workqueue: implement per-cwq active work limit

[linux-flexiantxendom0-natty.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>
#include <linux/idr.h>

/*
 * Structure fields follow one of the following exclusion rules.
 *
 * I: Set during initialization and read-only afterwards.
 *
 * L: cwq->lock protected.  Access with cwq->lock held.
 *
 * F: wq->flush_mutex protected.
 *
 * W: workqueue_lock protected.
 */

struct cpu_workqueue_struct;

struct worker {
        struct work_struct      *current_work;  /* L: work being processed */
        struct list_head        scheduled;      /* L: scheduled works */
        struct task_struct      *task;          /* I: worker task */
        struct cpu_workqueue_struct *cwq;       /* I: the associated cwq */
        int                     id;             /* I: worker id */
};

/*
 * The per-CPU workqueue (if single thread, we always use the first
 * possible cpu).  The lower WORK_STRUCT_FLAG_BITS of
 * work_struct->data are used for flags and thus cwqs need to be
 * aligned at two's power of the number of flag bits.
 */
struct cpu_workqueue_struct {

        spinlock_t lock;

        struct list_head worklist;
        wait_queue_head_t more_work;
        unsigned int            cpu;
        struct worker           *worker;

        struct workqueue_struct *wq;            /* I: the owning workqueue */
        int                     work_color;     /* L: current color */
        int                     flush_color;    /* L: flushing color */
        int                     nr_in_flight[WORK_NR_COLORS];
                                                /* L: nr of in_flight works */
        int                     nr_active;      /* L: nr of active works */
        int                     max_active;     /* I: max active works */
        struct list_head        delayed_works;  /* L: delayed works */
};

/*
 * Structure used to wait for workqueue flush.
 */
struct wq_flusher {
        struct list_head        list;           /* F: list of flushers */
        int                     flush_color;    /* F: flush color waiting for */
        struct completion       done;           /* flush completion */
};

/*
 * The externally visible workqueue abstraction is an array of
 * per-CPU workqueues:
 */
struct workqueue_struct {
        unsigned int            flags;          /* I: WQ_* flags */
        struct cpu_workqueue_struct *cpu_wq;    /* I: cwq's */
        struct list_head        list;           /* W: list of all workqueues */

        struct mutex            flush_mutex;    /* protects wq flushing */
        int                     work_color;     /* F: current work color */
        int                     flush_color;    /* F: current flush color */
        atomic_t                nr_cwqs_to_flush; /* flush in progress */
        struct wq_flusher       *first_flusher; /* F: first flusher */
        struct list_head        flusher_queue;  /* F: flush waiters */
        struct list_head        flusher_overflow; /* F: flush overflow list */

        const char              *name;          /* I: workqueue name */
#ifdef CONFIG_LOCKDEP
        struct lockdep_map      lockdep_map;
#endif
};

#ifdef CONFIG_DEBUG_OBJECTS_WORK

static struct debug_obj_descr work_debug_descr;

/*
 * fixup_init is called when:
 * - an active object is initialized
 */
static int work_fixup_init(void *addr, enum debug_obj_state state)
{
        struct work_struct *work = addr;

        switch (state) {
        case ODEBUG_STATE_ACTIVE:
                cancel_work_sync(work);
                debug_object_init(work, &work_debug_descr);
                return 1;
        default:
                return 0;
        }
}

/*
 * fixup_activate is called when:
 * - an active object is activated
 * - an unknown object is activated (might be a statically initialized object)
 */
static int work_fixup_activate(void *addr, enum debug_obj_state state)
{
        struct work_struct *work = addr;

        switch (state) {

        case ODEBUG_STATE_NOTAVAILABLE:
                /*
                 * This is not really a fixup. The work struct was
                 * statically initialized. We just make sure that it
                 * is tracked in the object tracker.
                 */
                if (test_bit(WORK_STRUCT_STATIC_BIT, work_data_bits(work))) {
                        debug_object_init(work, &work_debug_descr);
                        debug_object_activate(work, &work_debug_descr);
                        return 0;
                }
                WARN_ON_ONCE(1);
                return 0;

        case ODEBUG_STATE_ACTIVE:
                WARN_ON(1);

        default:
                return 0;
        }
}

/*
 * fixup_free is called when:
 * - an active object is freed
 */
static int work_fixup_free(void *addr, enum debug_obj_state state)
{
        struct work_struct *work = addr;

        switch (state) {
        case ODEBUG_STATE_ACTIVE:
                cancel_work_sync(work);
                debug_object_free(work, &work_debug_descr);
                return 1;
        default:
                return 0;
        }
}

static struct debug_obj_descr work_debug_descr = {
        .name           = "work_struct",
        .fixup_init     = work_fixup_init,
        .fixup_activate = work_fixup_activate,
        .fixup_free     = work_fixup_free,
};

static inline void debug_work_activate(struct work_struct *work)
{
        debug_object_activate(work, &work_debug_descr);
}

static inline void debug_work_deactivate(struct work_struct *work)
{
        debug_object_deactivate(work, &work_debug_descr);
}

void __init_work(struct work_struct *work, int onstack)
{
        if (onstack)
                debug_object_init_on_stack(work, &work_debug_descr);
        else
                debug_object_init(work, &work_debug_descr);
}
EXPORT_SYMBOL_GPL(__init_work);

void destroy_work_on_stack(struct work_struct *work)
{
        debug_object_free(work, &work_debug_descr);
}
EXPORT_SYMBOL_GPL(destroy_work_on_stack);

#else
static inline void debug_work_activate(struct work_struct *work) { }
static inline void debug_work_deactivate(struct work_struct *work) { }
#endif

/* Serializes the accesses to the list of workqueues. */
static DEFINE_SPINLOCK(workqueue_lock);
static LIST_HEAD(workqueues);
static DEFINE_PER_CPU(struct ida, worker_ida);

static int worker_thread(void *__worker);

static int singlethread_cpu __read_mostly;

static struct cpu_workqueue_struct *get_cwq(unsigned int cpu,
                                            struct workqueue_struct *wq)
{
        return per_cpu_ptr(wq->cpu_wq, cpu);
}

static struct cpu_workqueue_struct *target_cwq(unsigned int cpu,
                                               struct workqueue_struct *wq)
{
        if (unlikely(wq->flags & WQ_SINGLE_THREAD))
                cpu = singlethread_cpu;
        return get_cwq(cpu, wq);
}

static unsigned int work_color_to_flags(int color)
{
        return color << WORK_STRUCT_COLOR_SHIFT;
}

static int get_work_color(struct work_struct *work)
{
        return (*work_data_bits(work) >> WORK_STRUCT_COLOR_SHIFT) &
                ((1 << WORK_STRUCT_COLOR_BITS) - 1);
}

static int work_next_color(int color)
{
        return (color + 1) % WORK_NR_COLORS;
}

/*
 * 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 extra_flags)
{
        BUG_ON(!work_pending(work));

        atomic_long_set(&work->data, (unsigned long)cwq | work_static(work) |
                        WORK_STRUCT_PENDING | extra_flags);
}

/*
 * Clear WORK_STRUCT_PENDING and the workqueue on which it was queued.
 */
static inline void clear_wq_data(struct work_struct *work)
{
        atomic_long_set(&work->data, work_static(work));
}

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);
}

/**
 * insert_work - insert a work into cwq
 * @cwq: cwq @work belongs to
 * @work: work to insert
 * @head: insertion point
 * @extra_flags: extra WORK_STRUCT_* flags to set
 *
 * Insert @work into @cwq after @head.
 *
 * CONTEXT:
 * spin_lock_irq(cwq->lock).
 */
static void insert_work(struct cpu_workqueue_struct *cwq,
                        struct work_struct *work, struct list_head *head,
                        unsigned int extra_flags)
{
        /* we own @work, set data and link */
        set_wq_data(work, cwq, extra_flags);

        /*
         * 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(unsigned int cpu, struct workqueue_struct *wq,
                         struct work_struct *work)
{
        struct cpu_workqueue_struct *cwq = target_cwq(cpu, wq);
        struct list_head *worklist;
        unsigned long flags;

        debug_work_activate(work);

        spin_lock_irqsave(&cwq->lock, flags);
        BUG_ON(!list_empty(&work->entry));

        cwq->nr_in_flight[cwq->work_color]++;

        if (likely(cwq->nr_active < cwq->max_active)) {
                cwq->nr_active++;
                worklist = &cwq->worklist;
        } else
                worklist = &cwq->delayed_works;

        insert_work(cwq, work, worklist, work_color_to_flags(cwq->work_color));

        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_BIT, work_data_bits(work))) {
                __queue_work(cpu, wq, 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);

        __queue_work(smp_processor_id(), cwq->wq, &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_BIT, 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, target_cwq(raw_smp_processor_id(), wq), 0);
                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 struct worker *alloc_worker(void)
{
        struct worker *worker;

        worker = kzalloc(sizeof(*worker), GFP_KERNEL);
        if (worker)
                INIT_LIST_HEAD(&worker->scheduled);
        return worker;
}

/**
 * create_worker - create a new workqueue worker
 * @cwq: cwq the new worker will belong to
 * @bind: whether to set affinity to @cpu or not
 *
 * Create a new worker which is bound to @cwq.  The returned worker
 * can be started by calling start_worker() or destroyed using
 * destroy_worker().
 *
 * CONTEXT:
 * Might sleep.  Does GFP_KERNEL allocations.
 *
 * RETURNS:
 * Pointer to the newly created worker.
 */
static struct worker *create_worker(struct cpu_workqueue_struct *cwq, bool bind)
{
        int id = -1;
        struct worker *worker = NULL;

        spin_lock(&workqueue_lock);
        while (ida_get_new(&per_cpu(worker_ida, cwq->cpu), &id)) {
                spin_unlock(&workqueue_lock);
                if (!ida_pre_get(&per_cpu(worker_ida, cwq->cpu), GFP_KERNEL))
                        goto fail;
                spin_lock(&workqueue_lock);
        }
        spin_unlock(&workqueue_lock);

        worker = alloc_worker();
        if (!worker)
                goto fail;

        worker->cwq = cwq;
        worker->id = id;

        worker->task = kthread_create(worker_thread, worker, "kworker/%u:%d",
                                      cwq->cpu, id);
        if (IS_ERR(worker->task))
                goto fail;

        if (bind)
                kthread_bind(worker->task, cwq->cpu);

        return worker;
fail:
        if (id >= 0) {
                spin_lock(&workqueue_lock);
                ida_remove(&per_cpu(worker_ida, cwq->cpu), id);
                spin_unlock(&workqueue_lock);
        }
        kfree(worker);
        return NULL;
}

/**
 * start_worker - start a newly created worker
 * @worker: worker to start
 *
 * Start @worker.
 *
 * CONTEXT:
 * spin_lock_irq(cwq->lock).
 */
static void start_worker(struct worker *worker)
{
        wake_up_process(worker->task);
}

/**
 * destroy_worker - destroy a workqueue worker
 * @worker: worker to be destroyed
 *
 * Destroy @worker.
 */
static void destroy_worker(struct worker *worker)
{
        int cpu = worker->cwq->cpu;
        int id = worker->id;

        /* sanity check frenzy */
        BUG_ON(worker->current_work);
        BUG_ON(!list_empty(&worker->scheduled));

        kthread_stop(worker->task);
        kfree(worker);

        spin_lock(&workqueue_lock);
        ida_remove(&per_cpu(worker_ida, cpu), id);
        spin_unlock(&workqueue_lock);
}

/**
 * move_linked_works - move linked works to a list
 * @work: start of series of works to be scheduled
 * @head: target list to append @work to
 * @nextp: out paramter for nested worklist walking
 *
 * Schedule linked works starting from @work to @head.  Work series to
 * be scheduled starts at @work and includes any consecutive work with
 * WORK_STRUCT_LINKED set in its predecessor.
 *
 * If @nextp is not NULL, it's updated to point to the next work of
 * the last scheduled work.  This allows move_linked_works() to be
 * nested inside outer list_for_each_entry_safe().
 *
 * CONTEXT:
 * spin_lock_irq(cwq->lock).
 */
static void move_linked_works(struct work_struct *work, struct list_head *head,
                              struct work_struct **nextp)
{
        struct work_struct *n;

        /*
         * Linked worklist will always end before the end of the list,
         * use NULL for list head.
         */
        list_for_each_entry_safe_from(work, n, NULL, entry) {
                list_move_tail(&work->entry, head);
                if (!(*work_data_bits(work) & WORK_STRUCT_LINKED))
                        break;
        }

        /*
         * If we're already inside safe list traversal and have moved
         * multiple works to the scheduled queue, the next position
         * needs to be updated.
         */
        if (nextp)
                *nextp = n;
}

static void cwq_activate_first_delayed(struct cpu_workqueue_struct *cwq)
{
        struct work_struct *work = list_first_entry(&cwq->delayed_works,
                                                    struct work_struct, entry);

        move_linked_works(work, &cwq->worklist, NULL);
        cwq->nr_active++;
}

/**
 * cwq_dec_nr_in_flight - decrement cwq's nr_in_flight
 * @cwq: cwq of interest
 * @color: color of work which left the queue
 *
 * A work either has completed or is removed from pending queue,
 * decrement nr_in_flight of its cwq and handle workqueue flushing.
 *
 * CONTEXT:
 * spin_lock_irq(cwq->lock).
 */
static void cwq_dec_nr_in_flight(struct cpu_workqueue_struct *cwq, int color)
{
        /* ignore uncolored works */
        if (color == WORK_NO_COLOR)
                return;

        cwq->nr_in_flight[color]--;
        cwq->nr_active--;

        /* one down, submit a delayed one */
        if (!list_empty(&cwq->delayed_works) &&
            cwq->nr_active < cwq->max_active)
                cwq_activate_first_delayed(cwq);

        /* is flush in progress and are we at the flushing tip? */
        if (likely(cwq->flush_color != color))
                return;

        /* are there still in-flight works? */
        if (cwq->nr_in_flight[color])
                return;

        /* this cwq is done, clear flush_color */
        cwq->flush_color = -1;

        /*
         * If this was the last cwq, wake up the first flusher.  It
         * will handle the rest.
         */
        if (atomic_dec_and_test(&cwq->wq->nr_cwqs_to_flush))
                complete(&cwq->wq->first_flusher->done);
}

/**
 * process_one_work - process single work
 * @worker: self
 * @work: work to process
 *
 * Process @work.  This function contains all the logics necessary to
 * process a single work including synchronization against and
 * interaction with other workers on the same cpu, queueing and
 * flushing.  As long as context requirement is met, any worker can
 * call this function to process a work.
 *
 * CONTEXT:
 * spin_lock_irq(cwq->lock) which is released and regrabbed.
 */
static void process_one_work(struct worker *worker, struct work_struct *work)
{
        struct cpu_workqueue_struct *cwq = worker->cwq;
        work_func_t f = work->func;
        int work_color;
#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
        /* claim and process */
        debug_work_deactivate(work);
        worker->current_work = work;
        work_color = get_work_color(work);
        list_del_init(&work->entry);

        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);

        /* we're done with it, release */
        worker->current_work = NULL;
        cwq_dec_nr_in_flight(cwq, work_color);
}

/**
 * process_scheduled_works - process scheduled works
 * @worker: self
 *
 * Process all scheduled works.  Please note that the scheduled list
 * may change while processing a work, so this function repeatedly
 * fetches a work from the top and executes it.
 *
 * CONTEXT:
 * spin_lock_irq(cwq->lock) which may be released and regrabbed
 * multiple times.
 */
static void process_scheduled_works(struct worker *worker)
{
        while (!list_empty(&worker->scheduled)) {
                struct work_struct *work = list_first_entry(&worker->scheduled,
                                                struct work_struct, entry);
                process_one_work(worker, work);
        }
}

/**
 * worker_thread - the worker thread function
 * @__worker: self
 *
 * The cwq worker thread function.
 */
static int worker_thread(void *__worker)
{
        struct worker *worker = __worker;
        struct cpu_workqueue_struct *cwq = worker->cwq;
        DEFINE_WAIT(wait);

        if (cwq->wq->flags & 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;

                if (unlikely(!cpumask_equal(&worker->task->cpus_allowed,
                                            get_cpu_mask(cwq->cpu))))
                        set_cpus_allowed_ptr(worker->task,
                                             get_cpu_mask(cwq->cpu));

                spin_lock_irq(&cwq->lock);

                while (!list_empty(&cwq->worklist)) {
                        struct work_struct *work =
                                list_first_entry(&cwq->worklist,
                                                 struct work_struct, entry);

                        if (likely(!(*work_data_bits(work) &
                                     WORK_STRUCT_LINKED))) {
                                /* optimization path, not strictly necessary */
                                process_one_work(worker, work);
                                if (unlikely(!list_empty(&worker->scheduled)))
                                        process_scheduled_works(worker);
                        } else {
                                move_linked_works(work, &worker->scheduled,
                                                  NULL);
                                process_scheduled_works(worker);
                        }
                }

                spin_unlock_irq(&cwq->lock);
        }

        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);
}

/**
 * insert_wq_barrier - insert a barrier work
 * @cwq: cwq to insert barrier into
 * @barr: wq_barrier to insert
 * @target: target work to attach @barr to
 * @worker: worker currently executing @target, NULL if @target is not executing
 *
 * @barr is linked to @target such that @barr is completed only after
 * @target finishes execution.  Please note that the ordering
 * guarantee is observed only with respect to @target and on the local
 * cpu.
 *
 * Currently, a queued barrier can't be canceled.  This is because
 * try_to_grab_pending() can't determine whether the work to be
 * grabbed is at the head of the queue and thus can't clear LINKED
 * flag of the previous work while there must be a valid next work
 * after a work with LINKED flag set.
 *
 * Note that when @worker is non-NULL, @target may be modified
 * underneath us, so we can't reliably determine cwq from @target.
 *
 * CONTEXT:
 * spin_lock_irq(cwq->lock).
 */
static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
                              struct wq_barrier *barr,
                              struct work_struct *target, struct worker *worker)
{
        struct list_head *head;
        unsigned int linked = 0;

        /*
         * debugobject calls are safe here even with cwq->lock locked
         * as we know for sure that this will not trigger any of the
         * checks and call back into the fixup functions where we
         * might deadlock.
         */
        INIT_WORK_ON_STACK(&barr->work, wq_barrier_func);
        __set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
        init_completion(&barr->done);

        /*
         * If @target is currently being executed, schedule the
         * barrier to the worker; otherwise, put it after @target.
         */
        if (worker)
                head = worker->scheduled.next;
        else {
                unsigned long *bits = work_data_bits(target);

                head = target->entry.next;
                /* there can already be other linked works, inherit and set */
                linked = *bits & WORK_STRUCT_LINKED;
                __set_bit(WORK_STRUCT_LINKED_BIT, bits);
        }

        debug_work_activate(&barr->work);
        insert_work(cwq, &barr->work, head,
                    work_color_to_flags(WORK_NO_COLOR) | linked);
}

/**
 * flush_workqueue_prep_cwqs - prepare cwqs for workqueue flushing
 * @wq: workqueue being flushed
 * @flush_color: new flush color, < 0 for no-op
 * @work_color: new work color, < 0 for no-op
 *
 * Prepare cwqs for workqueue flushing.
 *
 * If @flush_color is non-negative, flush_color on all cwqs should be
 * -1.  If no cwq has in-flight commands at the specified color, all
 * cwq->flush_color's stay at -1 and %false is returned.  If any cwq
 * has in flight commands, its cwq->flush_color is set to
 * @flush_color, @wq->nr_cwqs_to_flush is updated accordingly, cwq
 * wakeup logic is armed and %true is returned.
 *
 * The caller should have initialized @wq->first_flusher prior to
 * calling this function with non-negative @flush_color.  If
 * @flush_color is negative, no flush color update is done and %false
 * is returned.
 *
 * If @work_color is non-negative, all cwqs should have the same
 * work_color which is previous to @work_color and all will be
 * advanced to @work_color.
 *
 * CONTEXT:
 * mutex_lock(wq->flush_mutex).
 *
 * RETURNS:
 * %true if @flush_color >= 0 and there's something to flush.  %false
 * otherwise.
 */
static bool flush_workqueue_prep_cwqs(struct workqueue_struct *wq,
                                      int flush_color, int work_color)
{
        bool wait = false;
        unsigned int cpu;

        if (flush_color >= 0) {
                BUG_ON(atomic_read(&wq->nr_cwqs_to_flush));
                atomic_set(&wq->nr_cwqs_to_flush, 1);
        }

        for_each_possible_cpu(cpu) {
                struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);

                spin_lock_irq(&cwq->lock);

                if (flush_color >= 0) {
                        BUG_ON(cwq->flush_color != -1);

                        if (cwq->nr_in_flight[flush_color]) {
                                cwq->flush_color = flush_color;
                                atomic_inc(&wq->nr_cwqs_to_flush);
                                wait = true;
                        }
                }

                if (work_color >= 0) {
                        BUG_ON(work_color != work_next_color(cwq->work_color));
                        cwq->work_color = work_color;
                }

                spin_unlock_irq(&cwq->lock);
        }

        if (flush_color >= 0 && atomic_dec_and_test(&wq->nr_cwqs_to_flush))
                complete(&wq->first_flusher->done);

        return wait;
}

/**
 * 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.
 */
void flush_workqueue(struct workqueue_struct *wq)
{
        struct wq_flusher this_flusher = {
                .list = LIST_HEAD_INIT(this_flusher.list),
                .flush_color = -1,
                .done = COMPLETION_INITIALIZER_ONSTACK(this_flusher.done),
        };
        int next_color;

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

        mutex_lock(&wq->flush_mutex);

        /*
         * Start-to-wait phase
         */
        next_color = work_next_color(wq->work_color);

        if (next_color != wq->flush_color) {
                /*
                 * Color space is not full.  The current work_color
                 * becomes our flush_color and work_color is advanced
                 * by one.
                 */
                BUG_ON(!list_empty(&wq->flusher_overflow));
                this_flusher.flush_color = wq->work_color;
                wq->work_color = next_color;

                if (!wq->first_flusher) {
                        /* no flush in progress, become the first flusher */
                        BUG_ON(wq->flush_color != this_flusher.flush_color);

                        wq->first_flusher = &this_flusher;

                        if (!flush_workqueue_prep_cwqs(wq, wq->flush_color,
                                                       wq->work_color)) {
                                /* nothing to flush, done */
                                wq->flush_color = next_color;
                                wq->first_flusher = NULL;
                                goto out_unlock;
                        }
                } else {
                        /* wait in queue */
                        BUG_ON(wq->flush_color == this_flusher.flush_color);
                        list_add_tail(&this_flusher.list, &wq->flusher_queue);
                        flush_workqueue_prep_cwqs(wq, -1, wq->work_color);
                }
        } else {
                /*
                 * Oops, color space is full, wait on overflow queue.
                 * The next flush completion will assign us
                 * flush_color and transfer to flusher_queue.
                 */
                list_add_tail(&this_flusher.list, &wq->flusher_overflow);
        }

        mutex_unlock(&wq->flush_mutex);

        wait_for_completion(&this_flusher.done);

        /*
         * Wake-up-and-cascade phase
         *
         * First flushers are responsible for cascading flushes and
         * handling overflow.  Non-first flushers can simply return.
         */
        if (wq->first_flusher != &this_flusher)
                return;

        mutex_lock(&wq->flush_mutex);

        wq->first_flusher = NULL;

        BUG_ON(!list_empty(&this_flusher.list));
        BUG_ON(wq->flush_color != this_flusher.flush_color);

        while (true) {
                struct wq_flusher *next, *tmp;

                /* complete all the flushers sharing the current flush color */
                list_for_each_entry_safe(next, tmp, &wq->flusher_queue, list) {
                        if (next->flush_color != wq->flush_color)
                                break;
                        list_del_init(&next->list);
                        complete(&next->done);
                }

                BUG_ON(!list_empty(&wq->flusher_overflow) &&
                       wq->flush_color != work_next_color(wq->work_color));

                /* this flush_color is finished, advance by one */
                wq->flush_color = work_next_color(wq->flush_color);

                /* one color has been freed, handle overflow queue */
                if (!list_empty(&wq->flusher_overflow)) {
                        /*
                         * Assign the same color to all overflowed
                         * flushers, advance work_color and append to
                         * flusher_queue.  This is the start-to-wait
                         * phase for these overflowed flushers.
                         */
                        list_for_each_entry(tmp, &wq->flusher_overflow, list)
                                tmp->flush_color = wq->work_color;

                        wq->work_color = work_next_color(wq->work_color);

                        list_splice_tail_init(&wq->flusher_overflow,
                                              &wq->flusher_queue);
                        flush_workqueue_prep_cwqs(wq, -1, wq->work_color);
                }

                if (list_empty(&wq->flusher_queue)) {
                        BUG_ON(wq->flush_color != wq->work_color);
                        break;
                }

                /*
                 * Need to flush more colors.  Make the next flusher
                 * the new first flusher and arm cwqs.
                 */
                BUG_ON(wq->flush_color == wq->work_color);
                BUG_ON(wq->flush_color != next->flush_color);

                list_del_init(&next->list);
                wq->first_flusher = next;

                if (flush_workqueue_prep_cwqs(wq, wq->flush_color, -1))
                        break;

                /*
                 * Meh... this color is already done, clear first
                 * flusher and repeat cascading.
                 */
                wq->first_flusher = NULL;
        }

out_unlock:
        mutex_unlock(&wq->flush_mutex);
}
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 worker *worker = NULL;
        struct cpu_workqueue_struct *cwq;
        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);

        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 already_gone;
        } else {
                if (cwq->worker && cwq->worker->current_work == work)
                        worker = cwq->worker;
                if (!worker)
                        goto already_gone;
        }

        insert_wq_barrier(cwq, &barr, work, worker);
        spin_unlock_irq(&cwq->lock);
        wait_for_completion(&barr.done);
        destroy_work_on_stack(&barr.work);
        return 1;
already_gone:
        spin_unlock_irq(&cwq->lock);
        return 0;
}
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_BIT, 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)) {
                        debug_work_deactivate(work);
                        list_del_init(&work->entry);
                        cwq_dec_nr_in_flight(cwq, get_work_color(work));
                        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;
        struct worker *worker;

        spin_lock_irq(&cwq->lock);

        worker = NULL;
        if (unlikely(cwq->worker && cwq->worker->current_work == work)) {
                worker = cwq->worker;
                insert_wq_barrier(cwq, &barr, work, worker);
        }

        spin_unlock_irq(&cwq->lock);

        if (unlikely(worker)) {
                wait_for_completion(&barr.done);
                destroy_work_on_stack(&barr.work);
        }
}

static void wait_on_work(struct work_struct *work)
{
        struct cpu_workqueue_struct *cwq;
        struct workqueue_struct *wq;
        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;

        for_each_possible_cpu(cpu)
                wait_on_cpu_work(get_cwq(cpu, wq), 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));

        clear_wq_data(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);

/**
 * flush_delayed_work - block until a dwork_struct's callback has terminated
 * @dwork: the delayed work which is to be flushed
 *
 * Any timeout is cancelled, and any pending work is run immediately.
 */
void flush_delayed_work(struct delayed_work *dwork)
{
        if (del_timer_sync(&dwork->timer)) {
                __queue_work(get_cpu(), get_wq_data(&dwork->work)->wq,
                             &dwork->work);
                put_cpu();
        }
        flush_work(&dwork->work);
}
EXPORT_SYMBOL(flush_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;
        int orig = -1;
        struct work_struct *works;

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

        get_online_cpus();

        /*
         * When running in keventd don't schedule a work item on
         * itself.  Can just call directly because the work queue is
         * already bound.  This also is faster.
         */
        if (current_is_keventd())
                orig = raw_smp_processor_id();

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

                INIT_WORK(work, func);
                if (cpu != orig)
                        schedule_work_on(cpu, work);
        }
        if (orig >= 0)
                func(per_cpu_ptr(works, orig));

        for_each_online_cpu(cpu)
                flush_work(per_cpu_ptr(works, cpu));

        put_online_cpus();
        free_percpu(works);
        return 0;
}

/**
 * flush_scheduled_work - ensure that any scheduled work has run to completion.
 *
 * Forces execution of the kernel-global workqueue and blocks until its
 * completion.
 *
 * Think twice before calling this function!  It's very easy to get into
 * trouble if you don't take great care.  Either of the following situations
 * will lead to deadlock:
 *
 *      One of the work items currently on the workqueue needs to acquire
 *      a lock held by your code or its caller.
 *
 *      Your code is running in the context of a work routine.
 *
 * They will be detected by lockdep when they occur, but the first might not
 * occur very often.  It depends on what work items are on the workqueue and
 * what locks they need, which you have no control over.
 *
 * In most situations flushing the entire workqueue is overkill; you merely
 * need to know that a particular work item isn't queued and isn't running.
 * In such cases you should use cancel_delayed_work_sync() or
 * cancel_work_sync() instead.
 */
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 = get_cwq(cpu, keventd_wq);
        if (current == cwq->worker->task)
                ret = 1;

        return ret;

}

static struct cpu_workqueue_struct *alloc_cwqs(void)
{
        /*
         * cwqs are forced aligned according to WORK_STRUCT_FLAG_BITS.
         * Make sure that the alignment isn't lower than that of
         * unsigned long long.
         */
        const size_t size = sizeof(struct cpu_workqueue_struct);
        const size_t align = max_t(size_t, 1 << WORK_STRUCT_FLAG_BITS,
                                   __alignof__(unsigned long long));
        struct cpu_workqueue_struct *cwqs;
#ifndef CONFIG_SMP
        void *ptr;

        /*
         * On UP, percpu allocator doesn't honor alignment parameter
         * and simply uses arch-dependent default.  Allocate enough
         * room to align cwq and put an extra pointer at the end
         * pointing back to the originally allocated pointer which
         * will be used for free.
         *
         * FIXME: This really belongs to UP percpu code.  Update UP
         * percpu code to honor alignment and remove this ugliness.
         */
        ptr = __alloc_percpu(size + align + sizeof(void *), 1);
        cwqs = PTR_ALIGN(ptr, align);
        *(void **)per_cpu_ptr(cwqs + 1, 0) = ptr;
#else
        /* On SMP, percpu allocator can do it itself */
        cwqs = __alloc_percpu(size, align);
#endif
        /* just in case, make sure it's actually aligned */
        BUG_ON(!IS_ALIGNED((unsigned long)cwqs, align));
        return cwqs;
}

static void free_cwqs(struct cpu_workqueue_struct *cwqs)
{
#ifndef CONFIG_SMP
        /* on UP, the pointer to free is stored right after the cwq */
        if (cwqs)
                free_percpu(*(void **)per_cpu_ptr(cwqs + 1, 0));
#else
        free_percpu(cwqs);
#endif
}

struct workqueue_struct *__create_workqueue_key(const char *name,
                                                unsigned int flags,
                                                int max_active,
                                                struct lock_class_key *key,
                                                const char *lock_name)
{
        bool singlethread = flags & WQ_SINGLE_THREAD;
        struct workqueue_struct *wq;
        bool failed = false;
        unsigned int cpu;

        max_active = clamp_val(max_active, 1, INT_MAX);

        wq = kzalloc(sizeof(*wq), GFP_KERNEL);
        if (!wq)
                goto err;

        wq->cpu_wq = alloc_cwqs();
        if (!wq->cpu_wq)
                goto err;

        wq->flags = flags;
        mutex_init(&wq->flush_mutex);
        atomic_set(&wq->nr_cwqs_to_flush, 0);
        INIT_LIST_HEAD(&wq->flusher_queue);
        INIT_LIST_HEAD(&wq->flusher_overflow);
        wq->name = name;
        lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
        INIT_LIST_HEAD(&wq->list);

        cpu_maps_update_begin();
        /*
         * 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) {
                struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);

                BUG_ON((unsigned long)cwq & WORK_STRUCT_FLAG_MASK);
                cwq->cpu = cpu;
                cwq->wq = wq;
                cwq->flush_color = -1;
                cwq->max_active = max_active;
                spin_lock_init(&cwq->lock);
                INIT_LIST_HEAD(&cwq->worklist);
                INIT_LIST_HEAD(&cwq->delayed_works);
                init_waitqueue_head(&cwq->more_work);

                if (failed)
                        continue;
                cwq->worker = create_worker(cwq,
                                            cpu_online(cpu) && !singlethread);
                if (cwq->worker)
                        start_worker(cwq->worker);
                else
                        failed = true;
        }

        spin_lock(&workqueue_lock);
        list_add(&wq->list, &workqueues);
        spin_unlock(&workqueue_lock);

        cpu_maps_update_done();

        if (failed) {
                destroy_workqueue(wq);
                wq = NULL;
        }
        return wq;
err:
        if (wq) {
                free_cwqs(wq->cpu_wq);
                kfree(wq);
        }
        return NULL;
}
EXPORT_SYMBOL_GPL(__create_workqueue_key);

/**
 * 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;

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

        flush_workqueue(wq);

        for_each_possible_cpu(cpu) {
                struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                int i;

                if (cwq->worker) {
                        destroy_worker(cwq->worker);
                        cwq->worker = NULL;
                }

                for (i = 0; i < WORK_NR_COLORS; i++)
                        BUG_ON(cwq->nr_in_flight[i]);
                BUG_ON(cwq->nr_active);
                BUG_ON(!list_empty(&cwq->delayed_works));
        }

        free_cwqs(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;

        action &= ~CPU_TASKS_FROZEN;

        list_for_each_entry(wq, &workqueues, list) {
                if (wq->flags & WQ_SINGLE_THREAD)
                        continue;

                cwq = get_cwq(cpu, wq);

                switch (action) {
                case CPU_POST_DEAD:
                        flush_workqueue(wq);
                        break;
                }
        }

        return notifier_from_errno(0);
}

#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)
{
        unsigned int cpu;

        for_each_possible_cpu(cpu)
                ida_init(&per_cpu(worker_ida, cpu));

        singlethread_cpu = cpumask_first(cpu_possible_mask);
        hotcpu_notifier(workqueue_cpu_callback, 0);
        keventd_wq = create_workqueue("events");
        BUG_ON(!keventd_wq);
}