cpu.c

/* 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/delay.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/scs.h>
#include <linux/percpu-rwsem.h>
#include <linux/cpuset.h>
#include <linux/random.h>
#include <linux/cc_platform.h>

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

#include "smpboot.h"

/**
 * struct cpuhp_cpu_state - Per cpu hotplug state storage
 * @state:	The current cpu state
 * @target:	The target state
 * @fail:	Current CPU hotplug callback 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
 * @node:	Remote CPU node; for multi-instance, do a
 *		single entry callback for install/remove
 * @last:	For multi-instance rollback, remember how far we got
 * @cb_state:	The state for a single callback (install/uninstall)
 * @result:	Result of the operation
 * @ap_sync_state:	State for AP synchronization
 * @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;
	atomic_t		ap_sync_state;
	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

/**
 * struct 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
 * @multi_instance:	State has multiple instances which get added afterwards
 */
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;
	/* private: */
	struct hlist_head	list;
	/* public: */
	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;
}

static bool cpuhp_step_empty(bool bringup, struct cpuhp_step *step)
{
	return bringup ? !step->startup.single : !step->teardown.single;
}

/**
 * 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.
 *
 * Return: %0 on success or a negative errno code
 */
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;
		return -EAGAIN;
	}

	if (cpuhp_step_empty(bringup, step)) {
		WARN_ON_ONCE(1);
		return 0;
	}

	if (!step->multi_instance) {
		WARN_ON_ONCE(lastp && *lastp);
		cb = bringup ? step->startup.single : step->teardown.single;

		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;

	/* 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;
}

/* Synchronization state management */
enum cpuhp_sync_state {
	SYNC_STATE_DEAD,
	SYNC_STATE_KICKED,
	SYNC_STATE_SHOULD_DIE,
	SYNC_STATE_ALIVE,
	SYNC_STATE_SHOULD_ONLINE,
	SYNC_STATE_ONLINE,
};

#ifdef CONFIG_HOTPLUG_CORE_SYNC
/**
 * cpuhp_ap_update_sync_state - Update synchronization state during bringup/teardown
 * @state:	The synchronization state to set
 *
 * No synchronization point. Just update of the synchronization state, but implies
 * a full barrier so that the AP changes are visible before the control CPU proceeds.
 */
static inline void cpuhp_ap_update_sync_state(enum cpuhp_sync_state state)
{
	atomic_t *st = this_cpu_ptr(&cpuhp_state.ap_sync_state);

	(void)atomic_xchg(st, state);
}

void __weak arch_cpuhp_sync_state_poll(void) { cpu_relax(); }

static bool cpuhp_wait_for_sync_state(unsigned int cpu, enum cpuhp_sync_state state,
				      enum cpuhp_sync_state next_state)
{
	atomic_t *st = per_cpu_ptr(&cpuhp_state.ap_sync_state, cpu);
	ktime_t now, end, start = ktime_get();
	int sync;

	end = start + 10ULL * NSEC_PER_SEC;

	sync = atomic_read(st);
	while (1) {
		if (sync == state) {
			if (!atomic_try_cmpxchg(st, &sync, next_state))
				continue;
			return true;
		}

		now = ktime_get();
		if (now > end) {
			/* Timeout. Leave the state unchanged */
			return false;
		} else if (now - start < NSEC_PER_MSEC) {
			/* Poll for one millisecond */
			arch_cpuhp_sync_state_poll();
		} else {
			usleep_range_state(USEC_PER_MSEC, 2 * USEC_PER_MSEC, TASK_UNINTERRUPTIBLE);
		}
		sync = atomic_read(st);
	}
	return true;
}
#else  /* CONFIG_HOTPLUG_CORE_SYNC */
static inline void cpuhp_ap_update_sync_state(enum cpuhp_sync_state state) { }
#endif /* !CONFIG_HOTPLUG_CORE_SYNC */

#ifdef CONFIG_HOTPLUG_CORE_SYNC_DEAD
/**
 * cpuhp_ap_report_dead - Update synchronization state to DEAD
 *
 * No synchronization point. Just update of the synchronization state.
 */
void cpuhp_ap_report_dead(void)
{
	cpuhp_ap_update_sync_state(SYNC_STATE_DEAD);
}

void __weak arch_cpuhp_cleanup_dead_cpu(unsigned int cpu) { }

/*
 * Late CPU shutdown synchronization point. Cannot use cpuhp_state::done_down
 * because the AP cannot issue complete() at this stage.
 */
static void cpuhp_bp_sync_dead(unsigned int cpu)
{
	atomic_t *st = per_cpu_ptr(&cpuhp_state.ap_sync_state, cpu);
	int sync = atomic_read(st);

	do {
		/* CPU can have reported dead already. Don't overwrite that! */
		if (sync == SYNC_STATE_DEAD)
			break;
	} while (!atomic_try_cmpxchg(st, &sync, SYNC_STATE_SHOULD_DIE));

	if (cpuhp_wait_for_sync_state(cpu, SYNC_STATE_DEAD, SYNC_STATE_DEAD)) {
		/* CPU reached dead state. Invoke the cleanup function */
		arch_cpuhp_cleanup_dead_cpu(cpu);
		return;
	}

	/* No further action possible. Emit message and give up. */
	pr_err("CPU%u failed to report dead state\n", cpu);
}
#else /* CONFIG_HOTPLUG_CORE_SYNC_DEAD */
static inline void cpuhp_bp_sync_dead(unsigned int cpu) { }
#endif /* !CONFIG_HOTPLUG_CORE_SYNC_DEAD */

#ifdef CONFIG_HOTPLUG_CORE_SYNC_FULL
/**
 * cpuhp_ap_sync_alive - Synchronize AP with the control CPU once it is alive
 *
 * Updates the AP synchronization state to SYNC_STATE_ALIVE and waits
 * for the BP to release it.
 */
void cpuhp_ap_sync_alive(void)
{
	atomic_t *st = this_cpu_ptr(&cpuhp_state.ap_sync_state);

	cpuhp_ap_update_sync_state(SYNC_STATE_ALIVE);

	/* Wait for the control CPU to release it. */
	while (atomic_read(st) != SYNC_STATE_SHOULD_ONLINE)
		cpu_relax();
}

static bool cpuhp_can_boot_ap(unsigned int cpu)
{
	atomic_t *st = per_cpu_ptr(&cpuhp_state.ap_sync_state, cpu);
	int sync = atomic_read(st);

again:
	switch (sync) {
	case SYNC_STATE_DEAD:
		/* CPU is properly dead */
		break;
	case SYNC_STATE_KICKED:
		/* CPU did not come up in previous attempt */
		break;
	case SYNC_STATE_ALIVE:
		/* CPU is stuck cpuhp_ap_sync_alive(). */
		break;
	default:
		/* CPU failed to report online or dead and is in limbo state. */
		return false;
	}

	/* Prepare for booting */
	if (!atomic_try_cmpxchg(st, &sync, SYNC_STATE_KICKED))
		goto again;

	return true;
}

void __weak arch_cpuhp_cleanup_kick_cpu(unsigned int cpu) { }

/*
 * Early CPU bringup synchronization point. Cannot use cpuhp_state::done_up
 * because the AP cannot issue complete() so early in the bringup.
 */
static int cpuhp_bp_sync_alive(unsigned int cpu)
{
	int ret = 0;

	if (!IS_ENABLED(CONFIG_HOTPLUG_CORE_SYNC_FULL))
		return 0;

	if (!cpuhp_wait_for_sync_state(cpu, SYNC_STATE_ALIVE, SYNC_STATE_SHOULD_ONLINE)) {
		pr_err("CPU%u failed to report alive state\n", cpu);
		ret = -EIO;
	}

	/* Let the architecture cleanup the kick alive mechanics. */
	arch_cpuhp_cleanup_kick_cpu(cpu);
	return ret;
}
#else /* CONFIG_HOTPLUG_CORE_SYNC_FULL */
static inline int cpuhp_bp_sync_alive(unsigned int cpu) { return 0; }
static inline bool cpuhp_can_boot_ap(unsigned int cpu) { return true; }
#endif /* !CONFIG_HOTPLUG_CORE_SYNC_FULL */

/* 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);
}

#ifdef CONFIG_LOCKDEP
int lockdep_is_cpus_held(void)
{
	return percpu_rwsem_is_held(&cpu_hotplug_lock);
}
#endif

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\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;
static unsigned int cpu_smt_max_threads __ro_after_init;
unsigned int cpu_smt_num_threads __read_mostly = UINT_MAX;

void __init cpu_smt_disable(bool force)
{
	if (!cpu_smt_possible())
		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;
	}
	cpu_smt_num_threads = 1;
}

/*
 * The decision whether SMT is supported can only be done after the full
 * CPU identification. Called from architecture code.
 */
void __init cpu_smt_set_num_threads(unsigned int num_threads,
				    unsigned int max_threads)
{
	WARN_ON(!num_threads || (num_threads > max_threads));

	if (max_threads == 1)
		cpu_smt_control = CPU_SMT_NOT_SUPPORTED;

	cpu_smt_max_threads = max_threads;

	/*
	 * If SMT has been disabled via the kernel command line or SMT is
	 * not supported, set cpu_smt_num_threads to 1 for consistency.
	 * If enabled, take the architecture requested number of threads
	 * to bring up into account.
	 */
	if (cpu_smt_control != CPU_SMT_ENABLED)
		cpu_smt_num_threads = 1;
	else if (num_threads < cpu_smt_num_threads)
		cpu_smt_num_threads = num_threads;
}

static int __init smt_cmdline_disable(char *str)
{
	cpu_smt_disable(str && !strcmp(str, "force"));
	return 0;
}
early_param("nosmt", smt_cmdline_disable);

/*
 * For Archicture supporting partial SMT states check if the thread is allowed.
 * Otherwise this has already been checked through cpu_smt_max_threads when
 * setting the SMT level.
 */
static inline bool cpu_smt_thread_allowed(unsigned int cpu)
{
#ifdef CONFIG_SMT_NUM_THREADS_DYNAMIC
	return topology_smt_thread_allowed(cpu);
#else
	return true;
#endif
}

static inline bool cpu_bootable(unsigned int cpu)
{
	if (cpu_smt_control == CPU_SMT_ENABLED && cpu_smt_thread_allowed(cpu))
		return true;

	/* All CPUs are bootable if controls are not configured */
	if (cpu_smt_control == CPU_SMT_NOT_IMPLEMENTED)
		return true;

	/* All CPUs are bootable if CPU is not SMT capable */
	if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
		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 supported and not 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_bootable(unsigned int cpu) { return true; }
#endif

static inline enum cpuhp_state
cpuhp_set_state(int cpu, struct cpuhp_cpu_state *st, enum cpuhp_state target)
{
	enum cpuhp_state prev_state = st->state;
	bool bringup = st->state < target;

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

	st->target = target;
	st->single = false;
	st->bringup = bringup;
	if (cpu_dying(cpu) != !bringup)
		set_cpu_dying(cpu, !bringup);

	return prev_state;
}

static inline void
cpuhp_reset_state(int cpu, struct cpuhp_cpu_state *st,
		  enum cpuhp_state prev_state)
{
	bool bringup = !st->bringup;

	st->target = prev_state;

	/*
	 * Already rolling back. No need invert the bringup value or to change
	 * the current state.
	 */
	if (st->rollback)
		return;

	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->bringup = bringup;
	if (cpu_dying(cpu) != !bringup)
		set_cpu_dying(cpu, !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(int cpu, struct cpuhp_cpu_state *st,
			 enum cpuhp_state target)
{
	enum cpuhp_state prev_state;
	int ret;

	prev_state = cpuhp_set_state(cpu, st, target);
	__cpuhp_kick_ap(st);
	if ((ret = st->result)) {
		cpuhp_reset_state(cpu, st, prev_state);
		__cpuhp_kick_ap(st);
	}

	return ret;
}

static int bringup_wait_for_ap_online(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_bootable() check will now return false if this is not the
	 * primary sibling.
	 */
	if (!cpu_bootable(cpu))
		return -ECANCELED;
	return 0;
}

#ifdef CONFIG_HOTPLUG_SPLIT_STARTUP
static int cpuhp_kick_ap_alive(unsigned int cpu)
{
	if (!cpuhp_can_boot_ap(cpu))
		return -EAGAIN;

	return arch_cpuhp_kick_ap_alive(cpu, idle_thread_get(cpu));
}

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

	ret = cpuhp_bp_sync_alive(cpu);
	if (ret)
		goto out_unlock;

	ret = bringup_wait_for_ap_online(cpu);
	if (ret)
		goto out_unlock;

	irq_unlock_sparse();

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

	return cpuhp_kick_ap(cpu, st, st->target);

out_unlock:
	irq_unlock_sparse();
	return ret;
}
#else
static int bringup_cpu(unsigned int cpu)
{
	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
	struct task_struct *idle = idle_thread_get(cpu);
	int ret;

	if (!cpuhp_can_boot_ap(cpu))
		return -EAGAIN;

	/*
	 * 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 by acquiring the
	 * sparse irq lock. Hold it until the upcoming CPU completes the
	 * startup in cpuhp_online_idle() which allows to avoid
	 * intermediate synchronization points in the architecture code.
	 */
	irq_lock_sparse();

	ret = __cpu_up(cpu, idle);
	if (ret)
		goto out_unlock;

	ret = cpuhp_bp_sync_alive(cpu);
	if (ret)
		goto out_unlock;

	ret = bringup_wait_for_ap_online(cpu);
	if (ret)
		goto out_unlock;

	irq_unlock_sparse();

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

	return cpuhp_kick_ap(cpu, st, st->target);

out_unlock:
	irq_unlock_sparse();
	return ret;
}
#endif

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_lazy_tlb(mm);
	return 0;
}

/*
 * Hotplug state machine related functions
 */

/*
 * Get the next state to run. Empty ones will be skipped. Returns true if a
 * state must be run.
 *
 * st->state will be modified ahead of time, to match state_to_run, as if it
 * has already ran.
 */
static bool cpuhp_next_state(bool bringup,
			     enum cpuhp_state *state_to_run,
			     struct cpuhp_cpu_state *st,
			     enum cpuhp_state target)
{
	do {
		if (bringup) {
			if (st->state >= target)
				return false;

			*state_to_run = ++st->state;
		} else {
			if (st->state <= target)
				return false;

			*state_to_run = st->state--;
		}

		if (!cpuhp_step_empty(bringup, cpuhp_get_step(*state_to_run)))
			break;
	} while (true);

	return true;
}

static int __cpuhp_invoke_callback_range(bool bringup,
					 unsigned int cpu,
					 struct cpuhp_cpu_state *st,
					 enum cpuhp_state target,
					 bool nofail)
{
	enum cpuhp_state state;
	int ret = 0;

	while (cpuhp_next_state(bringup, &state, st, target)) {
		int err;

		err = cpuhp_invoke_callback(cpu, state, bringup, NULL, NULL);
		if (!err)
			continue;

		if (nofail) {
			pr_warn("CPU %u %s state %s (%d) failed (%d)\n",
				cpu, bringup ? "UP" : "DOWN",
				cpuhp_get_step(st->state)->name,
				st->state, err);
			ret = -1;
		} else {
			ret = err;
			break;
		}
	}

	return ret;
}

static inline int cpuhp_invoke_callback_range(bool bringup,
					      unsigned int cpu,
					      struct cpuhp_cpu_state *st,
					      enum cpuhp_state target)
{
	return __cpuhp_invoke_callback_range(bringup, cpu, st, target, false);
}

static inline void cpuhp_invoke_callback_range_nofail(bool bringup,
						      unsigned int cpu,
						      struct cpuhp_cpu_state *st,
						      enum cpuhp_state target)
{
	__cpuhp_invoke_callback_range(bringup, cpu, st, target, true);
}

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.
	 */