xfs: always push the AIL to the target

[deliverable/linux.git] / fs / xfs / xfs_trans_ail.c

/*
 * 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_inum.h"
#include "xfs_trans.h"
#include "xfs_sb.h"
#include "xfs_ag.h"
#include "xfs_mount.h"
#include "xfs_trans_priv.h"
#include "xfs_error.h"

struct workqueue_struct *xfs_ail_wq;    /* AIL workqueue */

#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.
 */
static 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;
}

/*
 * AIL traversal cursor initialisation.
 *
 * The cursor keeps track of where our current traversal is up
 * to by tracking the next ƣtem 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.
 *
 * We don't link the push cursor because it is embedded in the struct
 * xfs_ail and hence easily findable.
 */
STATIC void
xfs_trans_ail_cursor_init(
        struct xfs_ail          *ailp,
        struct xfs_ail_cursor   *cur)
{
        cur->item = NULL;
        if (cur == &ailp->xa_cursors)
                return;

        cur->next = ailp->xa_cursors.next;
        ailp->xa_cursors.next = cur;
}

/*
 * Set the cursor to the next item, because when we look
 * up the cursor the current item may have been freed.
 */
STATIC void
xfs_trans_ail_cursor_set(
        struct xfs_ail          *ailp,
        struct xfs_ail_cursor   *cur,
        struct xfs_log_item     *lip)
{
        if (lip)
                cur->item = xfs_ail_next(ailp, lip);
}

/*
 * Get the next item in the traversal and advance the cursor.
 * If the cursor was invalidated (inidicated 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);
        xfs_trans_ail_cursor_set(ailp, cur, lip);
        return lip;
}

/*
 * Now that the traversal is complete, we need to remove the cursor
 * from the list of traversing cursors. Avoid removing the embedded
 * push cursor, but use the fact it is always present to make the
 * list deletion simple.
 */
void
xfs_trans_ail_cursor_done(
        struct xfs_ail          *ailp,
        struct xfs_ail_cursor   *done)
{
        struct xfs_ail_cursor   *prev = NULL;
        struct xfs_ail_cursor   *cur;

        done->item = NULL;
        if (done == &ailp->xa_cursors)
                return;
        prev = &ailp->xa_cursors;
        for (cur = prev->next; cur; prev = cur, cur = prev->next) {
                if (cur == done) {
                        prev->next = cur->next;
                        break;
                }
        }
        ASSERT(cur);
}

/*
 * 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
 * cursor item to a value of 1 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;

        /* need to search all cursors */
        for (cur = &ailp->xa_cursors; cur; cur = cur->next) {
                if (cur->item == lip)
                        cur->item = (struct xfs_log_item *)
                                        ((__psint_t)cur->item | 1);
        }
}

/*
 * Return the item in the AIL with the current lsn.
 * Return the current tree generation number for use
 * in calls to xfs_trans_next_ail().
 */
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);
        lip = xfs_ail_min(ailp);
        if (lsn == 0)
                goto out;

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

/*
 * splice the log item list into the AIL at the given LSN.
 */
static void
xfs_ail_splice(
        struct xfs_ail  *ailp,
        struct list_head *list,
        xfs_lsn_t       lsn)
{
        xfs_log_item_t  *next_lip;

        /* If the list is empty, just insert the item.  */
        if (list_empty(&ailp->xa_ail)) {
                list_splice(list, &ailp->xa_ail);
                return;
        }

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

        ASSERT(&next_lip->li_ail == &ailp->xa_ail ||
               XFS_LSN_CMP(next_lip->li_lsn, lsn) <= 0);

        list_splice_init(list, &next_lip->li_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);
}

/*
 * xfs_ail_worker does the work of pushing on the AIL. It will requeue itself
 * to run at a later time if there is more work to do to complete the push.
 */
STATIC void
xfs_ail_worker(
        struct work_struct      *work)
{
        struct xfs_ail          *ailp = container_of(to_delayed_work(work),
                                        struct xfs_ail, xa_work);
        xfs_mount_t             *mp = ailp->xa_mount;
        struct xfs_ail_cursor   *cur = &ailp->xa_cursors;
        xfs_log_item_t          *lip;
        xfs_lsn_t               lsn;
        xfs_lsn_t               target = ailp->xa_target;
        long                    tout = 10;
        int                     flush_log = 0;
        int                     stuck = 0;
        int                     count = 0;
        int                     push_xfsbufd = 0;

        spin_lock(&ailp->xa_lock);
        xfs_trans_ail_cursor_init(ailp, cur);
        lip = xfs_trans_ail_cursor_first(ailp, cur, ailp->xa_last_pushed_lsn);
        if (!lip || XFS_FORCED_SHUTDOWN(mp)) {
                /*
                 * AIL is empty or our push has reached the end.
                 */
                xfs_trans_ail_cursor_done(ailp, cur);
                spin_unlock(&ailp->xa_lock);
                goto out_done;
        }

        XFS_STATS_INC(xs_push_ail);

        /*
         * While the item we are looking at is below the given threshold
         * try to flush it out. We'd like not to stop until we've at least
         * tried to push on everything in the AIL with an LSN less than
         * the given threshold.
         *
         * However, we will stop after a certain number of pushes and wait
         * for a reduced timeout to fire before pushing further. This
         * prevents use from spinning when we can't do anything or there is
         * lots of contention on the AIL lists.
         */
        lsn = lip->li_lsn;
        while ((XFS_LSN_CMP(lip->li_lsn, target) <= 0)) {
                int     lock_result;
                /*
                 * If we can lock the item without sleeping, unlock the AIL
                 * lock and flush the item.  Then re-grab the AIL lock so we
                 * can look for the next item on the AIL. List changes are
                 * handled by the AIL lookup functions internally
                 *
                 * If we can't lock the item, either its holder will flush it
                 * or it is already being flushed or it is being relogged.  In
                 * any of these case it is being taken care of and we can just
                 * skip to the next item in the list.
                 */
                lock_result = IOP_TRYLOCK(lip);
                spin_unlock(&ailp->xa_lock);
                switch (lock_result) {
                case XFS_ITEM_SUCCESS:
                        XFS_STATS_INC(xs_push_ail_success);
                        IOP_PUSH(lip);
                        ailp->xa_last_pushed_lsn = lsn;
                        break;

                case XFS_ITEM_PUSHBUF:
                        XFS_STATS_INC(xs_push_ail_pushbuf);
                        IOP_PUSHBUF(lip);
                        ailp->xa_last_pushed_lsn = lsn;
                        push_xfsbufd = 1;
                        break;

                case XFS_ITEM_PINNED:
                        XFS_STATS_INC(xs_push_ail_pinned);
                        stuck++;
                        flush_log = 1;
                        break;

                case XFS_ITEM_LOCKED:
                        XFS_STATS_INC(xs_push_ail_locked);
                        ailp->xa_last_pushed_lsn = lsn;
                        stuck++;
                        break;

                default:
                        ASSERT(0);
                        break;
                }

                spin_lock(&ailp->xa_lock);
                /* should we bother continuing? */
                if (XFS_FORCED_SHUTDOWN(mp))
                        break;
                ASSERT(mp->m_log);

                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 (flush_log) {
                /*
                 * If something we need to push out was pinned, then
                 * push out the log so it will become unpinned and
                 * move forward in the AIL.
                 */
                XFS_STATS_INC(xs_push_ail_flush);
                xfs_log_force(mp, 0);
        }

        if (push_xfsbufd) {
                /* we've got delayed write buffers to flush */
                wake_up_process(mp->m_ddev_targp->bt_task);
        }

        /* assume we have more work to do in a short while */
out_done:
        if (!count) {
                /* We're past our target or empty, so idle */
                ailp->xa_last_pushed_lsn = 0;

                /*
                 * Check for an updated push target before clearing the
                 * XFS_AIL_PUSHING_BIT. If the target changed, we've got more
                 * work to do. Wait a bit longer before starting that work.
                 */
                smp_rmb();
                if (ailp->xa_target == target) {
                        clear_bit(XFS_AIL_PUSHING_BIT, &ailp->xa_flags);
                        return;
                }
                tout = 50;
        } else if (XFS_LSN_CMP(lsn, target) >= 0) {
                /*
                 * We reached the target 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 * 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
                 * continuing from where we were.
                 */
                tout = 20;
        }

        /* There is more to do, requeue us.  */
        queue_delayed_work(xfs_syncd_wq, &ailp->xa_work,
                                        msecs_to_jiffies(tout));
}

/*
 * 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();
        ailp->xa_target = threshold_lsn;
        if (!test_and_set_bit(XFS_AIL_PUSHING_BIT, &ailp->xa_flags))
                queue_delayed_work(xfs_syncd_wq, &ailp->xa_work, 0);
}

/*
 * 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);
}

/*
 * This is to be called when an item is unlocked that may have
 * been in the AIL.  It will wake up the first member of the AIL
 * wait list if this item's unlocking might allow it to progress.
 * If the item is in the AIL, then we need to get the AIL lock
 * while doing our checking so we don't race with someone going
 * to sleep waiting for this event in xfs_trans_push_ail().
 */
void
xfs_trans_unlocked_item(
        struct xfs_ail  *ailp,
        xfs_log_item_t  *lip)
{
        xfs_log_item_t  *min_lip;

        /*
         * If we're forcibly shutting down, we may have
         * unlocked log items arbitrarily. The last thing
         * we want to do is to move the tail of the log
         * over some potentially valid data.
         */
        if (!(lip->li_flags & XFS_LI_IN_AIL) ||
            XFS_FORCED_SHUTDOWN(ailp->xa_mount)) {
                return;
        }

        /*
         * This is the one case where we can call into xfs_ail_min()
         * without holding the AIL lock because we only care about the
         * case where we are at the tail of the AIL.  If the object isn't
         * at the tail, it doesn't matter what result we get back.  This
         * is slightly racy because since we were just unlocked, we could
         * go to sleep between the call to xfs_ail_min and the call to
         * xfs_log_move_tail, have someone else lock us, commit to us disk,
         * move us out of the tail of the AIL, and then we wake up.  However,
         * the call to xfs_log_move_tail() doesn't do anything if there's
         * not enough free space to wake people up so we're safe calling it.
         */
        min_lip = xfs_ail_min(ailp);

        if (min_lip == lip)
                xfs_log_move_tail(ailp->xa_mount, 1);
}       /* xfs_trans_unlocked_item */

/*
 * 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_log_item     **log_items,
        int                     nr_items,
        xfs_lsn_t               lsn) __releases(ailp->xa_lock)
{
        xfs_log_item_t          *mlip;
        xfs_lsn_t               tail_lsn;
        int                     mlip_changed = 0;
        int                     i;
        LIST_HEAD(tmp);

        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);
        }

        xfs_ail_splice(ailp, &tmp, lsn);

        if (!mlip_changed) {
                spin_unlock(&ailp->xa_lock);
                return;
        }

        /*
         * It is not safe to access mlip after the AIL lock is dropped, so we
         * must get a copy of li_lsn before we do so.  This is especially
         * important on 32-bit platforms where accessing and updating 64-bit
         * values like li_lsn is not atomic.
         */
        mlip = xfs_ail_min(ailp);
        tail_lsn = mlip->li_lsn;
        spin_unlock(&ailp->xa_lock);
        xfs_log_move_tail(ailp->xa_mount, tail_lsn);
}

/*
 * 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) __releases(ailp->xa_lock)
{
        xfs_log_item_t          *mlip;
        xfs_lsn_t               tail_lsn;
        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_CORRUPT_INCORE);
                        }
                        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) {
                spin_unlock(&ailp->xa_lock);
                return;
        }

        /*
         * It is not safe to access mlip after the AIL lock is dropped, so we
         * must get a copy of li_lsn before we do so.  This is especially
         * important on 32-bit platforms where accessing and updating 64-bit
         * values like li_lsn is not atomic. It is possible we've emptied the
         * AIL here, so if that is the case, pass an LSN of 0 to the tail move.
         */
        mlip = xfs_ail_min(ailp);
        tail_lsn = mlip ? mlip->li_lsn : 0;
        spin_unlock(&ailp->xa_lock);
        xfs_log_move_tail(ailp->xa_mount, tail_lsn);
}

/*
 * The active item list (AIL) is a doubly linked list of log
 * items sorted by ascending lsn.  The base of the list is
 * a forw/back pointer pair embedded in the xfs mount structure.
 * The base is initialized with both pointers pointing to the
 * base.  This case always needs to be distinguished, because
 * the base has no lsn to look at.  We almost always insert
 * at the end of the list, so on inserts we search from the
 * end of the list to find where the new item belongs.
 */

/*
 * Initialize the doubly linked list to point only to itself.
 */
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);
        spin_lock_init(&ailp->xa_lock);
        INIT_DELAYED_WORK(&ailp->xa_work, xfs_ail_worker);
        mp->m_ail = ailp;
        return 0;
}

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

        cancel_delayed_work_sync(&ailp->xa_work);
        kmem_free(ailp);
}