(32),	MASK_DECLARE_8(40),
  	MASK_DECLARE_8(48),	MASK_DECLARE_8(56),
  #endif
  };
  EXPORT_SYMBOL_GPL(cpu_bit_bitmap);
  
  const DECLARE_BITMAP(cpu_all_bits, NR_CPUS) = CPU_BITS_ALL;
  EXPORT_SYMBOL(cpu_all_bits);
  
  #ifdef CONFIG_INIT_ALL_POSSIBLE
  struct cpumask __cpu_possible_mask __read_mostly
  	= {CPU_BITS_ALL};
  #else
  struct cpumask __cpu_possible_mask __read_mostly;
  #endif
  EXPORT_SYMBOL(__cpu_possible_mask);
  
  struct cpumask __cpu_online_mask __read_mostly;
  EXPORT_SYMBOL(__cpu_online_mask);
  
  struct cpumask __cpu_present_mask __read_mostly;
  EXPORT_SYMBOL(__cpu_present_mask);
  
  struct cpumask __cpu_active_mask __read_mostly;
  EXPORT_SYMBOL(__cpu_active_mask);
  
  void init_cpu_present(const struct cpumask *src)
  {
  	cpumask_copy(&__cpu_present_mask, src);
  }
  
  void init_cpu_possible(const struct cpumask *src)
  {
  	cpumask_copy(&__cpu_possible_mask, src);
  }
  
  void init_cpu_online(const struct cpumask *src)
  {
  	cpumask_copy(&__cpu_online_mask, src);
  }
  
  /*
   * Activate the first processor.
   */
  void __init boot_cpu_init(void)
  {
  	int cpu = smp_processor_id();
  
  	/* Mark the boot cpu "present", "online" etc for SMP and UP case */
  	set_cpu_online(cpu, true);
  	set_cpu_active(cpu, true);
  	set_cpu_present(cpu, true);
  	set_cpu_possible(cpu, true);
  
  #ifdef CONFIG_SMP
  	__boot_cpu_id = cpu;
  #endif
  }
  
  /*
   * Must be called _AFTER_ setting up the per_cpu areas
   */
  void __init boot_cpu_hotplug_init(void)
  {
  #ifdef CONFIG_SMP
  	this_cpu_write(cpuhp_state.booted_once, true);
  #endif
  	this_cpu_write(cpuhp_state.state, CPUHP_ONLINE);
  }
  