kgdb(1): Add missing "

[dragonfly.git] / sys / kern / vfs_sync.c

/*
 * Copyright (c) 1989, 1993
 *      The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * 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 University 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 REGENTS 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 REGENTS 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.
 *
 *      @(#)vfs_subr.c  8.31 (Berkeley) 5/26/95
 * $FreeBSD: src/sys/kern/vfs_subr.c,v 1.249.2.30 2003/04/04 20:35:57 tegge Exp $
 */

/*
 * External virtual filesystem routines
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/buf.h>
#include <sys/conf.h>
#include <sys/dirent.h>
#include <sys/domain.h>
#include <sys/eventhandler.h>
#include <sys/fcntl.h>
#include <sys/kernel.h>
#include <sys/kthread.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/mount.h>
#include <sys/proc.h>
#include <sys/namei.h>
#include <sys/reboot.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/sysctl.h>
#include <sys/syslog.h>
#include <sys/vmmeter.h>
#include <sys/vnode.h>

#include <machine/limits.h>

#include <vm/vm.h>
#include <vm/vm_object.h>
#include <vm/vm_extern.h>
#include <vm/vm_kern.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_page.h>
#include <vm/vm_pager.h>
#include <vm/vnode_pager.h>

#include <sys/buf2.h>
#include <sys/thread2.h>

/*
 * The workitem queue.
 */
#define SYNCER_MAXDELAY         32
static int sysctl_kern_syncdelay(SYSCTL_HANDLER_ARGS);
time_t syncdelay = 30;          /* max time to delay syncing data */
SYSCTL_PROC(_kern, OID_AUTO, syncdelay, CTLTYPE_INT | CTLFLAG_RW, 0, 0,
                sysctl_kern_syncdelay, "I", "VFS data synchronization delay");
time_t filedelay = 30;          /* time to delay syncing files */
SYSCTL_INT(_kern, OID_AUTO, filedelay, CTLFLAG_RW,
                &filedelay, 0, "File synchronization delay");
time_t dirdelay = 29;           /* time to delay syncing directories */
SYSCTL_INT(_kern, OID_AUTO, dirdelay, CTLFLAG_RW,
                &dirdelay, 0, "Directory synchronization delay");
time_t metadelay = 28;          /* time to delay syncing metadata */
SYSCTL_INT(_kern, OID_AUTO, metadelay, CTLFLAG_RW,
                &metadelay, 0, "VFS metadata synchronization delay");
time_t retrydelay = 1;          /* retry delay after failure */
SYSCTL_INT(_kern, OID_AUTO, retrydelay, CTLFLAG_RW,
                &retrydelay, 0, "VFS retry synchronization delay");
static int rushjob;                     /* number of slots to run ASAP */
static int stat_rush_requests;  /* number of times I/O speeded up */
SYSCTL_INT(_debug, OID_AUTO, rush_requests, CTLFLAG_RW,
                &stat_rush_requests, 0, "");

LIST_HEAD(synclist, vnode);

#define SC_FLAG_EXIT            (0x1)           /* request syncer exit */
#define SC_FLAG_DONE            (0x2)           /* syncer confirm exit */

struct syncer_ctx {
        struct mount            *sc_mp;
        struct lwkt_token       sc_token;
        struct thread           *sc_thread;
        int                     sc_flags;
        struct synclist         *syncer_workitem_pending;
        long                    syncer_mask;
        int                     syncer_delayno;
        int                     syncer_forced;
        int                     syncer_rushjob;
        int                     syncer_unused01;
        long                    syncer_count;
};

static void syncer_thread(void *);

static int
sysctl_kern_syncdelay(SYSCTL_HANDLER_ARGS)
{
        int error;
        int v = syncdelay;

        error = sysctl_handle_int(oidp, &v, 0, req);
        if (error || !req->newptr)
                return (error);
        if (v < 1)
                v = 1;
        if (v > SYNCER_MAXDELAY)
                v = SYNCER_MAXDELAY;
        syncdelay = v;

        return(0);
}

/*
 * The workitem queue.
 * 
 * It is useful to delay writes of file data and filesystem metadata
 * for tens of seconds so that quickly created and deleted files need
 * not waste disk bandwidth being created and removed. To realize this,
 * we append vnodes to a "workitem" queue. When running with a soft
 * updates implementation, most pending metadata dependencies should
 * not wait for more than a few seconds. Thus, mounted on block devices
 * are delayed only about a half the time that file data is delayed.
 * Similarly, directory updates are more critical, so are only delayed
 * about a third the time that file data is delayed. Thus, there are
 * SYNCER_MAXDELAY queues that are processed round-robin at a rate of
 * one each second (driven off the filesystem syncer process). The
 * syncer_delayno variable indicates the next queue that is to be processed.
 * Items that need to be processed soon are placed in this queue:
 *
 *      syncer_workitem_pending[syncer_delayno]
 *
 * A delay of fifteen seconds is done by placing the request fifteen
 * entries later in the queue:
 *
 *      syncer_workitem_pending[(syncer_delayno + 15) & syncer_mask]
 *
 */

/*
 * Return the number of vnodes on the syncer's timed list.  This will
 * include the syncer vnode (mp->mnt_syncer) so if used, a minimum
 * value of 1 will be returned.
 */
long
vn_syncer_count(struct mount *mp)
{
        struct syncer_ctx *ctx;

        ctx = mp->mnt_syncer_ctx;
        if (ctx)
                return (ctx->syncer_count);
        return 0;
}

/*
 * Add an item to the syncer work queue.
 *
 * WARNING: Cannot get vp->v_token here if not already held, we must
 *          depend on the syncer_token (which might already be held by
 *          the caller) to protect v_synclist and VONWORKLST.
 *
 * MPSAFE
 */
void
vn_syncer_add(struct vnode *vp, int delay)
{
        struct syncer_ctx *ctx;
        int slot;

        ctx = vp->v_mount->mnt_syncer_ctx;
        lwkt_gettoken(&ctx->sc_token);

        if (vp->v_flag & VONWORKLST) {
                LIST_REMOVE(vp, v_synclist);
                --ctx->syncer_count;
        }
        if (delay <= 0) {
                slot = -delay & ctx->syncer_mask;
        } else {
                if (delay > SYNCER_MAXDELAY - 2)
                        delay = SYNCER_MAXDELAY - 2;
                slot = (ctx->syncer_delayno + delay) & ctx->syncer_mask;
        }

        LIST_INSERT_HEAD(&ctx->syncer_workitem_pending[slot], vp, v_synclist);
        vsetflags(vp, VONWORKLST);
        ++ctx->syncer_count;

        lwkt_reltoken(&ctx->sc_token);
}

/*
 * Removes the vnode from the syncer list.  Since we might block while
 * acquiring the syncer_token we have to [re]check conditions to determine
 * that it is ok to remove the vnode.
 *
 * Force removal if force != 0.  This can only occur during a forced unmount.
 *
 * vp->v_token held on call
 */
void
vn_syncer_remove(struct vnode *vp, int force)
{
        struct syncer_ctx *ctx;

        ctx = vp->v_mount->mnt_syncer_ctx;
        lwkt_gettoken(&ctx->sc_token);

        if ((vp->v_flag & (VISDIRTY | VONWORKLST | VOBJDIRTY)) == VONWORKLST &&
            RB_EMPTY(&vp->v_rbdirty_tree)) {
                vclrflags(vp, VONWORKLST);
                LIST_REMOVE(vp, v_synclist);
                --ctx->syncer_count;
        } else if (force && (vp->v_flag & VONWORKLST)) {
                vclrflags(vp, VONWORKLST);
                LIST_REMOVE(vp, v_synclist);
                --ctx->syncer_count;
        }

        lwkt_reltoken(&ctx->sc_token);
}

/*
 * vnode must be locked
 */
void
vclrisdirty(struct vnode *vp)
{
        vclrflags(vp, VISDIRTY);
        if (vp->v_flag & VONWORKLST)
                vn_syncer_remove(vp, 0);
}

void
vclrobjdirty(struct vnode *vp)
{
        vclrflags(vp, VOBJDIRTY);
        if (vp->v_flag & VONWORKLST)
                vn_syncer_remove(vp, 0);
}

/*
 * vnode must be stable
 */
void
vsetisdirty(struct vnode *vp)
{
        struct syncer_ctx *ctx;

        if ((vp->v_flag & VISDIRTY) == 0) {
                ctx = vp->v_mount->mnt_syncer_ctx;
                vsetflags(vp, VISDIRTY);
                lwkt_gettoken(&ctx->sc_token);
                if ((vp->v_flag & VONWORKLST) == 0)
                        vn_syncer_add(vp, syncdelay);
                lwkt_reltoken(&ctx->sc_token);
        }
}

void
vsetobjdirty(struct vnode *vp)
{
        struct syncer_ctx *ctx;

        if ((vp->v_flag & VOBJDIRTY) == 0) {
                ctx = vp->v_mount->mnt_syncer_ctx;
                vsetflags(vp, VOBJDIRTY);
                lwkt_gettoken(&ctx->sc_token);
                if ((vp->v_flag & VONWORKLST) == 0)
                        vn_syncer_add(vp, syncdelay);
                lwkt_reltoken(&ctx->sc_token);
        }
}

/*
 * Create per-filesystem syncer process
 */
void
vn_syncer_thr_create(struct mount *mp)
{
        struct syncer_ctx *ctx;
        static int syncalloc = 0;

        ctx = kmalloc(sizeof(struct syncer_ctx), M_TEMP, M_WAITOK | M_ZERO);
        ctx->sc_mp = mp;
        ctx->sc_flags = 0;
        ctx->syncer_workitem_pending = hashinit(SYNCER_MAXDELAY, M_DEVBUF,
                                                &ctx->syncer_mask);
        ctx->syncer_delayno = 0;
        lwkt_token_init(&ctx->sc_token, "syncer");
        mp->mnt_syncer_ctx = ctx;
        kthread_create(syncer_thread, ctx, &ctx->sc_thread,
                       "syncer%d", ++syncalloc & 0x7FFFFFFF);
}

/*
 * Stop per-filesystem syncer process
 */
void
vn_syncer_thr_stop(struct mount *mp)
{
        struct syncer_ctx *ctx;

        ctx = mp->mnt_syncer_ctx;
        if (ctx == NULL)
                return;

        lwkt_gettoken(&ctx->sc_token);

        /* Signal the syncer process to exit */
        ctx->sc_flags |= SC_FLAG_EXIT;
        wakeup(ctx);
        
        /* Wait till syncer process exits */
        while ((ctx->sc_flags & SC_FLAG_DONE) == 0) 
                tsleep(&ctx->sc_flags, 0, "syncexit", hz);

        mp->mnt_syncer_ctx = NULL;
        lwkt_reltoken(&ctx->sc_token);

        hashdestroy(ctx->syncer_workitem_pending, M_DEVBUF, ctx->syncer_mask);
        kfree(ctx, M_TEMP);
}

struct  thread *updatethread;

/*
 * System filesystem synchronizer daemon.
 */
static void
syncer_thread(void *_ctx)
{
        struct syncer_ctx *ctx = _ctx;
        struct synclist *slp;
        struct vnode *vp;
        long starttime;
        int *sc_flagsp;
        int sc_flags;
        int vnodes_synced = 0;
        int delta;
        int dummy = 0;

        for (;;) {
                kproc_suspend_loop();

                starttime = time_uptime;
                lwkt_gettoken(&ctx->sc_token);

                /*
                 * Push files whose dirty time has expired.  Be careful
                 * of interrupt race on slp queue.
                 *
                 * Note that vsyncscan() and vn_syncer_one() can pull items
                 * off the same list, so we shift vp's position in the
                 * list immediately.
                 */
                slp = &ctx->syncer_workitem_pending[ctx->syncer_delayno];

                while ((vp = LIST_FIRST(slp)) != NULL) {
                        vn_syncer_add(vp, retrydelay);
                        if (ctx->syncer_forced) {
                                if (vget(vp, LK_EXCLUSIVE) == 0) {
                                        VOP_FSYNC(vp, MNT_NOWAIT, 0);
                                        vput(vp);
                                        vnodes_synced++;
                                }
                        } else {
                                if (vget(vp, LK_EXCLUSIVE | LK_NOWAIT) == 0) {
                                        VOP_FSYNC(vp, MNT_LAZY, 0);
                                        vput(vp);
                                        vnodes_synced++;
                                }
                        }
                }

                /*
                 * Increment the slot upon completion.
                 */
                ctx->syncer_delayno = (ctx->syncer_delayno + 1) &
                                      ctx->syncer_mask;

                sc_flags = ctx->sc_flags;

                /* Exit on unmount */
                if (sc_flags & SC_FLAG_EXIT)
                        break;

                lwkt_reltoken(&ctx->sc_token);

                /*
                 * Do sync processing for each mount.
                 */
                if (ctx->sc_mp)
                        bio_ops_sync(ctx->sc_mp);

                /*
                 * The variable rushjob allows the kernel to speed up the
                 * processing of the filesystem syncer process. A rushjob
                 * value of N tells the filesystem syncer to process the next
                 * N seconds worth of work on its queue ASAP. Currently rushjob
                 * is used by the soft update code to speed up the filesystem
                 * syncer process when the incore state is getting so far
                 * ahead of the disk that the kernel memory pool is being
                 * threatened with exhaustion.
                 */
                delta = rushjob - ctx->syncer_rushjob;
                if ((u_int)delta > syncdelay / 2) {
                        ctx->syncer_rushjob = rushjob - syncdelay / 2;
                        tsleep(&dummy, 0, "rush", 1);
                        continue;
                }
                if (delta) {
                        ++ctx->syncer_rushjob;
                        tsleep(&dummy, 0, "rush", 1);
                        continue;
                }

                /*
                 * If it has taken us less than a second to process the
                 * current work, then wait. Otherwise start right over
                 * again. We can still lose time if any single round
                 * takes more than two seconds, but it does not really
                 * matter as we are just trying to generally pace the
                 * filesystem activity.
                 */
                if (time_uptime == starttime)
                        tsleep(ctx, 0, "syncer", hz);
        }

        /*
         * Unmount/exit path for per-filesystem syncers; sc_token held
         */
        ctx->sc_flags |= SC_FLAG_DONE;
        sc_flagsp = &ctx->sc_flags;
        lwkt_reltoken(&ctx->sc_token);
        wakeup(sc_flagsp);

        kthread_exit();
}

/*
 * This allows a filesystem to pro-actively request that a dirty
 * vnode be fsync()d.  This routine does not guarantee that one
 * will actually be fsynced.
 */
void
vn_syncer_one(struct mount *mp)
{
        struct syncer_ctx *ctx;
        struct synclist *slp;
        struct vnode *vp;
        int i;
        int n = syncdelay;

        ctx = mp->mnt_syncer_ctx;
        i = ctx->syncer_delayno & ctx->syncer_mask;
        cpu_ccfence();

        if (lwkt_trytoken(&ctx->sc_token) == 0)
                return;

        /*
         * Look ahead on our syncer time array.
         */
        do {
                slp = &ctx->syncer_workitem_pending[i];
                vp = LIST_FIRST(slp);
                if (vp && vp->v_type == VNON)
                        vp = LIST_NEXT(vp, v_synclist);
                if (vp)
                        break;
                i = (i + 1) & ctx->syncer_mask;
                /* i will be wrong if we stop here but vp is NULL so ok */
        } while(--n);

        /*
         * Process one vnode, skip the syncer vnode but also stop
         * if the syncer vnode is the only thing on this list.
         */
        if (vp) {
                vn_syncer_add(vp, retrydelay);
                if (vget(vp, LK_EXCLUSIVE | LK_NOWAIT) == 0) {
                        VOP_FSYNC(vp, MNT_LAZY, 0);
                        vput(vp);
                }
        }
        lwkt_reltoken(&ctx->sc_token);
}

/*
 * Request that the syncer daemon for a specific mount speed up its work.
 * If mp is NULL the caller generally wants to speed up all syncers.
 */
void
speedup_syncer(struct mount *mp)
{
        /*
         * Don't bother protecting the test.  unsleep_and_wakeup_thread()
         * will only do something real if the thread is in the right state.
         */
        atomic_add_int(&rushjob, 1);
        ++stat_rush_requests;
        if (mp)
                wakeup(mp->mnt_syncer_ctx);
}

/*
 * Routine to create and manage a filesystem syncer vnode.
 */
static int sync_close(struct vop_close_args *);
static int sync_fsync(struct vop_fsync_args *);
static int sync_inactive(struct vop_inactive_args *);
static int sync_reclaim (struct vop_reclaim_args *);
static int sync_print(struct vop_print_args *);

static struct vop_ops sync_vnode_vops = {
        .vop_default =  vop_eopnotsupp,
        .vop_close =    sync_close,
        .vop_fsync =    sync_fsync,
        .vop_inactive = sync_inactive,
        .vop_reclaim =  sync_reclaim,
        .vop_print =    sync_print,
};

static struct vop_ops *sync_vnode_vops_p = &sync_vnode_vops;

VNODEOP_SET(sync_vnode_vops);

/*
 * Create a new filesystem syncer vnode for the specified mount point.
 * This vnode is placed on the worklist and is responsible for sync'ing
 * the filesystem.
 *
 * NOTE: read-only mounts are also placed on the worklist.  The filesystem
 * sync code is also responsible for cleaning up vnodes.
 */
int
vfs_allocate_syncvnode(struct mount *mp)
{
        struct vnode *vp;
        static long start, incr, next;
        int error;

        /* Allocate a new vnode */
        error = getspecialvnode(VT_VFS, mp, &sync_vnode_vops_p, &vp, 0, 0);
        if (error) {
                mp->mnt_syncer = NULL;
                return (error);
        }
        vp->v_type = VNON;
        /*
         * Place the vnode onto the syncer worklist. We attempt to
         * scatter them about on the list so that they will go off
         * at evenly distributed times even if all the filesystems
         * are mounted at once.
         */
        next += incr;
        if (next == 0 || next > SYNCER_MAXDELAY) {
                start /= 2;
                incr /= 2;
                if (start == 0) {
                        start = SYNCER_MAXDELAY / 2;
                        incr = SYNCER_MAXDELAY;
                }
                next = start;
        }

        /*
         * Only put the syncer vnode onto the syncer list if we have a
         * syncer thread.  Some VFS's (aka NULLFS) don't need a syncer
         * thread.
         */
        if (mp->mnt_syncer_ctx)
                vn_syncer_add(vp, syncdelay > 0 ? next % syncdelay : 0);

        /*
         * The mnt_syncer field inherits the vnode reference, which is
         * held until later decomissioning.
         */
        mp->mnt_syncer = vp;
        vx_unlock(vp);
        return (0);
}

static int
sync_close(struct vop_close_args *ap)
{
        return (0);
}

/*
 * Do a lazy sync of the filesystem.
 *
 * sync_fsync { struct vnode *a_vp, int a_waitfor }
 */
static int
sync_fsync(struct vop_fsync_args *ap)
{
        struct vnode *syncvp = ap->a_vp;
        struct mount *mp = syncvp->v_mount;
        int asyncflag;

        /*
         * We only need to do something if this is a lazy evaluation.
         */
        if ((ap->a_waitfor & MNT_LAZY) == 0)
                return (0);

        /*
         * Move ourselves to the back of the sync list.
         */
        vn_syncer_add(syncvp, syncdelay);

        /*
         * Walk the list of vnodes pushing all that are dirty and
         * not already on the sync list, and freeing vnodes which have
         * no refs and whos VM objects are empty.  vfs_msync() handles
         * the VM issues and must be called whether the mount is readonly
         * or not.
         */
        if (vfs_busy(mp, LK_NOWAIT) != 0)
                return (0);
        if (mp->mnt_flag & MNT_RDONLY) {
                vfs_msync(mp, MNT_NOWAIT);
        } else {
                asyncflag = mp->mnt_flag & MNT_ASYNC;
                mp->mnt_flag &= ~MNT_ASYNC;     /* ZZZ hack */
                vfs_msync(mp, MNT_NOWAIT);
                VFS_SYNC(mp, MNT_NOWAIT | MNT_LAZY);
                if (asyncflag)
                        mp->mnt_flag |= MNT_ASYNC;
        }
        vfs_unbusy(mp);
        return (0);
}

/*
 * The syncer vnode is no longer referenced.
 *
 * sync_inactive { struct vnode *a_vp, struct proc *a_p }
 */
static int
sync_inactive(struct vop_inactive_args *ap)
{
        vgone_vxlocked(ap->a_vp);
        return (0);
}

/*
 * The syncer vnode is no longer needed and is being decommissioned.
 * This can only occur when the last reference has been released on
 * mp->mnt_syncer, so mp->mnt_syncer had better be NULL.
 *
 * Modifications to the worklist must be protected with a critical
 * section.
 *
 *      sync_reclaim { struct vnode *a_vp }
 */
static int
sync_reclaim(struct vop_reclaim_args *ap)
{
        struct vnode *vp = ap->a_vp;
        struct syncer_ctx *ctx;

        ctx = vp->v_mount->mnt_syncer_ctx;
        if (ctx) {
                lwkt_gettoken(&ctx->sc_token);
                KKASSERT(vp->v_mount->mnt_syncer != vp);
                if (vp->v_flag & VONWORKLST) {
                        LIST_REMOVE(vp, v_synclist);
                        vclrflags(vp, VONWORKLST);
                        --ctx->syncer_count;
                }
                lwkt_reltoken(&ctx->sc_token);
        } else {
                KKASSERT((vp->v_flag & VONWORKLST) == 0);
        }

        return (0);
}

/*
 * This is very similar to vmntvnodescan() but it only scans the
 * vnodes on the syncer list.  VFS's which support faster VFS_SYNC
 * operations use the VISDIRTY flag on the vnode to ensure that vnodes
 * with dirty inodes are added to the syncer in addition to vnodes
 * with dirty buffers, and can use this function instead of nmntvnodescan().
 *
 * This scan does not issue VOP_FSYNC()s.  The supplied callback is intended
 * to synchronize the file in the manner intended by the VFS using it.
 * 
 * This is important when a system has millions of vnodes.
 */
int
vsyncscan(
    struct mount *mp,
    int vmsc_flags,
    int (*slowfunc)(struct mount *mp, struct vnode *vp, void *data),
    void *data
) {
        struct syncer_ctx *ctx;
        struct synclist *slp;
        struct vnode *vp;
        int i;
        int count;
        int lkflags;

        if (vmsc_flags & VMSC_NOWAIT)
                lkflags = LK_NOWAIT;
        else
                lkflags = 0;

        /*
         * Syncer list context.  This API requires a dedicated syncer thread.
         * (MNTK_THR_SYNC).
         */
        KKASSERT(mp->mnt_kern_flag & MNTK_THR_SYNC);
        ctx = mp->mnt_syncer_ctx;
        lwkt_gettoken(&ctx->sc_token);

        /*
         * Setup for loop.  Allow races against the syncer thread but
         * require that the syncer thread no be lazy if we were told
         * not to be lazy.
         */
        i = ctx->syncer_delayno & ctx->syncer_mask;
        if ((vmsc_flags & VMSC_NOWAIT) == 0)
                ++ctx->syncer_forced;
        for (count = 0; count <= ctx->syncer_mask; ++count) {
                slp = &ctx->syncer_workitem_pending[i];

                while ((vp = LIST_FIRST(slp)) != NULL) {
                        KKASSERT(vp->v_mount == mp);
                        if (vmsc_flags & VMSC_GETVP) {
                                if (vget(vp, LK_EXCLUSIVE | lkflags) == 0) {
                                        slowfunc(mp, vp, data);
                                        vput(vp);
                                }
                        } else if (vmsc_flags & VMSC_GETVX) {
                                vx_get(vp);
                                slowfunc(mp, vp, data);
                                vx_put(vp);
                        } else {
                                vhold(vp);
                                slowfunc(mp, vp, data);
                                vdrop(vp);
                        }

                        /*
                         * vp could be invalid.  However, if vp is still at
                         * the head of the list it is clearly valid and we
                         * can safely move it.
                         */
                        if (LIST_FIRST(slp) == vp)
                                vn_syncer_add(vp, -(i + syncdelay));
                }
                i = (i + 1) & ctx->syncer_mask;
        }

        if ((vmsc_flags & VMSC_NOWAIT) == 0)
                --ctx->syncer_forced;
        lwkt_reltoken(&ctx->sc_token);
        return(0);
}

/*
 * Print out a syncer vnode.
 *
 *      sync_print { struct vnode *a_vp }
 */
static int
sync_print(struct vop_print_args *ap)
{
        struct vnode *vp = ap->a_vp;

        kprintf("syncer vnode");
        lockmgr_printinfo(&vp->v_lock);
        kprintf("\n");
        return (0);
}