xfs_trans_ail.c

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/*
 * Copyright (c) 2000-2002,2005 Silicon Graphics, Inc.
 * Copyright (c) 2008 Dave Chinner
 * 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 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would 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 the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_types.h"
#include "xfs_log.h"
#include "xfs_trans.h"
#include "xfs_sb.h"
#include "xfs_ag.h"
#include "xfs_mount.h"
#include "xfs_trans_priv.h"
#include "xfs_trace.h"
#include "xfs_error.h"

#ifdef DEBUG
/*
 * Check that the list is sorted as it should be.
 */
STATIC void
xfs_ail_check(
    struct xfs_ail  *ailp,
    xfs_log_item_t  *lip)
{
    xfs_log_item_t  *prev_lip;

    if (list_empty(&ailp->xa_ail))
        return;

    /*
     * Check the next and previous entries are valid.
     */
    ASSERT((lip->li_flags & XFS_LI_IN_AIL) != 0);
    prev_lip = list_entry(lip->li_ail.prev, xfs_log_item_t, li_ail);
    if (&prev_lip->li_ail != &ailp->xa_ail)
        ASSERT(XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) <= 0);

    prev_lip = list_entry(lip->li_ail.next, xfs_log_item_t, li_ail);
    if (&prev_lip->li_ail != &ailp->xa_ail)
        ASSERT(XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) >= 0);


#ifdef XFS_TRANS_DEBUG
    /*
     * Walk the list checking lsn ordering, and that every entry has the
     * XFS_LI_IN_AIL flag set. This is really expensive, so only do it
     * when specifically debugging the transaction subsystem.
     */
    prev_lip = list_entry(&ailp->xa_ail, xfs_log_item_t, li_ail);
    list_for_each_entry(lip, &ailp->xa_ail, li_ail) {
        if (&prev_lip->li_ail != &ailp->xa_ail)
            ASSERT(XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) <= 0);
        ASSERT((lip->li_flags & XFS_LI_IN_AIL) != 0);
        prev_lip = lip;
    }
#endif /* XFS_TRANS_DEBUG */
}
#else /* !DEBUG */
#define xfs_ail_check(a,l)
#endif /* DEBUG */

/*
 * Return a pointer to the first item in the AIL.  If the AIL is empty, then
 * return NULL.
 */
xfs_log_item_t *
xfs_ail_min(
    struct xfs_ail  *ailp)
{
    if (list_empty(&ailp->xa_ail))
        return NULL;

    return list_first_entry(&ailp->xa_ail, xfs_log_item_t, li_ail);
}

 /*
 * Return a pointer to the last item in the AIL.  If the AIL is empty, then
 * return NULL.
 */
static xfs_log_item_t *
xfs_ail_max(
    struct xfs_ail  *ailp)
{
    if (list_empty(&ailp->xa_ail))
        return NULL;

    return list_entry(ailp->xa_ail.prev, xfs_log_item_t, li_ail);
}

/*
 * Return a pointer to the item which follows the given item in the AIL.  If
 * the given item is the last item in the list, then return NULL.
 */
static xfs_log_item_t *
xfs_ail_next(
    struct xfs_ail  *ailp,
    xfs_log_item_t  *lip)
{
    if (lip->li_ail.next == &ailp->xa_ail)
        return NULL;

    return list_first_entry(&lip->li_ail, xfs_log_item_t, li_ail);
}

/*
 * This is called by the log manager code to determine the LSN of the tail of
 * the log.  This is exactly the LSN of the first item in the AIL.  If the AIL
 * is empty, then this function returns 0.
 *
 * We need the AIL lock in order to get a coherent read of the lsn of the last
 * item in the AIL.
 */
xfs_lsn_t
xfs_ail_min_lsn(
    struct xfs_ail  *ailp)
{
    xfs_lsn_t   lsn = 0;
    xfs_log_item_t  *lip;

    spin_lock(&ailp->xa_lock);
    lip = xfs_ail_min(ailp);
    if (lip)
        lsn = lip->li_lsn;
    spin_unlock(&ailp->xa_lock);

    return lsn;
}

/*
 * Return the maximum lsn held in the AIL, or zero if the AIL is empty.
 */
static xfs_lsn_t
xfs_ail_max_lsn(
    struct xfs_ail  *ailp)
{
    xfs_lsn_t       lsn = 0;
    xfs_log_item_t  *lip;

    spin_lock(&ailp->xa_lock);
    lip = xfs_ail_max(ailp);
    if (lip)
        lsn = lip->li_lsn;
    spin_unlock(&ailp->xa_lock);

    return lsn;
}

/*
 * The cursor keeps track of where our current traversal is up to by tracking
 * the next item in the list for us. However, for this to be safe, removing an
 * object from the AIL needs to invalidate any cursor that points to it. hence
 * the traversal cursor needs to be linked to the struct xfs_ail so that
 * deletion can search all the active cursors for invalidation.
 */
STATIC void
xfs_trans_ail_cursor_init(
    struct xfs_ail      *ailp,
    struct xfs_ail_cursor   *cur)
{
    cur->item = NULL;
    list_add_tail(&cur->list, &ailp->xa_cursors);
}

/*
 * Get the next item in the traversal and advance the cursor.  If the cursor
 * was invalidated (indicated by a lip of 1), restart the traversal.
 */
struct xfs_log_item *
xfs_trans_ail_cursor_next(
    struct xfs_ail      *ailp,
    struct xfs_ail_cursor   *cur)
{
    struct xfs_log_item *lip = cur->item;

    if ((__psint_t)lip & 1)
        lip = xfs_ail_min(ailp);
    if (lip)
        cur->item = xfs_ail_next(ailp, lip);
    return lip;
}

/*
 * When the traversal is complete, we need to remove the cursor from the list
 * of traversing cursors.
 */
void
xfs_trans_ail_cursor_done(
    struct xfs_ail      *ailp,
    struct xfs_ail_cursor   *cur)
{
    cur->item = NULL;
    list_del_init(&cur->list);
}

/*
 * Invalidate any cursor that is pointing to this item. This is called when an
 * item is removed from the AIL. Any cursor pointing to this object is now
 * invalid and the traversal needs to be terminated so it doesn't reference a
 * freed object. We set the low bit of the cursor item pointer so we can
 * distinguish between an invalidation and the end of the list when getting the
 * next item from the cursor.
 */
STATIC void
xfs_trans_ail_cursor_clear(
    struct xfs_ail      *ailp,
    struct xfs_log_item *lip)
{
    struct xfs_ail_cursor   *cur;

    list_for_each_entry(cur, &ailp->xa_cursors, list) {
        if (cur->item == lip)
            cur->item = (struct xfs_log_item *)
                    ((__psint_t)cur->item | 1);
    }
}

/*
 * Find the first item in the AIL with the given @lsn by searching in ascending
 * LSN order and initialise the cursor to point to the next item for a
 * ascending traversal.  Pass a @lsn of zero to initialise the cursor to the
 * first item in the AIL. Returns NULL if the list is empty.
 */
xfs_log_item_t *
xfs_trans_ail_cursor_first(
    struct xfs_ail      *ailp,
    struct xfs_ail_cursor   *cur,
    xfs_lsn_t       lsn)
{
    xfs_log_item_t      *lip;

    xfs_trans_ail_cursor_init(ailp, cur);

    if (lsn == 0) {
        lip = xfs_ail_min(ailp);
        goto out;
    }

    list_for_each_entry(lip, &ailp->xa_ail, li_ail) {
        if (XFS_LSN_CMP(lip->li_lsn, lsn) >= 0)
            goto out;
    }
    return NULL;

out:
    if (lip)
        cur->item = xfs_ail_next(ailp, lip);
    return lip;
}

static struct xfs_log_item *
__xfs_trans_ail_cursor_last(
    struct xfs_ail      *ailp,
    xfs_lsn_t       lsn)
{
    xfs_log_item_t      *lip;

    list_for_each_entry_reverse(lip, &ailp->xa_ail, li_ail) {
        if (XFS_LSN_CMP(lip->li_lsn, lsn) <= 0)
            return lip;
    }
    return NULL;
}

/*
 * Find the last item in the AIL with the given @lsn by searching in descending
 * LSN order and initialise the cursor to point to that item.  If there is no
 * item with the value of @lsn, then it sets the cursor to the last item with an
 * LSN lower than @lsn.  Returns NULL if the list is empty.
 */
struct xfs_log_item *
xfs_trans_ail_cursor_last(
    struct xfs_ail      *ailp,
    struct xfs_ail_cursor   *cur,
    xfs_lsn_t       lsn)
{
    xfs_trans_ail_cursor_init(ailp, cur);
    cur->item = __xfs_trans_ail_cursor_last(ailp, lsn);
    return cur->item;
}

/*
 * Splice the log item list into the AIL at the given LSN. We splice to the
 * tail of the given LSN to maintain insert order for push traversals. The
 * cursor is optional, allowing repeated updates to the same LSN to avoid
 * repeated traversals.  This should not be called with an empty list.
 */
static void
xfs_ail_splice(
    struct xfs_ail      *ailp,
    struct xfs_ail_cursor   *cur,
    struct list_head    *list,
    xfs_lsn_t       lsn)
{
    struct xfs_log_item *lip;

    ASSERT(!list_empty(list));

    /*
     * Use the cursor to determine the insertion point if one is
     * provided.  If not, or if the one we got is not valid,
     * find the place in the AIL where the items belong.
     */
    lip = cur ? cur->item : NULL;
    if (!lip || (__psint_t) lip & 1)
        lip = __xfs_trans_ail_cursor_last(ailp, lsn);

    /*
     * If a cursor is provided, we know we're processing the AIL
     * in lsn order, and future items to be spliced in will
     * follow the last one being inserted now.  Update the
     * cursor to point to that last item, now while we have a
     * reliable pointer to it.
     */
    if (cur)
        cur->item = list_entry(list->prev, struct xfs_log_item, li_ail);

    /*
     * Finally perform the splice.  Unless the AIL was empty,
     * lip points to the item in the AIL _after_ which the new
     * items should go.  If lip is null the AIL was empty, so
     * the new items go at the head of the AIL.
     */
    if (lip)
        list_splice(list, &lip->li_ail);
    else
        list_splice(list, &ailp->xa_ail);
}

/*
 * Delete the given item from the AIL.  Return a pointer to the item.
 */
static void
xfs_ail_delete(
    struct xfs_ail  *ailp,
    xfs_log_item_t  *lip)
{
    xfs_ail_check(ailp, lip);
    list_del(&lip->li_ail);
    xfs_trans_ail_cursor_clear(ailp, lip);
}

static long
xfsaild_push(
    struct xfs_ail      *ailp)
{
    xfs_mount_t     *mp = ailp->xa_mount;
    struct xfs_ail_cursor   cur;
    xfs_log_item_t      *lip;
    xfs_lsn_t       lsn;
    xfs_lsn_t       target;
    long            tout;
    int         stuck = 0;
    int         flushing = 0;
    int         count = 0;

    /*
     * If we encountered pinned items or did not finish writing out all
     * buffers the last time we ran, force the log first and wait for it
     * before pushing again.
     */
    if (ailp->xa_log_flush && ailp->xa_last_pushed_lsn == 0 &&
        (!list_empty_careful(&ailp->xa_buf_list) ||
         xfs_ail_min_lsn(ailp))) {
        ailp->xa_log_flush = 0;

        XFS_STATS_INC(xs_push_ail_flush);
        xfs_log_force(mp, XFS_LOG_SYNC);
    }

    spin_lock(&ailp->xa_lock);

    /* barrier matches the xa_target update in xfs_ail_push() */
    smp_rmb();
    target = ailp->xa_target;
    ailp->xa_target_prev = target;

    lip = xfs_trans_ail_cursor_first(ailp, &cur, ailp->xa_last_pushed_lsn);
    if (!lip) {
        /*
         * If the AIL is empty or our push has reached the end we are
         * done now.
         */
        xfs_trans_ail_cursor_done(ailp, &cur);
        spin_unlock(&ailp->xa_lock);
        goto out_done;
    }

    XFS_STATS_INC(xs_push_ail);

    lsn = lip->li_lsn;
    while ((XFS_LSN_CMP(lip->li_lsn, target) <= 0)) {
        int lock_result;

        /*
         * Note that IOP_PUSH may unlock and reacquire the AIL lock.  We
         * rely on the AIL cursor implementation to be able to deal with
         * the dropped lock.
         */
        lock_result = IOP_PUSH(lip, &ailp->xa_buf_list);
        switch (lock_result) {
        case XFS_ITEM_SUCCESS:
            XFS_STATS_INC(xs_push_ail_success);
            trace_xfs_ail_push(lip);

            ailp->xa_last_pushed_lsn = lsn;
            break;

        case XFS_ITEM_FLUSHING:
            /*
             * The item or its backing buffer is already beeing
             * flushed.  The typical reason for that is that an
             * inode buffer is locked because we already pushed the
             * updates to it as part of inode clustering.
             *
             * We do not want to to stop flushing just because lots
             * of items are already beeing flushed, but we need to
             * re-try the flushing relatively soon if most of the
             * AIL is beeing flushed.
             */
            XFS_STATS_INC(xs_push_ail_flushing);
            trace_xfs_ail_flushing(lip);

            flushing++;
            ailp->xa_last_pushed_lsn = lsn;
            break;

        case XFS_ITEM_PINNED:
            XFS_STATS_INC(xs_push_ail_pinned);
            trace_xfs_ail_pinned(lip);

            stuck++;
            ailp->xa_log_flush++;
            break;
        case XFS_ITEM_LOCKED:
            XFS_STATS_INC(xs_push_ail_locked);
            trace_xfs_ail_locked(lip);

            stuck++;
            break;
        default:
            ASSERT(0);
            break;
        }

        count++;

        /*
         * Are there too many items we can't do anything with?
         *
         * If we we are skipping too many items because we can't flush
         * them or they are already being flushed, we back off and
         * given them time to complete whatever operation is being
         * done. i.e. remove pressure from the AIL while we can't make
         * progress so traversals don't slow down further inserts and
         * removals to/from the AIL.
         *
         * The value of 100 is an arbitrary magic number based on
         * observation.
         */
        if (stuck > 100)
            break;

        lip = xfs_trans_ail_cursor_next(ailp, &cur);
        if (lip == NULL)
            break;
        lsn = lip->li_lsn;
    }
    xfs_trans_ail_cursor_done(ailp, &cur);
    spin_unlock(&ailp->xa_lock);

    if (xfs_buf_delwri_submit_nowait(&ailp->xa_buf_list))
        ailp->xa_log_flush++;

    if (!count || XFS_LSN_CMP(lsn, target) >= 0) {
out_done:
        /*
         * We reached the target or the AIL is empty, so wait a bit
         * longer for I/O to complete and remove pushed items from the
         * AIL before we start the next scan from the start of the AIL.
         */
        tout = 50;
        ailp->xa_last_pushed_lsn = 0;
    } else if (((stuck + flushing) * 100) / count > 90) {
        /*
         * Either there is a lot of contention on the AIL or we are
         * stuck due to operations in progress. "Stuck" in this case
         * is defined as >90% of the items we tried to push were stuck.
         *
         * Backoff a bit more to allow some I/O to complete before
         * restarting from the start of the AIL. This prevents us from
         * spinning on the same items, and if they are pinned will all
         * the restart to issue a log force to unpin the stuck items.
         */
        tout = 20;
        ailp->xa_last_pushed_lsn = 0;
    } else {
        /*
         * Assume we have more work to do in a short while.
         */
        tout = 10;
    }

    return tout;
}

static int
xfsaild(
    void        *data)
{
    struct xfs_ail  *ailp = data;
    long        tout = 0;   /* milliseconds */

    current->flags |= PF_MEMALLOC;

    while (!kthread_should_stop()) {
        if (tout && tout <= 20)
            __set_current_state(TASK_KILLABLE);
        else
            __set_current_state(TASK_INTERRUPTIBLE);

        spin_lock(&ailp->xa_lock);

        /*
         * Idle if the AIL is empty and we are not racing with a target
         * update. We check the AIL after we set the task to a sleep
         * state to guarantee that we either catch an xa_target update
         * or that a wake_up resets the state to TASK_RUNNING.
         * Otherwise, we run the risk of sleeping indefinitely.
         *
         * The barrier matches the xa_target update in xfs_ail_push().
         */
        smp_rmb();
        if (!xfs_ail_min(ailp) &&
            ailp->xa_target == ailp->xa_target_prev) {
            spin_unlock(&ailp->xa_lock);
            schedule();
            tout = 0;
            continue;
        }
        spin_unlock(&ailp->xa_lock);

        if (tout)
            schedule_timeout(msecs_to_jiffies(tout));

        __set_current_state(TASK_RUNNING);

        try_to_freeze();

        tout = xfsaild_push(ailp);
    }

    return 0;
}

/*
 * This routine is called to move the tail of the AIL forward.  It does this by
 * trying to flush items in the AIL whose lsns are below the given
 * threshold_lsn.
 *
 * The push is run asynchronously in a workqueue, which means the caller needs
 * to handle waiting on the async flush for space to become available.
 * We don't want to interrupt any push that is in progress, hence we only queue
 * work if we set the pushing bit approriately.
 *
 * We do this unlocked - we only need to know whether there is anything in the
 * AIL at the time we are called. We don't need to access the contents of
 * any of the objects, so the lock is not needed.
 */
void
xfs_ail_push(
    struct xfs_ail  *ailp,
    xfs_lsn_t   threshold_lsn)
{
    xfs_log_item_t  *lip;

    lip = xfs_ail_min(ailp);
    if (!lip || XFS_FORCED_SHUTDOWN(ailp->xa_mount) ||
        XFS_LSN_CMP(threshold_lsn, ailp->xa_target) <= 0)
        return;

    /*
     * Ensure that the new target is noticed in push code before it clears
     * the XFS_AIL_PUSHING_BIT.
     */
    smp_wmb();
    xfs_trans_ail_copy_lsn(ailp, &ailp->xa_target, &threshold_lsn);
    smp_wmb();

    wake_up_process(ailp->xa_task);
}

/*
 * Push out all items in the AIL immediately
 */
void
xfs_ail_push_all(
    struct xfs_ail  *ailp)
{
    xfs_lsn_t       threshold_lsn = xfs_ail_max_lsn(ailp);

    if (threshold_lsn)
        xfs_ail_push(ailp, threshold_lsn);
}

/*
 * Push out all items in the AIL immediately and wait until the AIL is empty.
 */
void
xfs_ail_push_all_sync(
    struct xfs_ail  *ailp)
{
    struct xfs_log_item *lip;
    DEFINE_WAIT(wait);

    spin_lock(&ailp->xa_lock);
    while ((lip = xfs_ail_max(ailp)) != NULL) {
        prepare_to_wait(&ailp->xa_empty, &wait, TASK_UNINTERRUPTIBLE);
        ailp->xa_target = lip->li_lsn;
        wake_up_process(ailp->xa_task);
        spin_unlock(&ailp->xa_lock);
        schedule();
        spin_lock(&ailp->xa_lock);
    }
    spin_unlock(&ailp->xa_lock);

    finish_wait(&ailp->xa_empty, &wait);
}

/*
 * xfs_trans_ail_update - bulk AIL insertion operation.
 *
 * @xfs_trans_ail_update takes an array of log items that all need to be
 * positioned at the same LSN in the AIL. If an item is not in the AIL, it will
 * be added.  Otherwise, it will be repositioned  by removing it and re-adding
 * it to the AIL. If we move the first item in the AIL, update the log tail to
 * match the new minimum LSN in the AIL.
 *
 * This function takes the AIL lock once to execute the update operations on
 * all the items in the array, and as such should not be called with the AIL
 * lock held. As a result, once we have the AIL lock, we need to check each log
 * item LSN to confirm it needs to be moved forward in the AIL.
 *
 * To optimise the insert operation, we delete all the items from the AIL in
 * the first pass, moving them into a temporary list, then splice the temporary
 * list into the correct position in the AIL. This avoids needing to do an
 * insert operation on every item.
 *
 * This function must be called with the AIL lock held.  The lock is dropped
 * before returning.
 */
void
xfs_trans_ail_update_bulk(
    struct xfs_ail      *ailp,
    struct xfs_ail_cursor   *cur,
    struct xfs_log_item **log_items,
    int         nr_items,
    xfs_lsn_t       lsn) __releases(ailp->xa_lock)
{
    xfs_log_item_t      *mlip;
    int         mlip_changed = 0;
    int         i;
    LIST_HEAD(tmp);

    ASSERT(nr_items > 0);       /* Not required, but true. */
    mlip = xfs_ail_min(ailp);

    for (i = 0; i < nr_items; i++) {
        struct xfs_log_item *lip = log_items[i];
        if (lip->li_flags & XFS_LI_IN_AIL) {
            /* check if we really need to move the item */
            if (XFS_LSN_CMP(lsn, lip->li_lsn) <= 0)
                continue;

            xfs_ail_delete(ailp, lip);
            if (mlip == lip)
                mlip_changed = 1;
        } else {
            lip->li_flags |= XFS_LI_IN_AIL;
        }
        lip->li_lsn = lsn;
        list_add(&lip->li_ail, &tmp);
    }

    if (!list_empty(&tmp))
        xfs_ail_splice(ailp, cur, &tmp, lsn);

    if (mlip_changed) {
        if (!XFS_FORCED_SHUTDOWN(ailp->xa_mount))
            xlog_assign_tail_lsn_locked(ailp->xa_mount);
        spin_unlock(&ailp->xa_lock);

        xfs_log_space_wake(ailp->xa_mount);
    } else {
        spin_unlock(&ailp->xa_lock);
    }
}

/*
 * xfs_trans_ail_delete_bulk - remove multiple log items from the AIL
 *
 * @xfs_trans_ail_delete_bulk takes an array of log items that all need to
 * removed from the AIL. The caller is already holding the AIL lock, and done
 * all the checks necessary to ensure the items passed in via @log_items are
 * ready for deletion. This includes checking that the items are in the AIL.
 *
 * For each log item to be removed, unlink it  from the AIL, clear the IN_AIL
 * flag from the item and reset the item's lsn to 0. If we remove the first
 * item in the AIL, update the log tail to match the new minimum LSN in the
 * AIL.
 *
 * This function will not drop the AIL lock until all items are removed from
 * the AIL to minimise the amount of lock traffic on the AIL. This does not
 * greatly increase the AIL hold time, but does significantly reduce the amount
 * of traffic on the lock, especially during IO completion.
 *
 * This function must be called with the AIL lock held.  The lock is dropped
 * before returning.
 */
void
xfs_trans_ail_delete_bulk(
    struct xfs_ail      *ailp,
    struct xfs_log_item **log_items,
    int         nr_items,
    int         shutdown_type) __releases(ailp->xa_lock)
{
    xfs_log_item_t      *mlip;
    int         mlip_changed = 0;
    int         i;

    mlip = xfs_ail_min(ailp);

    for (i = 0; i < nr_items; i++) {
        struct xfs_log_item *lip = log_items[i];
        if (!(lip->li_flags & XFS_LI_IN_AIL)) {
            struct xfs_mount    *mp = ailp->xa_mount;

            spin_unlock(&ailp->xa_lock);
            if (!XFS_FORCED_SHUTDOWN(mp)) {
                xfs_alert_tag(mp, XFS_PTAG_AILDELETE,
        "%s: attempting to delete a log item that is not in the AIL",
                        __func__);
                xfs_force_shutdown(mp, shutdown_type);
            }
            return;
        }

        xfs_ail_delete(ailp, lip);
        lip->li_flags &= ~XFS_LI_IN_AIL;
        lip->li_lsn = 0;
        if (mlip == lip)
            mlip_changed = 1;
    }

    if (mlip_changed) {
        if (!XFS_FORCED_SHUTDOWN(ailp->xa_mount))
            xlog_assign_tail_lsn_locked(ailp->xa_mount);
        if (list_empty(&ailp->xa_ail))
            wake_up_all(&ailp->xa_empty);
        spin_unlock(&ailp->xa_lock);

        xfs_log_space_wake(ailp->xa_mount);
    } else {
        spin_unlock(&ailp->xa_lock);
    }
}

int
xfs_trans_ail_init(
    xfs_mount_t *mp)
{
    struct xfs_ail  *ailp;

    ailp = kmem_zalloc(sizeof(struct xfs_ail), KM_MAYFAIL);
    if (!ailp)
        return ENOMEM;

    ailp->xa_mount = mp;
    INIT_LIST_HEAD(&ailp->xa_ail);
    INIT_LIST_HEAD(&ailp->xa_cursors);
    spin_lock_init(&ailp->xa_lock);
    INIT_LIST_HEAD(&ailp->xa_buf_list);
    init_waitqueue_head(&ailp->xa_empty);

    ailp->xa_task = kthread_run(xfsaild, ailp, "xfsaild/%s",
            ailp->xa_mount->m_fsname);
    if (IS_ERR(ailp->xa_task))
        goto out_free_ailp;

    mp->m_ail = ailp;
    return 0;

out_free_ailp:
    kmem_free(ailp);
    return ENOMEM;
}

void
xfs_trans_ail_destroy(
    xfs_mount_t *mp)
{
    struct xfs_ail  *ailp = mp->m_ail;

    kthread_stop(ailp->xa_task);
    kmem_free(ailp);
}