xfs: split out iclog writing from xfs_trans_commit()

[safe/jmp/linux-2.6] / fs / xfs / xfs_trans.c

/*
 * Copyright (c) 2000-2003,2005 Silicon Graphics, Inc.
 * 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_bit.h"
#include "xfs_log.h"
#include "xfs_inum.h"
#include "xfs_trans.h"
#include "xfs_sb.h"
#include "xfs_ag.h"
#include "xfs_dir2.h"
#include "xfs_dmapi.h"
#include "xfs_mount.h"
#include "xfs_error.h"
#include "xfs_da_btree.h"
#include "xfs_bmap_btree.h"
#include "xfs_alloc_btree.h"
#include "xfs_ialloc_btree.h"
#include "xfs_dir2_sf.h"
#include "xfs_attr_sf.h"
#include "xfs_dinode.h"
#include "xfs_inode.h"
#include "xfs_btree.h"
#include "xfs_ialloc.h"
#include "xfs_alloc.h"
#include "xfs_bmap.h"
#include "xfs_quota.h"
#include "xfs_trans_priv.h"
#include "xfs_trans_space.h"
#include "xfs_inode_item.h"


STATIC void     xfs_trans_apply_sb_deltas(xfs_trans_t *);
STATIC void     xfs_trans_uncommit(xfs_trans_t *, uint);
STATIC void     xfs_trans_committed(xfs_trans_t *, int);
STATIC void     xfs_trans_chunk_committed(xfs_log_item_chunk_t *, xfs_lsn_t, int);
STATIC void     xfs_trans_free(xfs_trans_t *);

kmem_zone_t     *xfs_trans_zone;


/*
 * Reservation functions here avoid a huge stack in xfs_trans_init
 * due to register overflow from temporaries in the calculations.
 */

STATIC uint
xfs_calc_write_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_WRITE_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_itruncate_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_ITRUNCATE_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_rename_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_RENAME_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_link_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_LINK_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_remove_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_REMOVE_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_symlink_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_SYMLINK_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_create_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_CREATE_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_mkdir_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_MKDIR_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_ifree_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_IFREE_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_ichange_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_ICHANGE_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_growdata_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_GROWDATA_LOG_RES(mp);
}

STATIC uint
xfs_calc_growrtalloc_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_GROWRTALLOC_LOG_RES(mp);
}

STATIC uint
xfs_calc_growrtzero_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_GROWRTZERO_LOG_RES(mp);
}

STATIC uint
xfs_calc_growrtfree_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_GROWRTFREE_LOG_RES(mp);
}

STATIC uint
xfs_calc_swrite_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_SWRITE_LOG_RES(mp);
}

STATIC uint
xfs_calc_writeid_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_WRITEID_LOG_RES(mp);
}

STATIC uint
xfs_calc_addafork_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_ADDAFORK_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_attrinval_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_ATTRINVAL_LOG_RES(mp);
}

STATIC uint
xfs_calc_attrset_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_ATTRSET_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_attrrm_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_ATTRRM_LOG_RES(mp) + XFS_DQUOT_LOGRES(mp);
}

STATIC uint
xfs_calc_clear_agi_bucket_reservation(xfs_mount_t *mp)
{
        return XFS_CALC_CLEAR_AGI_BUCKET_LOG_RES(mp);
}

/*
 * Initialize the precomputed transaction reservation values
 * in the mount structure.
 */
void
xfs_trans_init(
        xfs_mount_t     *mp)
{
        xfs_trans_reservations_t        *resp;

        resp = &(mp->m_reservations);
        resp->tr_write = xfs_calc_write_reservation(mp);
        resp->tr_itruncate = xfs_calc_itruncate_reservation(mp);
        resp->tr_rename = xfs_calc_rename_reservation(mp);
        resp->tr_link = xfs_calc_link_reservation(mp);
        resp->tr_remove = xfs_calc_remove_reservation(mp);
        resp->tr_symlink = xfs_calc_symlink_reservation(mp);
        resp->tr_create = xfs_calc_create_reservation(mp);
        resp->tr_mkdir = xfs_calc_mkdir_reservation(mp);
        resp->tr_ifree = xfs_calc_ifree_reservation(mp);
        resp->tr_ichange = xfs_calc_ichange_reservation(mp);
        resp->tr_growdata = xfs_calc_growdata_reservation(mp);
        resp->tr_swrite = xfs_calc_swrite_reservation(mp);
        resp->tr_writeid = xfs_calc_writeid_reservation(mp);
        resp->tr_addafork = xfs_calc_addafork_reservation(mp);
        resp->tr_attrinval = xfs_calc_attrinval_reservation(mp);
        resp->tr_attrset = xfs_calc_attrset_reservation(mp);
        resp->tr_attrrm = xfs_calc_attrrm_reservation(mp);
        resp->tr_clearagi = xfs_calc_clear_agi_bucket_reservation(mp);
        resp->tr_growrtalloc = xfs_calc_growrtalloc_reservation(mp);
        resp->tr_growrtzero = xfs_calc_growrtzero_reservation(mp);
        resp->tr_growrtfree = xfs_calc_growrtfree_reservation(mp);
}

/*
 * This routine is called to allocate a transaction structure.
 * The type parameter indicates the type of the transaction.  These
 * are enumerated in xfs_trans.h.
 *
 * Dynamically allocate the transaction structure from the transaction
 * zone, initialize it, and return it to the caller.
 */
xfs_trans_t *
xfs_trans_alloc(
        xfs_mount_t     *mp,
        uint            type)
{
        xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
        return _xfs_trans_alloc(mp, type, KM_SLEEP);
}

xfs_trans_t *
_xfs_trans_alloc(
        xfs_mount_t     *mp,
        uint            type,
        uint            memflags)
{
        xfs_trans_t     *tp;

        atomic_inc(&mp->m_active_trans);

        tp = kmem_zone_zalloc(xfs_trans_zone, memflags);
        tp->t_magic = XFS_TRANS_MAGIC;
        tp->t_type = type;
        tp->t_mountp = mp;
        tp->t_items_free = XFS_LIC_NUM_SLOTS;
        tp->t_busy_free = XFS_LBC_NUM_SLOTS;
        xfs_lic_init(&(tp->t_items));
        XFS_LBC_INIT(&(tp->t_busy));
        return tp;
}

/*
 * This is called to create a new transaction which will share the
 * permanent log reservation of the given transaction.  The remaining
 * unused block and rt extent reservations are also inherited.  This
 * implies that the original transaction is no longer allowed to allocate
 * blocks.  Locks and log items, however, are no inherited.  They must
 * be added to the new transaction explicitly.
 */
xfs_trans_t *
xfs_trans_dup(
        xfs_trans_t     *tp)
{
        xfs_trans_t     *ntp;

        ntp = kmem_zone_zalloc(xfs_trans_zone, KM_SLEEP);

        /*
         * Initialize the new transaction structure.
         */
        ntp->t_magic = XFS_TRANS_MAGIC;
        ntp->t_type = tp->t_type;
        ntp->t_mountp = tp->t_mountp;
        ntp->t_items_free = XFS_LIC_NUM_SLOTS;
        ntp->t_busy_free = XFS_LBC_NUM_SLOTS;
        xfs_lic_init(&(ntp->t_items));
        XFS_LBC_INIT(&(ntp->t_busy));

        ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
        ASSERT(tp->t_ticket != NULL);

        ntp->t_flags = XFS_TRANS_PERM_LOG_RES | (tp->t_flags & XFS_TRANS_RESERVE);
        ntp->t_ticket = xfs_log_ticket_get(tp->t_ticket);
        ntp->t_blk_res = tp->t_blk_res - tp->t_blk_res_used;
        tp->t_blk_res = tp->t_blk_res_used;
        ntp->t_rtx_res = tp->t_rtx_res - tp->t_rtx_res_used;
        tp->t_rtx_res = tp->t_rtx_res_used;
        ntp->t_pflags = tp->t_pflags;

        xfs_trans_dup_dqinfo(tp, ntp);

        atomic_inc(&tp->t_mountp->m_active_trans);
        return ntp;
}

/*
 * This is called to reserve free disk blocks and log space for the
 * given transaction.  This must be done before allocating any resources
 * within the transaction.
 *
 * This will return ENOSPC if there are not enough blocks available.
 * It will sleep waiting for available log space.
 * The only valid value for the flags parameter is XFS_RES_LOG_PERM, which
 * is used by long running transactions.  If any one of the reservations
 * fails then they will all be backed out.
 *
 * This does not do quota reservations. That typically is done by the
 * caller afterwards.
 */
int
xfs_trans_reserve(
        xfs_trans_t     *tp,
        uint            blocks,
        uint            logspace,
        uint            rtextents,
        uint            flags,
        uint            logcount)
{
        int             log_flags;
        int             error = 0;
        int             rsvd = (tp->t_flags & XFS_TRANS_RESERVE) != 0;

        /* Mark this thread as being in a transaction */
        current_set_flags_nested(&tp->t_pflags, PF_FSTRANS);

        /*
         * Attempt to reserve the needed disk blocks by decrementing
         * the number needed from the number available.  This will
         * fail if the count would go below zero.
         */
        if (blocks > 0) {
                error = xfs_mod_incore_sb(tp->t_mountp, XFS_SBS_FDBLOCKS,
                                          -((int64_t)blocks), rsvd);
                if (error != 0) {
                        current_restore_flags_nested(&tp->t_pflags, PF_FSTRANS);
                        return (XFS_ERROR(ENOSPC));
                }
                tp->t_blk_res += blocks;
        }

        /*
         * Reserve the log space needed for this transaction.
         */
        if (logspace > 0) {
                ASSERT((tp->t_log_res == 0) || (tp->t_log_res == logspace));
                ASSERT((tp->t_log_count == 0) ||
                        (tp->t_log_count == logcount));
                if (flags & XFS_TRANS_PERM_LOG_RES) {
                        log_flags = XFS_LOG_PERM_RESERV;
                        tp->t_flags |= XFS_TRANS_PERM_LOG_RES;
                } else {
                        ASSERT(tp->t_ticket == NULL);
                        ASSERT(!(tp->t_flags & XFS_TRANS_PERM_LOG_RES));
                        log_flags = 0;
                }

                error = xfs_log_reserve(tp->t_mountp, logspace, logcount,
                                        &tp->t_ticket,
                                        XFS_TRANSACTION, log_flags, tp->t_type);
                if (error) {
                        goto undo_blocks;
                }
                tp->t_log_res = logspace;
                tp->t_log_count = logcount;
        }

        /*
         * Attempt to reserve the needed realtime extents by decrementing
         * the number needed from the number available.  This will
         * fail if the count would go below zero.
         */
        if (rtextents > 0) {
                error = xfs_mod_incore_sb(tp->t_mountp, XFS_SBS_FREXTENTS,
                                          -((int64_t)rtextents), rsvd);
                if (error) {
                        error = XFS_ERROR(ENOSPC);
                        goto undo_log;
                }
                tp->t_rtx_res += rtextents;
        }

        return 0;

        /*
         * Error cases jump to one of these labels to undo any
         * reservations which have already been performed.
         */
undo_log:
        if (logspace > 0) {
                if (flags & XFS_TRANS_PERM_LOG_RES) {
                        log_flags = XFS_LOG_REL_PERM_RESERV;
                } else {
                        log_flags = 0;
                }
                xfs_log_done(tp->t_mountp, tp->t_ticket, NULL, log_flags);
                tp->t_ticket = NULL;
                tp->t_log_res = 0;
                tp->t_flags &= ~XFS_TRANS_PERM_LOG_RES;
        }

undo_blocks:
        if (blocks > 0) {
                (void) xfs_mod_incore_sb(tp->t_mountp, XFS_SBS_FDBLOCKS,
                                         (int64_t)blocks, rsvd);
                tp->t_blk_res = 0;
        }

        current_restore_flags_nested(&tp->t_pflags, PF_FSTRANS);

        return error;
}


/*
 * Record the indicated change to the given field for application
 * to the file system's superblock when the transaction commits.
 * For now, just store the change in the transaction structure.
 *
 * Mark the transaction structure to indicate that the superblock
 * needs to be updated before committing.
 *
 * Because we may not be keeping track of allocated/free inodes and
 * used filesystem blocks in the superblock, we do not mark the
 * superblock dirty in this transaction if we modify these fields.
 * We still need to update the transaction deltas so that they get
 * applied to the incore superblock, but we don't want them to
 * cause the superblock to get locked and logged if these are the
 * only fields in the superblock that the transaction modifies.
 */
void
xfs_trans_mod_sb(
        xfs_trans_t     *tp,
        uint            field,
        int64_t         delta)
{
        uint32_t        flags = (XFS_TRANS_DIRTY|XFS_TRANS_SB_DIRTY);
        xfs_mount_t     *mp = tp->t_mountp;

        switch (field) {
        case XFS_TRANS_SB_ICOUNT:
                tp->t_icount_delta += delta;
                if (xfs_sb_version_haslazysbcount(&mp->m_sb))
                        flags &= ~XFS_TRANS_SB_DIRTY;
                break;
        case XFS_TRANS_SB_IFREE:
                tp->t_ifree_delta += delta;
                if (xfs_sb_version_haslazysbcount(&mp->m_sb))
                        flags &= ~XFS_TRANS_SB_DIRTY;
                break;
        case XFS_TRANS_SB_FDBLOCKS:
                /*
                 * Track the number of blocks allocated in the
                 * transaction.  Make sure it does not exceed the
                 * number reserved.
                 */
                if (delta < 0) {
                        tp->t_blk_res_used += (uint)-delta;
                        ASSERT(tp->t_blk_res_used <= tp->t_blk_res);
                }
                tp->t_fdblocks_delta += delta;
                if (xfs_sb_version_haslazysbcount(&mp->m_sb))
                        flags &= ~XFS_TRANS_SB_DIRTY;
                break;
        case XFS_TRANS_SB_RES_FDBLOCKS:
                /*
                 * The allocation has already been applied to the
                 * in-core superblock's counter.  This should only
                 * be applied to the on-disk superblock.
                 */
                ASSERT(delta < 0);
                tp->t_res_fdblocks_delta += delta;
                if (xfs_sb_version_haslazysbcount(&mp->m_sb))
                        flags &= ~XFS_TRANS_SB_DIRTY;
                break;
        case XFS_TRANS_SB_FREXTENTS:
                /*
                 * Track the number of blocks allocated in the
                 * transaction.  Make sure it does not exceed the
                 * number reserved.
                 */
                if (delta < 0) {
                        tp->t_rtx_res_used += (uint)-delta;
                        ASSERT(tp->t_rtx_res_used <= tp->t_rtx_res);
                }
                tp->t_frextents_delta += delta;
                break;
        case XFS_TRANS_SB_RES_FREXTENTS:
                /*
                 * The allocation has already been applied to the
                 * in-core superblock's counter.  This should only
                 * be applied to the on-disk superblock.
                 */
                ASSERT(delta < 0);
                tp->t_res_frextents_delta += delta;
                break;
        case XFS_TRANS_SB_DBLOCKS:
                ASSERT(delta > 0);
                tp->t_dblocks_delta += delta;
                break;
        case XFS_TRANS_SB_AGCOUNT:
                ASSERT(delta > 0);
                tp->t_agcount_delta += delta;
                break;
        case XFS_TRANS_SB_IMAXPCT:
                tp->t_imaxpct_delta += delta;
                break;
        case XFS_TRANS_SB_REXTSIZE:
                tp->t_rextsize_delta += delta;
                break;
        case XFS_TRANS_SB_RBMBLOCKS:
                tp->t_rbmblocks_delta += delta;
                break;
        case XFS_TRANS_SB_RBLOCKS:
                tp->t_rblocks_delta += delta;
                break;
        case XFS_TRANS_SB_REXTENTS:
                tp->t_rextents_delta += delta;
                break;
        case XFS_TRANS_SB_REXTSLOG:
                tp->t_rextslog_delta += delta;
                break;
        default:
                ASSERT(0);
                return;
        }

        tp->t_flags |= flags;
}

/*
 * xfs_trans_apply_sb_deltas() is called from the commit code
 * to bring the superblock buffer into the current transaction
 * and modify it as requested by earlier calls to xfs_trans_mod_sb().
 *
 * For now we just look at each field allowed to change and change
 * it if necessary.
 */
STATIC void
xfs_trans_apply_sb_deltas(
        xfs_trans_t     *tp)
{
        xfs_dsb_t       *sbp;
        xfs_buf_t       *bp;
        int             whole = 0;

        bp = xfs_trans_getsb(tp, tp->t_mountp, 0);
        sbp = XFS_BUF_TO_SBP(bp);

        /*
         * Check that superblock mods match the mods made to AGF counters.
         */
        ASSERT((tp->t_fdblocks_delta + tp->t_res_fdblocks_delta) ==
               (tp->t_ag_freeblks_delta + tp->t_ag_flist_delta +
                tp->t_ag_btree_delta));

        /*
         * Only update the superblock counters if we are logging them
         */
        if (!xfs_sb_version_haslazysbcount(&(tp->t_mountp->m_sb))) {
                if (tp->t_icount_delta)
                        be64_add_cpu(&sbp->sb_icount, tp->t_icount_delta);
                if (tp->t_ifree_delta)
                        be64_add_cpu(&sbp->sb_ifree, tp->t_ifree_delta);
                if (tp->t_fdblocks_delta)
                        be64_add_cpu(&sbp->sb_fdblocks, tp->t_fdblocks_delta);
                if (tp->t_res_fdblocks_delta)
                        be64_add_cpu(&sbp->sb_fdblocks, tp->t_res_fdblocks_delta);
        }

        if (tp->t_frextents_delta)
                be64_add_cpu(&sbp->sb_frextents, tp->t_frextents_delta);
        if (tp->t_res_frextents_delta)
                be64_add_cpu(&sbp->sb_frextents, tp->t_res_frextents_delta);

        if (tp->t_dblocks_delta) {
                be64_add_cpu(&sbp->sb_dblocks, tp->t_dblocks_delta);
                whole = 1;
        }
        if (tp->t_agcount_delta) {
                be32_add_cpu(&sbp->sb_agcount, tp->t_agcount_delta);
                whole = 1;
        }
        if (tp->t_imaxpct_delta) {
                sbp->sb_imax_pct += tp->t_imaxpct_delta;
                whole = 1;
        }
        if (tp->t_rextsize_delta) {
                be32_add_cpu(&sbp->sb_rextsize, tp->t_rextsize_delta);
                whole = 1;
        }
        if (tp->t_rbmblocks_delta) {
                be32_add_cpu(&sbp->sb_rbmblocks, tp->t_rbmblocks_delta);
                whole = 1;
        }
        if (tp->t_rblocks_delta) {
                be64_add_cpu(&sbp->sb_rblocks, tp->t_rblocks_delta);
                whole = 1;
        }
        if (tp->t_rextents_delta) {
                be64_add_cpu(&sbp->sb_rextents, tp->t_rextents_delta);
                whole = 1;
        }
        if (tp->t_rextslog_delta) {
                sbp->sb_rextslog += tp->t_rextslog_delta;
                whole = 1;
        }

        if (whole)
                /*
                 * Log the whole thing, the fields are noncontiguous.
                 */
                xfs_trans_log_buf(tp, bp, 0, sizeof(xfs_dsb_t) - 1);
        else
                /*
                 * Since all the modifiable fields are contiguous, we
                 * can get away with this.
                 */
                xfs_trans_log_buf(tp, bp, offsetof(xfs_dsb_t, sb_icount),
                                  offsetof(xfs_dsb_t, sb_frextents) +
                                  sizeof(sbp->sb_frextents) - 1);
}

/*
 * xfs_trans_unreserve_and_mod_sb() is called to release unused reservations
 * and apply superblock counter changes to the in-core superblock.  The
 * t_res_fdblocks_delta and t_res_frextents_delta fields are explicitly NOT
 * applied to the in-core superblock.  The idea is that that has already been
 * done.
 *
 * This is done efficiently with a single call to xfs_mod_incore_sb_batch().
 * However, we have to ensure that we only modify each superblock field only
 * once because the application of the delta values may not be atomic. That can
 * lead to ENOSPC races occurring if we have two separate modifcations of the
 * free space counter to put back the entire reservation and then take away
 * what we used.
 *
 * If we are not logging superblock counters, then the inode allocated/free and
 * used block counts are not updated in the on disk superblock. In this case,
 * XFS_TRANS_SB_DIRTY will not be set when the transaction is updated but we
 * still need to update the incore superblock with the changes.
 */
STATIC void
xfs_trans_unreserve_and_mod_sb(
        xfs_trans_t     *tp)
{
        xfs_mod_sb_t    msb[14];        /* If you add cases, add entries */
        xfs_mod_sb_t    *msbp;
        xfs_mount_t     *mp = tp->t_mountp;
        /* REFERENCED */
        int             error;
        int             rsvd;
        int64_t         blkdelta = 0;
        int64_t         rtxdelta = 0;

        msbp = msb;
        rsvd = (tp->t_flags & XFS_TRANS_RESERVE) != 0;

        /* calculate free blocks delta */
        if (tp->t_blk_res > 0)
                blkdelta = tp->t_blk_res;

        if ((tp->t_fdblocks_delta != 0) &&
            (xfs_sb_version_haslazysbcount(&mp->m_sb) ||
             (tp->t_flags & XFS_TRANS_SB_DIRTY)))
                blkdelta += tp->t_fdblocks_delta;

        if (blkdelta != 0) {
                msbp->msb_field = XFS_SBS_FDBLOCKS;
                msbp->msb_delta = blkdelta;
                msbp++;
        }

        /* calculate free realtime extents delta */
        if (tp->t_rtx_res > 0)
                rtxdelta = tp->t_rtx_res;

        if ((tp->t_frextents_delta != 0) &&
            (tp->t_flags & XFS_TRANS_SB_DIRTY))
                rtxdelta += tp->t_frextents_delta;

        if (rtxdelta != 0) {
                msbp->msb_field = XFS_SBS_FREXTENTS;
                msbp->msb_delta = rtxdelta;
                msbp++;
        }

        /* apply remaining deltas */

        if (xfs_sb_version_haslazysbcount(&mp->m_sb) ||
             (tp->t_flags & XFS_TRANS_SB_DIRTY)) {
                if (tp->t_icount_delta != 0) {
                        msbp->msb_field = XFS_SBS_ICOUNT;
                        msbp->msb_delta = tp->t_icount_delta;
                        msbp++;
                }
                if (tp->t_ifree_delta != 0) {
                        msbp->msb_field = XFS_SBS_IFREE;
                        msbp->msb_delta = tp->t_ifree_delta;
                        msbp++;
                }
        }

        if (tp->t_flags & XFS_TRANS_SB_DIRTY) {
                if (tp->t_dblocks_delta != 0) {
                        msbp->msb_field = XFS_SBS_DBLOCKS;
                        msbp->msb_delta = tp->t_dblocks_delta;
                        msbp++;
                }
                if (tp->t_agcount_delta != 0) {
                        msbp->msb_field = XFS_SBS_AGCOUNT;
                        msbp->msb_delta = tp->t_agcount_delta;
                        msbp++;
                }
                if (tp->t_imaxpct_delta != 0) {
                        msbp->msb_field = XFS_SBS_IMAX_PCT;
                        msbp->msb_delta = tp->t_imaxpct_delta;
                        msbp++;
                }
                if (tp->t_rextsize_delta != 0) {
                        msbp->msb_field = XFS_SBS_REXTSIZE;
                        msbp->msb_delta = tp->t_rextsize_delta;
                        msbp++;
                }
                if (tp->t_rbmblocks_delta != 0) {
                        msbp->msb_field = XFS_SBS_RBMBLOCKS;
                        msbp->msb_delta = tp->t_rbmblocks_delta;
                        msbp++;
                }
                if (tp->t_rblocks_delta != 0) {
                        msbp->msb_field = XFS_SBS_RBLOCKS;
                        msbp->msb_delta = tp->t_rblocks_delta;
                        msbp++;
                }
                if (tp->t_rextents_delta != 0) {
                        msbp->msb_field = XFS_SBS_REXTENTS;
                        msbp->msb_delta = tp->t_rextents_delta;
                        msbp++;
                }
                if (tp->t_rextslog_delta != 0) {
                        msbp->msb_field = XFS_SBS_REXTSLOG;
                        msbp->msb_delta = tp->t_rextslog_delta;
                        msbp++;
                }
        }

        /*
         * If we need to change anything, do it.
         */
        if (msbp > msb) {
                error = xfs_mod_incore_sb_batch(tp->t_mountp, msb,
                        (uint)(msbp - msb), rsvd);
                ASSERT(error == 0);
        }
}

/*
 * Total up the number of log iovecs needed to commit this
 * transaction.  The transaction itself needs one for the
 * transaction header.  Ask each dirty item in turn how many
 * it needs to get the total.
 */
static uint
xfs_trans_count_vecs(
        xfs_trans_t     *tp)
{
        int                     nvecs;
        xfs_log_item_desc_t     *lidp;

        nvecs = 1;
        lidp = xfs_trans_first_item(tp);
        ASSERT(lidp != NULL);

        /* In the non-debug case we need to start bailing out if we
         * didn't find a log_item here, return zero and let trans_commit
         * deal with it.
         */
        if (lidp == NULL)
                return 0;

        while (lidp != NULL) {
                /*
                 * Skip items which aren't dirty in this transaction.
                 */
                if (!(lidp->lid_flags & XFS_LID_DIRTY)) {
                        lidp = xfs_trans_next_item(tp, lidp);
                        continue;
                }
                lidp->lid_size = IOP_SIZE(lidp->lid_item);
                nvecs += lidp->lid_size;
                lidp = xfs_trans_next_item(tp, lidp);
        }

        return nvecs;
}

/*
 * Fill in the vector with pointers to data to be logged
 * by this transaction.  The transaction header takes
 * the first vector, and then each dirty item takes the
 * number of vectors it indicated it needed in xfs_trans_count_vecs().
 *
 * As each item fills in the entries it needs, also pin the item
 * so that it cannot be flushed out until the log write completes.
 */
static void
xfs_trans_fill_vecs(
        struct xfs_trans        *tp,
        struct xfs_log_iovec    *log_vector)
{
        xfs_log_item_desc_t     *lidp;
        struct xfs_log_iovec    *vecp;
        uint                    nitems;

        /*
         * Skip over the entry for the transaction header, we'll
         * fill that in at the end.
         */
        vecp = log_vector + 1;

        nitems = 0;
        lidp = xfs_trans_first_item(tp);
        ASSERT(lidp);
        while (lidp) {
                /* Skip items which aren't dirty in this transaction. */
                if (!(lidp->lid_flags & XFS_LID_DIRTY)) {
                        lidp = xfs_trans_next_item(tp, lidp);
                        continue;
                }

                /*
                 * The item may be marked dirty but not log anything.  This can
                 * be used to get called when a transaction is committed.
                 */
                if (lidp->lid_size)
                        nitems++;
                IOP_FORMAT(lidp->lid_item, vecp);
                vecp += lidp->lid_size;
                IOP_PIN(lidp->lid_item);
                lidp = xfs_trans_next_item(tp, lidp);
        }

        /*
         * Now that we've counted the number of items in this transaction, fill
         * in the transaction header. Note that the transaction header does not
         * have a log item.
         */
        tp->t_header.th_magic = XFS_TRANS_HEADER_MAGIC;
        tp->t_header.th_type = tp->t_type;
        tp->t_header.th_num_items = nitems;
        log_vector->i_addr = (xfs_caddr_t)&tp->t_header;
        log_vector->i_len = sizeof(xfs_trans_header_t);
        log_vector->i_type = XLOG_REG_TYPE_TRANSHDR;
}

/*
 * Format the transaction direct to the iclog. This isolates the physical
 * transaction commit operation from the logical operation and hence allows
 * other methods to be introduced without affecting the existing commit path.
 */
static int
xfs_trans_commit_iclog(
        struct xfs_mount        *mp,
        struct xfs_trans        *tp,
        xfs_lsn_t               *commit_lsn,
        int                     flags)
{
        int                     shutdown;
        int                     error;
        int                     log_flags = 0;
        struct xlog_in_core     *commit_iclog;
#define XFS_TRANS_LOGVEC_COUNT  16
        struct xfs_log_iovec    log_vector_fast[XFS_TRANS_LOGVEC_COUNT];
        struct xfs_log_iovec    *log_vector;
        uint                    nvec;


        /*
         * Ask each log item how many log_vector entries it will
         * need so we can figure out how many to allocate.
         * Try to avoid the kmem_alloc() call in the common case
         * by using a vector from the stack when it fits.
         */
        nvec = xfs_trans_count_vecs(tp);
        if (nvec == 0) {
                return ENOMEM;  /* triggers a shutdown! */
        } else if (nvec <= XFS_TRANS_LOGVEC_COUNT) {
                log_vector = log_vector_fast;
        } else {
                log_vector = (xfs_log_iovec_t *)kmem_alloc(nvec *
                                                   sizeof(xfs_log_iovec_t),
                                                   KM_SLEEP);
        }

        /*
         * Fill in the log_vector and pin the logged items, and
         * then write the transaction to the log.
         */
        xfs_trans_fill_vecs(tp, log_vector);

        if (flags & XFS_TRANS_RELEASE_LOG_RES)
                log_flags = XFS_LOG_REL_PERM_RESERV;

        error = xfs_log_write(mp, log_vector, nvec, tp->t_ticket, &(tp->t_lsn));

        /*
         * The transaction is committed incore here, and can go out to disk
         * at any time after this call.  However, all the items associated
         * with the transaction are still locked and pinned in memory.
         */
        *commit_lsn = xfs_log_done(mp, tp->t_ticket, &commit_iclog, log_flags);

        tp->t_commit_lsn = *commit_lsn;
        if (nvec > XFS_TRANS_LOGVEC_COUNT)
                kmem_free(log_vector);

        /*
         * If we got a log write error. Unpin the logitems that we
         * had pinned, clean up, free trans structure, and return error.
         */
        if (error || *commit_lsn == -1) {
                current_restore_flags_nested(&tp->t_pflags, PF_FSTRANS);
                xfs_trans_uncommit(tp, flags|XFS_TRANS_ABORT);
                return XFS_ERROR(EIO);
        }

        /*
         * Once the transaction has committed, unused
         * reservations need to be released and changes to
         * the superblock need to be reflected in the in-core
         * version.  Do that now.
         */
        xfs_trans_unreserve_and_mod_sb(tp);

        /*
         * Tell the LM to call the transaction completion routine
         * when the log write with LSN commit_lsn completes (e.g.
         * when the transaction commit really hits the on-disk log).
         * After this call we cannot reference tp, because the call
         * can happen at any time and the call will free the transaction
         * structure pointed to by tp.  The only case where we call
         * the completion routine (xfs_trans_committed) directly is
         * if the log is turned off on a debug kernel or we're
         * running in simulation mode (the log is explicitly turned
         * off).
         */
        tp->t_logcb.cb_func = (void(*)(void*, int))xfs_trans_committed;
        tp->t_logcb.cb_arg = tp;

        /*
         * We need to pass the iclog buffer which was used for the
         * transaction commit record into this function, and attach
         * the callback to it. The callback must be attached before
         * the items are unlocked to avoid racing with other threads
         * waiting for an item to unlock.
         */
        shutdown = xfs_log_notify(mp, commit_iclog, &(tp->t_logcb));

        /*
         * Mark this thread as no longer being in a transaction
         */
        current_restore_flags_nested(&tp->t_pflags, PF_FSTRANS);

        /*
         * Once all the items of the transaction have been copied
         * to the in core log and the callback is attached, the
         * items can be unlocked.
         *
         * This will free descriptors pointing to items which were
         * not logged since there is nothing more to do with them.
         * For items which were logged, we will keep pointers to them
         * so they can be unpinned after the transaction commits to disk.
         * This will also stamp each modified meta-data item with
         * the commit lsn of this transaction for dependency tracking
         * purposes.
         */
        xfs_trans_unlock_items(tp, *commit_lsn);

        /*
         * If we detected a log error earlier, finish committing
         * the transaction now (unpin log items, etc).
         *
         * Order is critical here, to avoid using the transaction
         * pointer after its been freed (by xfs_trans_committed
         * either here now, or as a callback).  We cannot do this
         * step inside xfs_log_notify as was done earlier because
         * of this issue.
         */
        if (shutdown)
                xfs_trans_committed(tp, XFS_LI_ABORTED);

        /*
         * Now that the xfs_trans_committed callback has been attached,
         * and the items are released we can finally allow the iclog to
         * go to disk.
         */
        return xfs_log_release_iclog(mp, commit_iclog);
}


/*
 * xfs_trans_commit
 *
 * Commit the given transaction to the log a/synchronously.
 *
 * XFS disk error handling mechanism is not based on a typical
 * transaction abort mechanism. Logically after the filesystem
 * gets marked 'SHUTDOWN', we can't let any new transactions
 * be durable - ie. committed to disk - because some metadata might
 * be inconsistent. In such cases, this returns an error, and the
 * caller may assume that all locked objects joined to the transaction
 * have already been unlocked as if the commit had succeeded.
 * Do not reference the transaction structure after this call.
 */
 /*ARGSUSED*/
int
_xfs_trans_commit(
        xfs_trans_t     *tp,
        uint            flags,
        int             *log_flushed)
{
        xfs_mount_t             *mp = tp->t_mountp;
        xfs_lsn_t               commit_lsn = -1;
        int                     error;
        int                     log_flags = 0;
        int                     sync = tp->t_flags & XFS_TRANS_SYNC;
        int                     shutdown;

        /*
         * Determine whether this commit is releasing a permanent
         * log reservation or not.
         */
        if (flags & XFS_TRANS_RELEASE_LOG_RES) {
                ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
                log_flags = XFS_LOG_REL_PERM_RESERV;
        }

        /*
         * If there is nothing to be logged by the transaction,
         * then unlock all of the items associated with the
         * transaction and free the transaction structure.
         * Also make sure to return any reserved blocks to
         * the free pool.
         */
shut_us_down:
        shutdown = XFS_FORCED_SHUTDOWN(mp) ? EIO : 0;
        if (!(tp->t_flags & XFS_TRANS_DIRTY) || shutdown) {
                xfs_trans_unreserve_and_mod_sb(tp);
                /*
                 * It is indeed possible for the transaction to be
                 * not dirty but the dqinfo portion to be. All that
                 * means is that we have some (non-persistent) quota
                 * reservations that need to be unreserved.
                 */
                xfs_trans_unreserve_and_mod_dquots(tp);
                if (tp->t_ticket) {
                        commit_lsn = xfs_log_done(mp, tp->t_ticket,
                                                        NULL, log_flags);
                        if (commit_lsn == -1 && !shutdown)
                                shutdown = XFS_ERROR(EIO);
                }
                current_restore_flags_nested(&tp->t_pflags, PF_FSTRANS);
                xfs_trans_free_items(tp, shutdown? XFS_TRANS_ABORT : 0);
                xfs_trans_free_busy(tp);
                xfs_trans_free(tp);
                XFS_STATS_INC(xs_trans_empty);
                return (shutdown);
        }
        ASSERT(tp->t_ticket != NULL);

        /*
         * If we need to update the superblock, then do it now.
         */
        if (tp->t_flags & XFS_TRANS_SB_DIRTY)
                xfs_trans_apply_sb_deltas(tp);
        xfs_trans_apply_dquot_deltas(tp);

        error = xfs_trans_commit_iclog(mp, tp, &commit_lsn, flags);
        if (error == ENOMEM) {
                xfs_force_shutdown(mp, SHUTDOWN_LOG_IO_ERROR);
                goto shut_us_down;
        }

        /*
         * If the transaction needs to be synchronous, then force the
         * log out now and wait for it.
         */
        if (sync) {
                if (!error) {
                        error = _xfs_log_force_lsn(mp, commit_lsn,
                                      XFS_LOG_SYNC, log_flushed);
                }
                XFS_STATS_INC(xs_trans_sync);
        } else {
                XFS_STATS_INC(xs_trans_async);
        }

        return (error);
}

/*
 * Called from the trans_commit code when we notice that
 * the filesystem is in the middle of a forced shutdown.
 */
STATIC void
xfs_trans_uncommit(
        xfs_trans_t     *tp,
        uint            flags)
{
        xfs_log_item_desc_t     *lidp;

        for (lidp = xfs_trans_first_item(tp);
             lidp != NULL;
             lidp = xfs_trans_next_item(tp, lidp)) {
                /*
                 * Unpin all but those that aren't dirty.
                 */
                if (lidp->lid_flags & XFS_LID_DIRTY)
                        IOP_UNPIN_REMOVE(lidp->lid_item, tp);
        }

        xfs_trans_unreserve_and_mod_sb(tp);
        xfs_trans_unreserve_and_mod_dquots(tp);

        xfs_trans_free_items(tp, flags);
        xfs_trans_free_busy(tp);
        xfs_trans_free(tp);
}

/*
 * Unlock all of the transaction's items and free the transaction.
 * The transaction must not have modified any of its items, because
 * there is no way to restore them to their previous state.
 *
 * If the transaction has made a log reservation, make sure to release
 * it as well.
 */
void
xfs_trans_cancel(
        xfs_trans_t             *tp,
        int                     flags)
{
        int                     log_flags;
#ifdef DEBUG
        xfs_log_item_chunk_t    *licp;
        xfs_log_item_desc_t     *lidp;
        xfs_log_item_t          *lip;
        int                     i;
#endif
        xfs_mount_t             *mp = tp->t_mountp;

        /*
         * See if the caller is being too lazy to figure out if
         * the transaction really needs an abort.
         */
        if ((flags & XFS_TRANS_ABORT) && !(tp->t_flags & XFS_TRANS_DIRTY))
                flags &= ~XFS_TRANS_ABORT;
        /*
         * See if the caller is relying on us to shut down the
         * filesystem.  This happens in paths where we detect
         * corruption and decide to give up.
         */
        if ((tp->t_flags & XFS_TRANS_DIRTY) && !XFS_FORCED_SHUTDOWN(mp)) {
                XFS_ERROR_REPORT("xfs_trans_cancel", XFS_ERRLEVEL_LOW, mp);
                xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
        }
#ifdef DEBUG
        if (!(flags & XFS_TRANS_ABORT)) {
                licp = &(tp->t_items);
                while (licp != NULL) {
                        lidp = licp->lic_descs;
                        for (i = 0; i < licp->lic_unused; i++, lidp++) {
                                if (xfs_lic_isfree(licp, i)) {
                                        continue;
                                }

                                lip = lidp->lid_item;
                                if (!XFS_FORCED_SHUTDOWN(mp))
                                        ASSERT(!(lip->li_type == XFS_LI_EFD));
                        }
                        licp = licp->lic_next;
                }
        }
#endif
        xfs_trans_unreserve_and_mod_sb(tp);
        xfs_trans_unreserve_and_mod_dquots(tp);

        if (tp->t_ticket) {
                if (flags & XFS_TRANS_RELEASE_LOG_RES) {
                        ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
                        log_flags = XFS_LOG_REL_PERM_RESERV;
                } else {
                        log_flags = 0;
                }
                xfs_log_done(mp, tp->t_ticket, NULL, log_flags);
        }

        /* mark this thread as no longer being in a transaction */
        current_restore_flags_nested(&tp->t_pflags, PF_FSTRANS);

        xfs_trans_free_items(tp, flags);
        xfs_trans_free_busy(tp);
        xfs_trans_free(tp);
}


/*
 * Free the transaction structure.  If there is more clean up
 * to do when the structure is freed, add it here.
 */
STATIC void
xfs_trans_free(
        xfs_trans_t     *tp)
{
        atomic_dec(&tp->t_mountp->m_active_trans);
        xfs_trans_free_dqinfo(tp);
        kmem_zone_free(xfs_trans_zone, tp);
}

/*
 * Roll from one trans in the sequence of PERMANENT transactions to
 * the next: permanent transactions are only flushed out when
 * committed with XFS_TRANS_RELEASE_LOG_RES, but we still want as soon
 * as possible to let chunks of it go to the log. So we commit the
 * chunk we've been working on and get a new transaction to continue.
 */
int
xfs_trans_roll(
        struct xfs_trans        **tpp,
        struct xfs_inode        *dp)
{
        struct xfs_trans        *trans;
        unsigned int            logres, count;
        int                     error;

        /*
         * Ensure that the inode is always logged.
         */
        trans = *tpp;
        xfs_trans_log_inode(trans, dp, XFS_ILOG_CORE);

        /*
         * Copy the critical parameters from one trans to the next.
         */
        logres = trans->t_log_res;
        count = trans->t_log_count;
        *tpp = xfs_trans_dup(trans);

        /*
         * Commit the current transaction.
         * If this commit failed, then it'd just unlock those items that
         * are not marked ihold. That also means that a filesystem shutdown
         * is in progress. The caller takes the responsibility to cancel
         * the duplicate transaction that gets returned.
         */
        error = xfs_trans_commit(trans, 0);
        if (error)
                return (error);

        trans = *tpp;

        /*
         * transaction commit worked ok so we can drop the extra ticket
         * reference that we gained in xfs_trans_dup()
         */
        xfs_log_ticket_put(trans->t_ticket);


        /*
         * Reserve space in the log for th next transaction.
         * This also pushes items in the "AIL", the list of logged items,
         * out to disk if they are taking up space at the tail of the log
         * that we want to use.  This requires that either nothing be locked
         * across this call, or that anything that is locked be logged in
         * the prior and the next transactions.
         */
        error = xfs_trans_reserve(trans, 0, logres, 0,
                                  XFS_TRANS_PERM_LOG_RES, count);
        /*
         *  Ensure that the inode is in the new transaction and locked.
         */
        if (error)
                return error;

        xfs_trans_ijoin(trans, dp, XFS_ILOCK_EXCL);
        xfs_trans_ihold(trans, dp);
        return 0;
}

/*
 * THIS SHOULD BE REWRITTEN TO USE xfs_trans_next_item().
 *
 * This is typically called by the LM when a transaction has been fully
 * committed to disk.  It needs to unpin the items which have
 * been logged by the transaction and update their positions
 * in the AIL if necessary.
 * This also gets called when the transactions didn't get written out
 * because of an I/O error. Abortflag & XFS_LI_ABORTED is set then.
 *
 * Call xfs_trans_chunk_committed() to process the items in
 * each chunk.
 */
STATIC void
xfs_trans_committed(
        xfs_trans_t     *tp,
        int             abortflag)
{
        xfs_log_item_chunk_t    *licp;
        xfs_log_item_chunk_t    *next_licp;
        xfs_log_busy_chunk_t    *lbcp;
        xfs_log_busy_slot_t     *lbsp;
        int                     i;

        /*
         * Call the transaction's completion callback if there
         * is one.
         */
        if (tp->t_callback != NULL) {
                tp->t_callback(tp, tp->t_callarg);
        }

        /*
         * Special case the chunk embedded in the transaction.
         */
        licp = &(tp->t_items);
        if (!(xfs_lic_are_all_free(licp))) {
                xfs_trans_chunk_committed(licp, tp->t_lsn, abortflag);
        }

        /*
         * Process the items in each chunk in turn.
         */
        licp = licp->lic_next;
        while (licp != NULL) {
                ASSERT(!xfs_lic_are_all_free(licp));
                xfs_trans_chunk_committed(licp, tp->t_lsn, abortflag);
                next_licp = licp->lic_next;
                kmem_free(licp);
                licp = next_licp;
        }

        /*
         * Clear all the per-AG busy list items listed in this transaction
         */
        lbcp = &tp->t_busy;
        while (lbcp != NULL) {
                for (i = 0, lbsp = lbcp->lbc_busy; i < lbcp->lbc_unused; i++, lbsp++) {
                        if (!XFS_LBC_ISFREE(lbcp, i)) {
                                xfs_alloc_clear_busy(tp, lbsp->lbc_ag,
                                                     lbsp->lbc_idx);
                        }
                }
                lbcp = lbcp->lbc_next;
        }
        xfs_trans_free_busy(tp);

        /*
         * That's it for the transaction structure.  Free it.
         */
        xfs_trans_free(tp);
}

/*
 * This is called to perform the commit processing for each
 * item described by the given chunk.
 *
 * The commit processing consists of unlocking items which were
 * held locked with the SYNC_UNLOCK attribute, calling the committed
 * routine of each logged item, updating the item's position in the AIL
 * if necessary, and unpinning each item.  If the committed routine
 * returns -1, then do nothing further with the item because it
 * may have been freed.
 *
 * Since items are unlocked when they are copied to the incore
 * log, it is possible for two transactions to be completing
 * and manipulating the same item simultaneously.  The AIL lock
 * will protect the lsn field of each item.  The value of this
 * field can never go backwards.
 *
 * We unpin the items after repositioning them in the AIL, because
 * otherwise they could be immediately flushed and we'd have to race
 * with the flusher trying to pull the item from the AIL as we add it.
 */
STATIC void
xfs_trans_chunk_committed(
        xfs_log_item_chunk_t    *licp,
        xfs_lsn_t               lsn,
        int                     aborted)
{
        xfs_log_item_desc_t     *lidp;
        xfs_log_item_t          *lip;
        xfs_lsn_t               item_lsn;
        int                     i;

        lidp = licp->lic_descs;
        for (i = 0; i < licp->lic_unused; i++, lidp++) {
                struct xfs_ail          *ailp;

                if (xfs_lic_isfree(licp, i)) {
                        continue;
                }

                lip = lidp->lid_item;
                if (aborted)
                        lip->li_flags |= XFS_LI_ABORTED;

                /*
                 * Send in the ABORTED flag to the COMMITTED routine
                 * so that it knows whether the transaction was aborted
                 * or not.
                 */
                item_lsn = IOP_COMMITTED(lip, lsn);

                /*
                 * If the committed routine returns -1, make
                 * no more references to the item.
                 */
                if (XFS_LSN_CMP(item_lsn, (xfs_lsn_t)-1) == 0) {
                        continue;
                }

                /*
                 * If the returned lsn is greater than what it
                 * contained before, update the location of the
                 * item in the AIL.  If it is not, then do nothing.
                 * Items can never move backwards in the AIL.
                 *
                 * While the new lsn should usually be greater, it
                 * is possible that a later transaction completing
                 * simultaneously with an earlier one using the
                 * same item could complete first with a higher lsn.
                 * This would cause the earlier transaction to fail
                 * the test below.
                 */
                ailp = lip->li_ailp;
                spin_lock(&ailp->xa_lock);
                if (XFS_LSN_CMP(item_lsn, lip->li_lsn) > 0) {
                        /*
                         * This will set the item's lsn to item_lsn
                         * and update the position of the item in
                         * the AIL.
                         *
                         * xfs_trans_ail_update() drops the AIL lock.
                         */
                        xfs_trans_ail_update(ailp, lip, item_lsn);
                } else {
                        spin_unlock(&ailp->xa_lock);
                }

                /*
                 * Now that we've repositioned the item in the AIL,
                 * unpin it so it can be flushed. Pass information
                 * about buffer stale state down from the log item
                 * flags, if anyone else stales the buffer we do not
                 * want to pay any attention to it.
                 */
                IOP_UNPIN(lip);
        }
}