SLOW_WORK: Allow a requeueable work item to sleep till the thread is needed

[safe/jmp/linux-2.6] / kernel / slow-work.c

/* Worker thread pool for slow items, such as filesystem lookups or mkdirs
 *
 * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 *
 * See Documentation/slow-work.txt
 */

#include <linux/module.h>
#include <linux/slow-work.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/wait.h>
#include <linux/proc_fs.h>
#include "slow-work.h"

static void slow_work_cull_timeout(unsigned long);
static void slow_work_oom_timeout(unsigned long);

#ifdef CONFIG_SYSCTL
static int slow_work_min_threads_sysctl(struct ctl_table *, int,
                                        void __user *, size_t *, loff_t *);

static int slow_work_max_threads_sysctl(struct ctl_table *, int ,
                                        void __user *, size_t *, loff_t *);
#endif

/*
 * The pool of threads has at least min threads in it as long as someone is
 * using the facility, and may have as many as max.
 *
 * A portion of the pool may be processing very slow operations.
 */
static unsigned slow_work_min_threads = 2;
static unsigned slow_work_max_threads = 4;
static unsigned vslow_work_proportion = 50; /* % of threads that may process
                                             * very slow work */

#ifdef CONFIG_SYSCTL
static const int slow_work_min_min_threads = 2;
static int slow_work_max_max_threads = SLOW_WORK_THREAD_LIMIT;
static const int slow_work_min_vslow = 1;
static const int slow_work_max_vslow = 99;

ctl_table slow_work_sysctls[] = {
        {
                .ctl_name       = CTL_UNNUMBERED,
                .procname       = "min-threads",
                .data           = &slow_work_min_threads,
                .maxlen         = sizeof(unsigned),
                .mode           = 0644,
                .proc_handler   = slow_work_min_threads_sysctl,
                .extra1         = (void *) &slow_work_min_min_threads,
                .extra2         = &slow_work_max_threads,
        },
        {
                .ctl_name       = CTL_UNNUMBERED,
                .procname       = "max-threads",
                .data           = &slow_work_max_threads,
                .maxlen         = sizeof(unsigned),
                .mode           = 0644,
                .proc_handler   = slow_work_max_threads_sysctl,
                .extra1         = &slow_work_min_threads,
                .extra2         = (void *) &slow_work_max_max_threads,
        },
        {
                .ctl_name       = CTL_UNNUMBERED,
                .procname       = "vslow-percentage",
                .data           = &vslow_work_proportion,
                .maxlen         = sizeof(unsigned),
                .mode           = 0644,
                .proc_handler   = &proc_dointvec_minmax,
                .extra1         = (void *) &slow_work_min_vslow,
                .extra2         = (void *) &slow_work_max_vslow,
        },
        { .ctl_name = 0 }
};
#endif

/*
 * The active state of the thread pool
 */
static atomic_t slow_work_thread_count;
static atomic_t vslow_work_executing_count;

static bool slow_work_may_not_start_new_thread;
static bool slow_work_cull; /* cull a thread due to lack of activity */
static DEFINE_TIMER(slow_work_cull_timer, slow_work_cull_timeout, 0, 0);
static DEFINE_TIMER(slow_work_oom_timer, slow_work_oom_timeout, 0, 0);
static struct slow_work slow_work_new_thread; /* new thread starter */

/*
 * slow work ID allocation (use slow_work_queue_lock)
 */
static DECLARE_BITMAP(slow_work_ids, SLOW_WORK_THREAD_LIMIT);

/*
 * Unregistration tracking to prevent put_ref() from disappearing during module
 * unload
 */
#ifdef CONFIG_MODULES
static struct module *slow_work_thread_processing[SLOW_WORK_THREAD_LIMIT];
static struct module *slow_work_unreg_module;
static struct slow_work *slow_work_unreg_work_item;
static DECLARE_WAIT_QUEUE_HEAD(slow_work_unreg_wq);
static DEFINE_MUTEX(slow_work_unreg_sync_lock);
#endif

/*
 * Data for tracking currently executing items for indication through /proc
 */
#ifdef CONFIG_SLOW_WORK_PROC
struct slow_work *slow_work_execs[SLOW_WORK_THREAD_LIMIT];
pid_t slow_work_pids[SLOW_WORK_THREAD_LIMIT];
DEFINE_RWLOCK(slow_work_execs_lock);
#endif

/*
 * The queues of work items and the lock governing access to them.  These are
 * shared between all the CPUs.  It doesn't make sense to have per-CPU queues
 * as the number of threads bears no relation to the number of CPUs.
 *
 * There are two queues of work items: one for slow work items, and one for
 * very slow work items.
 */
LIST_HEAD(slow_work_queue);
LIST_HEAD(vslow_work_queue);
DEFINE_SPINLOCK(slow_work_queue_lock);

/*
 * The following are two wait queues that get pinged when a work item is placed
 * on an empty queue.  These allow work items that are hogging a thread by
 * sleeping in a way that could be deferred to yield their thread and enqueue
 * themselves.
 */
static DECLARE_WAIT_QUEUE_HEAD(slow_work_queue_waits_for_occupation);
static DECLARE_WAIT_QUEUE_HEAD(vslow_work_queue_waits_for_occupation);

/*
 * The thread controls.  A variable used to signal to the threads that they
 * should exit when the queue is empty, a waitqueue used by the threads to wait
 * for signals, and a completion set by the last thread to exit.
 */
static bool slow_work_threads_should_exit;
static DECLARE_WAIT_QUEUE_HEAD(slow_work_thread_wq);
static DECLARE_COMPLETION(slow_work_last_thread_exited);

/*
 * The number of users of the thread pool and its lock.  Whilst this is zero we
 * have no threads hanging around, and when this reaches zero, we wait for all
 * active or queued work items to complete and kill all the threads we do have.
 */
static int slow_work_user_count;
static DEFINE_MUTEX(slow_work_user_lock);

static inline int slow_work_get_ref(struct slow_work *work)
{
        if (work->ops->get_ref)
                return work->ops->get_ref(work);

        return 0;
}

static inline void slow_work_put_ref(struct slow_work *work)
{
        if (work->ops->put_ref)
                work->ops->put_ref(work);
}

/*
 * Calculate the maximum number of active threads in the pool that are
 * permitted to process very slow work items.
 *
 * The answer is rounded up to at least 1, but may not equal or exceed the
 * maximum number of the threads in the pool.  This means we always have at
 * least one thread that can process slow work items, and we always have at
 * least one thread that won't get tied up doing so.
 */
static unsigned slow_work_calc_vsmax(void)
{
        unsigned vsmax;

        vsmax = atomic_read(&slow_work_thread_count) * vslow_work_proportion;
        vsmax /= 100;
        vsmax = max(vsmax, 1U);
        return min(vsmax, slow_work_max_threads - 1);
}

/*
 * Attempt to execute stuff queued on a slow thread.  Return true if we managed
 * it, false if there was nothing to do.
 */
static noinline bool slow_work_execute(int id)
{
#ifdef CONFIG_MODULES
        struct module *module;
#endif
        struct slow_work *work = NULL;
        unsigned vsmax;
        bool very_slow;

        vsmax = slow_work_calc_vsmax();

        /* see if we can schedule a new thread to be started if we're not
         * keeping up with the work */
        if (!waitqueue_active(&slow_work_thread_wq) &&
            (!list_empty(&slow_work_queue) || !list_empty(&vslow_work_queue)) &&
            atomic_read(&slow_work_thread_count) < slow_work_max_threads &&
            !slow_work_may_not_start_new_thread)
                slow_work_enqueue(&slow_work_new_thread);

        /* find something to execute */
        spin_lock_irq(&slow_work_queue_lock);
        if (!list_empty(&vslow_work_queue) &&
            atomic_read(&vslow_work_executing_count) < vsmax) {
                work = list_entry(vslow_work_queue.next,
                                  struct slow_work, link);
                if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
                        BUG();
                list_del_init(&work->link);
                atomic_inc(&vslow_work_executing_count);
                very_slow = true;
        } else if (!list_empty(&slow_work_queue)) {
                work = list_entry(slow_work_queue.next,
                                  struct slow_work, link);
                if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
                        BUG();
                list_del_init(&work->link);
                very_slow = false;
        } else {
                very_slow = false; /* avoid the compiler warning */
        }

#ifdef CONFIG_MODULES
        if (work)
                slow_work_thread_processing[id] = work->owner;
#endif
        if (work) {
                slow_work_mark_time(work);
                slow_work_begin_exec(id, work);
        }

        spin_unlock_irq(&slow_work_queue_lock);

        if (!work)
                return false;

        if (!test_and_clear_bit(SLOW_WORK_PENDING, &work->flags))
                BUG();

        /* don't execute if the work is in the process of being cancelled */
        if (!test_bit(SLOW_WORK_CANCELLING, &work->flags))
                work->ops->execute(work);

        if (very_slow)
                atomic_dec(&vslow_work_executing_count);
        clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);

        /* wake up anyone waiting for this work to be complete */
        wake_up_bit(&work->flags, SLOW_WORK_EXECUTING);

        slow_work_end_exec(id, work);

        /* if someone tried to enqueue the item whilst we were executing it,
         * then it'll be left unenqueued to avoid multiple threads trying to
         * execute it simultaneously
         *
         * there is, however, a race between us testing the pending flag and
         * getting the spinlock, and between the enqueuer setting the pending
         * flag and getting the spinlock, so we use a deferral bit to tell us
         * if the enqueuer got there first
         */
        if (test_bit(SLOW_WORK_PENDING, &work->flags)) {
                spin_lock_irq(&slow_work_queue_lock);

                if (!test_bit(SLOW_WORK_EXECUTING, &work->flags) &&
                    test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags))
                        goto auto_requeue;

                spin_unlock_irq(&slow_work_queue_lock);
        }

        /* sort out the race between module unloading and put_ref() */
        slow_work_put_ref(work);

#ifdef CONFIG_MODULES
        module = slow_work_thread_processing[id];
        slow_work_thread_processing[id] = NULL;
        smp_mb();
        if (slow_work_unreg_work_item == work ||
            slow_work_unreg_module == module)
                wake_up_all(&slow_work_unreg_wq);
#endif

        return true;

auto_requeue:
        /* we must complete the enqueue operation
         * - we transfer our ref on the item back to the appropriate queue
         * - don't wake another thread up as we're awake already
         */
        slow_work_mark_time(work);
        if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
                list_add_tail(&work->link, &vslow_work_queue);
        else
                list_add_tail(&work->link, &slow_work_queue);
        spin_unlock_irq(&slow_work_queue_lock);
        slow_work_thread_processing[id] = NULL;
        return true;
}

/**
 * slow_work_sleep_till_thread_needed - Sleep till thread needed by other work
 * work: The work item under execution that wants to sleep
 * _timeout: Scheduler sleep timeout
 *
 * Allow a requeueable work item to sleep on a slow-work processor thread until
 * that thread is needed to do some other work or the sleep is interrupted by
 * some other event.
 *
 * The caller must set up a wake up event before calling this and must have set
 * the appropriate sleep mode (such as TASK_UNINTERRUPTIBLE) and tested its own
 * condition before calling this function as no test is made here.
 *
 * False is returned if there is nothing on the queue; true is returned if the
 * work item should be requeued
 */
bool slow_work_sleep_till_thread_needed(struct slow_work *work,
                                        signed long *_timeout)
{
        wait_queue_head_t *wfo_wq;
        struct list_head *queue;

        DEFINE_WAIT(wait);

        if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
                wfo_wq = &vslow_work_queue_waits_for_occupation;
                queue = &vslow_work_queue;
        } else {
                wfo_wq = &slow_work_queue_waits_for_occupation;
                queue = &slow_work_queue;
        }

        if (!list_empty(queue))
                return true;

        add_wait_queue_exclusive(wfo_wq, &wait);
        if (list_empty(queue))
                *_timeout = schedule_timeout(*_timeout);
        finish_wait(wfo_wq, &wait);

        return !list_empty(queue);
}
EXPORT_SYMBOL(slow_work_sleep_till_thread_needed);

/**
 * slow_work_enqueue - Schedule a slow work item for processing
 * @work: The work item to queue
 *
 * Schedule a slow work item for processing.  If the item is already undergoing
 * execution, this guarantees not to re-enter the execution routine until the
 * first execution finishes.
 *
 * The item is pinned by this function as it retains a reference to it, managed
 * through the item operations.  The item is unpinned once it has been
 * executed.
 *
 * An item may hog the thread that is running it for a relatively large amount
 * of time, sufficient, for example, to perform several lookup, mkdir, create
 * and setxattr operations.  It may sleep on I/O and may sleep to obtain locks.
 *
 * Conversely, if a number of items are awaiting processing, it may take some
 * time before any given item is given attention.  The number of threads in the
 * pool may be increased to deal with demand, but only up to a limit.
 *
 * If SLOW_WORK_VERY_SLOW is set on the work item, then it will be placed in
 * the very slow queue, from which only a portion of the threads will be
 * allowed to pick items to execute.  This ensures that very slow items won't
 * overly block ones that are just ordinarily slow.
 *
 * Returns 0 if successful, -EAGAIN if not (or -ECANCELED if cancelled work is
 * attempted queued)
 */
int slow_work_enqueue(struct slow_work *work)
{
        wait_queue_head_t *wfo_wq;
        struct list_head *queue;
        unsigned long flags;
        int ret;

        if (test_bit(SLOW_WORK_CANCELLING, &work->flags))
                return -ECANCELED;

        BUG_ON(slow_work_user_count <= 0);
        BUG_ON(!work);
        BUG_ON(!work->ops);

        /* when honouring an enqueue request, we only promise that we will run
         * the work function in the future; we do not promise to run it once
         * per enqueue request
         *
         * we use the PENDING bit to merge together repeat requests without
         * having to disable IRQs and take the spinlock, whilst still
         * maintaining our promise
         */
        if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
                if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
                        wfo_wq = &vslow_work_queue_waits_for_occupation;
                        queue = &vslow_work_queue;
                } else {
                        wfo_wq = &slow_work_queue_waits_for_occupation;
                        queue = &slow_work_queue;
                }

                spin_lock_irqsave(&slow_work_queue_lock, flags);

                if (unlikely(test_bit(SLOW_WORK_CANCELLING, &work->flags)))
                        goto cancelled;

                /* we promise that we will not attempt to execute the work
                 * function in more than one thread simultaneously
                 *
                 * this, however, leaves us with a problem if we're asked to
                 * enqueue the work whilst someone is executing the work
                 * function as simply queueing the work immediately means that
                 * another thread may try executing it whilst it is already
                 * under execution
                 *
                 * to deal with this, we set the ENQ_DEFERRED bit instead of
                 * enqueueing, and the thread currently executing the work
                 * function will enqueue the work item when the work function
                 * returns and it has cleared the EXECUTING bit
                 */
                if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
                        set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
                } else {
                        ret = slow_work_get_ref(work);
                        if (ret < 0)
                                goto failed;
                        slow_work_mark_time(work);
                        list_add_tail(&work->link, queue);
                        wake_up(&slow_work_thread_wq);

                        /* if someone who could be requeued is sleeping on a
                         * thread, then ask them to yield their thread */
                        if (work->link.prev == queue)
                                wake_up(wfo_wq);
                }

                spin_unlock_irqrestore(&slow_work_queue_lock, flags);
        }
        return 0;

cancelled:
        ret = -ECANCELED;
failed:
        spin_unlock_irqrestore(&slow_work_queue_lock, flags);
        return ret;
}
EXPORT_SYMBOL(slow_work_enqueue);

static int slow_work_wait(void *word)
{
        schedule();
        return 0;
}

/**
 * slow_work_cancel - Cancel a slow work item
 * @work: The work item to cancel
 *
 * This function will cancel a previously enqueued work item. If we cannot
 * cancel the work item, it is guarenteed to have run when this function
 * returns.
 */
void slow_work_cancel(struct slow_work *work)
{
        bool wait = true, put = false;

        set_bit(SLOW_WORK_CANCELLING, &work->flags);
        smp_mb();

        /* if the work item is a delayed work item with an active timer, we
         * need to wait for the timer to finish _before_ getting the spinlock,
         * lest we deadlock against the timer routine
         *
         * the timer routine will leave DELAYED set if it notices the
         * CANCELLING flag in time
         */
        if (test_bit(SLOW_WORK_DELAYED, &work->flags)) {
                struct delayed_slow_work *dwork =
                        container_of(work, struct delayed_slow_work, work);
                del_timer_sync(&dwork->timer);
        }

        spin_lock_irq(&slow_work_queue_lock);

        if (test_bit(SLOW_WORK_DELAYED, &work->flags)) {
                /* the timer routine aborted or never happened, so we are left
                 * holding the timer's reference on the item and should just
                 * drop the pending flag and wait for any ongoing execution to
                 * finish */
                struct delayed_slow_work *dwork =
                        container_of(work, struct delayed_slow_work, work);

                BUG_ON(timer_pending(&dwork->timer));
                BUG_ON(!list_empty(&work->link));

                clear_bit(SLOW_WORK_DELAYED, &work->flags);
                put = true;
                clear_bit(SLOW_WORK_PENDING, &work->flags);

        } else if (test_bit(SLOW_WORK_PENDING, &work->flags) &&
                   !list_empty(&work->link)) {
                /* the link in the pending queue holds a reference on the item
                 * that we will need to release */
                list_del_init(&work->link);
                wait = false;
                put = true;
                clear_bit(SLOW_WORK_PENDING, &work->flags);

        } else if (test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags)) {
                /* the executor is holding our only reference on the item, so
                 * we merely need to wait for it to finish executing */
                clear_bit(SLOW_WORK_PENDING, &work->flags);
        }

        spin_unlock_irq(&slow_work_queue_lock);

        /* the EXECUTING flag is set by the executor whilst the spinlock is set
         * and before the item is dequeued - so assuming the above doesn't
         * actually dequeue it, simply waiting for the EXECUTING flag to be
         * released here should be sufficient */
        if (wait)
                wait_on_bit(&work->flags, SLOW_WORK_EXECUTING, slow_work_wait,
                            TASK_UNINTERRUPTIBLE);

        clear_bit(SLOW_WORK_CANCELLING, &work->flags);
        if (put)
                slow_work_put_ref(work);
}
EXPORT_SYMBOL(slow_work_cancel);

/*
 * Handle expiry of the delay timer, indicating that a delayed slow work item
 * should now be queued if not cancelled
 */
static void delayed_slow_work_timer(unsigned long data)
{
        wait_queue_head_t *wfo_wq;
        struct list_head *queue;
        struct slow_work *work = (struct slow_work *) data;
        unsigned long flags;
        bool queued = false, put = false, first = false;

        if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
                wfo_wq = &vslow_work_queue_waits_for_occupation;
                queue = &vslow_work_queue;
        } else {
                wfo_wq = &slow_work_queue_waits_for_occupation;
                queue = &slow_work_queue;
        }

        spin_lock_irqsave(&slow_work_queue_lock, flags);
        if (likely(!test_bit(SLOW_WORK_CANCELLING, &work->flags))) {
                clear_bit(SLOW_WORK_DELAYED, &work->flags);

                if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
                        /* we discard the reference the timer was holding in
                         * favour of the one the executor holds */
                        set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
                        put = true;
                } else {
                        slow_work_mark_time(work);
                        list_add_tail(&work->link, queue);
                        queued = true;
                        if (work->link.prev == queue)
                                first = true;
                }
        }

        spin_unlock_irqrestore(&slow_work_queue_lock, flags);
        if (put)
                slow_work_put_ref(work);
        if (first)
                wake_up(wfo_wq);
        if (queued)
                wake_up(&slow_work_thread_wq);
}

/**
 * delayed_slow_work_enqueue - Schedule a delayed slow work item for processing
 * @dwork: The delayed work item to queue
 * @delay: When to start executing the work, in jiffies from now
 *
 * This is similar to slow_work_enqueue(), but it adds a delay before the work
 * is actually queued for processing.
 *
 * The item can have delayed processing requested on it whilst it is being
 * executed.  The delay will begin immediately, and if it expires before the
 * item finishes executing, the item will be placed back on the queue when it
 * has done executing.
 */
int delayed_slow_work_enqueue(struct delayed_slow_work *dwork,
                              unsigned long delay)
{
        struct slow_work *work = &dwork->work;
        unsigned long flags;
        int ret;

        if (delay == 0)
                return slow_work_enqueue(&dwork->work);

        BUG_ON(slow_work_user_count <= 0);
        BUG_ON(!work);
        BUG_ON(!work->ops);

        if (test_bit(SLOW_WORK_CANCELLING, &work->flags))
                return -ECANCELED;

        if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
                spin_lock_irqsave(&slow_work_queue_lock, flags);

                if (test_bit(SLOW_WORK_CANCELLING, &work->flags))
                        goto cancelled;

                /* the timer holds a reference whilst it is pending */
                ret = work->ops->get_ref(work);
                if (ret < 0)
                        goto cant_get_ref;

                if (test_and_set_bit(SLOW_WORK_DELAYED, &work->flags))
                        BUG();
                dwork->timer.expires = jiffies + delay;
                dwork->timer.data = (unsigned long) work;
                dwork->timer.function = delayed_slow_work_timer;
                add_timer(&dwork->timer);

                spin_unlock_irqrestore(&slow_work_queue_lock, flags);
        }

        return 0;

cancelled:
        ret = -ECANCELED;
cant_get_ref:
        spin_unlock_irqrestore(&slow_work_queue_lock, flags);
        return ret;
}
EXPORT_SYMBOL(delayed_slow_work_enqueue);

/*
 * Schedule a cull of the thread pool at some time in the near future
 */
static void slow_work_schedule_cull(void)
{
        mod_timer(&slow_work_cull_timer,
                  round_jiffies(jiffies + SLOW_WORK_CULL_TIMEOUT));
}

/*
 * Worker thread culling algorithm
 */
static bool slow_work_cull_thread(void)
{
        unsigned long flags;
        bool do_cull = false;

        spin_lock_irqsave(&slow_work_queue_lock, flags);

        if (slow_work_cull) {
                slow_work_cull = false;

                if (list_empty(&slow_work_queue) &&
                    list_empty(&vslow_work_queue) &&
                    atomic_read(&slow_work_thread_count) >
                    slow_work_min_threads) {
                        slow_work_schedule_cull();
                        do_cull = true;
                }
        }

        spin_unlock_irqrestore(&slow_work_queue_lock, flags);
        return do_cull;
}

/*
 * Determine if there is slow work available for dispatch
 */
static inline bool slow_work_available(int vsmax)
{
        return !list_empty(&slow_work_queue) ||
                (!list_empty(&vslow_work_queue) &&
                 atomic_read(&vslow_work_executing_count) < vsmax);
}

/*
 * Worker thread dispatcher
 */
static int slow_work_thread(void *_data)
{
        int vsmax, id;

        DEFINE_WAIT(wait);

        set_freezable();
        set_user_nice(current, -5);

        /* allocate ourselves an ID */
        spin_lock_irq(&slow_work_queue_lock);
        id = find_first_zero_bit(slow_work_ids, SLOW_WORK_THREAD_LIMIT);
        BUG_ON(id < 0 || id >= SLOW_WORK_THREAD_LIMIT);
        __set_bit(id, slow_work_ids);
        slow_work_set_thread_pid(id, current->pid);
        spin_unlock_irq(&slow_work_queue_lock);

        sprintf(current->comm, "kslowd%03u", id);

        for (;;) {
                vsmax = vslow_work_proportion;
                vsmax *= atomic_read(&slow_work_thread_count);
                vsmax /= 100;

                prepare_to_wait_exclusive(&slow_work_thread_wq, &wait,
                                          TASK_INTERRUPTIBLE);
                if (!freezing(current) &&
                    !slow_work_threads_should_exit &&
                    !slow_work_available(vsmax) &&
                    !slow_work_cull)
                        schedule();
                finish_wait(&slow_work_thread_wq, &wait);

                try_to_freeze();

                vsmax = vslow_work_proportion;
                vsmax *= atomic_read(&slow_work_thread_count);
                vsmax /= 100;

                if (slow_work_available(vsmax) && slow_work_execute(id)) {
                        cond_resched();
                        if (list_empty(&slow_work_queue) &&
                            list_empty(&vslow_work_queue) &&
                            atomic_read(&slow_work_thread_count) >
                            slow_work_min_threads)
                                slow_work_schedule_cull();
                        continue;
                }

                if (slow_work_threads_should_exit)
                        break;

                if (slow_work_cull && slow_work_cull_thread())
                        break;
        }

        spin_lock_irq(&slow_work_queue_lock);
        slow_work_set_thread_pid(id, 0);
        __clear_bit(id, slow_work_ids);
        spin_unlock_irq(&slow_work_queue_lock);

        if (atomic_dec_and_test(&slow_work_thread_count))
                complete_and_exit(&slow_work_last_thread_exited, 0);
        return 0;
}

/*
 * Handle thread cull timer expiration
 */
static void slow_work_cull_timeout(unsigned long data)
{
        slow_work_cull = true;
        wake_up(&slow_work_thread_wq);
}

/*
 * Start a new slow work thread
 */
static void slow_work_new_thread_execute(struct slow_work *work)
{
        struct task_struct *p;

        if (slow_work_threads_should_exit)
                return;

        if (atomic_read(&slow_work_thread_count) >= slow_work_max_threads)
                return;

        if (!mutex_trylock(&slow_work_user_lock))
                return;

        slow_work_may_not_start_new_thread = true;
        atomic_inc(&slow_work_thread_count);
        p = kthread_run(slow_work_thread, NULL, "kslowd");
        if (IS_ERR(p)) {
                printk(KERN_DEBUG "Slow work thread pool: OOM\n");
                if (atomic_dec_and_test(&slow_work_thread_count))
                        BUG(); /* we're running on a slow work thread... */
                mod_timer(&slow_work_oom_timer,
                          round_jiffies(jiffies + SLOW_WORK_OOM_TIMEOUT));
        } else {
                /* ratelimit the starting of new threads */
                mod_timer(&slow_work_oom_timer, jiffies + 1);
        }

        mutex_unlock(&slow_work_user_lock);
}

static const struct slow_work_ops slow_work_new_thread_ops = {
        .owner          = THIS_MODULE,
        .execute        = slow_work_new_thread_execute,
#ifdef CONFIG_SLOW_WORK_PROC
        .desc           = slow_work_new_thread_desc,
#endif
};

/*
 * post-OOM new thread start suppression expiration
 */
static void slow_work_oom_timeout(unsigned long data)
{
        slow_work_may_not_start_new_thread = false;
}

#ifdef CONFIG_SYSCTL
/*
 * Handle adjustment of the minimum number of threads
 */
static int slow_work_min_threads_sysctl(struct ctl_table *table, int write,
                                        void __user *buffer,
                                        size_t *lenp, loff_t *ppos)
{
        int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
        int n;

        if (ret == 0) {
                mutex_lock(&slow_work_user_lock);
                if (slow_work_user_count > 0) {
                        /* see if we need to start or stop threads */
                        n = atomic_read(&slow_work_thread_count) -
                                slow_work_min_threads;

                        if (n < 0 && !slow_work_may_not_start_new_thread)
                                slow_work_enqueue(&slow_work_new_thread);
                        else if (n > 0)
                                slow_work_schedule_cull();
                }
                mutex_unlock(&slow_work_user_lock);
        }

        return ret;
}

/*
 * Handle adjustment of the maximum number of threads
 */
static int slow_work_max_threads_sysctl(struct ctl_table *table, int write,
                                        void __user *buffer,
                                        size_t *lenp, loff_t *ppos)
{
        int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
        int n;

        if (ret == 0) {
                mutex_lock(&slow_work_user_lock);
                if (slow_work_user_count > 0) {
                        /* see if we need to stop threads */
                        n = slow_work_max_threads -
                                atomic_read(&slow_work_thread_count);

                        if (n < 0)
                                slow_work_schedule_cull();
                }
                mutex_unlock(&slow_work_user_lock);
        }

        return ret;
}
#endif /* CONFIG_SYSCTL */

/**
 * slow_work_register_user - Register a user of the facility
 * @module: The module about to make use of the facility
 *
 * Register a user of the facility, starting up the initial threads if there
 * aren't any other users at this point.  This will return 0 if successful, or
 * an error if not.
 */
int slow_work_register_user(struct module *module)
{
        struct task_struct *p;
        int loop;

        mutex_lock(&slow_work_user_lock);

        if (slow_work_user_count == 0) {
                printk(KERN_NOTICE "Slow work thread pool: Starting up\n");
                init_completion(&slow_work_last_thread_exited);

                slow_work_threads_should_exit = false;
                slow_work_init(&slow_work_new_thread,
                               &slow_work_new_thread_ops);
                slow_work_may_not_start_new_thread = false;
                slow_work_cull = false;

                /* start the minimum number of threads */
                for (loop = 0; loop < slow_work_min_threads; loop++) {
                        atomic_inc(&slow_work_thread_count);
                        p = kthread_run(slow_work_thread, NULL, "kslowd");
                        if (IS_ERR(p))
                                goto error;
                }
                printk(KERN_NOTICE "Slow work thread pool: Ready\n");
        }

        slow_work_user_count++;
        mutex_unlock(&slow_work_user_lock);
        return 0;

error:
        if (atomic_dec_and_test(&slow_work_thread_count))
                complete(&slow_work_last_thread_exited);
        if (loop > 0) {
                printk(KERN_ERR "Slow work thread pool:"
                       " Aborting startup on ENOMEM\n");
                slow_work_threads_should_exit = true;
                wake_up_all(&slow_work_thread_wq);
                wait_for_completion(&slow_work_last_thread_exited);
                printk(KERN_ERR "Slow work thread pool: Aborted\n");
        }
        mutex_unlock(&slow_work_user_lock);
        return PTR_ERR(p);
}
EXPORT_SYMBOL(slow_work_register_user);

/*
 * wait for all outstanding items from the calling module to complete
 * - note that more items may be queued whilst we're waiting
 */
static void slow_work_wait_for_items(struct module *module)
{
        DECLARE_WAITQUEUE(myself, current);
        struct slow_work *work;
        int loop;

        mutex_lock(&slow_work_unreg_sync_lock);
        add_wait_queue(&slow_work_unreg_wq, &myself);

        for (;;) {
                spin_lock_irq(&slow_work_queue_lock);

                /* first of all, we wait for the last queued item in each list
                 * to be processed */
                list_for_each_entry_reverse(work, &vslow_work_queue, link) {
                        if (work->owner == module) {
                                set_current_state(TASK_UNINTERRUPTIBLE);
                                slow_work_unreg_work_item = work;
                                goto do_wait;
                        }
                }
                list_for_each_entry_reverse(work, &slow_work_queue, link) {
                        if (work->owner == module) {
                                set_current_state(TASK_UNINTERRUPTIBLE);
                                slow_work_unreg_work_item = work;
                                goto do_wait;
                        }
                }

                /* then we wait for the items being processed to finish */
                slow_work_unreg_module = module;
                smp_mb();
                for (loop = 0; loop < SLOW_WORK_THREAD_LIMIT; loop++) {
                        if (slow_work_thread_processing[loop] == module)
                                goto do_wait;
                }
                spin_unlock_irq(&slow_work_queue_lock);
                break; /* okay, we're done */

        do_wait:
                spin_unlock_irq(&slow_work_queue_lock);
                schedule();
                slow_work_unreg_work_item = NULL;
                slow_work_unreg_module = NULL;
        }

        remove_wait_queue(&slow_work_unreg_wq, &myself);
        mutex_unlock(&slow_work_unreg_sync_lock);
}

/**
 * slow_work_unregister_user - Unregister a user of the facility
 * @module: The module whose items should be cleared
 *
 * Unregister a user of the facility, killing all the threads if this was the
 * last one.
 *
 * This waits for all the work items belonging to the nominated module to go
 * away before proceeding.
 */
void slow_work_unregister_user(struct module *module)
{
        /* first of all, wait for all outstanding items from the calling module
         * to complete */
        if (module)
                slow_work_wait_for_items(module);

        /* then we can actually go about shutting down the facility if need
         * be */
        mutex_lock(&slow_work_user_lock);

        BUG_ON(slow_work_user_count <= 0);

        slow_work_user_count--;
        if (slow_work_user_count == 0) {
                printk(KERN_NOTICE "Slow work thread pool: Shutting down\n");
                slow_work_threads_should_exit = true;
                del_timer_sync(&slow_work_cull_timer);
                del_timer_sync(&slow_work_oom_timer);
                wake_up_all(&slow_work_thread_wq);
                wait_for_completion(&slow_work_last_thread_exited);
                printk(KERN_NOTICE "Slow work thread pool:"
                       " Shut down complete\n");
        }

        mutex_unlock(&slow_work_user_lock);
}
EXPORT_SYMBOL(slow_work_unregister_user);

/*
 * Initialise the slow work facility
 */
static int __init init_slow_work(void)
{
        unsigned nr_cpus = num_possible_cpus();

        if (slow_work_max_threads < nr_cpus)
                slow_work_max_threads = nr_cpus;
#ifdef CONFIG_SYSCTL
        if (slow_work_max_max_threads < nr_cpus * 2)
                slow_work_max_max_threads = nr_cpus * 2;
#endif
#ifdef CONFIG_SLOW_WORK_PROC
        proc_create("slow_work_rq", S_IFREG | 0400, NULL,
                    &slow_work_runqueue_fops);
#endif
        return 0;
}

subsys_initcall(init_slow_work);