HAMMER VFS - Major retooling of the refcount mechanics, and fix a deadlock

[dragonfly.git] / sys / vfs / hammer / hammer_flusher.c

/*
 * Copyright (c) 2008 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * 
 * $DragonFly: src/sys/vfs/hammer/hammer_flusher.c,v 1.45 2008/07/31 04:42:04 dillon Exp $
 */
/*
 * HAMMER dependancy flusher thread
 *
 * Meta data updates create buffer dependancies which are arranged as a
 * hierarchy of lists.
 */

#include "hammer.h"

static void hammer_flusher_master_thread(void *arg);
static void hammer_flusher_slave_thread(void *arg);
static void hammer_flusher_flush(hammer_mount_t hmp);
static void hammer_flusher_flush_inode(hammer_inode_t ip,
                                        hammer_transaction_t trans);

RB_GENERATE(hammer_fls_rb_tree, hammer_inode, rb_flsnode,
              hammer_ino_rb_compare);

/*
 * Inodes are sorted and assigned to slave threads in groups of 128.
 * We want a flush group size large enough such that the slave threads
 * are not likely to interfere with each other when accessing the B-Tree,
 * but not so large that we lose concurrency.
 */
#define HAMMER_FLUSH_GROUP_SIZE 128

/*
 * Support structures for the flusher threads.
 */
struct hammer_flusher_info {
        TAILQ_ENTRY(hammer_flusher_info) entry;
        struct hammer_mount *hmp;
        thread_t        td;
        int             runstate;
        int             count;
        hammer_flush_group_t flg;
        hammer_inode_t  work_array[HAMMER_FLUSH_GROUP_SIZE];
};

typedef struct hammer_flusher_info *hammer_flusher_info_t;

/*
 * Sync all inodes pending on the flusher.
 *
 * All flush groups will be flushed.  This does not queue dirty inodes
 * to the flush groups, it just flushes out what has already been queued!
 */
void
hammer_flusher_sync(hammer_mount_t hmp)
{
        int seq;

        seq = hammer_flusher_async(hmp, NULL);
        hammer_flusher_wait(hmp, seq);
}

/*
 * Sync all inodes pending on the flusher - return immediately.
 *
 * All flush groups will be flushed.
 */
int
hammer_flusher_async(hammer_mount_t hmp, hammer_flush_group_t close_flg)
{
        hammer_flush_group_t flg;
        int seq = hmp->flusher.next;

        TAILQ_FOREACH(flg, &hmp->flush_group_list, flush_entry) {
                if (flg->running == 0)
                        ++seq;
                flg->closed = 1;
                if (flg == close_flg)
                        break;
        }
        if (hmp->flusher.td) {
                if (hmp->flusher.signal++ == 0)
                        wakeup(&hmp->flusher.signal);
        } else {
                seq = hmp->flusher.done;
        }
        return(seq);
}

int
hammer_flusher_async_one(hammer_mount_t hmp)
{
        int seq;

        if (hmp->flusher.td) {
                seq = hmp->flusher.next;
                if (hmp->flusher.signal++ == 0)
                        wakeup(&hmp->flusher.signal);
        } else {
                seq = hmp->flusher.done;
        }
        return(seq);
}

/*
 * Wait for the flusher to get to the specified sequence number.
 * Signal the flusher as often as necessary to keep it going.
 */
void
hammer_flusher_wait(hammer_mount_t hmp, int seq)
{
        while ((int)(seq - hmp->flusher.done) > 0) {
                if (hmp->flusher.act != seq) {
                        if (hmp->flusher.signal++ == 0)
                                wakeup(&hmp->flusher.signal);
                }
                tsleep(&hmp->flusher.done, 0, "hmrfls", 0);
        }
}

void
hammer_flusher_wait_next(hammer_mount_t hmp)
{
        int seq;

        seq = hammer_flusher_async_one(hmp);
        hammer_flusher_wait(hmp, seq);
}

void
hammer_flusher_create(hammer_mount_t hmp)
{
        hammer_flusher_info_t info;
        int i;

        hmp->flusher.signal = 0;
        hmp->flusher.act = 0;
        hmp->flusher.done = 0;
        hmp->flusher.next = 1;
        hammer_ref(&hmp->flusher.finalize_lock);
        TAILQ_INIT(&hmp->flusher.run_list);
        TAILQ_INIT(&hmp->flusher.ready_list);

        lwkt_create(hammer_flusher_master_thread, hmp,
                    &hmp->flusher.td, NULL, 0, -1, "hammer-M");
        for (i = 0; i < HAMMER_MAX_FLUSHERS; ++i) {
                info = kmalloc(sizeof(*info), hmp->m_misc, M_WAITOK|M_ZERO);
                info->hmp = hmp;
                TAILQ_INSERT_TAIL(&hmp->flusher.ready_list, info, entry);
                lwkt_create(hammer_flusher_slave_thread, info,
                            &info->td, NULL, 0, -1, "hammer-S%d", i);
        }
}

void
hammer_flusher_destroy(hammer_mount_t hmp)
{
        hammer_flusher_info_t info;

        /*
         * Kill the master
         */
        hmp->flusher.exiting = 1;
        while (hmp->flusher.td) {
                ++hmp->flusher.signal;
                wakeup(&hmp->flusher.signal);
                tsleep(&hmp->flusher.exiting, 0, "hmrwex", hz);
        }

        /*
         * Kill the slaves
         */
        while ((info = TAILQ_FIRST(&hmp->flusher.ready_list)) != NULL) {
                KKASSERT(info->runstate == 0);
                TAILQ_REMOVE(&hmp->flusher.ready_list, info, entry);
                info->runstate = -1;
                wakeup(&info->runstate);
                while (info->td)
                        tsleep(&info->td, 0, "hmrwwc", 0);
                kfree(info, hmp->m_misc);
        }
}

/*
 * The master flusher thread manages the flusher sequence id and
 * synchronization with the slave work threads.
 */
static void
hammer_flusher_master_thread(void *arg)
{
        hammer_flush_group_t flg;
        hammer_mount_t hmp;

        hmp = arg;

        for (;;) {
                /*
                 * Do at least one flush cycle.  We may have to update the
                 * UNDO FIFO even if no inodes are queued.
                 */
                for (;;) {
                        while (hmp->flusher.group_lock)
                                tsleep(&hmp->flusher.group_lock, 0, "hmrhld", 0);
                        hmp->flusher.act = hmp->flusher.next;
                        ++hmp->flusher.next;
                        hammer_flusher_clean_loose_ios(hmp);
                        hammer_flusher_flush(hmp);
                        hmp->flusher.done = hmp->flusher.act;
                        wakeup(&hmp->flusher.done);
                        flg = TAILQ_FIRST(&hmp->flush_group_list);
                        if (flg == NULL || flg->closed == 0)
                                break;
                        if (hmp->flags & HAMMER_MOUNT_CRITICAL_ERROR)
                                break;
                }

                /*
                 * Wait for activity.
                 */
                if (hmp->flusher.exiting && TAILQ_EMPTY(&hmp->flush_group_list))
                        break;
                while (hmp->flusher.signal == 0)
                        tsleep(&hmp->flusher.signal, 0, "hmrwwa", 0);

                /*
                 * Flush for each count on signal but only allow one extra
                 * flush request to build up.
                 */
                if (--hmp->flusher.signal != 0)
                        hmp->flusher.signal = 1;
        }

        /*
         * And we are done.
         */
        hmp->flusher.td = NULL;
        wakeup(&hmp->flusher.exiting);
        lwkt_exit();
}

/*
 * Flush all inodes in the current flush group.
 */
static void
hammer_flusher_flush(hammer_mount_t hmp)
{
        hammer_flusher_info_t info;
        hammer_flush_group_t flg;
        hammer_reserve_t resv;
        hammer_inode_t ip;
        hammer_inode_t next_ip;
        int slave_index;
        int count;

        /*
         * Just in-case there's a flush race on mount
         */
        if (TAILQ_FIRST(&hmp->flusher.ready_list) == NULL)
                return;

        /*
         * We only do one flg but we may have to loop/retry.
         */
        count = 0;
        while ((flg = TAILQ_FIRST(&hmp->flush_group_list)) != NULL) {
                ++count;
                if (hammer_debug_general & 0x0001) {
                        kprintf("hammer_flush %d ttl=%d recs=%d\n",
                                hmp->flusher.act,
                                flg->total_count, flg->refs);
                }
                if (hmp->flags & HAMMER_MOUNT_CRITICAL_ERROR)
                        break;
                hammer_start_transaction_fls(&hmp->flusher.trans, hmp);

                /*
                 * If the previous flush cycle just about exhausted our
                 * UNDO space we may have to do a dummy cycle to move the
                 * first_offset up before actually digging into a new cycle,
                 * or the new cycle will not have sufficient undo space.
                 */
                if (hammer_flusher_undo_exhausted(&hmp->flusher.trans, 3))
                        hammer_flusher_finalize(&hmp->flusher.trans, 0);

                /*
                 * Ok, we are running this flush group now (this prevents new
                 * additions to it).
                 */
                flg->running = 1;
                if (hmp->next_flush_group == flg)
                        hmp->next_flush_group = TAILQ_NEXT(flg, flush_entry);

                /*
                 * Iterate the inodes in the flg's flush_tree and assign
                 * them to slaves.
                 */
                slave_index = 0;
                info = TAILQ_FIRST(&hmp->flusher.ready_list);
                next_ip = RB_FIRST(hammer_fls_rb_tree, &flg->flush_tree);

                while ((ip = next_ip) != NULL) {
                        next_ip = RB_NEXT(hammer_fls_rb_tree,
                                          &flg->flush_tree, ip);

                        if (++hmp->check_yield > hammer_yield_check) {
                                hmp->check_yield = 0;
                                lwkt_user_yield();
                        }

                        /*
                         * Add ip to the slave's work array.  The slave is
                         * not currently running.
                         */
                        info->work_array[info->count++] = ip;
                        if (info->count != HAMMER_FLUSH_GROUP_SIZE)
                                continue;

                        /*
                         * Get the slave running
                         */
                        TAILQ_REMOVE(&hmp->flusher.ready_list, info, entry);
                        TAILQ_INSERT_TAIL(&hmp->flusher.run_list, info, entry);
                        info->flg = flg;
                        info->runstate = 1;
                        wakeup(&info->runstate);

                        /*
                         * Get a new slave.  We may have to wait for one to
                         * finish running.
                         */
                        while ((info = TAILQ_FIRST(&hmp->flusher.ready_list)) == NULL) {
                                tsleep(&hmp->flusher.ready_list, 0, "hmrfcc", 0);
                        }
                }

                /*
                 * Run the current slave if necessary
                 */
                if (info->count) {
                        TAILQ_REMOVE(&hmp->flusher.ready_list, info, entry);
                        TAILQ_INSERT_TAIL(&hmp->flusher.run_list, info, entry);
                        info->flg = flg;
                        info->runstate = 1;
                        wakeup(&info->runstate);
                }

                /*
                 * Wait for all slaves to finish running
                 */
                while (TAILQ_FIRST(&hmp->flusher.run_list) != NULL)
                        tsleep(&hmp->flusher.ready_list, 0, "hmrfcc", 0);

                /*
                 * Do the final finalization, clean up
                 */
                hammer_flusher_finalize(&hmp->flusher.trans, 1);
                hmp->flusher.tid = hmp->flusher.trans.tid;

                hammer_done_transaction(&hmp->flusher.trans);

                /*
                 * Loop up on the same flg.  If the flg is done clean it up
                 * and break out.  We only flush one flg.
                 */
                if (RB_EMPTY(&flg->flush_tree)) {
                        KKASSERT(flg->refs == 0);
                        TAILQ_REMOVE(&hmp->flush_group_list, flg, flush_entry);
                        kfree(flg, hmp->m_misc);
                        break;
                }
        }

        /*
         * We may have pure meta-data to flush, or we may have to finish
         * cycling the UNDO FIFO, even if there were no flush groups.
         */
        if (count == 0 && hammer_flusher_haswork(hmp)) {
                hammer_start_transaction_fls(&hmp->flusher.trans, hmp);
                hammer_flusher_finalize(&hmp->flusher.trans, 1);
                hammer_done_transaction(&hmp->flusher.trans);
        }

        /*
         * Clean up any freed big-blocks (typically zone-2). 
         * resv->flush_group is typically set several flush groups ahead
         * of the free to ensure that the freed block is not reused until
         * it can no longer be reused.
         */
        while ((resv = TAILQ_FIRST(&hmp->delay_list)) != NULL) {
                if (resv->flush_group != hmp->flusher.act)
                        break;
                hammer_reserve_clrdelay(hmp, resv);
        }
}


/*
 * The slave flusher thread pulls work off the master flush list until no
 * work is left.
 */
static void
hammer_flusher_slave_thread(void *arg)
{
        hammer_flush_group_t flg;
        hammer_flusher_info_t info;
        hammer_mount_t hmp;
        hammer_inode_t ip;
        int i;

        info = arg;
        hmp = info->hmp;

        for (;;) {
                while (info->runstate == 0)
                        tsleep(&info->runstate, 0, "hmrssw", 0);
                if (info->runstate < 0)
                        break;
                flg = info->flg;

                for (i = 0; i < info->count; ++i) {
                        ip = info->work_array[i];
                        hammer_flusher_flush_inode(ip, &hmp->flusher.trans);
                        ++hammer_stats_inode_flushes;
                }
                info->count = 0;
                info->runstate = 0;
                TAILQ_REMOVE(&hmp->flusher.run_list, info, entry);
                TAILQ_INSERT_TAIL(&hmp->flusher.ready_list, info, entry);
                wakeup(&hmp->flusher.ready_list);
        }
        info->td = NULL;
        wakeup(&info->td);
        lwkt_exit();
}

void
hammer_flusher_clean_loose_ios(hammer_mount_t hmp)
{
        hammer_buffer_t buffer;
        hammer_io_t io;

        /*
         * loose ends - buffers without bp's aren't tracked by the kernel
         * and can build up, so clean them out.  This can occur when an
         * IO completes on a buffer with no references left.
         */
        if ((io = TAILQ_FIRST(&hmp->lose_list)) != NULL) {
                crit_enter();   /* biodone() race */
                while ((io = TAILQ_FIRST(&hmp->lose_list)) != NULL) {
                        KKASSERT(io->mod_list == &hmp->lose_list);
                        TAILQ_REMOVE(&hmp->lose_list, io, mod_entry);
                        io->mod_list = NULL;
                        hammer_ref(&io->lock);
                        buffer = (void *)io;
                        hammer_rel_buffer(buffer, 0);
                }
                crit_exit();
        }
}

/*
 * Flush a single inode that is part of a flush group.
 *
 * Flusher errors are extremely serious, even ENOSPC shouldn't occur because
 * the front-end should have reserved sufficient space on the media.  Any
 * error other then EWOULDBLOCK will force the mount to be read-only.
 */
static
void
hammer_flusher_flush_inode(hammer_inode_t ip, hammer_transaction_t trans)
{
        hammer_mount_t hmp = ip->hmp;
        int error;

        hammer_flusher_clean_loose_ios(hmp);
        error = hammer_sync_inode(trans, ip);

        /*
         * EWOULDBLOCK can happen under normal operation, all other errors
         * are considered extremely serious.  We must set WOULDBLOCK
         * mechanics to deal with the mess left over from the abort of the
         * previous flush.
         */
        if (error) {
                ip->flags |= HAMMER_INODE_WOULDBLOCK;
                if (error == EWOULDBLOCK)
                        error = 0;
        }
        hammer_flush_inode_done(ip, error);
        while (hmp->flusher.finalize_want)
                tsleep(&hmp->flusher.finalize_want, 0, "hmrsxx", 0);
        if (hammer_flusher_undo_exhausted(trans, 1)) {
                kprintf("HAMMER: Warning: UNDO area too small!\n");
                hammer_flusher_finalize(trans, 1);
        } else if (hammer_flusher_meta_limit(trans->hmp)) {
                hammer_flusher_finalize(trans, 0);
        }
}

/*
 * Return non-zero if the UNDO area has less then (QUARTER / 4) of its
 * space left.
 *
 * 1/4 - Emergency free undo space level.  Below this point the flusher
 *       will finalize even if directory dependancies have not been resolved.
 *
 * 2/4 - Used by the pruning and reblocking code.  These functions may be
 *       running in parallel with a flush and cannot be allowed to drop
 *       available undo space to emergency levels.
 *
 * 3/4 - Used at the beginning of a flush to force-sync the volume header
 *       to give the flush plenty of runway to work in.
 */
int
hammer_flusher_undo_exhausted(hammer_transaction_t trans, int quarter)
{
        if (hammer_undo_space(trans) <
            hammer_undo_max(trans->hmp) * quarter / 4) {
                return(1);
        } else {
                return(0);
        }
}

/*
 * Flush all pending UNDOs, wait for write completion, update the volume
 * header with the new UNDO end position, and flush it.  Then
 * asynchronously flush the meta-data.
 *
 * If this is the last finalization in a flush group we also synchronize
 * our cached blockmap and set hmp->flusher_undo_start and our cached undo
 * fifo first_offset so the next flush resets the FIFO pointers.
 *
 * If this is not final it is being called because too many dirty meta-data
 * buffers have built up and must be flushed with UNDO synchronization to
 * avoid a buffer cache deadlock.
 */
void
hammer_flusher_finalize(hammer_transaction_t trans, int final)
{
        hammer_volume_t root_volume;
        hammer_blockmap_t cundomap, dundomap;
        hammer_mount_t hmp;
        hammer_io_t io;
        hammer_off_t save_undo_next_offset;
        int count;
        int i;

        hmp = trans->hmp;
        root_volume = trans->rootvol;

        /*
         * Exclusively lock the flusher.  This guarantees that all dirty
         * buffers will be idled (have a mod-count of 0).
         */
        ++hmp->flusher.finalize_want;
        hammer_lock_ex(&hmp->flusher.finalize_lock);

        /*
         * If this isn't the final sync several threads may have hit the
         * meta-limit at the same time and raced.  Only sync if we really
         * have to, after acquiring the lock.
         */
        if (final == 0 && !hammer_flusher_meta_limit(hmp))
                goto done;

        if (hmp->flags & HAMMER_MOUNT_CRITICAL_ERROR)
                goto done;

        /*
         * Flush data buffers.  This can occur asynchronously and at any
         * time.  We must interlock against the frontend direct-data write
         * but do not have to acquire the sync-lock yet.
         *
         * These data buffers have already been collected prior to the
         * related inode(s) getting queued to the flush group.
         */
        count = 0;
        while ((io = TAILQ_FIRST(&hmp->data_list)) != NULL) {
                if (io->ioerror)
                        break;
                hammer_ref(&io->lock);
                hammer_io_write_interlock(io);
                KKASSERT(io->type != HAMMER_STRUCTURE_VOLUME);
                hammer_io_flush(io, 0);
                hammer_io_done_interlock(io);
                hammer_rel_buffer((hammer_buffer_t)io, 0);
                ++count;
        }

        /*
         * The sync-lock is required for the remaining sequence.  This lock
         * prevents meta-data from being modified.
         */
        hammer_sync_lock_ex(trans);

        /*
         * If we have been asked to finalize the volume header sync the
         * cached blockmap to the on-disk blockmap.  Generate an UNDO
         * record for the update.
         */
        if (final) {
                cundomap = &hmp->blockmap[0];
                dundomap = &root_volume->ondisk->vol0_blockmap[0];
                if (root_volume->io.modified) {
                        hammer_modify_volume(trans, root_volume,
                                             dundomap, sizeof(hmp->blockmap));
                        for (i = 0; i < HAMMER_MAX_ZONES; ++i)
                                hammer_crc_set_blockmap(&cundomap[i]);
                        bcopy(cundomap, dundomap, sizeof(hmp->blockmap));
                        hammer_modify_volume_done(root_volume);
                }
        }

        /*
         * Flush UNDOs.  This can occur concurrently with the data flush
         * because data writes never overwrite.
         *
         * This also waits for I/Os to complete and flushes the cache on
         * the target disk.
         *
         * Record the UNDO append point as this can continue to change
         * after we have flushed the UNDOs.
         */
        cundomap = &hmp->blockmap[HAMMER_ZONE_UNDO_INDEX];
        hammer_lock_ex(&hmp->undo_lock);
        save_undo_next_offset = cundomap->next_offset;
        hammer_unlock(&hmp->undo_lock);
        hammer_flusher_flush_undos(hmp, HAMMER_FLUSH_UNDOS_FORCED);

        if (hmp->flags & HAMMER_MOUNT_CRITICAL_ERROR)
                goto failed;

        /*
         * HAMMER VERSION < 4:
         *      Update the on-disk volume header with new UNDO FIFO end
         *      position (do not generate new UNDO records for this change).
         *      We have to do this for the UNDO FIFO whether (final) is
         *      set or not in order for the UNDOs to be recognized on
         *      recovery.
         *
         * HAMMER VERSION >= 4:
         *      The UNDO FIFO data written above will be recognized on
         *      recovery without us having to sync the volume header.
         *
         * Also update the on-disk next_tid field.  This does not require
         * an UNDO.  However, because our TID is generated before we get
         * the sync lock another sync may have beat us to the punch.
         *
         * This also has the side effect of updating first_offset based on
         * a prior finalization when the first finalization of the next flush
         * cycle occurs, removing any undo info from the prior finalization
         * from consideration.
         *
         * The volume header will be flushed out synchronously.
         */
        dundomap = &root_volume->ondisk->vol0_blockmap[HAMMER_ZONE_UNDO_INDEX];
        cundomap = &hmp->blockmap[HAMMER_ZONE_UNDO_INDEX];

        if (dundomap->first_offset != cundomap->first_offset ||
                   dundomap->next_offset != save_undo_next_offset) {
                hammer_modify_volume(NULL, root_volume, NULL, 0);
                dundomap->first_offset = cundomap->first_offset;
                dundomap->next_offset = save_undo_next_offset;
                hammer_crc_set_blockmap(dundomap);
                hammer_modify_volume_done(root_volume);
        }

        /*
         * vol0_next_tid is used for TID selection and is updated without
         * an UNDO so we do not reuse a TID that may have been rolled-back.
         *
         * vol0_last_tid is the highest fully-synchronized TID.  It is
         * set-up when the UNDO fifo is fully synced, later on (not here).
         *
         * The root volume can be open for modification by other threads
         * generating UNDO or REDO records.  For example, reblocking,
         * pruning, REDO mode fast-fsyncs, so the write interlock is
         * mandatory.
         */
        if (root_volume->io.modified) {
                hammer_modify_volume(NULL, root_volume, NULL, 0);
                if (root_volume->ondisk->vol0_next_tid < trans->tid)
                        root_volume->ondisk->vol0_next_tid = trans->tid;
                hammer_crc_set_volume(root_volume->ondisk);
                hammer_modify_volume_done(root_volume);
                hammer_io_write_interlock(&root_volume->io);
                hammer_io_flush(&root_volume->io, 0);
                hammer_io_done_interlock(&root_volume->io);
        }

        /*
         * Wait for I/Os to complete.
         *
         * For HAMMER VERSION 4+ filesystems we do not have to wait for
         * the I/O to complete as the new UNDO FIFO entries are recognized
         * even without the volume header update.  This allows the volume
         * header to flushed along with meta-data, significantly reducing
         * flush overheads.
         */
        hammer_flusher_clean_loose_ios(hmp);
        if (hmp->version < HAMMER_VOL_VERSION_FOUR)
                hammer_io_wait_all(hmp, "hmrfl3", 1);

        if (hmp->flags & HAMMER_MOUNT_CRITICAL_ERROR)
                goto failed;

        /*
         * Flush meta-data.  The meta-data will be undone if we crash
         * so we can safely flush it asynchronously.  There is no need
         * to wait for I/O to complete (or issue a synchronous disk flush).
         *
         * In fact, even if we did wait the meta-data will still be undone
         * by a crash up until the next flush cycle due to the first_offset
         * in the volume header for the UNDO FIFO not being adjusted until
         * the following flush cycle.
         */
        count = 0;
        while ((io = TAILQ_FIRST(&hmp->meta_list)) != NULL) {
                if (io->ioerror)
                        break;
                KKASSERT(io->modify_refs == 0);
                hammer_ref(&io->lock);
                KKASSERT(io->type != HAMMER_STRUCTURE_VOLUME);
                hammer_io_flush(io, 0);
                hammer_rel_buffer((hammer_buffer_t)io, 0);
                ++count;
        }

        /*
         * If this is the final finalization for the flush group set
         * up for the next sequence by setting a new first_offset in
         * our cached blockmap and clearing the undo history.
         *
         * Even though we have updated our cached first_offset, the on-disk
         * first_offset still governs available-undo-space calculations.
         *
         * We synchronize to save_undo_next_offset rather than
         * cundomap->next_offset because that is what we flushed out
         * above.
         *
         * NOTE! UNDOs can only be added with the sync_lock held
         *       so we can clear the undo history without racing.
         *       REDOs can be added at any time which is why we
         *       have to be careful and use save_undo_next_offset
         *       when setting the new first_offset.
         */
        if (final) {
                cundomap = &hmp->blockmap[HAMMER_ZONE_UNDO_INDEX];
                if (cundomap->first_offset != save_undo_next_offset) {
                        cundomap->first_offset = save_undo_next_offset;
                        hmp->hflags |= HMNT_UNDO_DIRTY;
                } else if (cundomap->first_offset != cundomap->next_offset) {
                        hmp->hflags |= HMNT_UNDO_DIRTY;
                } else {
                        hmp->hflags &= ~HMNT_UNDO_DIRTY;
                }
                hammer_clear_undo_history(hmp);

                /*
                 * Flush tid sequencing.  flush_tid1 is fully synchronized,
                 * meaning a crash will not roll it back.  flush_tid2 has
                 * been written out asynchronously and a crash will roll
                 * it back.  flush_tid1 is used for all mirroring masters.
                 */
                if (hmp->flush_tid1 != hmp->flush_tid2) {
                        hmp->flush_tid1 = hmp->flush_tid2;
                        wakeup(&hmp->flush_tid1);
                }
                hmp->flush_tid2 = trans->tid;

                /*
                 * Clear the REDO SYNC flag.  This flag is used to ensure
                 * that the recovery span in the UNDO/REDO FIFO contains
                 * at least one REDO SYNC record.
                 */
                hmp->flags &= ~HAMMER_MOUNT_REDO_SYNC;
        }

        /*
         * Cleanup.  Report any critical errors.
         */
failed:
        hammer_sync_unlock(trans);

        if (hmp->flags & HAMMER_MOUNT_CRITICAL_ERROR) {
                kprintf("HAMMER(%s): Critical write error during flush, "
                        "refusing to sync UNDO FIFO\n",
                        root_volume->ondisk->vol_name);
        }

done:
        hammer_unlock(&hmp->flusher.finalize_lock);

        if (--hmp->flusher.finalize_want == 0)
                wakeup(&hmp->flusher.finalize_want);
        hammer_stats_commits += final;
}

/*
 * Flush UNDOs.
 */
void
hammer_flusher_flush_undos(hammer_mount_t hmp, int mode)
{
        hammer_io_t io;
        int count;

        count = 0;
        while ((io = TAILQ_FIRST(&hmp->undo_list)) != NULL) {
                if (io->ioerror)
                        break;
                hammer_ref(&io->lock);
                KKASSERT(io->type != HAMMER_STRUCTURE_VOLUME);
                hammer_io_write_interlock(io);
                hammer_io_flush(io, hammer_undo_reclaim(io));
                hammer_io_done_interlock(io);
                hammer_rel_buffer((hammer_buffer_t)io, 0);
                ++count;
        }
        hammer_flusher_clean_loose_ios(hmp);
        if (mode == HAMMER_FLUSH_UNDOS_FORCED ||
            (mode == HAMMER_FLUSH_UNDOS_AUTO && count)) {
                hammer_io_wait_all(hmp, "hmrfl1", 1);
        } else {
                hammer_io_wait_all(hmp, "hmrfl2", 0);
        }
}

/*
 * Return non-zero if too many dirty meta-data buffers have built up.
 *
 * Since we cannot allow such buffers to flush until we have dealt with
 * the UNDOs, we risk deadlocking the kernel's buffer cache.
 */
int
hammer_flusher_meta_limit(hammer_mount_t hmp)
{
        if (hmp->locked_dirty_space + hmp->io_running_space >
            hammer_limit_dirtybufspace) {
                return(1);
        }
        return(0);
}

/*
 * Return non-zero if too many dirty meta-data buffers have built up.
 *
 * This version is used by background operations (mirror, prune, reblock)
 * to leave room for foreground operations.
 */
int
hammer_flusher_meta_halflimit(hammer_mount_t hmp)
{
        if (hmp->locked_dirty_space + hmp->io_running_space >
            hammer_limit_dirtybufspace / 2) {
                return(1);
        }
        return(0);
}

/*
 * Return non-zero if the flusher still has something to flush.
 */
int
hammer_flusher_haswork(hammer_mount_t hmp)
{
        if (hmp->flags & HAMMER_MOUNT_CRITICAL_ERROR)
                return(0);
        if (TAILQ_FIRST(&hmp->flush_group_list) ||      /* dirty inodes */
            TAILQ_FIRST(&hmp->volu_list) ||             /* dirty buffers */
            TAILQ_FIRST(&hmp->undo_list) ||
            TAILQ_FIRST(&hmp->data_list) ||
            TAILQ_FIRST(&hmp->meta_list) ||
            (hmp->hflags & HMNT_UNDO_DIRTY)             /* UNDO FIFO sync */
        ) {
                return(1);
        }
        return(0);
}