MFC 248571,248976,249004,249042,249188,249195-249196,249206,249207,249319,

[FreeBSD/stable/9.git] / sys / cddl / contrib / opensolaris / uts / common / fs / 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 <mm@FreeBSD.org>
 * Copyright (c) 2013 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>

/*
 * ZFS Transaction Groups
 * ----------------------
 *
 * ZFS transaction groups are, as the name implies, groups of transactions
 * that act on persistent state. ZFS asserts consistency at the granularity of
 * these transaction groups. Each successive transaction group (txg) is
 * assigned a 64-bit consecutive identifier. There are three active
 * transaction group states: open, quiescing, or syncing. At any given time,
 * there may be an active txg associated with each state; each active txg may
 * either be processing, or blocked waiting to enter the next state. There may
 * be up to three active txgs, and there is always a txg in the open state
 * (though it may be blocked waiting to enter the quiescing state). In broad
 * strokes, transactions — operations that change in-memory structures — are
 * accepted into the txg in the open state, and are completed while the txg is
 * in the open or quiescing states. The accumulated changes are written to
 * disk in the syncing state.
 *
 * Open
 *
 * When a new txg becomes active, it first enters the open state. New
 * transactions — updates to in-memory structures — are assigned to the
 * currently open txg. There is always a txg in the open state so that ZFS can
 * accept new changes (though the txg may refuse new changes if it has hit
 * some limit). ZFS advances the open txg to the next state for a variety of
 * reasons such as it hitting a time or size threshold, or the execution of an
 * administrative action that must be completed in the syncing state.
 *
 * Quiescing
 *
 * After a txg exits the open state, it enters the quiescing state. The
 * quiescing state is intended to provide a buffer between accepting new
 * transactions in the open state and writing them out to stable storage in
 * the syncing state. While quiescing, transactions can continue their
 * operation without delaying either of the other states. Typically, a txg is
 * in the quiescing state very briefly since the operations are bounded by
 * software latencies rather than, say, slower I/O latencies. After all
 * transactions complete, the txg is ready to enter the next state.
 *
 * Syncing
 *
 * In the syncing state, the in-memory state built up during the open and (to
 * a lesser degree) the quiescing states is written to stable storage. The
 * process of writing out modified data can, in turn modify more data. For
 * example when we write new blocks, we need to allocate space for them; those
 * allocations modify metadata (space maps)... which themselves must be
 * written to stable storage. During the sync state, ZFS iterates, writing out
 * data until it converges and all in-memory changes have been written out.
 * The first such pass is the largest as it encompasses all the modified user
 * data (as opposed to filesystem metadata). Subsequent passes typically have
 * far less data to write as they consist exclusively of filesystem metadata.
 *
 * To ensure convergence, after a certain number of passes ZFS begins
 * overwriting locations on stable storage that had been allocated earlier in
 * the syncing state (and subsequently freed). ZFS usually allocates new
 * blocks to optimize for large, continuous, writes. For the syncing state to
 * converge however it must complete a pass where no new blocks are allocated
 * since each allocation requires a modification of persistent metadata.
 * Further, to hasten convergence, after a prescribed number of passes, ZFS
 * also defers frees, and stops compressing.
 *
 * In addition to writing out user data, we must also execute synctasks during
 * the syncing context. A synctask is the mechanism by which some
 * administrative activities work such as creating and destroying snapshots or
 * datasets. Note that when a synctask is initiated it enters the open txg,
 * and ZFS then pushes that txg as quickly as possible to completion of the
 * syncing state in order to reduce the latency of the administrative
 * activity. To complete the syncing state, ZFS writes out a new uberblock,
 * the root of the tree of blocks that comprise all state stored on the ZFS
 * pool. Finally, if there is a quiesced txg waiting, we signal that it can
 * now transition to the syncing state.
 */

static void txg_sync_thread(void *arg);
static void txg_quiesce_thread(void *arg);

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

SYSCTL_DECL(_vfs_zfs);
SYSCTL_NODE(_vfs_zfs, OID_AUTO, txg, CTLFLAG_RW, 0, "ZFS TXG");
TUNABLE_INT("vfs.zfs.txg.timeout", &zfs_txg_timeout);
SYSCTL_INT(_vfs_zfs_txg, OID_AUTO, timeout, CTLFLAG_RW, &zfs_txg_timeout, 0,
    "Maximum 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 = kmem_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);

        kmem_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(cv, &tx->tx_sync_lock, time);
        else
                cv_wait(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 = &tx->tx_cpu[CPU_SEQID];
        uint64_t txg;

        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)
{
        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++;

        /*
         * 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().
         */
        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);
        }
}

static void
txg_do_callbacks(void *arg)
{
        list_t *cb_list = arg;

        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",
                            max_ncpus, minclsyspri, max_ncpus, max_ncpus * 2,
                            TASKQ_PREPOPULATE);
                }

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

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

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

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

        txg_thread_enter(tx, &cpr);

        start = delta = 0;
        for (;;) {
                uint64_t timer, timeout = zfs_txg_timeout * hz;
                uint64_t txg;

                /*
                 * 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);

                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();
                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);
        }
}

static void
txg_quiesce_thread(void *arg)
{
        dsl_pool_t *dp = arg;
        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 - ddi_get_lbolt());

        mutex_exit(&tx->tx_sync_lock);
}

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

        ASSERT(!dsl_pool_config_held(dp));

        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;

        ASSERT(!dsl_pool_config_held(dp));

        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 (unless it's already on the list).
 * Returns B_TRUE if it was actually added.
 */
boolean_t
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);
        boolean_t add;

        mutex_enter(&tl->tl_lock);
        add = (tn->tn_member[t] == 0);
        if (add) {
                tn->tn_member[t] = 1;
                tn->tn_next[t] = tl->tl_head[t];
                tl->tl_head[t] = tn;
        }
        mutex_exit(&tl->tl_lock);

        return (add);
}

/*
 * Add an entry to the end of the list, unless it's already on the list.
 * (walks list to find end)
 * Returns B_TRUE if it was actually added.
 */
boolean_t
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);
        boolean_t add;

        mutex_enter(&tl->tl_lock);
        add = (tn->tn_member[t] == 0);
        if (add) {
                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 (add);
}

/*
 * 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);
}

boolean_t
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] != 0);
}

/*
 * 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);
}