On Tue, Nov 06, 2007 at 02:33:53AM -0800, akpm@linux-foundation.org wrote:

[mmotm.git] / 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
 *   Kai Petzke <wpp@marie.physik.tu-berlin.de>
 *   Theodore Ts'o <tytso@mit.edu>
 *
 * Made to use alloc_percpu by Christoph Lameter.
 */

#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>
#include <linux/lockdep.h>
#define CREATE_TRACE_POINTS
#include <trace/events/workqueue.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 work_struct *current_work;

        struct workqueue_struct *wq;
        struct task_struct *thread;
} ____cacheline_aligned;

/*
 * The externally visible workqueue abstraction is an array of
 * per-CPU workqueues:
 */
struct workqueue_struct {
        struct cpu_workqueue_struct *cpu_wq;
        struct list_head list;
        const char *name;
        int singlethread;
        int freezeable;         /* Freeze threads during suspend */
        int rt;
#ifdef CONFIG_LOCKDEP
        struct lockdep_map lockdep_map;
#endif
};

/* Serializes the accesses to the list of workqueues. */
static DEFINE_SPINLOCK(workqueue_lock);
static LIST_HEAD(workqueues);

static int singlethread_cpu __read_mostly;
static const struct cpumask *cpu_singlethread_map __read_mostly;
/*
 * _cpu_down() first removes CPU from cpu_online_map, then CPU_DEAD
 * flushes cwq->worklist. This means that flush_workqueue/wait_on_work
 * which comes in between can't use for_each_online_cpu(). We could
 * use cpu_possible_map, the cpumask below is more a documentation
 * than optimization.
 */
static cpumask_var_t cpu_populated_map __read_mostly;

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

static const struct cpumask *wq_cpu_map(struct workqueue_struct *wq)
{
        return is_wq_single_threaded(wq)
                ? cpu_singlethread_map : cpu_populated_map;
}

static
struct cpu_workqueue_struct *wq_per_cpu(struct workqueue_struct *wq, int cpu)
{
        if (unlikely(is_wq_single_threaded(wq)))
                cpu = singlethread_cpu;
        return per_cpu_ptr(wq->cpu_wq, cpu);
}

/*
 * 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,
                                struct cpu_workqueue_struct *cwq)
{
        unsigned long new;

        BUG_ON(!work_pending(work));

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

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

static void insert_work(struct cpu_workqueue_struct *cwq,
                        struct work_struct *work, struct list_head *head)
{
        trace_workqueue_insertion(cwq->thread, work);

        set_wq_data(work, cwq);
        /*
         * Ensure that we get the right work->data if we see the
         * result of list_add() below, see try_to_grab_pending().
         */
        smp_wmb();
        list_add_tail(&work->entry, head);
        wake_up(&cwq->more_work);
}

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, &cwq->worklist);
        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 on which it was submitted, but if the CPU dies
 * it can be processed by another CPU.
 */
int queue_work(struct workqueue_struct *wq, struct work_struct *work)
{
        int ret;

        ret = queue_work_on(get_cpu(), wq, work);
        put_cpu();

        return ret;
}
EXPORT_SYMBOL_GPL(queue_work);

/**
 * queue_work_on - queue work on specific cpu
 * @cpu: CPU number to execute work on
 * @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 a specific CPU, the caller must ensure it
 * can't go away.
 */
int
queue_work_on(int cpu, struct workqueue_struct *wq, struct work_struct *work)
{
        int ret = 0;

        if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) {
                BUG_ON(!list_empty(&work->entry));
                __queue_work(wq_per_cpu(wq, cpu), work);
                ret = 1;
        }
        return ret;
}
EXPORT_SYMBOL_GPL(queue_work_on);

static void delayed_work_timer_fn(unsigned long __data)
{
        struct delayed_work *dwork = (struct delayed_work *)__data;
        struct cpu_workqueue_struct *cwq = get_wq_data(&dwork->work);
        struct workqueue_struct *wq = cwq->wq;

        __queue_work(wq_per_cpu(wq, smp_processor_id()), &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 queue_delayed_work(struct workqueue_struct *wq,
                        struct delayed_work *dwork, unsigned long delay)
{
        if (delay == 0)
                return queue_work(wq, &dwork->work);

        return queue_delayed_work_on(-1, wq, dwork, delay);
}
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));

                timer_stats_timer_set_start_info(&dwork->timer);

                /* This stores cwq for the moment, for the timer_fn */
                set_wq_data(work, wq_per_cpu(wq, raw_smp_processor_id()));
                timer->expires = jiffies + delay;
                timer->data = (unsigned long)dwork;
                timer->function = delayed_work_timer_fn;

                if (unlikely(cpu >= 0))
                        add_timer_on(timer, cpu);
                else
                        add_timer(timer);
                ret = 1;
        }
        return ret;
}
EXPORT_SYMBOL_GPL(queue_delayed_work_on);

static void run_workqueue(struct cpu_workqueue_struct *cwq)
{
        spin_lock_irq(&cwq->lock);
        while (!list_empty(&cwq->worklist)) {
                struct work_struct *work = list_entry(cwq->worklist.next,
                                                struct work_struct, entry);
                work_func_t f = work->func;
#ifdef CONFIG_LOCKDEP
                /*
                 * It is permissible to free the struct work_struct
                 * from inside the function that is called from it,
                 * this we need to take into account for lockdep too.
                 * To avoid bogus "held lock freed" warnings as well
                 * as problems when looking into work->lockdep_map,
                 * make a copy and use that here.
                 */
                struct lockdep_map lockdep_map = work->lockdep_map;
#endif
                trace_workqueue_execution(cwq->thread, work);
                cwq->current_work = work;
                list_del_init(cwq->worklist.next);
                spin_unlock_irq(&cwq->lock);

                BUG_ON(get_wq_data(work) != cwq);
                work_clear_pending(work);
                lock_map_acquire(&cwq->wq->lockdep_map);
                lock_map_acquire(&lockdep_map);
                f(work);
                lock_map_release(&lockdep_map);
                lock_map_release(&cwq->wq->lockdep_map);

                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(),
                                        task_pid_nr(current));
                        printk(KERN_ERR "    last function: ");
                        print_symbol("%s\n", (unsigned long)f);
                        debug_show_held_locks(current);
                        dump_stack();
                }

                spin_lock_irq(&cwq->lock);
                cwq->current_work = NULL;
        }
        spin_unlock_irq(&cwq->lock);
}

static int worker_thread(void *__cwq)
{
        struct cpu_workqueue_struct *cwq = __cwq;
        DEFINE_WAIT(wait);

        if (cwq->wq->freezeable)
                set_freezable();

        for (;;) {
                prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE);
                if (!freezing(current) &&
                    !kthread_should_stop() &&
                    list_empty(&cwq->worklist))
                        schedule();
                finish_wait(&cwq->more_work, &wait);

                try_to_freeze();

                if (kthread_should_stop())
                        break;

                run_workqueue(cwq);
        }

        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, struct list_head *head)
{
        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, head);
}

static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
{
        int active = 0;
        struct wq_barrier barr;

        WARN_ON(cwq->thread == current);

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

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

        return active;
}

/**
 * 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 flush_workqueue(struct workqueue_struct *wq)
{
        const struct cpumask *cpu_map = wq_cpu_map(wq);
        int cpu;

        might_sleep();
        lock_map_acquire(&wq->lockdep_map);
        lock_map_release(&wq->lockdep_map);
        for_each_cpu(cpu, cpu_map)
                flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
}
EXPORT_SYMBOL_GPL(flush_workqueue);

/**
 * flush_work - block until a work_struct's callback has terminated
 * @work: the work which is to be flushed
 *
 * Returns false if @work has already terminated.
 *
 * It is expected that, prior to calling flush_work(), the caller has
 * arranged for the work to not be requeued, otherwise it doesn't make
 * sense to use this function.
 */
int flush_work(struct work_struct *work)
{
        struct cpu_workqueue_struct *cwq;
        struct list_head *prev;
        struct wq_barrier barr;

        might_sleep();
        cwq = get_wq_data(work);
        if (!cwq)
                return 0;

        lock_map_acquire(&cwq->wq->lockdep_map);
        lock_map_release(&cwq->wq->lockdep_map);

        prev = NULL;
        spin_lock_irq(&cwq->lock);
        if (!list_empty(&work->entry)) {
                /*
                 * See the comment near try_to_grab_pending()->smp_rmb().
                 * If it was re-queued under us we are not going to wait.
                 */
                smp_rmb();
                if (unlikely(cwq != get_wq_data(work)))
                        goto out;
                prev = &work->entry;
        } else {
                if (cwq->current_work != work)
                        goto out;
                prev = &cwq->worklist;
        }
        insert_wq_barrier(cwq, &barr, prev->next);
out:
        spin_unlock_irq(&cwq->lock);
        if (!prev)
                return 0;

        wait_for_completion(&barr.done);
        return 1;
}
EXPORT_SYMBOL_GPL(flush_work);

/*
 * Upon a successful return (>= 0), the caller "owns" WORK_STRUCT_PENDING bit,
 * so this work can't be re-armed in any way.
 */
static int try_to_grab_pending(struct work_struct *work)
{
        struct cpu_workqueue_struct *cwq;
        int ret = -1;

        if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work)))
                return 0;

        /*
         * The queueing is in progress, or it is already queued. Try to
         * steal it from ->worklist without clearing WORK_STRUCT_PENDING.
         */

        cwq = get_wq_data(work);
        if (!cwq)
                return ret;

        spin_lock_irq(&cwq->lock);
        if (!list_empty(&work->entry)) {
                /*
                 * This work is queued, but perhaps we locked the wrong cwq.
                 * In that case we must see the new value after rmb(), see
                 * insert_work()->wmb().
                 */
                smp_rmb();
                if (cwq == get_wq_data(work)) {
                        list_del_init(&work->entry);
                        ret = 1;
                }
        }
        spin_unlock_irq(&cwq->lock);

        return ret;
}

static void wait_on_cpu_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, cwq->worklist.next);
                running = 1;
        }
        spin_unlock_irq(&cwq->lock);

        if (unlikely(running))
                wait_for_completion(&barr.done);
}

static void wait_on_work(struct work_struct *work)
{
        struct cpu_workqueue_struct *cwq;
        struct workqueue_struct *wq;
        const struct cpumask *cpu_map;
        int cpu;

        might_sleep();

        lock_map_acquire(&work->lockdep_map);
        lock_map_release(&work->lockdep_map);

        cwq = get_wq_data(work);
        if (!cwq)
                return;

        wq = cwq->wq;
        cpu_map = wq_cpu_map(wq);

        for_each_cpu(cpu, cpu_map)
                wait_on_cpu_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
}

static int __cancel_work_timer(struct work_struct *work,
                                struct timer_list* timer)
{
        int ret;

        do {
                ret = (timer && likely(del_timer(timer)));
                if (!ret)
                        ret = try_to_grab_pending(work);
                wait_on_work(work);
        } while (unlikely(ret < 0));

        work_clear_pending(work);
        return ret;
}

/**
 * cancel_work_sync - block until a work_struct's callback has terminated
 * @work: the work which is to be flushed
 *
 * Returns true if @work was pending.
 *
 * cancel_work_sync() will cancel the work if it is queued. If the work's
 * callback appears to be running, cancel_work_sync() will block until it
 * has completed.
 *
 * It is possible to use this function if the work re-queues itself. It can
 * cancel the work even if it migrates to another workqueue, however in that
 * case it only guarantees that work->func() has completed on the last queued
 * workqueue.
 *
 * cancel_work_sync(&delayed_work->work) should be used only if ->timer is not
 * pending, otherwise it goes into a busy-wait loop until the timer expires.
 *
 * The caller must ensure that workqueue_struct on which this work was last
 * queued can't be destroyed before this function returns.
 */
int cancel_work_sync(struct work_struct *work)
{
        return __cancel_work_timer(work, NULL);
}
EXPORT_SYMBOL_GPL(cancel_work_sync);

/**
 * cancel_delayed_work_sync - reliably kill off a delayed work.
 * @dwork: the delayed work struct
 *
 * Returns true if @dwork was pending.
 *
 * It is possible to use this function if @dwork rearms itself via queue_work()
 * or queue_delayed_work(). See also the comment for cancel_work_sync().
 */
int cancel_delayed_work_sync(struct delayed_work *dwork)
{
        return __cancel_work_timer(&dwork->work, &dwork->timer);
}
EXPORT_SYMBOL(cancel_delayed_work_sync);

static struct workqueue_struct *keventd_wq __read_mostly;

/**
 * schedule_work - put work task in global workqueue
 * @work: job to be done
 *
 * Returns zero if @work was already on the kernel-global workqueue and
 * non-zero otherwise.
 *
 * This puts a job in the kernel-global workqueue if it was not already
 * queued and leaves it in the same position on the kernel-global
 * workqueue otherwise.
 */
int schedule_work(struct work_struct *work)
{
        return queue_work(keventd_wq, work);
}
EXPORT_SYMBOL(schedule_work);

/*
 * schedule_work_on - put work task on a specific cpu
 * @cpu: cpu to put the work task on
 * @work: job to be done
 *
 * This puts a job on a specific cpu
 */
int schedule_work_on(int cpu, struct work_struct *work)
{
        return queue_work_on(cpu, keventd_wq, work);
}
EXPORT_SYMBOL(schedule_work_on);

/**
 * 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 schedule_delayed_work(struct delayed_work *dwork,
                                        unsigned long delay)
{
        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.
 *
 * 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;

        get_online_cpus();
        for_each_online_cpu(cpu) {
                struct work_struct *work = per_cpu_ptr(works, cpu);

                INIT_WORK(work, func);
                schedule_work_on(cpu, work);
        }
        for_each_online_cpu(cpu)
                flush_work(per_cpu_ptr(works, cpu));
        put_online_cpus();
        free_percpu(works);
        return 0;
}

void flush_scheduled_work(void)
{
        flush_workqueue(keventd_wq);
}
EXPORT_SYMBOL(flush_scheduled_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 = raw_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;

}

static struct cpu_workqueue_struct *
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);

        return cwq;
}

static int create_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
{
        struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
        struct workqueue_struct *wq = cwq->wq;
        const char *fmt = is_wq_single_threaded(wq) ? "%s" : "%s/%d";
        struct task_struct *p;

        p = kthread_create(worker_thread, cwq, fmt, wq->name, cpu);
        /*
         * Nobody can add the work_struct to this cwq,
         *      if (caller is __create_workqueue)
         *              nobody should see this wq
         *      else // caller is CPU_UP_PREPARE
         *              cpu is not on cpu_online_map
         * so we can abort safely.
         */
        if (IS_ERR(p))
                return PTR_ERR(p);
        if (cwq->wq->rt)
                sched_setscheduler_nocheck(p, SCHED_FIFO, &param);
        cwq->thread = p;

        trace_workqueue_creation(cwq->thread, cpu);

        return 0;
}

static void start_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
{
        struct task_struct *p = cwq->thread;

        if (p != NULL) {
                if (cpu >= 0)
                        kthread_bind(p, cpu);
                wake_up_process(p);
        }
}

struct workqueue_struct *__create_workqueue_key(const char *name,
                                                int singlethread,
                                                int freezeable,
                                                int rt,
                                                struct lock_class_key *key,
                                                const char *lock_name)
{
        struct workqueue_struct *wq;
        struct cpu_workqueue_struct *cwq;
        int err = 0, cpu;

        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;
        lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
        wq->singlethread = singlethread;
        wq->freezeable = freezeable;
        wq->rt = rt;
        INIT_LIST_HEAD(&wq->list);

        if (singlethread) {
                cwq = init_cpu_workqueue(wq, singlethread_cpu);
                err = create_workqueue_thread(cwq, singlethread_cpu);
                start_workqueue_thread(cwq, -1);
        } else {
                cpu_maps_update_begin();
                /*
                 * We must place this wq on list even if the code below fails.
                 * cpu_down(cpu) can remove cpu from cpu_populated_map before
                 * destroy_workqueue() takes the lock, in that case we leak
                 * cwq[cpu]->thread.
                 */
                spin_lock(&workqueue_lock);
                list_add(&wq->list, &workqueues);
                spin_unlock(&workqueue_lock);
                /*
                 * We must initialize cwqs for each possible cpu even if we
                 * are going to call destroy_workqueue() finally. Otherwise
                 * cpu_up() can hit the uninitialized cwq once we drop the
                 * lock.
                 */
                for_each_possible_cpu(cpu) {
                        cwq = init_cpu_workqueue(wq, cpu);
                        if (err || !cpu_online(cpu))
                                continue;
                        err = create_workqueue_thread(cwq, cpu);
                        start_workqueue_thread(cwq, cpu);
                }
                cpu_maps_update_done();
        }

        if (err) {
                destroy_workqueue(wq);
                wq = NULL;
        }
        return wq;
}
EXPORT_SYMBOL_GPL(__create_workqueue_key);

static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq)
{
        /*
         * Our caller is either destroy_workqueue() or CPU_POST_DEAD,
         * cpu_add_remove_lock protects cwq->thread.
         */
        if (cwq->thread == NULL)
                return;

        lock_map_acquire(&cwq->wq->lockdep_map);
        lock_map_release(&cwq->wq->lockdep_map);

        flush_cpu_workqueue(cwq);
        /*
         * If the caller is CPU_POST_DEAD and cwq->worklist was not empty,
         * a concurrent flush_workqueue() can insert a barrier after us.
         * However, in that case run_workqueue() won't return and check
         * kthread_should_stop() until it flushes all work_struct's.
         * When ->worklist becomes empty it is safe to exit because no
         * more work_structs can be queued on this cwq: flush_workqueue
         * checks list_empty(), and a "normal" queue_work() can't use
         * a dead CPU.
         */
        trace_workqueue_destruction(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)
{
        const struct cpumask *cpu_map = wq_cpu_map(wq);
        int cpu;

        cpu_maps_update_begin();
        spin_lock(&workqueue_lock);
        list_del(&wq->list);
        spin_unlock(&workqueue_lock);

        for_each_cpu(cpu, cpu_map)
                cleanup_workqueue_thread(per_cpu_ptr(wq->cpu_wq, cpu));
        cpu_maps_update_done();

        free_percpu(wq->cpu_wq);
        kfree(wq);
}
EXPORT_SYMBOL_GPL(destroy_workqueue);

static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
                                                unsigned long action,
                                                void *hcpu)
{
        unsigned int cpu = (unsigned long)hcpu;
        struct cpu_workqueue_struct *cwq;
        struct workqueue_struct *wq;
        int ret = NOTIFY_OK;

        action &= ~CPU_TASKS_FROZEN;

        switch (action) {
        case CPU_UP_PREPARE:
                cpumask_set_cpu(cpu, cpu_populated_map);
        }
undo:
        list_for_each_entry(wq, &workqueues, list) {
                cwq = per_cpu_ptr(wq->cpu_wq, cpu);

                switch (action) {
                case CPU_UP_PREPARE:
                        if (!create_workqueue_thread(cwq, cpu))
                                break;
                        printk(KERN_ERR "workqueue [%s] for %i failed\n",
                                wq->name, cpu);
                        action = CPU_UP_CANCELED;
                        ret = NOTIFY_BAD;
                        goto undo;

                case CPU_ONLINE:
                        start_workqueue_thread(cwq, cpu);
                        break;

                case CPU_UP_CANCELED:
                        start_workqueue_thread(cwq, -1);
                case CPU_POST_DEAD:
                        cleanup_workqueue_thread(cwq);
                        break;
                }
        }

        switch (action) {
        case CPU_UP_CANCELED:
        case CPU_POST_DEAD:
                cpumask_clear_cpu(cpu, cpu_populated_map);
        }

        return ret;
}

#ifdef CONFIG_SMP

struct work_for_cpu {
        struct completion completion;
        long (*fn)(void *);
        void *arg;
        long ret;
};

static int do_work_for_cpu(void *_wfc)
{
        struct work_for_cpu *wfc = _wfc;
        wfc->ret = wfc->fn(wfc->arg);
        complete(&wfc->completion);
        return 0;
}

/**
 * work_on_cpu - run a function in user context on a particular cpu
 * @cpu: the cpu to run on
 * @fn: the function to run
 * @arg: the function arg
 *
 * This will return the value @fn returns.
 * It is up to the caller to ensure that the cpu doesn't go offline.
 * The caller must not hold any locks which would prevent @fn from completing.
 */
long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
{
        struct task_struct *sub_thread;
        struct work_for_cpu wfc = {
                .completion = COMPLETION_INITIALIZER_ONSTACK(wfc.completion),
                .fn = fn,
                .arg = arg,
        };

        sub_thread = kthread_create(do_work_for_cpu, &wfc, "work_for_cpu");
        if (IS_ERR(sub_thread))
                return PTR_ERR(sub_thread);
        kthread_bind(sub_thread, cpu);
        wake_up_process(sub_thread);
        wait_for_completion(&wfc.completion);
        return wfc.ret;
}
EXPORT_SYMBOL_GPL(work_on_cpu);
#endif /* CONFIG_SMP */

void __init init_workqueues(void)
{
        alloc_cpumask_var(&cpu_populated_map, GFP_KERNEL);

        cpumask_copy(cpu_populated_map, cpu_online_mask);
        singlethread_cpu = cpumask_first(cpu_possible_mask);
        cpu_singlethread_map = cpumask_of(singlethread_cpu);
        hotcpu_notifier(workqueue_cpu_callback, 0);
        keventd_wq = create_workqueue("events");
        BUG_ON(!keventd_wq);
}