workqueue: don't clear cwq->thread until it exits

[safe/jmp/linux-2.6] / kernel / workqueue.c

/*
 * linux/kernel/workqueue.c
 *
 * Generic mechanism for defining kernel helper threads for running
 * arbitrary tasks in process context.
 *
 * Started by Ingo Molnar, Copyright (C) 2002
 *
 * Derived from the taskqueue/keventd code by:
 *
 *   David Woodhouse <dwmw2@infradead.org>
 *   Andrew Morton <andrewm@uow.edu.au>
 *   Kai Petzke <wpp@marie.physik.tu-berlin.de>
 *   Theodore Ts'o <tytso@mit.edu>
 *
 * Made to use alloc_percpu by Christoph Lameter <clameter@sgi.com>.
 */

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/signal.h>
#include <linux/completion.h>
#include <linux/workqueue.h>
#include <linux/slab.h>
#include <linux/cpu.h>
#include <linux/notifier.h>
#include <linux/kthread.h>
#include <linux/hardirq.h>
#include <linux/mempolicy.h>
#include <linux/freezer.h>
#include <linux/kallsyms.h>
#include <linux/debug_locks.h>

/*
 * The per-CPU workqueue (if single thread, we always use the first
 * possible cpu).
 */
struct cpu_workqueue_struct {

        spinlock_t lock;

        struct list_head worklist;
        wait_queue_head_t more_work;

        struct workqueue_struct *wq;
        struct task_struct *thread;
        struct work_struct *current_work;

        int run_depth;          /* Detect run_workqueue() recursion depth */
} ____cacheline_aligned;

/*
 * The externally visible workqueue abstraction is an array of
 * per-CPU workqueues:
 */
struct workqueue_struct {
        struct cpu_workqueue_struct *cpu_wq;
        const char *name;
        struct list_head list;  /* Empty if single thread */
        int freezeable;         /* Freeze threads during suspend */
};

/* All the per-cpu workqueues on the system, for hotplug cpu to add/remove
   threads to each one as cpus come/go. */
static long migrate_sequence __read_mostly;
static DEFINE_MUTEX(workqueue_mutex);
static LIST_HEAD(workqueues);

static int singlethread_cpu;

/* If it's single threaded, it isn't in the list of workqueues. */
static inline int is_single_threaded(struct workqueue_struct *wq)
{
        return list_empty(&wq->list);
}

/*
 * Set the workqueue on which a work item is to be run
 * - Must *only* be called if the pending flag is set
 */
static inline void set_wq_data(struct work_struct *work, void *wq)
{
        unsigned long new;

        BUG_ON(!work_pending(work));

        new = (unsigned long) wq | (1UL << WORK_STRUCT_PENDING);
        new |= WORK_STRUCT_FLAG_MASK & *work_data_bits(work);
        atomic_long_set(&work->data, new);
}

static inline void *get_wq_data(struct work_struct *work)
{
        return (void *) (atomic_long_read(&work->data) & WORK_STRUCT_WQ_DATA_MASK);
}

static int __run_work(struct cpu_workqueue_struct *cwq, struct work_struct *work)
{
        int ret = 0;
        unsigned long flags;

        spin_lock_irqsave(&cwq->lock, flags);
        /*
         * We need to re-validate the work info after we've gotten
         * the cpu_workqueue lock. We can run the work now iff:
         *
         *  - the wq_data still matches the cpu_workqueue_struct
         *  - AND the work is still marked pending
         *  - AND the work is still on a list (which will be this
         *    workqueue_struct list)
         *
         * All these conditions are important, because we
         * need to protect against the work being run right
         * now on another CPU (all but the last one might be
         * true if it's currently running and has not been
         * released yet, for example).
         */
        if (get_wq_data(work) == cwq
            && work_pending(work)
            && !list_empty(&work->entry)) {
                work_func_t f = work->func;
                cwq->current_work = work;
                list_del_init(&work->entry);
                spin_unlock_irqrestore(&cwq->lock, flags);

                if (!test_bit(WORK_STRUCT_NOAUTOREL, work_data_bits(work)))
                        work_release(work);
                f(work);

                spin_lock_irqsave(&cwq->lock, flags);
                cwq->current_work = NULL;
                ret = 1;
        }
        spin_unlock_irqrestore(&cwq->lock, flags);
        return ret;
}

/**
 * run_scheduled_work - run scheduled work synchronously
 * @work: work to run
 *
 * This checks if the work was pending, and runs it
 * synchronously if so. It returns a boolean to indicate
 * whether it had any scheduled work to run or not.
 *
 * NOTE! This _only_ works for normal work_structs. You
 * CANNOT use this for delayed work, because the wq data
 * for delayed work will not point properly to the per-
 * CPU workqueue struct, but will change!
 */
int fastcall run_scheduled_work(struct work_struct *work)
{
        for (;;) {
                struct cpu_workqueue_struct *cwq;

                if (!work_pending(work))
                        return 0;
                if (list_empty(&work->entry))
                        return 0;
                /* NOTE! This depends intimately on __queue_work! */
                cwq = get_wq_data(work);
                if (!cwq)
                        return 0;
                if (__run_work(cwq, work))
                        return 1;
        }
}
EXPORT_SYMBOL(run_scheduled_work);

static void insert_work(struct cpu_workqueue_struct *cwq,
                                struct work_struct *work, int tail)
{
        set_wq_data(work, cwq);
        if (tail)
                list_add_tail(&work->entry, &cwq->worklist);
        else
                list_add(&work->entry, &cwq->worklist);
        wake_up(&cwq->more_work);
}

/* Preempt must be disabled. */
static void __queue_work(struct cpu_workqueue_struct *cwq,
                         struct work_struct *work)
{
        unsigned long flags;

        spin_lock_irqsave(&cwq->lock, flags);
        insert_work(cwq, work, 1);
        spin_unlock_irqrestore(&cwq->lock, flags);
}

/**
 * queue_work - queue work on a workqueue
 * @wq: workqueue to use
 * @work: work to queue
 *
 * Returns 0 if @work was already on a queue, non-zero otherwise.
 *
 * We queue the work to the CPU it was submitted, but there is no
 * guarantee that it will be processed by that CPU.
 */
int fastcall queue_work(struct workqueue_struct *wq, struct work_struct *work)
{
        int ret = 0, cpu = get_cpu();

        if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) {
                if (unlikely(is_single_threaded(wq)))
                        cpu = singlethread_cpu;
                BUG_ON(!list_empty(&work->entry));
                __queue_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
                ret = 1;
        }
        put_cpu();
        return ret;
}
EXPORT_SYMBOL_GPL(queue_work);

void delayed_work_timer_fn(unsigned long __data)
{
        struct delayed_work *dwork = (struct delayed_work *)__data;
        struct workqueue_struct *wq = get_wq_data(&dwork->work);
        int cpu = smp_processor_id();

        if (unlikely(is_single_threaded(wq)))
                cpu = singlethread_cpu;

        __queue_work(per_cpu_ptr(wq->cpu_wq, cpu), &dwork->work);
}

/**
 * queue_delayed_work - queue work on a workqueue after delay
 * @wq: workqueue to use
 * @dwork: delayable work to queue
 * @delay: number of jiffies to wait before queueing
 *
 * Returns 0 if @work was already on a queue, non-zero otherwise.
 */
int fastcall queue_delayed_work(struct workqueue_struct *wq,
                        struct delayed_work *dwork, unsigned long delay)
{
        int ret = 0;
        struct timer_list *timer = &dwork->timer;
        struct work_struct *work = &dwork->work;

        timer_stats_timer_set_start_info(timer);
        if (delay == 0)
                return queue_work(wq, work);

        if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) {
                BUG_ON(timer_pending(timer));
                BUG_ON(!list_empty(&work->entry));

                /* This stores wq for the moment, for the timer_fn */
                set_wq_data(work, wq);
                timer->expires = jiffies + delay;
                timer->data = (unsigned long)dwork;
                timer->function = delayed_work_timer_fn;
                add_timer(timer);
                ret = 1;
        }
        return ret;
}
EXPORT_SYMBOL_GPL(queue_delayed_work);

/**
 * queue_delayed_work_on - queue work on specific CPU after delay
 * @cpu: CPU number to execute work on
 * @wq: workqueue to use
 * @dwork: work to queue
 * @delay: number of jiffies to wait before queueing
 *
 * Returns 0 if @work was already on a queue, non-zero otherwise.
 */
int queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
                        struct delayed_work *dwork, unsigned long delay)
{
        int ret = 0;
        struct timer_list *timer = &dwork->timer;
        struct work_struct *work = &dwork->work;

        if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) {
                BUG_ON(timer_pending(timer));
                BUG_ON(!list_empty(&work->entry));

                /* This stores wq for the moment, for the timer_fn */
                set_wq_data(work, wq);
                timer->expires = jiffies + delay;
                timer->data = (unsigned long)dwork;
                timer->function = delayed_work_timer_fn;
                add_timer_on(timer, cpu);
                ret = 1;
        }
        return ret;
}
EXPORT_SYMBOL_GPL(queue_delayed_work_on);

static void run_workqueue(struct cpu_workqueue_struct *cwq)
{
        unsigned long flags;

        /*
         * Keep taking off work from the queue until
         * done.
         */
        spin_lock_irqsave(&cwq->lock, flags);
        cwq->run_depth++;
        if (cwq->run_depth > 3) {
                /* morton gets to eat his hat */
                printk("%s: recursion depth exceeded: %d\n",
                        __FUNCTION__, cwq->run_depth);
                dump_stack();
        }
        while (!list_empty(&cwq->worklist)) {
                struct work_struct *work = list_entry(cwq->worklist.next,
                                                struct work_struct, entry);
                work_func_t f = work->func;

                cwq->current_work = work;
                list_del_init(cwq->worklist.next);
                spin_unlock_irqrestore(&cwq->lock, flags);

                BUG_ON(get_wq_data(work) != cwq);
                if (!test_bit(WORK_STRUCT_NOAUTOREL, work_data_bits(work)))
                        work_release(work);
                f(work);

                if (unlikely(in_atomic() || lockdep_depth(current) > 0)) {
                        printk(KERN_ERR "BUG: workqueue leaked lock or atomic: "
                                        "%s/0x%08x/%d\n",
                                        current->comm, preempt_count(),
                                        current->pid);
                        printk(KERN_ERR "    last function: ");
                        print_symbol("%s\n", (unsigned long)f);
                        debug_show_held_locks(current);
                        dump_stack();
                }

                spin_lock_irqsave(&cwq->lock, flags);
                cwq->current_work = NULL;
        }
        cwq->run_depth--;
        spin_unlock_irqrestore(&cwq->lock, flags);
}

static int worker_thread(void *__cwq)
{
        struct cpu_workqueue_struct *cwq = __cwq;
        DECLARE_WAITQUEUE(wait, current);
        struct k_sigaction sa;
        sigset_t blocked;

        if (!cwq->wq->freezeable)
                current->flags |= PF_NOFREEZE;

        set_user_nice(current, -5);

        /* Block and flush all signals */
        sigfillset(&blocked);
        sigprocmask(SIG_BLOCK, &blocked, NULL);
        flush_signals(current);

        /*
         * We inherited MPOL_INTERLEAVE from the booting kernel.
         * Set MPOL_DEFAULT to insure node local allocations.
         */
        numa_default_policy();

        /* SIG_IGN makes children autoreap: see do_notify_parent(). */
        sa.sa.sa_handler = SIG_IGN;
        sa.sa.sa_flags = 0;
        siginitset(&sa.sa.sa_mask, sigmask(SIGCHLD));
        do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);

        set_current_state(TASK_INTERRUPTIBLE);
        while (!kthread_should_stop()) {
                if (cwq->wq->freezeable)
                        try_to_freeze();

                add_wait_queue(&cwq->more_work, &wait);
                if (list_empty(&cwq->worklist))
                        schedule();
                else
                        __set_current_state(TASK_RUNNING);
                remove_wait_queue(&cwq->more_work, &wait);

                if (!list_empty(&cwq->worklist))
                        run_workqueue(cwq);
                set_current_state(TASK_INTERRUPTIBLE);
        }
        __set_current_state(TASK_RUNNING);
        return 0;
}

struct wq_barrier {
        struct work_struct      work;
        struct completion       done;
};

static void wq_barrier_func(struct work_struct *work)
{
        struct wq_barrier *barr = container_of(work, struct wq_barrier, work);
        complete(&barr->done);
}

static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
                                        struct wq_barrier *barr, int tail)
{
        INIT_WORK(&barr->work, wq_barrier_func);
        __set_bit(WORK_STRUCT_PENDING, work_data_bits(&barr->work));

        init_completion(&barr->done);

        insert_work(cwq, &barr->work, tail);
}

static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
{
        if (cwq->thread == current) {
                /*
                 * Probably keventd trying to flush its own queue. So simply run
                 * it by hand rather than deadlocking.
                 */
                run_workqueue(cwq);
        } else {
                struct wq_barrier barr;
                int active = 0;

                spin_lock_irq(&cwq->lock);
                if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) {
                        insert_wq_barrier(cwq, &barr, 1);
                        active = 1;
                }
                spin_unlock_irq(&cwq->lock);

                if (active)
                        wait_for_completion(&barr.done);
        }
}

/**
 * flush_workqueue - ensure that any scheduled work has run to completion.
 * @wq: workqueue to flush
 *
 * Forces execution of the workqueue and blocks until its completion.
 * This is typically used in driver shutdown handlers.
 *
 * We sleep until all works which were queued on entry have been handled,
 * but we are not livelocked by new incoming ones.
 *
 * This function used to run the workqueues itself.  Now we just wait for the
 * helper threads to do it.
 */
void fastcall flush_workqueue(struct workqueue_struct *wq)
{
        if (is_single_threaded(wq)) {
                /* Always use first cpu's area. */
                flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));
        } else {
                long sequence;
                int cpu;
again:
                sequence = migrate_sequence;

                for_each_possible_cpu(cpu)
                        flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));

                if (unlikely(sequence != migrate_sequence))
                        goto again;
        }
}
EXPORT_SYMBOL_GPL(flush_workqueue);

static void wait_on_work(struct cpu_workqueue_struct *cwq,
                                struct work_struct *work)
{
        struct wq_barrier barr;
        int running = 0;

        spin_lock_irq(&cwq->lock);
        if (unlikely(cwq->current_work == work)) {
                insert_wq_barrier(cwq, &barr, 0);
                running = 1;
        }
        spin_unlock_irq(&cwq->lock);

        if (unlikely(running)) {
                mutex_unlock(&workqueue_mutex);
                wait_for_completion(&barr.done);
                mutex_lock(&workqueue_mutex);
        }
}

/**
 * flush_work - block until a work_struct's callback has terminated
 * @wq: the workqueue on which the work is queued
 * @work: the work which is to be flushed
 *
 * flush_work() will attempt to cancel the work if it is queued.  If the work's
 * callback appears to be running, flush_work() will block until it has
 * completed.
 *
 * flush_work() is designed to be used when the caller is tearing down data
 * structures which the callback function operates upon.  It is expected that,
 * prior to calling flush_work(), the caller has arranged for the work to not
 * be requeued.
 */
void flush_work(struct workqueue_struct *wq, struct work_struct *work)
{
        struct cpu_workqueue_struct *cwq;

        mutex_lock(&workqueue_mutex);
        cwq = get_wq_data(work);
        /* Was it ever queued ? */
        if (!cwq)
                goto out;

        /*
         * This work can't be re-queued, and the lock above protects us
         * from take_over_work(), no need to re-check that get_wq_data()
         * is still the same when we take cwq->lock.
         */
        spin_lock_irq(&cwq->lock);
        list_del_init(&work->entry);
        work_release(work);
        spin_unlock_irq(&cwq->lock);

        if (is_single_threaded(wq)) {
                /* Always use first cpu's area. */
                wait_on_work(per_cpu_ptr(wq->cpu_wq, singlethread_cpu), work);
        } else {
                int cpu;

                for_each_online_cpu(cpu)
                        wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
        }
out:
        mutex_unlock(&workqueue_mutex);
}
EXPORT_SYMBOL_GPL(flush_work);

static void init_cpu_workqueue(struct workqueue_struct *wq, int cpu)
{
        struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);

        cwq->wq = wq;
        spin_lock_init(&cwq->lock);
        INIT_LIST_HEAD(&cwq->worklist);
        init_waitqueue_head(&cwq->more_work);
}

static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,
                                                        int cpu)
{
        struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
        struct task_struct *p;

        if (is_single_threaded(wq))
                p = kthread_create(worker_thread, cwq, "%s", wq->name);
        else
                p = kthread_create(worker_thread, cwq, "%s/%d", wq->name, cpu);
        if (IS_ERR(p))
                return NULL;
        cwq->thread = p;
        return p;
}

struct workqueue_struct *__create_workqueue(const char *name,
                                            int singlethread, int freezeable)
{
        int cpu, destroy = 0;
        struct workqueue_struct *wq;
        struct task_struct *p;

        wq = kzalloc(sizeof(*wq), GFP_KERNEL);
        if (!wq)
                return NULL;

        wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct);
        if (!wq->cpu_wq) {
                kfree(wq);
                return NULL;
        }

        wq->name = name;
        wq->freezeable = freezeable;

        mutex_lock(&workqueue_mutex);
        if (singlethread) {
                INIT_LIST_HEAD(&wq->list);
                init_cpu_workqueue(wq, singlethread_cpu);
                p = create_workqueue_thread(wq, singlethread_cpu);
                if (!p)
                        destroy = 1;
                else
                        wake_up_process(p);
        } else {
                list_add(&wq->list, &workqueues);
                for_each_possible_cpu(cpu) {
                        init_cpu_workqueue(wq, cpu);
                        if (!cpu_online(cpu))
                                continue;

                        p = create_workqueue_thread(wq, cpu);
                        if (p) {
                                kthread_bind(p, cpu);
                                wake_up_process(p);
                        } else
                                destroy = 1;
                }
        }
        mutex_unlock(&workqueue_mutex);

        /*
         * Was there any error during startup? If yes then clean up:
         */
        if (destroy) {
                destroy_workqueue(wq);
                wq = NULL;
        }
        return wq;
}
EXPORT_SYMBOL_GPL(__create_workqueue);

static void cleanup_workqueue_thread(struct workqueue_struct *wq, int cpu)
{
        struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);

        if (cwq->thread) {
                kthread_stop(cwq->thread);
                cwq->thread = NULL;
        }
}

/**
 * destroy_workqueue - safely terminate a workqueue
 * @wq: target workqueue
 *
 * Safely destroy a workqueue. All work currently pending will be done first.
 */
void destroy_workqueue(struct workqueue_struct *wq)
{
        int cpu;

        flush_workqueue(wq);

        /* We don't need the distraction of CPUs appearing and vanishing. */
        mutex_lock(&workqueue_mutex);
        if (is_single_threaded(wq))
                cleanup_workqueue_thread(wq, singlethread_cpu);
        else {
                for_each_online_cpu(cpu)
                        cleanup_workqueue_thread(wq, cpu);
                list_del(&wq->list);
        }
        mutex_unlock(&workqueue_mutex);
        free_percpu(wq->cpu_wq);
        kfree(wq);
}
EXPORT_SYMBOL_GPL(destroy_workqueue);

static struct workqueue_struct *keventd_wq;

/**
 * schedule_work - put work task in global workqueue
 * @work: job to be done
 *
 * This puts a job in the kernel-global workqueue.
 */
int fastcall schedule_work(struct work_struct *work)
{
        return queue_work(keventd_wq, work);
}
EXPORT_SYMBOL(schedule_work);

/**
 * schedule_delayed_work - put work task in global workqueue after delay
 * @dwork: job to be done
 * @delay: number of jiffies to wait or 0 for immediate execution
 *
 * After waiting for a given time this puts a job in the kernel-global
 * workqueue.
 */
int fastcall schedule_delayed_work(struct delayed_work *dwork,
                                        unsigned long delay)
{
        timer_stats_timer_set_start_info(&dwork->timer);
        return queue_delayed_work(keventd_wq, dwork, delay);
}
EXPORT_SYMBOL(schedule_delayed_work);

/**
 * schedule_delayed_work_on - queue work in global workqueue on CPU after delay
 * @cpu: cpu to use
 * @dwork: job to be done
 * @delay: number of jiffies to wait
 *
 * After waiting for a given time this puts a job in the kernel-global
 * workqueue on the specified CPU.
 */
int schedule_delayed_work_on(int cpu,
                        struct delayed_work *dwork, unsigned long delay)
{
        return queue_delayed_work_on(cpu, keventd_wq, dwork, delay);
}
EXPORT_SYMBOL(schedule_delayed_work_on);

/**
 * schedule_on_each_cpu - call a function on each online CPU from keventd
 * @func: the function to call
 *
 * Returns zero on success.
 * Returns -ve errno on failure.
 *
 * Appears to be racy against CPU hotplug.
 *
 * schedule_on_each_cpu() is very slow.
 */
int schedule_on_each_cpu(work_func_t func)
{
        int cpu;
        struct work_struct *works;

        works = alloc_percpu(struct work_struct);
        if (!works)
                return -ENOMEM;

        preempt_disable();              /* CPU hotplug */
        for_each_online_cpu(cpu) {
                struct work_struct *work = per_cpu_ptr(works, cpu);

                INIT_WORK(work, func);
                set_bit(WORK_STRUCT_PENDING, work_data_bits(work));
                __queue_work(per_cpu_ptr(keventd_wq->cpu_wq, cpu), work);
        }
        preempt_enable();
        flush_workqueue(keventd_wq);
        free_percpu(works);
        return 0;
}

void flush_scheduled_work(void)
{
        flush_workqueue(keventd_wq);
}
EXPORT_SYMBOL(flush_scheduled_work);

void flush_work_keventd(struct work_struct *work)
{
        flush_work(keventd_wq, work);
}
EXPORT_SYMBOL(flush_work_keventd);

/**
 * cancel_rearming_delayed_workqueue - reliably kill off a delayed work whose handler rearms the delayed work.
 * @wq:   the controlling workqueue structure
 * @dwork: the delayed work struct
 */
void cancel_rearming_delayed_workqueue(struct workqueue_struct *wq,
                                       struct delayed_work *dwork)
{
        while (!cancel_delayed_work(dwork))
                flush_workqueue(wq);
}
EXPORT_SYMBOL(cancel_rearming_delayed_workqueue);

/**
 * cancel_rearming_delayed_work - reliably kill off a delayed keventd work whose handler rearms the delayed work.
 * @dwork: the delayed work struct
 */
void cancel_rearming_delayed_work(struct delayed_work *dwork)
{
        cancel_rearming_delayed_workqueue(keventd_wq, dwork);
}
EXPORT_SYMBOL(cancel_rearming_delayed_work);

/**
 * execute_in_process_context - reliably execute the routine with user context
 * @fn:         the function to execute
 * @ew:         guaranteed storage for the execute work structure (must
 *              be available when the work executes)
 *
 * Executes the function immediately if process context is available,
 * otherwise schedules the function for delayed execution.
 *
 * Returns:     0 - function was executed
 *              1 - function was scheduled for execution
 */
int execute_in_process_context(work_func_t fn, struct execute_work *ew)
{
        if (!in_interrupt()) {
                fn(&ew->work);
                return 0;
        }

        INIT_WORK(&ew->work, fn);
        schedule_work(&ew->work);

        return 1;
}
EXPORT_SYMBOL_GPL(execute_in_process_context);

int keventd_up(void)
{
        return keventd_wq != NULL;
}

int current_is_keventd(void)
{
        struct cpu_workqueue_struct *cwq;
        int cpu = smp_processor_id();   /* preempt-safe: keventd is per-cpu */
        int ret = 0;

        BUG_ON(!keventd_wq);

        cwq = per_cpu_ptr(keventd_wq->cpu_wq, cpu);
        if (current == cwq->thread)
                ret = 1;

        return ret;

}

/* Take the work from this (downed) CPU. */
static void take_over_work(struct workqueue_struct *wq, unsigned int cpu)
{
        struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
        struct list_head list;
        struct work_struct *work;

        spin_lock_irq(&cwq->lock);
        list_replace_init(&cwq->worklist, &list);
        migrate_sequence++;

        while (!list_empty(&list)) {
                printk("Taking work for %s\n", wq->name);
                work = list_entry(list.next,struct work_struct,entry);
                list_del(&work->entry);
                __queue_work(per_cpu_ptr(wq->cpu_wq, smp_processor_id()), work);
        }
        spin_unlock_irq(&cwq->lock);
}

/* We're holding the cpucontrol mutex here */
static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
                                  unsigned long action,
                                  void *hcpu)
{
        unsigned int hotcpu = (unsigned long)hcpu;
        struct workqueue_struct *wq;

        switch (action) {
        case CPU_UP_PREPARE:
                mutex_lock(&workqueue_mutex);
                /* Create a new workqueue thread for it. */
                list_for_each_entry(wq, &workqueues, list) {
                        if (!create_workqueue_thread(wq, hotcpu)) {
                                printk("workqueue for %i failed\n", hotcpu);
                                return NOTIFY_BAD;
                        }
                }
                break;

        case CPU_ONLINE:
                /* Kick off worker threads. */
                list_for_each_entry(wq, &workqueues, list) {
                        struct cpu_workqueue_struct *cwq;

                        cwq = per_cpu_ptr(wq->cpu_wq, hotcpu);
                        kthread_bind(cwq->thread, hotcpu);
                        wake_up_process(cwq->thread);
                }
                mutex_unlock(&workqueue_mutex);
                break;

        case CPU_UP_CANCELED:
                list_for_each_entry(wq, &workqueues, list) {
                        if (!per_cpu_ptr(wq->cpu_wq, hotcpu)->thread)
                                continue;
                        /* Unbind so it can run. */
                        kthread_bind(per_cpu_ptr(wq->cpu_wq, hotcpu)->thread,
                                     any_online_cpu(cpu_online_map));
                        cleanup_workqueue_thread(wq, hotcpu);
                }
                mutex_unlock(&workqueue_mutex);
                break;

        case CPU_DOWN_PREPARE:
                mutex_lock(&workqueue_mutex);
                break;

        case CPU_DOWN_FAILED:
                mutex_unlock(&workqueue_mutex);
                break;

        case CPU_DEAD:
                list_for_each_entry(wq, &workqueues, list)
                        cleanup_workqueue_thread(wq, hotcpu);
                list_for_each_entry(wq, &workqueues, list)
                        take_over_work(wq, hotcpu);
                mutex_unlock(&workqueue_mutex);
                break;
        }

        return NOTIFY_OK;
}

void init_workqueues(void)
{
        singlethread_cpu = first_cpu(cpu_possible_map);
        hotcpu_notifier(workqueue_cpu_callback, 0);
        keventd_wq = create_workqueue("events");
        BUG_ON(!keventd_wq);
}