kernel/stop_machine.c - maze/linux - Git at Google

 /*
  * kernel/stop_machine.c
  *
  * Copyright (C) 2008, 2005	IBM Corporation.
  * Copyright (C) 2008, 2005	Rusty Russell rusty@rustcorp.com.au
  * Copyright (C) 2010		SUSE Linux Products GmbH
  * Copyright (C) 2010		Tejun Heo <tj@kernel.org>
  *
  * This file is released under the GPLv2 and any later version.
  */
 #include <linux/completion.h>
 #include <linux/cpu.h>
 #include <linux/init.h>
 #include <linux/kthread.h>
 #include <linux/module.h>
 #include <linux/percpu.h>
 #include <linux/sched.h>
 #include <linux/stop_machine.h>
 #include <linux/interrupt.h>
 #include <linux/kallsyms.h>

 #include <linux/atomic.h>

 /*
  * Structure to determine completion condition and record errors.  May
  * be shared by works on different cpus.
  */
 struct cpu_stop_done {
 	atomic_t		nr_todo;	/* nr left to execute */
 	bool			executed;	/* actually executed? */
 	int			ret;		/* collected return value */
 	struct completion	completion;	/* fired if nr_todo reaches 0 */
 };

 /* the actual stopper, one per every possible cpu, enabled on online cpus */
 struct cpu_stopper {
 	spinlock_t		lock;
 	bool			enabled;	/* is this stopper enabled? */
 	struct list_head	works;		/* list of pending works */
 	struct task_struct	*thread;	/* stopper thread */
 };

 static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);

 static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
 {
 	memset(done, 0, sizeof(*done));
 	atomic_set(&done->nr_todo, nr_todo);
 	init_completion(&done->completion);
 }

 /* signal completion unless @done is NULL */
 static void cpu_stop_signal_done(struct cpu_stop_done *done, bool executed)
 {
 	if (done) {
 		if (executed)
 			done->executed = true;
 		if (atomic_dec_and_test(&done->nr_todo))
 			complete(&done->completion);
 	}
 }

 /* queue @work to @stopper.  if offline, @work is completed immediately */
 static void cpu_stop_queue_work(struct cpu_stopper *stopper,
 				struct cpu_stop_work *work)
 {
 	unsigned long flags;

 	spin_lock_irqsave(&stopper->lock, flags);

 	if (stopper->enabled) {
 		list_add_tail(&work->list, &stopper->works);
 		wake_up_process(stopper->thread);
 	} else
 		cpu_stop_signal_done(work->done, false);

 	spin_unlock_irqrestore(&stopper->lock, flags);
 }

 /**
  * stop_one_cpu - stop a cpu
  * @cpu: cpu to stop
  * @fn: function to execute
  * @arg: argument to @fn
  *
  * Execute @fn(@arg) on @cpu.  @fn is run in a process context with
  * the highest priority preempting any task on the cpu and
  * monopolizing it.  This function returns after the execution is
  * complete.
  *
  * This function doesn't guarantee @cpu stays online till @fn
  * completes.  If @cpu goes down in the middle, execution may happen
  * partially or fully on different cpus.  @fn should either be ready
  * for that or the caller should ensure that @cpu stays online until
  * this function completes.
  *
  * CONTEXT:
  * Might sleep.
  *
  * RETURNS:
  * -ENOENT if @fn(@arg) was not executed because @cpu was offline;
  * otherwise, the return value of @fn.
  */
 int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
 {
 	struct cpu_stop_done done;
 	struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = &done };

 	cpu_stop_init_done(&done, 1);
 	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), &work);
 	wait_for_completion(&done.completion);
 	return done.executed ? done.ret : -ENOENT;
 }

 /**
  * stop_one_cpu_nowait - stop a cpu but don't wait for completion
  * @cpu: cpu to stop
  * @fn: function to execute
  * @arg: argument to @fn
  *
  * Similar to stop_one_cpu() but doesn't wait for completion.  The
  * caller is responsible for ensuring @work_buf is currently unused
  * and will remain untouched until stopper starts executing @fn.
  *
  * CONTEXT:
  * Don't care.
  */
 void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
 			struct cpu_stop_work *work_buf)
 {
 	*work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
 	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), work_buf);
 }

 /* static data for stop_cpus */
 static DEFINE_MUTEX(stop_cpus_mutex);
 static DEFINE_PER_CPU(struct cpu_stop_work, stop_cpus_work);

 static void queue_stop_cpus_work(const struct cpumask *cpumask,
 				 cpu_stop_fn_t fn, void *arg,
 				 struct cpu_stop_done *done)
 {
 	struct cpu_stop_work *work;
 	unsigned int cpu;

 	/* initialize works and done */
 	for_each_cpu(cpu, cpumask) {
 		work = &per_cpu(stop_cpus_work, cpu);
 		work->fn = fn;
 		work->arg = arg;
 		work->done = done;
 	}

 	/*
 	 * Disable preemption while queueing to avoid getting
 	 * preempted by a stopper which might wait for other stoppers
 	 * to enter @fn which can lead to deadlock.
 	 */
 	preempt_disable();
 	for_each_cpu(cpu, cpumask)
 		cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu),
 				    &per_cpu(stop_cpus_work, cpu));
 	preempt_enable();
 }

 static int __stop_cpus(const struct cpumask *cpumask,
 		       cpu_stop_fn_t fn, void *arg)
 {
 	struct cpu_stop_done done;

 	cpu_stop_init_done(&done, cpumask_weight(cpumask));
 	queue_stop_cpus_work(cpumask, fn, arg, &done);
 	wait_for_completion(&done.completion);
 	return done.executed ? done.ret : -ENOENT;
 }

 /**
  * stop_cpus - stop multiple cpus
  * @cpumask: cpus to stop
  * @fn: function to execute
  * @arg: argument to @fn
  *
  * Execute @fn(@arg) on online cpus in @cpumask.  On each target cpu,
  * @fn is run in a process context with the highest priority
  * preempting any task on the cpu and monopolizing it.  This function
  * returns after all executions are complete.
  *
  * This function doesn't guarantee the cpus in @cpumask stay online
  * till @fn completes.  If some cpus go down in the middle, execution
  * on the cpu may happen partially or fully on different cpus.  @fn
  * should either be ready for that or the caller should ensure that
  * the cpus stay online until this function completes.
  *
  * All stop_cpus() calls are serialized making it safe for @fn to wait
  * for all cpus to start executing it.
  *
  * CONTEXT:
  * Might sleep.
  *
  * RETURNS:
  * -ENOENT if @fn(@arg) was not executed at all because all cpus in
  * @cpumask were offline; otherwise, 0 if all executions of @fn
  * returned 0, any non zero return value if any returned non zero.
  */
 int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 {
 	int ret;

 	/* static works are used, process one request at a time */
 	mutex_lock(&stop_cpus_mutex);
 	ret = __stop_cpus(cpumask, fn, arg);
 	mutex_unlock(&stop_cpus_mutex);
 	return ret;
 }

 /**
  * try_stop_cpus - try to stop multiple cpus
  * @cpumask: cpus to stop
  * @fn: function to execute
  * @arg: argument to @fn
  *
  * Identical to stop_cpus() except that it fails with -EAGAIN if
  * someone else is already using the facility.
  *
  * CONTEXT:
  * Might sleep.
  *
  * RETURNS:
  * -EAGAIN if someone else is already stopping cpus, -ENOENT if
  * @fn(@arg) was not executed at all because all cpus in @cpumask were
  * offline; otherwise, 0 if all executions of @fn returned 0, any non
  * zero return value if any returned non zero.
  */
 int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 {
 	int ret;

 	/* static works are used, process one request at a time */
 	if (!mutex_trylock(&stop_cpus_mutex))
 		return -EAGAIN;
 	ret = __stop_cpus(cpumask, fn, arg);
 	mutex_unlock(&stop_cpus_mutex);
 	return ret;
 }

 static int cpu_stopper_thread(void *data)
 {
 	struct cpu_stopper *stopper = data;
 	struct cpu_stop_work *work;
 	int ret;

 repeat:
 	set_current_state(TASK_INTERRUPTIBLE);	/* mb paired w/ kthread_stop */

 	if (kthread_should_stop()) {
 		__set_current_state(TASK_RUNNING);
 		return 0;
 	}

 	work = NULL;
 	spin_lock_irq(&stopper->lock);
 	if (!list_empty(&stopper->works)) {
 		work = list_first_entry(&stopper->works,
 					struct cpu_stop_work, list);
 		list_del_init(&work->list);
 	}
 	spin_unlock_irq(&stopper->lock);

 	if (work) {
 		cpu_stop_fn_t fn = work->fn;
 		void *arg = work->arg;
 		struct cpu_stop_done *done = work->done;
 		char ksym_buf[KSYM_NAME_LEN] __maybe_unused;

 		__set_current_state(TASK_RUNNING);

 		/* cpu stop callbacks are not allowed to sleep */
 		preempt_disable();

 		ret = fn(arg);
 		if (ret)
 			done->ret = ret;

 		/* restore preemption and check it's still balanced */
 		preempt_enable();
 		WARN_ONCE(preempt_count(),
 			  "cpu_stop: %s(%p) leaked preempt count\n",
 			  kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
 					  ksym_buf), arg);

 		cpu_stop_signal_done(done, true);
 	} else
 		schedule();

 	goto repeat;
 }

 extern void sched_set_stop_task(int cpu, struct task_struct *stop);

 /* manage stopper for a cpu, mostly lifted from sched migration thread mgmt */
 static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,
 					   unsigned long action, void *hcpu)
 {
 	unsigned int cpu = (unsigned long)hcpu;
 	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 	struct task_struct *p;

 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_UP_PREPARE:
 		BUG_ON(stopper->thread || stopper->enabled ||
 		       !list_empty(&stopper->works));
 		p = kthread_create_on_node(cpu_stopper_thread,
 					   stopper,
 					   cpu_to_node(cpu),
 					   "migration/%d", cpu);
 		if (IS_ERR(p))
 			return notifier_from_errno(PTR_ERR(p));
 		get_task_struct(p);
 		kthread_bind(p, cpu);
 		sched_set_stop_task(cpu, p);
 		stopper->thread = p;
 		break;

 	case CPU_ONLINE:
 		/* strictly unnecessary, as first user will wake it */
 		wake_up_process(stopper->thread);
 		/* mark enabled */
 		spin_lock_irq(&stopper->lock);
 		stopper->enabled = true;
 		spin_unlock_irq(&stopper->lock);
 		break;

 #ifdef CONFIG_HOTPLUG_CPU
 	case CPU_UP_CANCELED:
 	case CPU_POST_DEAD:
 	{
 		struct cpu_stop_work *work;

 		sched_set_stop_task(cpu, NULL);
 		/* kill the stopper */
 		kthread_stop(stopper->thread);
 		/* drain remaining works */
 		spin_lock_irq(&stopper->lock);
 		list_for_each_entry(work, &stopper->works, list)
 			cpu_stop_signal_done(work->done, false);
 		stopper->enabled = false;
 		spin_unlock_irq(&stopper->lock);
 		/* release the stopper */
 		put_task_struct(stopper->thread);
 		stopper->thread = NULL;
 		break;
 	}
 #endif
 	}

 	return NOTIFY_OK;
 }

 /*
  * Give it a higher priority so that cpu stopper is available to other
  * cpu notifiers.  It currently shares the same priority as sched
  * migration_notifier.
  */
 static struct notifier_block __cpuinitdata cpu_stop_cpu_notifier = {
 	.notifier_call	= cpu_stop_cpu_callback,
 	.priority	= 10,
 };

 static int __init cpu_stop_init(void)
 {
 	void *bcpu = (void *)(long)smp_processor_id();
 	unsigned int cpu;
 	int err;

 	for_each_possible_cpu(cpu) {
 		struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

 		spin_lock_init(&stopper->lock);
 		INIT_LIST_HEAD(&stopper->works);
 	}

 	/* start one for the boot cpu */
 	err = cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_UP_PREPARE,
 				    bcpu);
 	BUG_ON(err != NOTIFY_OK);
 	cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_ONLINE, bcpu);
 	register_cpu_notifier(&cpu_stop_cpu_notifier);

 	return 0;
 }
 early_initcall(cpu_stop_init);

 #ifdef CONFIG_STOP_MACHINE

 /* This controls the threads on each CPU. */
 enum stopmachine_state {
 	/* Dummy starting state for thread. */
 	STOPMACHINE_NONE,
 	/* Awaiting everyone to be scheduled. */
 	STOPMACHINE_PREPARE,
 	/* Disable interrupts. */
 	STOPMACHINE_DISABLE_IRQ,
 	/* Run the function */
 	STOPMACHINE_RUN,
 	/* Exit */
 	STOPMACHINE_EXIT,
 };

 struct stop_machine_data {
 	int			(*fn)(void *);
 	void			*data;
 	/* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
 	unsigned int		num_threads;
 	const struct cpumask	*active_cpus;

 	enum stopmachine_state	state;
 	atomic_t		thread_ack;
 };

 static void set_state(struct stop_machine_data *smdata,
 		      enum stopmachine_state newstate)
 {
 	/* Reset ack counter. */
 	atomic_set(&smdata->thread_ack, smdata->num_threads);
 	smp_wmb();
 	smdata->state = newstate;
 }

 /* Last one to ack a state moves to the next state. */
 static void ack_state(struct stop_machine_data *smdata)
 {
 	if (atomic_dec_and_test(&smdata->thread_ack))
 		set_state(smdata, smdata->state + 1);
 }

 /* This is the cpu_stop function which stops the CPU. */
 static int stop_machine_cpu_stop(void *data)
 {
 	struct stop_machine_data *smdata = data;
 	enum stopmachine_state curstate = STOPMACHINE_NONE;
 	int cpu = smp_processor_id(), err = 0;
 	unsigned long flags;
 	bool is_active;

 	/*
 	 * When called from stop_machine_from_inactive_cpu(), irq might
 	 * already be disabled.  Save the state and restore it on exit.
 	 */
 	local_save_flags(flags);

 	if (!smdata->active_cpus)
 		is_active = cpu == cpumask_first(cpu_online_mask);
 	else
 		is_active = cpumask_test_cpu(cpu, smdata->active_cpus);

 	/* Simple state machine */
 	do {
 		/* Chill out and ensure we re-read stopmachine_state. */
 		cpu_relax();
 		if (smdata->state != curstate) {
 			curstate = smdata->state;
 			switch (curstate) {
 			case STOPMACHINE_DISABLE_IRQ:
 				local_irq_disable();
 				hard_irq_disable();
 				break;
 			case STOPMACHINE_RUN:
 				if (is_active)
 					err = smdata->fn(smdata->data);
 				break;
 			default:
 				break;
 			}
 			ack_state(smdata);
 		}
 	} while (curstate != STOPMACHINE_EXIT);

 	local_irq_restore(flags);
 	return err;
 }

 int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 {
 	struct stop_machine_data smdata = { .fn = fn, .data = data,
 					    .num_threads = num_online_cpus(),
 					    .active_cpus = cpus };

 	/* Set the initial state and stop all online cpus. */
 	set_state(&smdata, STOPMACHINE_PREPARE);
 	return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);
 }

 int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 {
 	int ret;

 	/* No CPUs can come up or down during this. */
 	get_online_cpus();
 	ret = __stop_machine(fn, data, cpus);
 	put_online_cpus();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(stop_machine);

 /**
  * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
  * @fn: the function to run
  * @data: the data ptr for the @fn()
  * @cpus: the cpus to run the @fn() on (NULL = any online cpu)
  *
  * This is identical to stop_machine() but can be called from a CPU which
  * is not active.  The local CPU is in the process of hotplug (so no other
  * CPU hotplug can start) and not marked active and doesn't have enough
  * context to sleep.
  *
  * This function provides stop_machine() functionality for such state by
  * using busy-wait for synchronization and executing @fn directly for local
  * CPU.
  *
  * CONTEXT:
  * Local CPU is inactive.  Temporarily stops all active CPUs.
  *
  * RETURNS:
  * 0 if all executions of @fn returned 0, any non zero return value if any
  * returned non zero.
  */
 int stop_machine_from_inactive_cpu(int (*fn)(void *), void *data,
 				  const struct cpumask *cpus)
 {
 	struct stop_machine_data smdata = { .fn = fn, .data = data,
 					    .active_cpus = cpus };
 	struct cpu_stop_done done;
 	int ret;

 	/* Local CPU must be inactive and CPU hotplug in progress. */
 	BUG_ON(cpu_active(raw_smp_processor_id()));
 	smdata.num_threads = num_active_cpus() + 1;	/* +1 for local */

 	/* No proper task established and can't sleep - busy wait for lock. */
 	while (!mutex_trylock(&stop_cpus_mutex))
 		cpu_relax();

 	/* Schedule work on other CPUs and execute directly for local CPU */
 	set_state(&smdata, STOPMACHINE_PREPARE);
 	cpu_stop_init_done(&done, num_active_cpus());
 	queue_stop_cpus_work(cpu_active_mask, stop_machine_cpu_stop, &smdata,
 			     &done);
 	ret = stop_machine_cpu_stop(&smdata);

 	/* Busy wait for completion. */
 	while (!completion_done(&done.completion))
 		cpu_relax();

 	mutex_unlock(&stop_cpus_mutex);
 	return ret ?: done.ret;
 }

 #endif	/* CONFIG_STOP_MACHINE */
	/*
	* kernel/stop_machine.c
	*
	* Copyright (C) 2008, 2005 IBM Corporation.
	* Copyright (C) 2008, 2005 Rusty Russell rusty@rustcorp.com.au
	* Copyright (C) 2010 SUSE Linux Products GmbH
	* Copyright (C) 2010 Tejun Heo <tj@kernel.org>
	*
	* This file is released under the GPLv2 and any later version.
	*/
	#include <linux/completion.h>
	#include <linux/cpu.h>
	#include <linux/init.h>
	#include <linux/kthread.h>
	#include <linux/module.h>
	#include <linux/percpu.h>
	#include <linux/sched.h>
	#include <linux/stop_machine.h>
	#include <linux/interrupt.h>
	#include <linux/kallsyms.h>

	#include <linux/atomic.h>

	/*
	* Structure to determine completion condition and record errors. May
	* be shared by works on different cpus.
	*/
	struct cpu_stop_done {
	atomic_t nr_todo; /* nr left to execute */
	bool executed; /* actually executed? */
	int ret; /* collected return value */
	struct completion completion; /* fired if nr_todo reaches 0 */
	};

	/* the actual stopper, one per every possible cpu, enabled on online cpus */
	struct cpu_stopper {
	spinlock_t lock;
	bool enabled; /* is this stopper enabled? */
	struct list_head works; /* list of pending works */
	struct task_struct thread; / stopper thread */
	};

	static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);

	static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
	{
	memset(done, 0, sizeof(*done));
	atomic_set(&done->nr_todo, nr_todo);
	init_completion(&done->completion);
	}

	/* signal completion unless @done is NULL */
	static void cpu_stop_signal_done(struct cpu_stop_done *done, bool executed)
	{
	if (done) {
	if (executed)
	done->executed = true;
	if (atomic_dec_and_test(&done->nr_todo))
	complete(&done->completion);
	}
	}

	/* queue @work to @stopper. if offline, @work is completed immediately */
	static void cpu_stop_queue_work(struct cpu_stopper *stopper,
	struct cpu_stop_work *work)
	{
	unsigned long flags;

	spin_lock_irqsave(&stopper->lock, flags);

	if (stopper->enabled) {
	list_add_tail(&work->list, &stopper->works);
	wake_up_process(stopper->thread);
	} else
	cpu_stop_signal_done(work->done, false);

	spin_unlock_irqrestore(&stopper->lock, flags);
	}

	/**
	* stop_one_cpu - stop a cpu
	* @cpu: cpu to stop
	* @fn: function to execute
	* @arg: argument to @fn
	*
	* Execute @fn(@arg) on @cpu. @fn is run in a process context with
	* the highest priority preempting any task on the cpu and
	* monopolizing it. This function returns after the execution is
	* complete.
	*
	* This function doesn't guarantee @cpu stays online till @fn
	* completes. If @cpu goes down in the middle, execution may happen
	* partially or fully on different cpus. @fn should either be ready
	* for that or the caller should ensure that @cpu stays online until
	* this function completes.
	*
	* CONTEXT:
	* Might sleep.
	*
	* RETURNS:
	* -ENOENT if @fn(@arg) was not executed because @cpu was offline;
	* otherwise, the return value of @fn.
	*/
	int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
	{
	struct cpu_stop_done done;
	struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = &done };

	cpu_stop_init_done(&done, 1);
	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), &work);
	wait_for_completion(&done.completion);
	return done.executed ? done.ret : -ENOENT;
	}

	/**
	* stop_one_cpu_nowait - stop a cpu but don't wait for completion
	* @cpu: cpu to stop
	* @fn: function to execute
	* @arg: argument to @fn
	*
	* Similar to stop_one_cpu() but doesn't wait for completion. The
	* caller is responsible for ensuring @work_buf is currently unused
	* and will remain untouched until stopper starts executing @fn.
	*
	* CONTEXT:
	* Don't care.
	*/
	void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
	struct cpu_stop_work *work_buf)
	{
	*work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), work_buf);
	}

	/* static data for stop_cpus */
	static DEFINE_MUTEX(stop_cpus_mutex);
	static DEFINE_PER_CPU(struct cpu_stop_work, stop_cpus_work);

	static void queue_stop_cpus_work(const struct cpumask *cpumask,
	cpu_stop_fn_t fn, void *arg,
	struct cpu_stop_done *done)
	{
	struct cpu_stop_work *work;
	unsigned int cpu;

	/* initialize works and done */
	for_each_cpu(cpu, cpumask) {
	work = &per_cpu(stop_cpus_work, cpu);
	work->fn = fn;
	work->arg = arg;
	work->done = done;
	}

	/*
	* Disable preemption while queueing to avoid getting
	* preempted by a stopper which might wait for other stoppers
	* to enter @fn which can lead to deadlock.
	*/
	preempt_disable();
	for_each_cpu(cpu, cpumask)
	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu),
	&per_cpu(stop_cpus_work, cpu));
	preempt_enable();
	}

	static int __stop_cpus(const struct cpumask *cpumask,
	cpu_stop_fn_t fn, void *arg)
	{
	struct cpu_stop_done done;

	cpu_stop_init_done(&done, cpumask_weight(cpumask));
	queue_stop_cpus_work(cpumask, fn, arg, &done);
	wait_for_completion(&done.completion);
	return done.executed ? done.ret : -ENOENT;
	}

	/**
	* stop_cpus - stop multiple cpus
	* @cpumask: cpus to stop
	* @fn: function to execute
	* @arg: argument to @fn
	*
	* Execute @fn(@arg) on online cpus in @cpumask. On each target cpu,
	* @fn is run in a process context with the highest priority
	* preempting any task on the cpu and monopolizing it. This function
	* returns after all executions are complete.
	*
	* This function doesn't guarantee the cpus in @cpumask stay online
	* till @fn completes. If some cpus go down in the middle, execution
	* on the cpu may happen partially or fully on different cpus. @fn
	* should either be ready for that or the caller should ensure that
	* the cpus stay online until this function completes.
	*
	* All stop_cpus() calls are serialized making it safe for @fn to wait
	* for all cpus to start executing it.
	*
	* CONTEXT:
	* Might sleep.
	*
	* RETURNS:
	* -ENOENT if @fn(@arg) was not executed at all because all cpus in
	* @cpumask were offline; otherwise, 0 if all executions of @fn
	* returned 0, any non zero return value if any returned non zero.
	*/
	int stop_cpus(const struct cpumask cpumask, cpu_stop_fn_t fn, void arg)
	{
	int ret;

	/* static works are used, process one request at a time */
	mutex_lock(&stop_cpus_mutex);
	ret = __stop_cpus(cpumask, fn, arg);
	mutex_unlock(&stop_cpus_mutex);
	return ret;
	}

	/**
	* try_stop_cpus - try to stop multiple cpus
	* @cpumask: cpus to stop
	* @fn: function to execute
	* @arg: argument to @fn
	*
	* Identical to stop_cpus() except that it fails with -EAGAIN if
	* someone else is already using the facility.
	*
	* CONTEXT:
	* Might sleep.
	*
	* RETURNS:
	* -EAGAIN if someone else is already stopping cpus, -ENOENT if
	* @fn(@arg) was not executed at all because all cpus in @cpumask were
	* offline; otherwise, 0 if all executions of @fn returned 0, any non
	* zero return value if any returned non zero.
	*/
	int try_stop_cpus(const struct cpumask cpumask, cpu_stop_fn_t fn, void arg)
	{
	int ret;

	/* static works are used, process one request at a time */
	if (!mutex_trylock(&stop_cpus_mutex))
	return -EAGAIN;
	ret = __stop_cpus(cpumask, fn, arg);
	mutex_unlock(&stop_cpus_mutex);
	return ret;
	}

	static int cpu_stopper_thread(void *data)
	{
	struct cpu_stopper *stopper = data;
	struct cpu_stop_work *work;
	int ret;

	repeat:
	set_current_state(TASK_INTERRUPTIBLE); /* mb paired w/ kthread_stop */

	if (kthread_should_stop()) {
	__set_current_state(TASK_RUNNING);
	return 0;
	}

	work = NULL;
	spin_lock_irq(&stopper->lock);
	if (!list_empty(&stopper->works)) {
	work = list_first_entry(&stopper->works,
	struct cpu_stop_work, list);
	list_del_init(&work->list);
	}
	spin_unlock_irq(&stopper->lock);

	if (work) {
	cpu_stop_fn_t fn = work->fn;
	void *arg = work->arg;
	struct cpu_stop_done *done = work->done;
	char ksym_buf[KSYM_NAME_LEN] __maybe_unused;

	__set_current_state(TASK_RUNNING);

	/* cpu stop callbacks are not allowed to sleep */
	preempt_disable();

	ret = fn(arg);
	if (ret)
	done->ret = ret;

	/* restore preemption and check it's still balanced */
	preempt_enable();
	WARN_ONCE(preempt_count(),
	"cpu_stop: %s(%p) leaked preempt count\n",
	kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
	ksym_buf), arg);

	cpu_stop_signal_done(done, true);
	} else
	schedule();

	goto repeat;
	}

	extern void sched_set_stop_task(int cpu, struct task_struct *stop);

	/* manage stopper for a cpu, mostly lifted from sched migration thread mgmt */
	static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,
	unsigned long action, void *hcpu)
	{
	unsigned int cpu = (unsigned long)hcpu;
	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
	struct task_struct *p;

	switch (action & ~CPU_TASKS_FROZEN) {
	case CPU_UP_PREPARE:
	BUG_ON(stopper->thread \|\| stopper->enabled \|\|
	!list_empty(&stopper->works));
	p = kthread_create_on_node(cpu_stopper_thread,
	stopper,
	cpu_to_node(cpu),
	"migration/%d", cpu);
	if (IS_ERR(p))
	return notifier_from_errno(PTR_ERR(p));
	get_task_struct(p);
	kthread_bind(p, cpu);
	sched_set_stop_task(cpu, p);
	stopper->thread = p;
	break;

	case CPU_ONLINE:
	/* strictly unnecessary, as first user will wake it */
	wake_up_process(stopper->thread);
	/* mark enabled */
	spin_lock_irq(&stopper->lock);
	stopper->enabled = true;
	spin_unlock_irq(&stopper->lock);
	break;

	#ifdef CONFIG_HOTPLUG_CPU
	case CPU_UP_CANCELED:
	case CPU_POST_DEAD:
	{
	struct cpu_stop_work *work;

	sched_set_stop_task(cpu, NULL);
	/* kill the stopper */
	kthread_stop(stopper->thread);
	/* drain remaining works */
	spin_lock_irq(&stopper->lock);
	list_for_each_entry(work, &stopper->works, list)
	cpu_stop_signal_done(work->done, false);
	stopper->enabled = false;
	spin_unlock_irq(&stopper->lock);
	/* release the stopper */
	put_task_struct(stopper->thread);
	stopper->thread = NULL;
	break;
	}
	#endif
	}

	return NOTIFY_OK;
	}

	/*
	* Give it a higher priority so that cpu stopper is available to other
	* cpu notifiers. It currently shares the same priority as sched
	* migration_notifier.
	*/
	static struct notifier_block __cpuinitdata cpu_stop_cpu_notifier = {
	.notifier_call = cpu_stop_cpu_callback,
	.priority = 10,
	};

	static int __init cpu_stop_init(void)
	{
	void bcpu = (void )(long)smp_processor_id();
	unsigned int cpu;
	int err;

	for_each_possible_cpu(cpu) {
	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

	spin_lock_init(&stopper->lock);
	INIT_LIST_HEAD(&stopper->works);
	}

	/* start one for the boot cpu */
	err = cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_UP_PREPARE,
	bcpu);
	BUG_ON(err != NOTIFY_OK);
	cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_ONLINE, bcpu);
	register_cpu_notifier(&cpu_stop_cpu_notifier);

	return 0;
	}
	early_initcall(cpu_stop_init);

	#ifdef CONFIG_STOP_MACHINE

	/* This controls the threads on each CPU. */
	enum stopmachine_state {
	/* Dummy starting state for thread. */
	STOPMACHINE_NONE,
	/* Awaiting everyone to be scheduled. */
	STOPMACHINE_PREPARE,
	/* Disable interrupts. */
	STOPMACHINE_DISABLE_IRQ,
	/* Run the function */
	STOPMACHINE_RUN,
	/* Exit */
	STOPMACHINE_EXIT,
	};

	struct stop_machine_data {
	int (fn)(void );
	void *data;
	/* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
	unsigned int num_threads;
	const struct cpumask *active_cpus;

	enum stopmachine_state state;
	atomic_t thread_ack;
	};

	static void set_state(struct stop_machine_data *smdata,
	enum stopmachine_state newstate)
	{
	/* Reset ack counter. */
	atomic_set(&smdata->thread_ack, smdata->num_threads);
	smp_wmb();
	smdata->state = newstate;
	}

	/* Last one to ack a state moves to the next state. */
	static void ack_state(struct stop_machine_data *smdata)
	{
	if (atomic_dec_and_test(&smdata->thread_ack))
	set_state(smdata, smdata->state + 1);
	}

	/* This is the cpu_stop function which stops the CPU. */
	static int stop_machine_cpu_stop(void *data)
	{
	struct stop_machine_data *smdata = data;
	enum stopmachine_state curstate = STOPMACHINE_NONE;
	int cpu = smp_processor_id(), err = 0;
	unsigned long flags;
	bool is_active;

	/*
	* When called from stop_machine_from_inactive_cpu(), irq might
	* already be disabled. Save the state and restore it on exit.
	*/
	local_save_flags(flags);

	if (!smdata->active_cpus)
	is_active = cpu == cpumask_first(cpu_online_mask);
	else
	is_active = cpumask_test_cpu(cpu, smdata->active_cpus);

	/* Simple state machine */
	do {
	/* Chill out and ensure we re-read stopmachine_state. */
	cpu_relax();
	if (smdata->state != curstate) {
	curstate = smdata->state;
	switch (curstate) {
	case STOPMACHINE_DISABLE_IRQ:
	local_irq_disable();
	hard_irq_disable();
	break;
	case STOPMACHINE_RUN:
	if (is_active)
	err = smdata->fn(smdata->data);
	break;
	default:
	break;
	}
	ack_state(smdata);
	}
	} while (curstate != STOPMACHINE_EXIT);

	local_irq_restore(flags);
	return err;
	}

	int __stop_machine(int (fn)(void ), void data, const struct cpumask cpus)
	{
	struct stop_machine_data smdata = { .fn = fn, .data = data,
	.num_threads = num_online_cpus(),
	.active_cpus = cpus };

	/* Set the initial state and stop all online cpus. */
	set_state(&smdata, STOPMACHINE_PREPARE);
	return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);
	}

	int stop_machine(int (fn)(void ), void data, const struct cpumask cpus)
	{
	int ret;

	/* No CPUs can come up or down during this. */
	get_online_cpus();
	ret = __stop_machine(fn, data, cpus);
	put_online_cpus();
	return ret;
	}
	EXPORT_SYMBOL_GPL(stop_machine);

	/**
	* stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
	* @fn: the function to run
	* @data: the data ptr for the @fn()
	* @cpus: the cpus to run the @fn() on (NULL = any online cpu)
	*
	* This is identical to stop_machine() but can be called from a CPU which
	* is not active. The local CPU is in the process of hotplug (so no other
	* CPU hotplug can start) and not marked active and doesn't have enough
	* context to sleep.
	*
	* This function provides stop_machine() functionality for such state by
	* using busy-wait for synchronization and executing @fn directly for local
	* CPU.
	*
	* CONTEXT:
	* Local CPU is inactive. Temporarily stops all active CPUs.
	*
	* RETURNS:
	* 0 if all executions of @fn returned 0, any non zero return value if any
	* returned non zero.
	*/
	int stop_machine_from_inactive_cpu(int (fn)(void ), void *data,
	const struct cpumask *cpus)
	{
	struct stop_machine_data smdata = { .fn = fn, .data = data,
	.active_cpus = cpus };
	struct cpu_stop_done done;
	int ret;

	/* Local CPU must be inactive and CPU hotplug in progress. */
	BUG_ON(cpu_active(raw_smp_processor_id()));
	smdata.num_threads = num_active_cpus() + 1; /* +1 for local */

	/* No proper task established and can't sleep - busy wait for lock. */
	while (!mutex_trylock(&stop_cpus_mutex))
	cpu_relax();

	/* Schedule work on other CPUs and execute directly for local CPU */
	set_state(&smdata, STOPMACHINE_PREPARE);
	cpu_stop_init_done(&done, num_active_cpus());
	queue_stop_cpus_work(cpu_active_mask, stop_machine_cpu_stop, &smdata,
	&done);
	ret = stop_machine_cpu_stop(&smdata);

	/* Busy wait for completion. */
	while (!completion_done(&done.completion))
	cpu_relax();

	mutex_unlock(&stop_cpus_mutex);
	return ret ?: done.ret;
	}

	#endif /* CONFIG_STOP_MACHINE */