[deliverable/linux.git] / fs / btrfs / async-thread.c

/*
 * Copyright (C) 2007 Oracle.  All rights reserved.
 * Copyright (C) 2014 Fujitsu.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include <linux/kthread.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/freezer.h>
#include <linux/workqueue.h>
#include "async-thread.h"

#define WORK_QUEUED_BIT 0
#define WORK_DONE_BIT 1
#define WORK_ORDER_DONE_BIT 2
#define WORK_HIGH_PRIO_BIT 3

#define NO_THRESHOLD (-1)
#define DFT_THRESHOLD (32)

/*
 * container for the kthread task pointer and the list of pending work
 * One of these is allocated per thread.
 */
struct btrfs_worker_thread {
        /* pool we belong to */
        struct btrfs_workers *workers;

        /* list of struct btrfs_work that are waiting for service */
        struct list_head pending;
        struct list_head prio_pending;

        /* list of worker threads from struct btrfs_workers */
        struct list_head worker_list;

        /* kthread */
        struct task_struct *task;

        /* number of things on the pending list */
        atomic_t num_pending;

        /* reference counter for this struct */
        atomic_t refs;

        unsigned long sequence;

        /* protects the pending list. */
        spinlock_t lock;

        /* set to non-zero when this thread is already awake and kicking */
        int working;

        /* are we currently idle */
        int idle;
};

static int __btrfs_start_workers(struct btrfs_workers *workers);

/*
 * btrfs_start_workers uses kthread_run, which can block waiting for memory
 * for a very long time.  It will actually throttle on page writeback,
 * and so it may not make progress until after our btrfs worker threads
 * process all of the pending work structs in their queue
 *
 * This means we can't use btrfs_start_workers from inside a btrfs worker
 * thread that is used as part of cleaning dirty memory, which pretty much
 * involves all of the worker threads.
 *
 * Instead we have a helper queue who never has more than one thread
 * where we scheduler thread start operations.  This worker_start struct
 * is used to contain the work and hold a pointer to the queue that needs
 * another worker.
 */
struct worker_start {
        struct btrfs_work work;
        struct btrfs_workers *queue;
};

static void start_new_worker_func(struct btrfs_work *work)
{
        struct worker_start *start;
        start = container_of(work, struct worker_start, work);
        __btrfs_start_workers(start->queue);
        kfree(start);
}

/*
 * helper function to move a thread onto the idle list after it
 * has finished some requests.
 */
static void check_idle_worker(struct btrfs_worker_thread *worker)
{
        if (!worker->idle && atomic_read(&worker->num_pending) <
            worker->workers->idle_thresh / 2) {
                unsigned long flags;
                spin_lock_irqsave(&worker->workers->lock, flags);
                worker->idle = 1;

                /* the list may be empty if the worker is just starting */
                if (!list_empty(&worker->worker_list) &&
                    !worker->workers->stopping) {
                        list_move(&worker->worker_list,
                                 &worker->workers->idle_list);
                }
                spin_unlock_irqrestore(&worker->workers->lock, flags);
        }
}

/*
 * helper function to move a thread off the idle list after new
 * pending work is added.
 */
static void check_busy_worker(struct btrfs_worker_thread *worker)
{
        if (worker->idle && atomic_read(&worker->num_pending) >=
            worker->workers->idle_thresh) {
                unsigned long flags;
                spin_lock_irqsave(&worker->workers->lock, flags);
                worker->idle = 0;

                if (!list_empty(&worker->worker_list) &&
                    !worker->workers->stopping) {
                        list_move_tail(&worker->worker_list,
                                      &worker->workers->worker_list);
                }
                spin_unlock_irqrestore(&worker->workers->lock, flags);
        }
}

static void check_pending_worker_creates(struct btrfs_worker_thread *worker)
{
        struct btrfs_workers *workers = worker->workers;
        struct worker_start *start;
        unsigned long flags;

        rmb();
        if (!workers->atomic_start_pending)
                return;

        start = kzalloc(sizeof(*start), GFP_NOFS);
        if (!start)
                return;

        start->work.func = start_new_worker_func;
        start->queue = workers;

        spin_lock_irqsave(&workers->lock, flags);
        if (!workers->atomic_start_pending)
                goto out;

        workers->atomic_start_pending = 0;
        if (workers->num_workers + workers->num_workers_starting >=
            workers->max_workers)
                goto out;

        workers->num_workers_starting += 1;
        spin_unlock_irqrestore(&workers->lock, flags);
        btrfs_queue_worker(workers->atomic_worker_start, &start->work);
        return;

out:
        kfree(start);
        spin_unlock_irqrestore(&workers->lock, flags);
}

static noinline void run_ordered_completions(struct btrfs_workers *workers,
                                            struct btrfs_work *work)
{
        if (!workers->ordered)
                return;

        set_bit(WORK_DONE_BIT, &work->flags);

        spin_lock(&workers->order_lock);

        while (1) {
                if (!list_empty(&workers->prio_order_list)) {
                        work = list_entry(workers->prio_order_list.next,
                                          struct btrfs_work, order_list);
                } else if (!list_empty(&workers->order_list)) {
                        work = list_entry(workers->order_list.next,
                                          struct btrfs_work, order_list);
                } else {
                        break;
                }
                if (!test_bit(WORK_DONE_BIT, &work->flags))
                        break;

                /* we are going to call the ordered done function, but
                 * we leave the work item on the list as a barrier so
                 * that later work items that are done don't have their
                 * functions called before this one returns
                 */
                if (test_and_set_bit(WORK_ORDER_DONE_BIT, &work->flags))
                        break;

                spin_unlock(&workers->order_lock);

                work->ordered_func(work);

                /* now take the lock again and drop our item from the list */
                spin_lock(&workers->order_lock);
                list_del(&work->order_list);
                spin_unlock(&workers->order_lock);

                /*
                 * we don't want to call the ordered free functions
                 * with the lock held though
                 */
                work->ordered_free(work);
                spin_lock(&workers->order_lock);
        }

        spin_unlock(&workers->order_lock);
}

static void put_worker(struct btrfs_worker_thread *worker)
{
        if (atomic_dec_and_test(&worker->refs))
                kfree(worker);
}

static int try_worker_shutdown(struct btrfs_worker_thread *worker)
{
        int freeit = 0;

        spin_lock_irq(&worker->lock);
        spin_lock(&worker->workers->lock);
        if (worker->workers->num_workers > 1 &&
            worker->idle &&
            !worker->working &&
            !list_empty(&worker->worker_list) &&
            list_empty(&worker->prio_pending) &&
            list_empty(&worker->pending) &&
            atomic_read(&worker->num_pending) == 0) {
                freeit = 1;
                list_del_init(&worker->worker_list);
                worker->workers->num_workers--;
        }
        spin_unlock(&worker->workers->lock);
        spin_unlock_irq(&worker->lock);

        if (freeit)
                put_worker(worker);
        return freeit;
}

static struct btrfs_work *get_next_work(struct btrfs_worker_thread *worker,
                                        struct list_head *prio_head,
                                        struct list_head *head)
{
        struct btrfs_work *work = NULL;
        struct list_head *cur = NULL;

        if (!list_empty(prio_head)) {
                cur = prio_head->next;
                goto out;
        }

        smp_mb();
        if (!list_empty(&worker->prio_pending))
                goto refill;

        if (!list_empty(head)) {
                cur = head->next;
                goto out;
        }

refill:
        spin_lock_irq(&worker->lock);
        list_splice_tail_init(&worker->prio_pending, prio_head);
        list_splice_tail_init(&worker->pending, head);

        if (!list_empty(prio_head))
                cur = prio_head->next;
        else if (!list_empty(head))
                cur = head->next;
        spin_unlock_irq(&worker->lock);

        if (!cur)
                goto out_fail;

out:
        work = list_entry(cur, struct btrfs_work, list);

out_fail:
        return work;
}

/*
 * main loop for servicing work items
 */
static int worker_loop(void *arg)
{
        struct btrfs_worker_thread *worker = arg;
        struct list_head head;
        struct list_head prio_head;
        struct btrfs_work *work;

        INIT_LIST_HEAD(&head);
        INIT_LIST_HEAD(&prio_head);

        do {
again:
                while (1) {


                        work = get_next_work(worker, &prio_head, &head);
                        if (!work)
                                break;

                        list_del(&work->list);
                        clear_bit(WORK_QUEUED_BIT, &work->flags);

                        work->worker = worker;

                        work->func(work);

                        atomic_dec(&worker->num_pending);
                        /*
                         * unless this is an ordered work queue,
                         * 'work' was probably freed by func above.
                         */
                        run_ordered_completions(worker->workers, work);

                        check_pending_worker_creates(worker);
                        cond_resched();
                }

                spin_lock_irq(&worker->lock);
                check_idle_worker(worker);

                if (freezing(current)) {
                        worker->working = 0;
                        spin_unlock_irq(&worker->lock);
                        try_to_freeze();
                } else {
                        spin_unlock_irq(&worker->lock);
                        if (!kthread_should_stop()) {
                                cpu_relax();
                                /*
                                 * we've dropped the lock, did someone else
                                 * jump_in?
                                 */
                                smp_mb();
                                if (!list_empty(&worker->pending) ||
                                    !list_empty(&worker->prio_pending))
                                        continue;

                                /*
                                 * this short schedule allows more work to
                                 * come in without the queue functions
                                 * needing to go through wake_up_process()
                                 *
                                 * worker->working is still 1, so nobody
                                 * is going to try and wake us up
                                 */
                                schedule_timeout(1);
                                smp_mb();
                                if (!list_empty(&worker->pending) ||
                                    !list_empty(&worker->prio_pending))
                                        continue;

                                if (kthread_should_stop())
                                        break;

                                /* still no more work?, sleep for real */
                                spin_lock_irq(&worker->lock);
                                set_current_state(TASK_INTERRUPTIBLE);
                                if (!list_empty(&worker->pending) ||
                                    !list_empty(&worker->prio_pending)) {
                                        spin_unlock_irq(&worker->lock);
                                        set_current_state(TASK_RUNNING);
                                        goto again;
                                }

                                /*
                                 * this makes sure we get a wakeup when someone
                                 * adds something new to the queue
                                 */
                                worker->working = 0;
                                spin_unlock_irq(&worker->lock);

                                if (!kthread_should_stop()) {
                                        schedule_timeout(HZ * 120);
                                        if (!worker->working &&
                                            try_worker_shutdown(worker)) {
                                                return 0;
                                        }
                                }
                        }
                        __set_current_state(TASK_RUNNING);
                }
        } while (!kthread_should_stop());
        return 0;
}

/*
 * this will wait for all the worker threads to shutdown
 */
void btrfs_stop_workers(struct btrfs_workers *workers)
{
        struct list_head *cur;
        struct btrfs_worker_thread *worker;
        int can_stop;

        spin_lock_irq(&workers->lock);
        workers->stopping = 1;
        list_splice_init(&workers->idle_list, &workers->worker_list);
        while (!list_empty(&workers->worker_list)) {
                cur = workers->worker_list.next;
                worker = list_entry(cur, struct btrfs_worker_thread,
                                    worker_list);

                atomic_inc(&worker->refs);
                workers->num_workers -= 1;
                if (!list_empty(&worker->worker_list)) {
                        list_del_init(&worker->worker_list);
                        put_worker(worker);
                        can_stop = 1;
                } else
                        can_stop = 0;
                spin_unlock_irq(&workers->lock);
                if (can_stop)
                        kthread_stop(worker->task);
                spin_lock_irq(&workers->lock);
                put_worker(worker);
        }
        spin_unlock_irq(&workers->lock);
}

/*
 * simple init on struct btrfs_workers
 */
void btrfs_init_workers(struct btrfs_workers *workers, char *name, int max,
                        struct btrfs_workers *async_helper)
{
        workers->num_workers = 0;
        workers->num_workers_starting = 0;
        INIT_LIST_HEAD(&workers->worker_list);
        INIT_LIST_HEAD(&workers->idle_list);
        INIT_LIST_HEAD(&workers->order_list);
        INIT_LIST_HEAD(&workers->prio_order_list);
        spin_lock_init(&workers->lock);
        spin_lock_init(&workers->order_lock);
        workers->max_workers = max;
        workers->idle_thresh = 32;
        workers->name = name;
        workers->ordered = 0;
        workers->atomic_start_pending = 0;
        workers->atomic_worker_start = async_helper;
        workers->stopping = 0;
}

/*
 * starts new worker threads.  This does not enforce the max worker
 * count in case you need to temporarily go past it.
 */
static int __btrfs_start_workers(struct btrfs_workers *workers)
{
        struct btrfs_worker_thread *worker;
        int ret = 0;

        worker = kzalloc(sizeof(*worker), GFP_NOFS);
        if (!worker) {
                ret = -ENOMEM;
                goto fail;
        }

        INIT_LIST_HEAD(&worker->pending);
        INIT_LIST_HEAD(&worker->prio_pending);
        INIT_LIST_HEAD(&worker->worker_list);
        spin_lock_init(&worker->lock);

        atomic_set(&worker->num_pending, 0);
        atomic_set(&worker->refs, 1);
        worker->workers = workers;
        worker->task = kthread_create(worker_loop, worker,
                                      "btrfs-%s-%d", workers->name,
                                      workers->num_workers + 1);
        if (IS_ERR(worker->task)) {
                ret = PTR_ERR(worker->task);
                goto fail;
        }

        spin_lock_irq(&workers->lock);
        if (workers->stopping) {
                spin_unlock_irq(&workers->lock);
                ret = -EINVAL;
                goto fail_kthread;
        }
        list_add_tail(&worker->worker_list, &workers->idle_list);
        worker->idle = 1;
        workers->num_workers++;
        workers->num_workers_starting--;
        WARN_ON(workers->num_workers_starting < 0);
        spin_unlock_irq(&workers->lock);

        wake_up_process(worker->task);
        return 0;

fail_kthread:
        kthread_stop(worker->task);
fail:
        kfree(worker);
        spin_lock_irq(&workers->lock);
        workers->num_workers_starting--;
        spin_unlock_irq(&workers->lock);
        return ret;
}

int btrfs_start_workers(struct btrfs_workers *workers)
{
        spin_lock_irq(&workers->lock);
        workers->num_workers_starting++;
        spin_unlock_irq(&workers->lock);
        return __btrfs_start_workers(workers);
}

/*
 * run through the list and find a worker thread that doesn't have a lot
 * to do right now.  This can return null if we aren't yet at the thread
 * count limit and all of the threads are busy.
 */
static struct btrfs_worker_thread *next_worker(struct btrfs_workers *workers)
{
        struct btrfs_worker_thread *worker;
        struct list_head *next;
        int enforce_min;

        enforce_min = (workers->num_workers + workers->num_workers_starting) <
                workers->max_workers;

        /*
         * if we find an idle thread, don't move it to the end of the
         * idle list.  This improves the chance that the next submission
         * will reuse the same thread, and maybe catch it while it is still
         * working
         */
        if (!list_empty(&workers->idle_list)) {
                next = workers->idle_list.next;
                worker = list_entry(next, struct btrfs_worker_thread,
                                    worker_list);
                return worker;
        }
        if (enforce_min || list_empty(&workers->worker_list))
                return NULL;

        /*
         * if we pick a busy task, move the task to the end of the list.
         * hopefully this will keep things somewhat evenly balanced.
         * Do the move in batches based on the sequence number.  This groups
         * requests submitted at roughly the same time onto the same worker.
         */
        next = workers->worker_list.next;
        worker = list_entry(next, struct btrfs_worker_thread, worker_list);
        worker->sequence++;

        if (worker->sequence % workers->idle_thresh == 0)
                list_move_tail(next, &workers->worker_list);
        return worker;
}

/*
 * selects a worker thread to take the next job.  This will either find
 * an idle worker, start a new worker up to the max count, or just return
 * one of the existing busy workers.
 */
static struct btrfs_worker_thread *find_worker(struct btrfs_workers *workers)
{
        struct btrfs_worker_thread *worker;
        unsigned long flags;
        struct list_head *fallback;
        int ret;

        spin_lock_irqsave(&workers->lock, flags);
again:
        worker = next_worker(workers);

        if (!worker) {
                if (workers->num_workers + workers->num_workers_starting >=
                    workers->max_workers) {
                        goto fallback;
                } else if (workers->atomic_worker_start) {
                        workers->atomic_start_pending = 1;
                        goto fallback;
                } else {
                        workers->num_workers_starting++;
                        spin_unlock_irqrestore(&workers->lock, flags);
                        /* we're below the limit, start another worker */
                        ret = __btrfs_start_workers(workers);
                        spin_lock_irqsave(&workers->lock, flags);
                        if (ret)
                                goto fallback;
                        goto again;
                }
        }
        goto found;

fallback:
        fallback = NULL;
        /*
         * we have failed to find any workers, just
         * return the first one we can find.
         */
        if (!list_empty(&workers->worker_list))
                fallback = workers->worker_list.next;
        if (!list_empty(&workers->idle_list))
                fallback = workers->idle_list.next;
        BUG_ON(!fallback);
        worker = list_entry(fallback,
                  struct btrfs_worker_thread, worker_list);
found:
        /*
         * this makes sure the worker doesn't exit before it is placed
         * onto a busy/idle list
         */
        atomic_inc(&worker->num_pending);
        spin_unlock_irqrestore(&workers->lock, flags);
        return worker;
}

/*
 * btrfs_requeue_work just puts the work item back on the tail of the list
 * it was taken from.  It is intended for use with long running work functions
 * that make some progress and want to give the cpu up for others.
 */
void btrfs_requeue_work(struct btrfs_work *work)
{
        struct btrfs_worker_thread *worker = work->worker;
        unsigned long flags;
        int wake = 0;

        if (test_and_set_bit(WORK_QUEUED_BIT, &work->flags))
                return;

        spin_lock_irqsave(&worker->lock, flags);
        if (test_bit(WORK_HIGH_PRIO_BIT, &work->flags))
                list_add_tail(&work->list, &worker->prio_pending);
        else
                list_add_tail(&work->list, &worker->pending);
        atomic_inc(&worker->num_pending);

        /* by definition we're busy, take ourselves off the idle
         * list
         */
        if (worker->idle) {
                spin_lock(&worker->workers->lock);
                worker->idle = 0;
                list_move_tail(&worker->worker_list,
                              &worker->workers->worker_list);
                spin_unlock(&worker->workers->lock);
        }
        if (!worker->working) {
                wake = 1;
                worker->working = 1;
        }

        if (wake)
                wake_up_process(worker->task);
        spin_unlock_irqrestore(&worker->lock, flags);
}

void btrfs_set_work_high_prio(struct btrfs_work *work)
{
        set_bit(WORK_HIGH_PRIO_BIT, &work->flags);
}

/*
 * places a struct btrfs_work into the pending queue of one of the kthreads
 */
void btrfs_queue_worker(struct btrfs_workers *workers, struct btrfs_work *work)
{
        struct btrfs_worker_thread *worker;
        unsigned long flags;
        int wake = 0;

        /* don't requeue something already on a list */
        if (test_and_set_bit(WORK_QUEUED_BIT, &work->flags))
                return;

        worker = find_worker(workers);
        if (workers->ordered) {
                /*
                 * you're not allowed to do ordered queues from an
                 * interrupt handler
                 */
                spin_lock(&workers->order_lock);
                if (test_bit(WORK_HIGH_PRIO_BIT, &work->flags)) {
                        list_add_tail(&work->order_list,
                                      &workers->prio_order_list);
                } else {
                        list_add_tail(&work->order_list, &workers->order_list);
                }
                spin_unlock(&workers->order_lock);
        } else {
                INIT_LIST_HEAD(&work->order_list);
        }

        spin_lock_irqsave(&worker->lock, flags);

        if (test_bit(WORK_HIGH_PRIO_BIT, &work->flags))
                list_add_tail(&work->list, &worker->prio_pending);
        else
                list_add_tail(&work->list, &worker->pending);
        check_busy_worker(worker);

        /*
         * avoid calling into wake_up_process if this thread has already
         * been kicked
         */
        if (!worker->working)
                wake = 1;
        worker->working = 1;

        if (wake)
                wake_up_process(worker->task);
        spin_unlock_irqrestore(&worker->lock, flags);
}

struct __btrfs_workqueue_struct {
        struct workqueue_struct *normal_wq;
        /* List head pointing to ordered work list */
        struct list_head ordered_list;

        /* Spinlock for ordered_list */
        spinlock_t list_lock;

        /* Thresholding related variants */
        atomic_t pending;
        int max_active;
        int current_max;
        int thresh;
        unsigned int count;
        spinlock_t thres_lock;
};

struct btrfs_workqueue_struct {
        struct __btrfs_workqueue_struct *normal;
        struct __btrfs_workqueue_struct *high;
};

static inline struct __btrfs_workqueue_struct
*__btrfs_alloc_workqueue(char *name, int flags, int max_active, int thresh)
{
        struct __btrfs_workqueue_struct *ret = kzalloc(sizeof(*ret), GFP_NOFS);

        if (unlikely(!ret))
                return NULL;

        ret->max_active = max_active;
        atomic_set(&ret->pending, 0);
        if (thresh == 0)
                thresh = DFT_THRESHOLD;
        /* For low threshold, disabling threshold is a better choice */
        if (thresh < DFT_THRESHOLD) {
                ret->current_max = max_active;
                ret->thresh = NO_THRESHOLD;
        } else {
                ret->current_max = 1;
                ret->thresh = thresh;
        }

        if (flags & WQ_HIGHPRI)
                ret->normal_wq = alloc_workqueue("%s-%s-high", flags,
                                                 ret->max_active,
                                                 "btrfs", name);
        else
                ret->normal_wq = alloc_workqueue("%s-%s", flags,
                                                 ret->max_active, "btrfs",
                                                 name);
        if (unlikely(!ret->normal_wq)) {
                kfree(ret);
                return NULL;
        }

        INIT_LIST_HEAD(&ret->ordered_list);
        spin_lock_init(&ret->list_lock);
        spin_lock_init(&ret->thres_lock);
        return ret;
}

static inline void
__btrfs_destroy_workqueue(struct __btrfs_workqueue_struct *wq);

struct btrfs_workqueue_struct *btrfs_alloc_workqueue(char *name,
                                                     int flags,
                                                     int max_active,
                                                     int thresh)
{
        struct btrfs_workqueue_struct *ret = kzalloc(sizeof(*ret), GFP_NOFS);

        if (unlikely(!ret))
                return NULL;

        ret->normal = __btrfs_alloc_workqueue(name, flags & ~WQ_HIGHPRI,
                                              max_active, thresh);
        if (unlikely(!ret->normal)) {
                kfree(ret);
                return NULL;
        }

        if (flags & WQ_HIGHPRI) {
                ret->high = __btrfs_alloc_workqueue(name, flags, max_active,
                                                    thresh);
                if (unlikely(!ret->high)) {
                        __btrfs_destroy_workqueue(ret->normal);
                        kfree(ret);
                        return NULL;
                }
        }
        return ret;
}

/*
 * Hook for threshold which will be called in btrfs_queue_work.
 * This hook WILL be called in IRQ handler context,
 * so workqueue_set_max_active MUST NOT be called in this hook
 */
static inline void thresh_queue_hook(struct __btrfs_workqueue_struct *wq)
{
        if (wq->thresh == NO_THRESHOLD)
                return;
        atomic_inc(&wq->pending);
}

/*
 * Hook for threshold which will be called before executing the work,
 * This hook is called in kthread content.
 * So workqueue_set_max_active is called here.
 */
static inline void thresh_exec_hook(struct __btrfs_workqueue_struct *wq)
{
        int new_max_active;
        long pending;
        int need_change = 0;

        if (wq->thresh == NO_THRESHOLD)
                return;

        atomic_dec(&wq->pending);
        spin_lock(&wq->thres_lock);
        /*
         * Use wq->count to limit the calling frequency of
         * workqueue_set_max_active.
         */
        wq->count++;
        wq->count %= (wq->thresh / 4);
        if (!wq->count)
                goto  out;
        new_max_active = wq->current_max;

        /*
         * pending may be changed later, but it's OK since we really
         * don't need it so accurate to calculate new_max_active.
         */
        pending = atomic_read(&wq->pending);
        if (pending > wq->thresh)
                new_max_active++;
        if (pending < wq->thresh / 2)
                new_max_active--;
        new_max_active = clamp_val(new_max_active, 1, wq->max_active);
        if (new_max_active != wq->current_max)  {
                need_change = 1;
                wq->current_max = new_max_active;
        }
out:
        spin_unlock(&wq->thres_lock);

        if (need_change) {
                workqueue_set_max_active(wq->normal_wq, wq->current_max);
        }
}

static void run_ordered_work(struct __btrfs_workqueue_struct *wq)
{
        struct list_head *list = &wq->ordered_list;
        struct btrfs_work_struct *work;
        spinlock_t *lock = &wq->list_lock;
        unsigned long flags;

        while (1) {
                spin_lock_irqsave(lock, flags);
                if (list_empty(list))
                        break;
                work = list_entry(list->next, struct btrfs_work_struct,
                                  ordered_list);
                if (!test_bit(WORK_DONE_BIT, &work->flags))
                        break;

                /*
                 * we are going to call the ordered done function, but
                 * we leave the work item on the list as a barrier so
                 * that later work items that are done don't have their
                 * functions called before this one returns
                 */
                if (test_and_set_bit(WORK_ORDER_DONE_BIT, &work->flags))
                        break;
                spin_unlock_irqrestore(lock, flags);
                work->ordered_func(work);

                /* now take the lock again and drop our item from the list */
                spin_lock_irqsave(lock, flags);
                list_del(&work->ordered_list);
                spin_unlock_irqrestore(lock, flags);

                /*
                 * we don't want to call the ordered free functions
                 * with the lock held though
                 */
                work->ordered_free(work);
        }
        spin_unlock_irqrestore(lock, flags);
}

static void normal_work_helper(struct work_struct *arg)
{
        struct btrfs_work_struct *work;
        struct __btrfs_workqueue_struct *wq;
        int need_order = 0;

        work = container_of(arg, struct btrfs_work_struct, normal_work);
        /*
         * We should not touch things inside work in the following cases:
         * 1) after work->func() if it has no ordered_free
         *    Since the struct is freed in work->func().
         * 2) after setting WORK_DONE_BIT
         *    The work may be freed in other threads almost instantly.
         * So we save the needed things here.
         */
        if (work->ordered_func)
                need_order = 1;
        wq = work->wq;

        thresh_exec_hook(wq);
        work->func(work);
        if (need_order) {
                set_bit(WORK_DONE_BIT, &work->flags);
                run_ordered_work(wq);
        }
}

void btrfs_init_work(struct btrfs_work_struct *work,
                     void (*func)(struct btrfs_work_struct *),
                     void (*ordered_func)(struct btrfs_work_struct *),
                     void (*ordered_free)(struct btrfs_work_struct *))
{
        work->func = func;
        work->ordered_func = ordered_func;
        work->ordered_free = ordered_free;
        INIT_WORK(&work->normal_work, normal_work_helper);
        INIT_LIST_HEAD(&work->ordered_list);
        work->flags = 0;
}

static inline void __btrfs_queue_work(struct __btrfs_workqueue_struct *wq,
                                      struct btrfs_work_struct *work)
{
        unsigned long flags;

        work->wq = wq;
        thresh_queue_hook(wq);
        if (work->ordered_func) {
                spin_lock_irqsave(&wq->list_lock, flags);
                list_add_tail(&work->ordered_list, &wq->ordered_list);
                spin_unlock_irqrestore(&wq->list_lock, flags);
        }
        queue_work(wq->normal_wq, &work->normal_work);
}

void btrfs_queue_work(struct btrfs_workqueue_struct *wq,
                      struct btrfs_work_struct *work)
{
        struct __btrfs_workqueue_struct *dest_wq;

        if (test_bit(WORK_HIGH_PRIO_BIT, &work->flags) && wq->high)
                dest_wq = wq->high;
        else
                dest_wq = wq->normal;
        __btrfs_queue_work(dest_wq, work);
}

static inline void
__btrfs_destroy_workqueue(struct __btrfs_workqueue_struct *wq)
{
        destroy_workqueue(wq->normal_wq);
        kfree(wq);
}

void btrfs_destroy_workqueue(struct btrfs_workqueue_struct *wq)
{
        if (!wq)
                return;
        if (wq->high)
                __btrfs_destroy_workqueue(wq->high);
        __btrfs_destroy_workqueue(wq->normal);
}

void btrfs_workqueue_set_max(struct btrfs_workqueue_struct *wq, int max)
{
        wq->normal->max_active = max;
        if (wq->high)
                wq->high->max_active = max;
}

void btrfs_set_work_high_priority(struct btrfs_work_struct *work)
{
        set_bit(WORK_HIGH_PRIO_BIT, &work->flags);
}