stop_machine.c

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/*
 * kernel/stop_machine.c
 *
 * Copyright (C) 2008, 2005 IBM Corporation.
 * Copyright (C) 2008, 2005 Rusty Russell [email protected]
 * Copyright (C) 2010       SUSE Linux Products GmbH
 * Copyright (C) 2010       Tejun Heo <[email protected]>
 *
 * 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/export.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 bool stop_machine_initialized = false;

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);
}

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;
}

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;
}

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;
}

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);

    stop_machine_initialized = true;

    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 };

    if (!stop_machine_initialized) {
        /*
         * Handle the case where stop_machine() is called
         * early in boot before stop_machine() has been
         * initialized.
         */
        unsigned long flags;
        int ret;

        WARN_ON_ONCE(smdata.num_threads != 1);

        local_irq_save(flags);
        hard_irq_disable();
        ret = (*fn)(data);
        local_irq_restore(flags);

        return ret;
    }

    /* 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);

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 */