Illumos #3086: unnecessarily setting DS_FLAG_INCONSISTENT on async

[mirror_zfs.git] / module / zfs / txg.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Portions Copyright 2011 Martin Matuska
 * Copyright (c) 2012 by Delphix. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/txg_impl.h>
#include <sys/dmu_impl.h>
#include <sys/dmu_tx.h>
#include <sys/dsl_pool.h>
#include <sys/dsl_scan.h>
#include <sys/callb.h>
#include <sys/spa_impl.h>

/*
 * Pool-wide transaction groups.
 */

static void txg_sync_thread(dsl_pool_t *dp);
static void txg_quiesce_thread(dsl_pool_t *dp);

int zfs_txg_timeout = 5;        /* max seconds worth of delta per txg */

/*
 * Prepare the txg subsystem.
 */
void
txg_init(dsl_pool_t *dp, uint64_t txg)
{
        tx_state_t *tx = &dp->dp_tx;
        int c;
        bzero(tx, sizeof (tx_state_t));

        tx->tx_cpu = vmem_zalloc(max_ncpus * sizeof (tx_cpu_t), KM_SLEEP);

        for (c = 0; c < max_ncpus; c++) {
                int i;

                mutex_init(&tx->tx_cpu[c].tc_lock, NULL, MUTEX_DEFAULT, NULL);
                for (i = 0; i < TXG_SIZE; i++) {
                        cv_init(&tx->tx_cpu[c].tc_cv[i], NULL, CV_DEFAULT,
                            NULL);
                        list_create(&tx->tx_cpu[c].tc_callbacks[i],
                            sizeof (dmu_tx_callback_t),
                            offsetof(dmu_tx_callback_t, dcb_node));
                }
        }

        mutex_init(&tx->tx_sync_lock, NULL, MUTEX_DEFAULT, NULL);

        cv_init(&tx->tx_sync_more_cv, NULL, CV_DEFAULT, NULL);
        cv_init(&tx->tx_sync_done_cv, NULL, CV_DEFAULT, NULL);
        cv_init(&tx->tx_quiesce_more_cv, NULL, CV_DEFAULT, NULL);
        cv_init(&tx->tx_quiesce_done_cv, NULL, CV_DEFAULT, NULL);
        cv_init(&tx->tx_exit_cv, NULL, CV_DEFAULT, NULL);

        tx->tx_open_txg = txg;
}

/*
 * Close down the txg subsystem.
 */
void
txg_fini(dsl_pool_t *dp)
{
        tx_state_t *tx = &dp->dp_tx;
        int c;

        ASSERT(tx->tx_threads == 0);

        mutex_destroy(&tx->tx_sync_lock);

        cv_destroy(&tx->tx_sync_more_cv);
        cv_destroy(&tx->tx_sync_done_cv);
        cv_destroy(&tx->tx_quiesce_more_cv);
        cv_destroy(&tx->tx_quiesce_done_cv);
        cv_destroy(&tx->tx_exit_cv);

        for (c = 0; c < max_ncpus; c++) {
                int i;

                mutex_destroy(&tx->tx_cpu[c].tc_lock);
                for (i = 0; i < TXG_SIZE; i++) {
                        cv_destroy(&tx->tx_cpu[c].tc_cv[i]);
                        list_destroy(&tx->tx_cpu[c].tc_callbacks[i]);
                }
        }

        if (tx->tx_commit_cb_taskq != NULL)
                taskq_destroy(tx->tx_commit_cb_taskq);

        vmem_free(tx->tx_cpu, max_ncpus * sizeof (tx_cpu_t));

        bzero(tx, sizeof (tx_state_t));
}

/*
 * Start syncing transaction groups.
 */
void
txg_sync_start(dsl_pool_t *dp)
{
        tx_state_t *tx = &dp->dp_tx;

        mutex_enter(&tx->tx_sync_lock);

        dprintf("pool %p\n", dp);

        ASSERT(tx->tx_threads == 0);

        tx->tx_threads = 2;

        tx->tx_quiesce_thread = thread_create(NULL, 0, txg_quiesce_thread,
            dp, 0, &p0, TS_RUN, minclsyspri);

        /*
         * The sync thread can need a larger-than-default stack size on
         * 32-bit x86.  This is due in part to nested pools and
         * scrub_visitbp() recursion.
         */
        tx->tx_sync_thread = thread_create(NULL, 32<<10, txg_sync_thread,
            dp, 0, &p0, TS_RUN, minclsyspri);

        mutex_exit(&tx->tx_sync_lock);
}

static void
txg_thread_enter(tx_state_t *tx, callb_cpr_t *cpr)
{
        CALLB_CPR_INIT(cpr, &tx->tx_sync_lock, callb_generic_cpr, FTAG);
        mutex_enter(&tx->tx_sync_lock);
}

static void
txg_thread_exit(tx_state_t *tx, callb_cpr_t *cpr, kthread_t **tpp)
{
        ASSERT(*tpp != NULL);
        *tpp = NULL;
        tx->tx_threads--;
        cv_broadcast(&tx->tx_exit_cv);
        CALLB_CPR_EXIT(cpr);            /* drops &tx->tx_sync_lock */
        thread_exit();
}

static void
txg_thread_wait(tx_state_t *tx, callb_cpr_t *cpr, kcondvar_t *cv, uint64_t time)
{
        CALLB_CPR_SAFE_BEGIN(cpr);

        if (time)
                (void) cv_timedwait_interruptible(cv, &tx->tx_sync_lock,
                    ddi_get_lbolt() + time);
        else
                cv_wait_interruptible(cv, &tx->tx_sync_lock);

        CALLB_CPR_SAFE_END(cpr, &tx->tx_sync_lock);
}

/*
 * Stop syncing transaction groups.
 */
void
txg_sync_stop(dsl_pool_t *dp)
{
        tx_state_t *tx = &dp->dp_tx;

        dprintf("pool %p\n", dp);
        /*
         * Finish off any work in progress.
         */
        ASSERT(tx->tx_threads == 2);

        /*
         * We need to ensure that we've vacated the deferred space_maps.
         */
        txg_wait_synced(dp, tx->tx_open_txg + TXG_DEFER_SIZE);

        /*
         * Wake all sync threads and wait for them to die.
         */
        mutex_enter(&tx->tx_sync_lock);

        ASSERT(tx->tx_threads == 2);

        tx->tx_exiting = 1;

        cv_broadcast(&tx->tx_quiesce_more_cv);
        cv_broadcast(&tx->tx_quiesce_done_cv);
        cv_broadcast(&tx->tx_sync_more_cv);

        while (tx->tx_threads != 0)
                cv_wait(&tx->tx_exit_cv, &tx->tx_sync_lock);

        tx->tx_exiting = 0;

        mutex_exit(&tx->tx_sync_lock);
}

uint64_t
txg_hold_open(dsl_pool_t *dp, txg_handle_t *th)
{
        tx_state_t *tx = &dp->dp_tx;
        tx_cpu_t *tc;
        uint64_t txg;

        /*
         * It appears the processor id is simply used as a "random"
         * number to index into the array, and there isn't any other
         * significance to the chosen tx_cpu. Because.. Why not use
         * the current cpu to index into the array?
         */
        kpreempt_disable();
        tc = &tx->tx_cpu[CPU_SEQID];
        kpreempt_enable();

        mutex_enter(&tc->tc_lock);

        txg = tx->tx_open_txg;
        tc->tc_count[txg & TXG_MASK]++;

        th->th_cpu = tc;
        th->th_txg = txg;

        return (txg);
}

void
txg_rele_to_quiesce(txg_handle_t *th)
{
        tx_cpu_t *tc = th->th_cpu;

        mutex_exit(&tc->tc_lock);
}

void
txg_register_callbacks(txg_handle_t *th, list_t *tx_callbacks)
{
        tx_cpu_t *tc = th->th_cpu;
        int g = th->th_txg & TXG_MASK;

        mutex_enter(&tc->tc_lock);
        list_move_tail(&tc->tc_callbacks[g], tx_callbacks);
        mutex_exit(&tc->tc_lock);
}

void
txg_rele_to_sync(txg_handle_t *th)
{
        tx_cpu_t *tc = th->th_cpu;
        int g = th->th_txg & TXG_MASK;

        mutex_enter(&tc->tc_lock);
        ASSERT(tc->tc_count[g] != 0);
        if (--tc->tc_count[g] == 0)
                cv_broadcast(&tc->tc_cv[g]);
        mutex_exit(&tc->tc_lock);

        th->th_cpu = NULL;      /* defensive */
}

static void
txg_quiesce(dsl_pool_t *dp, uint64_t txg)
{
        hrtime_t start;
        txg_history_t *th;
        tx_state_t *tx = &dp->dp_tx;
        int g = txg & TXG_MASK;
        int c;

        /*
         * Grab all tx_cpu locks so nobody else can get into this txg.
         */
        for (c = 0; c < max_ncpus; c++)
                mutex_enter(&tx->tx_cpu[c].tc_lock);

        ASSERT(txg == tx->tx_open_txg);
        tx->tx_open_txg++;

        /*
         * Measure how long the txg was open and replace the kstat.
         */
        th = dsl_pool_txg_history_get(dp, txg);
        th->th_kstat.open_time = gethrtime() - th->th_kstat.birth;
        th->th_kstat.state = TXG_STATE_QUIESCING;
        dsl_pool_txg_history_put(th);
        dsl_pool_txg_history_add(dp, tx->tx_open_txg);

        /*
         * Now that we've incremented tx_open_txg, we can let threads
         * enter the next transaction group.
         */
        for (c = 0; c < max_ncpus; c++)
                mutex_exit(&tx->tx_cpu[c].tc_lock);

        /*
         * Quiesce the transaction group by waiting for everyone to txg_exit().
         */
        start = gethrtime();

        for (c = 0; c < max_ncpus; c++) {
                tx_cpu_t *tc = &tx->tx_cpu[c];
                mutex_enter(&tc->tc_lock);
                while (tc->tc_count[g] != 0)
                        cv_wait(&tc->tc_cv[g], &tc->tc_lock);
                mutex_exit(&tc->tc_lock);
        }

        /*
         * Measure how long the txg took to quiesce.
         */
        th = dsl_pool_txg_history_get(dp, txg);
        th->th_kstat.quiesce_time = gethrtime() - start;
        dsl_pool_txg_history_put(th);
}

static void
txg_do_callbacks(list_t *cb_list)
{
        dmu_tx_do_callbacks(cb_list, 0);

        list_destroy(cb_list);

        kmem_free(cb_list, sizeof (list_t));
}

/*
 * Dispatch the commit callbacks registered on this txg to worker threads.
 */
static void
txg_dispatch_callbacks(dsl_pool_t *dp, uint64_t txg)
{
        int c;
        tx_state_t *tx = &dp->dp_tx;
        list_t *cb_list;

        for (c = 0; c < max_ncpus; c++) {
                tx_cpu_t *tc = &tx->tx_cpu[c];
                /* No need to lock tx_cpu_t at this point */

                int g = txg & TXG_MASK;

                if (list_is_empty(&tc->tc_callbacks[g]))
                        continue;

                if (tx->tx_commit_cb_taskq == NULL) {
                        /*
                         * Commit callback taskq hasn't been created yet.
                         */
                        tx->tx_commit_cb_taskq = taskq_create("tx_commit_cb",
                            100, minclsyspri, max_ncpus, INT_MAX,
                            TASKQ_THREADS_CPU_PCT | TASKQ_PREPOPULATE);
                }

                cb_list = kmem_alloc(sizeof (list_t), KM_PUSHPAGE);
                list_create(cb_list, sizeof (dmu_tx_callback_t),
                    offsetof(dmu_tx_callback_t, dcb_node));

                list_move_tail(cb_list, &tc->tc_callbacks[g]);

                (void) taskq_dispatch(tx->tx_commit_cb_taskq, (task_func_t *)
                    txg_do_callbacks, cb_list, TQ_SLEEP);
        }
}

/*
 * Wait for pending commit callbacks of already-synced transactions to finish
 * processing.
 * Calling this function from within a commit callback will deadlock.
 */
void
txg_wait_callbacks(dsl_pool_t *dp)
{
        tx_state_t *tx = &dp->dp_tx;

        if (tx->tx_commit_cb_taskq != NULL)
                taskq_wait(tx->tx_commit_cb_taskq);
}

static void
txg_sync_thread(dsl_pool_t *dp)
{
        spa_t *spa = dp->dp_spa;
        tx_state_t *tx = &dp->dp_tx;
        callb_cpr_t cpr;
        uint64_t start, delta;

#ifdef _KERNEL
        /*
         * Annotate this process with a flag that indicates that it is
         * unsafe to use KM_SLEEP during memory allocations due to the
         * potential for a deadlock.  KM_PUSHPAGE should be used instead.
         */
        current->flags |= PF_NOFS;
#endif /* _KERNEL */

        txg_thread_enter(tx, &cpr);

        start = delta = 0;
        for (;;) {
                hrtime_t hrstart;
                txg_history_t *th;
                uint64_t timer, timeout;
                uint64_t txg;

                timeout = zfs_txg_timeout * hz;

                /*
                 * We sync when we're scanning, there's someone waiting
                 * on us, or the quiesce thread has handed off a txg to
                 * us, or we have reached our timeout.
                 */
                timer = (delta >= timeout ? 0 : timeout - delta);
                while (!dsl_scan_active(dp->dp_scan) &&
                    !tx->tx_exiting && timer > 0 &&
                    tx->tx_synced_txg >= tx->tx_sync_txg_waiting &&
                    tx->tx_quiesced_txg == 0) {
                        dprintf("waiting; tx_synced=%llu waiting=%llu dp=%p\n",
                            tx->tx_synced_txg, tx->tx_sync_txg_waiting, dp);
                        txg_thread_wait(tx, &cpr, &tx->tx_sync_more_cv, timer);
                        delta = ddi_get_lbolt() - start;
                        timer = (delta > timeout ? 0 : timeout - delta);
                }

                /*
                 * Wait until the quiesce thread hands off a txg to us,
                 * prompting it to do so if necessary.
                 */
                while (!tx->tx_exiting && tx->tx_quiesced_txg == 0) {
                        if (tx->tx_quiesce_txg_waiting < tx->tx_open_txg+1)
                                tx->tx_quiesce_txg_waiting = tx->tx_open_txg+1;
                        cv_broadcast(&tx->tx_quiesce_more_cv);
                        txg_thread_wait(tx, &cpr, &tx->tx_quiesce_done_cv, 0);
                }

                if (tx->tx_exiting)
                        txg_thread_exit(tx, &cpr, &tx->tx_sync_thread);

                /*
                 * Consume the quiesced txg which has been handed off to
                 * us.  This may cause the quiescing thread to now be
                 * able to quiesce another txg, so we must signal it.
                 */
                txg = tx->tx_quiesced_txg;
                tx->tx_quiesced_txg = 0;
                tx->tx_syncing_txg = txg;
                cv_broadcast(&tx->tx_quiesce_more_cv);

                th = dsl_pool_txg_history_get(dp, txg);
                th->th_kstat.state = TXG_STATE_SYNCING;
                vdev_get_stats(spa->spa_root_vdev, &th->th_vs1);
                dsl_pool_txg_history_put(th);

                dprintf("txg=%llu quiesce_txg=%llu sync_txg=%llu\n",
                    txg, tx->tx_quiesce_txg_waiting, tx->tx_sync_txg_waiting);
                mutex_exit(&tx->tx_sync_lock);

                start = ddi_get_lbolt();
                hrstart = gethrtime();
                spa_sync(spa, txg);
                delta = ddi_get_lbolt() - start;

                mutex_enter(&tx->tx_sync_lock);
                tx->tx_synced_txg = txg;
                tx->tx_syncing_txg = 0;
                cv_broadcast(&tx->tx_sync_done_cv);

                /*
                 * Dispatch commit callbacks to worker threads.
                 */
                txg_dispatch_callbacks(dp, txg);

                /*
                 * Measure the txg sync time determine the amount of I/O done.
                 */
                th = dsl_pool_txg_history_get(dp, txg);
                vdev_get_stats(spa->spa_root_vdev, &th->th_vs2);
                th->th_kstat.sync_time = gethrtime() - hrstart;
                th->th_kstat.nread = th->th_vs2.vs_bytes[ZIO_TYPE_READ] -
                    th->th_vs1.vs_bytes[ZIO_TYPE_READ];
                th->th_kstat.nwritten = th->th_vs2.vs_bytes[ZIO_TYPE_WRITE] -
                    th->th_vs1.vs_bytes[ZIO_TYPE_WRITE];
                th->th_kstat.reads = th->th_vs2.vs_ops[ZIO_TYPE_READ] -
                    th->th_vs1.vs_ops[ZIO_TYPE_READ];
                th->th_kstat.writes = th->th_vs2.vs_ops[ZIO_TYPE_WRITE] -
                    th->th_vs1.vs_ops[ZIO_TYPE_WRITE];
                th->th_kstat.state = TXG_STATE_COMMITTED;
                dsl_pool_txg_history_put(th);
        }
}

static void
txg_quiesce_thread(dsl_pool_t *dp)
{
        tx_state_t *tx = &dp->dp_tx;
        callb_cpr_t cpr;

        txg_thread_enter(tx, &cpr);

        for (;;) {
                uint64_t txg;

                /*
                 * We quiesce when there's someone waiting on us.
                 * However, we can only have one txg in "quiescing" or
                 * "quiesced, waiting to sync" state.  So we wait until
                 * the "quiesced, waiting to sync" txg has been consumed
                 * by the sync thread.
                 */
                while (!tx->tx_exiting &&
                    (tx->tx_open_txg >= tx->tx_quiesce_txg_waiting ||
                    tx->tx_quiesced_txg != 0))
                        txg_thread_wait(tx, &cpr, &tx->tx_quiesce_more_cv, 0);

                if (tx->tx_exiting)
                        txg_thread_exit(tx, &cpr, &tx->tx_quiesce_thread);

                txg = tx->tx_open_txg;
                dprintf("txg=%llu quiesce_txg=%llu sync_txg=%llu\n",
                    txg, tx->tx_quiesce_txg_waiting,
                    tx->tx_sync_txg_waiting);
                mutex_exit(&tx->tx_sync_lock);
                txg_quiesce(dp, txg);
                mutex_enter(&tx->tx_sync_lock);

                /*
                 * Hand this txg off to the sync thread.
                 */
                dprintf("quiesce done, handing off txg %llu\n", txg);
                tx->tx_quiesced_txg = txg;
                cv_broadcast(&tx->tx_sync_more_cv);
                cv_broadcast(&tx->tx_quiesce_done_cv);
        }
}

/*
 * Delay this thread by 'ticks' if we are still in the open transaction
 * group and there is already a waiting txg quiesing or quiesced.  Abort
 * the delay if this txg stalls or enters the quiesing state.
 */
void
txg_delay(dsl_pool_t *dp, uint64_t txg, int ticks)
{
        tx_state_t *tx = &dp->dp_tx;
        clock_t timeout = ddi_get_lbolt() + ticks;

        /* don't delay if this txg could transition to quiesing immediately */
        if (tx->tx_open_txg > txg ||
            tx->tx_syncing_txg == txg-1 || tx->tx_synced_txg == txg-1)
                return;

        mutex_enter(&tx->tx_sync_lock);
        if (tx->tx_open_txg > txg || tx->tx_synced_txg == txg-1) {
                mutex_exit(&tx->tx_sync_lock);
                return;
        }

        while (ddi_get_lbolt() < timeout &&
            tx->tx_syncing_txg < txg-1 && !txg_stalled(dp))
                (void) cv_timedwait(&tx->tx_quiesce_more_cv, &tx->tx_sync_lock,
                    timeout);

        DMU_TX_STAT_BUMP(dmu_tx_delay);

        mutex_exit(&tx->tx_sync_lock);
}

void
txg_wait_synced(dsl_pool_t *dp, uint64_t txg)
{
        tx_state_t *tx = &dp->dp_tx;

        mutex_enter(&tx->tx_sync_lock);
        ASSERT(tx->tx_threads == 2);
        if (txg == 0)
                txg = tx->tx_open_txg + TXG_DEFER_SIZE;
        if (tx->tx_sync_txg_waiting < txg)
                tx->tx_sync_txg_waiting = txg;
        dprintf("txg=%llu quiesce_txg=%llu sync_txg=%llu\n",
            txg, tx->tx_quiesce_txg_waiting, tx->tx_sync_txg_waiting);
        while (tx->tx_synced_txg < txg) {
                dprintf("broadcasting sync more "
                    "tx_synced=%llu waiting=%llu dp=%p\n",
                    tx->tx_synced_txg, tx->tx_sync_txg_waiting, dp);
                cv_broadcast(&tx->tx_sync_more_cv);
                cv_wait(&tx->tx_sync_done_cv, &tx->tx_sync_lock);
        }
        mutex_exit(&tx->tx_sync_lock);
}

void
txg_wait_open(dsl_pool_t *dp, uint64_t txg)
{
        tx_state_t *tx = &dp->dp_tx;

        mutex_enter(&tx->tx_sync_lock);
        ASSERT(tx->tx_threads == 2);
        if (txg == 0)
                txg = tx->tx_open_txg + 1;
        if (tx->tx_quiesce_txg_waiting < txg)
                tx->tx_quiesce_txg_waiting = txg;
        dprintf("txg=%llu quiesce_txg=%llu sync_txg=%llu\n",
            txg, tx->tx_quiesce_txg_waiting, tx->tx_sync_txg_waiting);
        while (tx->tx_open_txg < txg) {
                cv_broadcast(&tx->tx_quiesce_more_cv);
                cv_wait(&tx->tx_quiesce_done_cv, &tx->tx_sync_lock);
        }
        mutex_exit(&tx->tx_sync_lock);
}

boolean_t
txg_stalled(dsl_pool_t *dp)
{
        tx_state_t *tx = &dp->dp_tx;
        return (tx->tx_quiesce_txg_waiting > tx->tx_open_txg);
}

boolean_t
txg_sync_waiting(dsl_pool_t *dp)
{
        tx_state_t *tx = &dp->dp_tx;

        return (tx->tx_syncing_txg <= tx->tx_sync_txg_waiting ||
            tx->tx_quiesced_txg != 0);
}

/*
 * Per-txg object lists.
 */
void
txg_list_create(txg_list_t *tl, size_t offset)
{
        int t;

        mutex_init(&tl->tl_lock, NULL, MUTEX_DEFAULT, NULL);

        tl->tl_offset = offset;

        for (t = 0; t < TXG_SIZE; t++)
                tl->tl_head[t] = NULL;
}

void
txg_list_destroy(txg_list_t *tl)
{
        int t;

        for (t = 0; t < TXG_SIZE; t++)
                ASSERT(txg_list_empty(tl, t));

        mutex_destroy(&tl->tl_lock);
}

boolean_t
txg_list_empty(txg_list_t *tl, uint64_t txg)
{
        return (tl->tl_head[txg & TXG_MASK] == NULL);
}

/*
 * Add an entry to the list.
 * Returns 0 if it's a new entry, 1 if it's already there.
 */
int
txg_list_add(txg_list_t *tl, void *p, uint64_t txg)
{
        int t = txg & TXG_MASK;
        txg_node_t *tn = (txg_node_t *)((char *)p + tl->tl_offset);
        int already_on_list;

        mutex_enter(&tl->tl_lock);
        already_on_list = tn->tn_member[t];
        if (!already_on_list) {
                tn->tn_member[t] = 1;
                tn->tn_next[t] = tl->tl_head[t];
                tl->tl_head[t] = tn;
        }
        mutex_exit(&tl->tl_lock);

        return (already_on_list);
}

/*
 * Add an entry to the end of the list (walks list to find end).
 * Returns 0 if it's a new entry, 1 if it's already there.
 */
int
txg_list_add_tail(txg_list_t *tl, void *p, uint64_t txg)
{
        int t = txg & TXG_MASK;
        txg_node_t *tn = (txg_node_t *)((char *)p + tl->tl_offset);
        int already_on_list;

        mutex_enter(&tl->tl_lock);
        already_on_list = tn->tn_member[t];
        if (!already_on_list) {
                txg_node_t **tp;

                for (tp = &tl->tl_head[t]; *tp != NULL; tp = &(*tp)->tn_next[t])
                        continue;

                tn->tn_member[t] = 1;
                tn->tn_next[t] = NULL;
                *tp = tn;
        }
        mutex_exit(&tl->tl_lock);

        return (already_on_list);
}

/*
 * Remove the head of the list and return it.
 */
void *
txg_list_remove(txg_list_t *tl, uint64_t txg)
{
        int t = txg & TXG_MASK;
        txg_node_t *tn;
        void *p = NULL;

        mutex_enter(&tl->tl_lock);
        if ((tn = tl->tl_head[t]) != NULL) {
                p = (char *)tn - tl->tl_offset;
                tl->tl_head[t] = tn->tn_next[t];
                tn->tn_next[t] = NULL;
                tn->tn_member[t] = 0;
        }
        mutex_exit(&tl->tl_lock);

        return (p);
}

/*
 * Remove a specific item from the list and return it.
 */
void *
txg_list_remove_this(txg_list_t *tl, void *p, uint64_t txg)
{
        int t = txg & TXG_MASK;
        txg_node_t *tn, **tp;

        mutex_enter(&tl->tl_lock);

        for (tp = &tl->tl_head[t]; (tn = *tp) != NULL; tp = &tn->tn_next[t]) {
                if ((char *)tn - tl->tl_offset == p) {
                        *tp = tn->tn_next[t];
                        tn->tn_next[t] = NULL;
                        tn->tn_member[t] = 0;
                        mutex_exit(&tl->tl_lock);
                        return (p);
                }
        }

        mutex_exit(&tl->tl_lock);

        return (NULL);
}

int
txg_list_member(txg_list_t *tl, void *p, uint64_t txg)
{
        int t = txg & TXG_MASK;
        txg_node_t *tn = (txg_node_t *)((char *)p + tl->tl_offset);

        return (tn->tn_member[t]);
}

/*
 * Walk a txg list -- only safe if you know it's not changing.
 */
void *
txg_list_head(txg_list_t *tl, uint64_t txg)
{
        int t = txg & TXG_MASK;
        txg_node_t *tn = tl->tl_head[t];

        return (tn == NULL ? NULL : (char *)tn - tl->tl_offset);
}

void *
txg_list_next(txg_list_t *tl, void *p, uint64_t txg)
{
        int t = txg & TXG_MASK;
        txg_node_t *tn = (txg_node_t *)((char *)p + tl->tl_offset);

        tn = tn->tn_next[t];

        return (tn == NULL ? NULL : (char *)tn - tl->tl_offset);
}

#if defined(_KERNEL) && defined(HAVE_SPL)
EXPORT_SYMBOL(txg_init);
EXPORT_SYMBOL(txg_fini);
EXPORT_SYMBOL(txg_sync_start);
EXPORT_SYMBOL(txg_sync_stop);
EXPORT_SYMBOL(txg_hold_open);
EXPORT_SYMBOL(txg_rele_to_quiesce);
EXPORT_SYMBOL(txg_rele_to_sync);
EXPORT_SYMBOL(txg_register_callbacks);
EXPORT_SYMBOL(txg_delay);
EXPORT_SYMBOL(txg_wait_synced);
EXPORT_SYMBOL(txg_wait_open);
EXPORT_SYMBOL(txg_wait_callbacks);
EXPORT_SYMBOL(txg_stalled);
EXPORT_SYMBOL(txg_sync_waiting);

module_param(zfs_txg_timeout, int, 0644);
MODULE_PARM_DESC(zfs_txg_timeout, "Max seconds worth of delta per txg");
#endif