stop_machine: Don't do for_each_cpu() twice in queue_stop_cpus_work()

[mirror_ubuntu-zesty-kernel.git] / kernel / stop_machine.c

/*
 * 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/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/smpboot.h>
#include <linux/atomic.h>
#include <linux/lglock.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 {
        struct task_struct      *thread;

        spinlock_t              lock;
        bool                    enabled;        /* is this stopper enabled? */
        struct list_head        works;          /* list of pending works */

        struct cpu_stop_work    stop_work;      /* for stop_cpus */
};

static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
static bool stop_machine_initialized = false;

/*
 * Avoids a race between stop_two_cpus and global stop_cpus, where
 * the stoppers could get queued up in reverse order, leading to
 * system deadlock. Using an lglock means stop_two_cpus remains
 * relatively cheap.
 */
DEFINE_STATIC_LGLOCK(stop_cpus_lock);

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(unsigned int cpu, struct cpu_stop_work *work)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

        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(cpu, &work);
        wait_for_completion(&done.completion);
        return done.executed ? done.ret : -ENOENT;
}

/* This controls the threads on each CPU. */
enum multi_stop_state {
        /* Dummy starting state for thread. */
        MULTI_STOP_NONE,
        /* Awaiting everyone to be scheduled. */
        MULTI_STOP_PREPARE,
        /* Disable interrupts. */
        MULTI_STOP_DISABLE_IRQ,
        /* Run the function */
        MULTI_STOP_RUN,
        /* Exit */
        MULTI_STOP_EXIT,
};

struct multi_stop_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 multi_stop_state   state;
        atomic_t                thread_ack;
};

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

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

/* This is the cpu_stop function which stops the CPU. */
static int multi_cpu_stop(void *data)
{
        struct multi_stop_data *msdata = data;
        enum multi_stop_state curstate = MULTI_STOP_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 (!msdata->active_cpus)
                is_active = cpu == cpumask_first(cpu_online_mask);
        else
                is_active = cpumask_test_cpu(cpu, msdata->active_cpus);

        /* Simple state machine */
        do {
                /* Chill out and ensure we re-read multi_stop_state. */
                cpu_relax();
                if (msdata->state != curstate) {
                        curstate = msdata->state;
                        switch (curstate) {
                        case MULTI_STOP_DISABLE_IRQ:
                                local_irq_disable();
                                hard_irq_disable();
                                break;
                        case MULTI_STOP_RUN:
                                if (is_active)
                                        err = msdata->fn(msdata->data);
                                break;
                        default:
                                break;
                        }
                        ack_state(msdata);
                }
        } while (curstate != MULTI_STOP_EXIT);

        local_irq_restore(flags);
        return err;
}

/**
 * stop_two_cpus - stops two cpus
 * @cpu1: the cpu to stop
 * @cpu2: the other cpu to stop
 * @fn: function to execute
 * @arg: argument to @fn
 *
 * Stops both the current and specified CPU and runs @fn on one of them.
 *
 * returns when both are completed.
 */
int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
{
        struct cpu_stop_done done;
        struct cpu_stop_work work1, work2;
        struct multi_stop_data msdata;

        preempt_disable();
        msdata = (struct multi_stop_data){
                .fn = fn,
                .data = arg,
                .num_threads = 2,
                .active_cpus = cpumask_of(cpu1),
        };

        work1 = work2 = (struct cpu_stop_work){
                .fn = multi_cpu_stop,
                .arg = &msdata,
                .done = &done
        };

        cpu_stop_init_done(&done, 2);
        set_state(&msdata, MULTI_STOP_PREPARE);

        /*
         * If we observe both CPUs active we know _cpu_down() cannot yet have
         * queued its stop_machine works and therefore ours will get executed
         * first. Or its not either one of our CPUs that's getting unplugged,
         * in which case we don't care.
         *
         * This relies on the stopper workqueues to be FIFO.
         */
        if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
                preempt_enable();
                return -ENOENT;
        }

        lg_double_lock(&stop_cpus_lock, cpu1, cpu2);
        cpu_stop_queue_work(cpu1, &work1);
        cpu_stop_queue_work(cpu2, &work2);
        lg_double_unlock(&stop_cpus_lock, cpu1, cpu2);

        preempt_enable();

        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
 * @work_buf: pointer to cpu_stop_work structure
 *
 * 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(cpu, work_buf);
}

/* static data for stop_cpus */
static DEFINE_MUTEX(stop_cpus_mutex);

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;

        /*
         * 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.
         */
        lg_global_lock(&stop_cpus_lock);
        for_each_cpu(cpu, cpumask) {
                work = &per_cpu(cpu_stopper.stop_work, cpu);
                work->fn = fn;
                work->arg = arg;
                work->done = done;
                cpu_stop_queue_work(cpu, work);
        }
        lg_global_unlock(&stop_cpus_lock);
}

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_stop_should_run(unsigned int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        unsigned long flags;
        int run;

        spin_lock_irqsave(&stopper->lock, flags);
        run = !list_empty(&stopper->works);
        spin_unlock_irqrestore(&stopper->lock, flags);
        return run;
}

static void cpu_stopper_thread(unsigned int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        struct cpu_stop_work *work;
        int ret;

repeat:
        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;

                /* 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);
                goto repeat;
        }
}

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

static void cpu_stop_create(unsigned int cpu)
{
        sched_set_stop_task(cpu, per_cpu(cpu_stopper.thread, cpu));
}

static void cpu_stop_park(unsigned int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        struct cpu_stop_work *work;
        unsigned long flags;

        /* drain remaining works */
        spin_lock_irqsave(&stopper->lock, flags);
        list_for_each_entry(work, &stopper->works, list)
                cpu_stop_signal_done(work->done, false);
        stopper->enabled = false;
        spin_unlock_irqrestore(&stopper->lock, flags);
}

static void cpu_stop_unpark(unsigned int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

        spin_lock_irq(&stopper->lock);
        stopper->enabled = true;
        spin_unlock_irq(&stopper->lock);
}

static struct smp_hotplug_thread cpu_stop_threads = {
        .store                  = &cpu_stopper.thread,
        .thread_should_run      = cpu_stop_should_run,
        .thread_fn              = cpu_stopper_thread,
        .thread_comm            = "migration/%u",
        .create                 = cpu_stop_create,
        .setup                  = cpu_stop_unpark,
        .park                   = cpu_stop_park,
        .pre_unpark             = cpu_stop_unpark,
        .selfparking            = true,
};

static int __init cpu_stop_init(void)
{
        unsigned int cpu;

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

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

        BUG_ON(smpboot_register_percpu_thread(&cpu_stop_threads));
        stop_machine_initialized = true;
        return 0;
}
early_initcall(cpu_stop_init);

#ifdef CONFIG_STOP_MACHINE

int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
{
        struct multi_stop_data msdata = {
                .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(msdata.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(&msdata, MULTI_STOP_PREPARE);
        return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);
}

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 multi_stop_data msdata = { .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()));
        msdata.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(&msdata, MULTI_STOP_PREPARE);
        cpu_stop_init_done(&done, num_active_cpus());
        queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop, &msdata,
                             &done);
        ret = multi_cpu_stop(&msdata);

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

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

#endif  /* CONFIG_STOP_MACHINE */