kernel - Attempt to fix cluster pbuf deadlock on recursive filesystems

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

/*
 * Copyright (c) 2004,2013 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.
 */

/*
 * External lock/ref-related vnode functions
 *
 * vs_state transition locking requirements:
 *
 *      INACTIVE -> CACHED|DYING        vx_lock(excl) + vfs_spin
 *      DYING    -> CACHED              vx_lock(excl)
 *      ACTIVE   -> INACTIVE            (none)       + v_spin + vfs_spin
 *      INACTIVE -> ACTIVE              vn_lock(any) + v_spin + vfs_spin
 *      CACHED   -> ACTIVE              vn_lock(any) + v_spin + vfs_spin
 *
 * NOTE: Switching to/from ACTIVE/INACTIVE requires v_spin and vfs_spin,
 *
 *       Switching into ACTIVE also requires a vref and vnode lock, however
 *       the vnode lock is allowed to be SHARED.
 *
 *       Switching into a CACHED or DYING state requires an exclusive vnode
 *       lock or vx_lock (which is almost the same thing).
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/malloc.h>
#include <sys/mount.h>
#include <sys/proc.h>
#include <sys/vnode.h>
#include <sys/buf.h>
#include <sys/sysctl.h>

#include <machine/limits.h>

#include <vm/vm.h>
#include <vm/vm_object.h>

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

#define VACT_MAX        10
#define VACT_INC        2

static void vnode_terminate(struct vnode *vp);

static MALLOC_DEFINE(M_VNODE, "vnodes", "vnode structures");

/*
 * The vnode free list hold inactive vnodes.  Aged inactive vnodes
 * are inserted prior to the mid point, and otherwise inserted
 * at the tail.
 */
TAILQ_HEAD(freelst, vnode);
static struct freelst   vnode_active_list;
static struct freelst   vnode_inactive_list;
static struct vnode     vnode_active_rover;
static struct spinlock  vfs_spin = SPINLOCK_INITIALIZER(vfs_spin, "vfs_spin");

int  activevnodes = 0;
SYSCTL_INT(_debug, OID_AUTO, activevnodes, CTLFLAG_RD,
        &activevnodes, 0, "Number of active nodes");
int  cachedvnodes = 0;
SYSCTL_INT(_debug, OID_AUTO, cachedvnodes, CTLFLAG_RD,
        &cachedvnodes, 0, "Number of total cached nodes");
int  inactivevnodes = 0;
SYSCTL_INT(_debug, OID_AUTO, inactivevnodes, CTLFLAG_RD,
        &inactivevnodes, 0, "Number of inactive nodes");
static int batchfreevnodes = 5;
SYSCTL_INT(_debug, OID_AUTO, batchfreevnodes, CTLFLAG_RW,
        &batchfreevnodes, 0, "Number of vnodes to free at once");
#ifdef TRACKVNODE
static u_long trackvnode;
SYSCTL_ULONG(_debug, OID_AUTO, trackvnode, CTLFLAG_RW,
                &trackvnode, 0, "");
#endif

/*
 * Called from vfsinit()
 */
void
vfs_lock_init(void)
{
        TAILQ_INIT(&vnode_inactive_list);
        TAILQ_INIT(&vnode_active_list);
        TAILQ_INSERT_TAIL(&vnode_active_list, &vnode_active_rover, v_list);
        spin_init(&vfs_spin, "vfslock");
        kmalloc_raise_limit(M_VNODE, 0);        /* unlimited */
}

/*
 * Misc functions
 */
static __inline
void
_vsetflags(struct vnode *vp, int flags)
{
        atomic_set_int(&vp->v_flag, flags);
}

static __inline
void
_vclrflags(struct vnode *vp, int flags)
{
        atomic_clear_int(&vp->v_flag, flags);
}

void
vsetflags(struct vnode *vp, int flags)
{
        _vsetflags(vp, flags);
}

void
vclrflags(struct vnode *vp, int flags)
{
        _vclrflags(vp, flags);
}

/*
 * Place the vnode on the active list.
 *
 * Caller must hold vp->v_spin
 */
static __inline 
void
_vactivate(struct vnode *vp)
{
#ifdef TRACKVNODE
        if ((u_long)vp == trackvnode)
                kprintf("_vactivate %p %08x\n", vp, vp->v_flag);
#endif
        spin_lock(&vfs_spin);

        switch(vp->v_state) {
        case VS_ACTIVE:
                panic("_vactivate: already active");
                /* NOT REACHED */
                spin_unlock(&vfs_spin);
                return;
        case VS_INACTIVE:
                TAILQ_REMOVE(&vnode_inactive_list, vp, v_list);
                --inactivevnodes;
                break;
        case VS_CACHED:
        case VS_DYING:
                break;
        }
        TAILQ_INSERT_TAIL(&vnode_active_list, vp, v_list);
        vp->v_state = VS_ACTIVE;
        ++activevnodes;

        spin_unlock(&vfs_spin);
}

/*
 * Put a vnode on the inactive list.
 *
 * Caller must hold v_spin
 */
static __inline
void
_vinactive(struct vnode *vp)
{
#ifdef TRACKVNODE
        if ((u_long)vp == trackvnode) {
                kprintf("_vinactive %p %08x\n", vp, vp->v_flag);
                print_backtrace(-1);
        }
#endif
        spin_lock(&vfs_spin);

        /*
         * Remove from active list if it is sitting on it
         */
        switch(vp->v_state) {
        case VS_ACTIVE:
                TAILQ_REMOVE(&vnode_active_list, vp, v_list);
                --activevnodes;
                break;
        case VS_INACTIVE:
                panic("_vinactive: already inactive");
                /* NOT REACHED */
                spin_unlock(&vfs_spin);
                return;
        case VS_CACHED:
        case VS_DYING:
                break;
        }

        /*
         * Distinguish between basically dead vnodes, vnodes with cached
         * data, and vnodes without cached data.  A rover will shift the
         * vnodes around as their cache status is lost.
         */
        if (vp->v_flag & VRECLAIMED) {
                TAILQ_INSERT_HEAD(&vnode_inactive_list, vp, v_list);
        } else {
                TAILQ_INSERT_TAIL(&vnode_inactive_list, vp, v_list);
        }
        ++inactivevnodes;
        vp->v_state = VS_INACTIVE;

        spin_unlock(&vfs_spin);
}

static __inline
void
_vinactive_tail(struct vnode *vp)
{
        spin_lock(&vfs_spin);

        /*
         * Remove from active list if it is sitting on it
         */
        switch(vp->v_state) {
        case VS_ACTIVE:
                TAILQ_REMOVE(&vnode_active_list, vp, v_list);
                --activevnodes;
                break;
        case VS_INACTIVE:
                panic("_vinactive_tail: already inactive");
                /* NOT REACHED */
                spin_unlock(&vfs_spin);
                return;
        case VS_CACHED:
        case VS_DYING:
                break;
        }

        TAILQ_INSERT_TAIL(&vnode_inactive_list, vp, v_list);
        ++inactivevnodes;
        vp->v_state = VS_INACTIVE;

        spin_unlock(&vfs_spin);
}

/*
 * Add a ref to an active vnode.  This function should never be called
 * with an inactive vnode (use vget() instead), but might be called
 * with other states.
 */
void
vref(struct vnode *vp)
{
        KASSERT((VREFCNT(vp) > 0 && vp->v_state != VS_INACTIVE),
                ("vref: bad refcnt %08x %d", vp->v_refcnt, vp->v_state));
        atomic_add_int(&vp->v_refcnt, 1);
}

/*
 * Release a ref on an active or inactive vnode.
 *
 * Caller has no other requirements.
 *
 * If VREF_FINALIZE is set this will deactivate the vnode on the 1->0
 * transition, otherwise we leave the vnode in the active list and
 * do a lockless transition to 0, which is very important for the
 * critical path.
 *
 * (vrele() is not called when a vnode is being destroyed w/kfree)
 */
void
vrele(struct vnode *vp)
{
        for (;;) {
                int count = vp->v_refcnt;
                cpu_ccfence();
                KKASSERT((count & VREF_MASK) > 0);
                KKASSERT(vp->v_state == VS_ACTIVE ||
                         vp->v_state == VS_INACTIVE);

                /*
                 * 2+ case
                 */
                if ((count & VREF_MASK) > 1) {
                        if (atomic_cmpset_int(&vp->v_refcnt, count, count - 1))
                                break;
                        continue;
                }

                /*
                 * 1->0 transition case must handle possible finalization.
                 * When finalizing we transition 1->0x40000000.  Note that
                 * cachedvnodes is only adjusted on transitions to ->0.
                 *
                 * WARNING! VREF_TERMINATE can be cleared at any point
                 *          when the refcnt is non-zero (by vget()) and
                 *          the vnode has not been reclaimed.  Thus
                 *          transitions out of VREF_TERMINATE do not have
                 *          to mess with cachedvnodes.
                 */
                if (count & VREF_FINALIZE) {
                        vx_lock(vp);
                        if (atomic_cmpset_int(&vp->v_refcnt,
                                              count, VREF_TERMINATE)) {
                                vnode_terminate(vp);
                                break;
                        }
                        vx_unlock(vp);
                } else {
                        if (atomic_cmpset_int(&vp->v_refcnt, count, 0)) {
                                atomic_add_int(&cachedvnodes, 1);
                                break;
                        }
                }
                /* retry */
        }
}

/*
 * Add an auxiliary data structure reference to the vnode.  Auxiliary
 * references do not change the state of the vnode or prevent deactivation
 * or reclamation of the vnode, but will prevent the vnode from being
 * destroyed (kfree()'d).
 *
 * WARNING!  vhold() must not acquire v_spin.  The spinlock may or may not
 *           already be held by the caller.  vdrop() will clean up the
 *           free list state.
 */
void
vhold(struct vnode *vp)
{
        atomic_add_int(&vp->v_auxrefs, 1);
}

/*
 * Remove an auxiliary reference from the vnode.
 */
void
vdrop(struct vnode *vp)
{
        atomic_add_int(&vp->v_auxrefs, -1);
}

/*
 * This function is called on the 1->0 transition (which is actually
 * 1->VREF_TERMINATE) when VREF_FINALIZE is set, forcing deactivation
 * of the vnode.
 *
 * Additional vrefs are allowed to race but will not result in a reentrant
 * call to vnode_terminate() due to refcnt being VREF_TERMINATE.  This
 * prevents additional 1->0 transitions.
 *
 * ONLY A VGET() CAN REACTIVATE THE VNODE.
 *
 * Caller must hold the VX lock.
 *
 * NOTE: v_mount may be NULL due to assigmment to dead_vnode_vops
 *
 * NOTE: The vnode may be marked inactive with dirty buffers
 *       or dirty pages in its cached VM object still present.
 *
 * NOTE: VS_FREE should not be set on entry (the vnode was expected to
 *       previously be active).  We lose control of the vnode the instant
 *       it is placed on the free list.
 *
 *       The VX lock is required when transitioning to VS_CACHED but is
 *       not sufficient for the vshouldfree() interlocked test or when
 *       transitioning away from VS_CACHED.  v_spin is also required for
 *       those cases.
 */
static
void
vnode_terminate(struct vnode *vp)
{
        KKASSERT(vp->v_state == VS_ACTIVE);

        if ((vp->v_flag & VINACTIVE) == 0) {
                _vsetflags(vp, VINACTIVE);
                if (vp->v_mount)
                        VOP_INACTIVE(vp);
                /* might deactivate page */
        }
        spin_lock(&vp->v_spin);
        _vinactive(vp);
        spin_unlock(&vp->v_spin);

        vx_unlock(vp);
}

/****************************************************************
 *                      VX LOCKING FUNCTIONS                    *
 ****************************************************************
 *
 * These functions lock vnodes for reclamation and deactivation related
 * activities.  The caller must already be holding some sort of reference
 * on the vnode.
 */
void
vx_lock(struct vnode *vp)
{
        lockmgr(&vp->v_lock, LK_EXCLUSIVE);
}

void
vx_unlock(struct vnode *vp)
{
        lockmgr(&vp->v_lock, LK_RELEASE);
}

/****************************************************************
 *                      VNODE ACQUISITION FUNCTIONS             *
 ****************************************************************
 *
 * These functions must be used when accessing a vnode that has no
 * chance of being destroyed in a SMP race.  That means the caller will
 * usually either hold an auxiliary reference (such as the namecache)
 * or hold some other lock that ensures that the vnode cannot be destroyed.
 *
 * These functions are MANDATORY for any code chain accessing a vnode
 * whos activation state is not known.
 *
 * vget() can be called with LK_NOWAIT and will return EBUSY if the
 * lock cannot be immediately acquired.
 *
 * vget()/vput() are used when reactivation is desired.
 *
 * vx_get() and vx_put() are used when reactivation is not desired.
 */
int
vget(struct vnode *vp, int flags)
{
        int error;

        /*
         * A lock type must be passed
         */
        if ((flags & LK_TYPE_MASK) == 0) {
                panic("vget() called with no lock specified!");
                /* NOT REACHED */
        }

        /*
         * Reference the structure and then acquire the lock.
         *
         * NOTE: The requested lock might be a shared lock and does
         *       not protect our access to the refcnt or other fields.
         */
        if ((atomic_fetchadd_int(&vp->v_refcnt, 1) & VREF_MASK) == 0)
                atomic_add_int(&cachedvnodes, -1);

        if ((error = vn_lock(vp, flags | LK_FAILRECLAIM)) != 0) {
                /*
                 * The lock failed, undo and return an error.  This will not
                 * normally trigger a termination.
                 */
                vrele(vp);
        } else if (vp->v_flag & VRECLAIMED) {
                /*
                 * The node is being reclaimed and cannot be reactivated
                 * any more, undo and return ENOENT.
                 */
                vn_unlock(vp);
                vrele(vp);
                error = ENOENT;
        } else if (vp->v_state == VS_ACTIVE) {
                /*
                 * A VS_ACTIVE vnode coupled with the fact that we have
                 * a vnode lock (even if shared) prevents v_state from
                 * changing.  Since the vnode is not in a VRECLAIMED state,
                 * we can safely clear VINACTIVE.
                 *
                 * NOTE! Multiple threads may clear VINACTIVE if this is
                 *       shared lock.  This race is allowed.
                 */
                _vclrflags(vp, VINACTIVE);      /* SMP race ok */
                vp->v_act += VACT_INC;
                if (vp->v_act > VACT_MAX)       /* SMP race ok */
                        vp->v_act = VACT_MAX;
                error = 0;
        } else {
                /*
                 * If the vnode is not VS_ACTIVE it must be reactivated
                 * in addition to clearing VINACTIVE.  An exclusive spin_lock
                 * is needed to manipulate the vnode's list.
                 *
                 * Because the lockmgr lock might be shared, we might race
                 * another reactivation, which we handle.  In this situation,
                 * however, the refcnt prevents other v_state races.
                 *
                 * As with above, clearing VINACTIVE is allowed to race other
                 * clearings of VINACTIVE.
                 *
                 * VREF_TERMINATE and VREF_FINALIZE can only be cleared when
                 * the refcnt is non-zero and the vnode has not been
                 * reclaimed.  This also means that the transitions do
                 * not affect cachedvnodes.
                 */
                _vclrflags(vp, VINACTIVE);
                vp->v_act += VACT_INC;
                if (vp->v_act > VACT_MAX)       /* SMP race ok */
                        vp->v_act = VACT_MAX;
                spin_lock(&vp->v_spin);

                switch(vp->v_state) {
                case VS_INACTIVE:
                        _vactivate(vp);
                        atomic_clear_int(&vp->v_refcnt, VREF_TERMINATE |
                                                        VREF_FINALIZE);
                        spin_unlock(&vp->v_spin);
                        break;
                case VS_CACHED:
                        _vactivate(vp);
                        atomic_clear_int(&vp->v_refcnt, VREF_TERMINATE |
                                                        VREF_FINALIZE);
                        spin_unlock(&vp->v_spin);
                        break;
                case VS_ACTIVE:
                        atomic_clear_int(&vp->v_refcnt, VREF_FINALIZE);
                        spin_unlock(&vp->v_spin);
                        break;
                case VS_DYING:
                        spin_unlock(&vp->v_spin);
                        panic("Impossible VS_DYING state");
                        break;
                }
                error = 0;
        }
        return(error);
}

#ifdef DEBUG_VPUT

void
debug_vput(struct vnode *vp, const char *filename, int line)
{
        kprintf("vput(%p) %s:%d\n", vp, filename, line);
        vn_unlock(vp);
        vrele(vp);
}

#else

void
vput(struct vnode *vp)
{
        vn_unlock(vp);
        vrele(vp);
}

#endif

/*
 * Acquire the vnode lock unguarded.
 *
 * The non-blocking version also uses a slightly different mechanic.
 * This function will explicitly fail not only if it cannot acquire
 * the lock normally, but also if the caller already holds a lock.
 *
 * The adjusted mechanic is used to close a loophole where complex
 * VOP_RECLAIM code can circle around recursively and allocate the
 * same vnode it is trying to destroy from the freelist.
 *
 * Any filesystem (aka UFS) which puts LK_CANRECURSE in lk_flags can
 * cause the incorrect behavior to occur.  If not for that lockmgr()
 * would do the right thing.
 *
 * XXX The vx_*() locks should use auxrefs, not the main reference counter.
 */
void
vx_get(struct vnode *vp)
{
        if ((atomic_fetchadd_int(&vp->v_refcnt, 1) & VREF_MASK) == 0)
                atomic_add_int(&cachedvnodes, -1);
        lockmgr(&vp->v_lock, LK_EXCLUSIVE);
}

int
vx_get_nonblock(struct vnode *vp)
{
        int error;

        if (lockcountnb(&vp->v_lock))
                return(EBUSY);
        error = lockmgr(&vp->v_lock, LK_EXCLUSIVE | LK_NOWAIT);
        if (error == 0) {
                if ((atomic_fetchadd_int(&vp->v_refcnt, 1) & VREF_MASK) == 0)
                        atomic_add_int(&cachedvnodes, -1);
        }
        return(error);
}

/*
 * Release a VX lock that also held a ref on the vnode.  vrele() will handle
 * any needed state transitions.
 *
 * However, filesystems use this function to get rid of unwanted new vnodes
 * so try to get the vnode on the correct queue in that case.
 */
void
vx_put(struct vnode *vp)
{
        if (vp->v_type == VNON || vp->v_type == VBAD)
                atomic_set_int(&vp->v_refcnt, VREF_FINALIZE);
        lockmgr(&vp->v_lock, LK_RELEASE);
        vrele(vp);
}

/*
 * Try to reuse a vnode from the free list.  This function is somewhat
 * advisory in that NULL can be returned as a normal case, even if free
 * vnodes are present.
 *
 * The scan is limited because it can result in excessive CPU use during
 * periods of extreme vnode use.
 *
 * NOTE: The returned vnode is not completely initialized.
 */
static
struct vnode *
cleanfreevnode(int maxcount)
{
        struct vnode *vp;
        int count;
        int trigger = (long)vmstats.v_page_count / (activevnodes * 2 + 1);

        /*
         * Try to deactivate some vnodes cached on the active list.
         */
        if (cachedvnodes < inactivevnodes)
                goto skip;

        for (count = 0; count < maxcount * 2; count++) {
                spin_lock(&vfs_spin);

                vp = TAILQ_NEXT(&vnode_active_rover, v_list);
                TAILQ_REMOVE(&vnode_active_list, &vnode_active_rover, v_list);
                if (vp == NULL) {
                        TAILQ_INSERT_HEAD(&vnode_active_list,
                                          &vnode_active_rover, v_list);
                } else {
                        TAILQ_INSERT_AFTER(&vnode_active_list, vp,
                                           &vnode_active_rover, v_list);
                }
                if (vp == NULL) {
                        spin_unlock(&vfs_spin);
                        continue;
                }
                if ((vp->v_refcnt & VREF_MASK) != 0) {
                        spin_unlock(&vfs_spin);
                        vp->v_act += VACT_INC;
                        if (vp->v_act > VACT_MAX)       /* SMP race ok */
                                vp->v_act = VACT_MAX;
                        continue;
                }

                /*
                 * decrement by less if the vnode's object has a lot of
                 * VM pages.  XXX possible SMP races.
                 */
                if (vp->v_act > 0) {
                        vm_object_t obj;
                        if ((obj = vp->v_object) != NULL &&
                            obj->resident_page_count >= trigger) {
                                vp->v_act -= 1;
                        } else {
                                vp->v_act -= VACT_INC;
                        }
                        if (vp->v_act < 0)
                                vp->v_act = 0;
                        spin_unlock(&vfs_spin);
                        continue;
                }

                /*
                 * Try to deactivate the vnode.
                 */
                if ((atomic_fetchadd_int(&vp->v_refcnt, 1) & VREF_MASK) == 0)
                        atomic_add_int(&cachedvnodes, -1);
                atomic_set_int(&vp->v_refcnt, VREF_FINALIZE);

                spin_unlock(&vfs_spin);
                vrele(vp);
        }

skip:
        /*
         * Loop trying to lock the first vnode on the free list.
         * Cycle if we can't.
         */
        for (count = 0; count < maxcount; count++) {
                spin_lock(&vfs_spin);

                vp = TAILQ_FIRST(&vnode_inactive_list);
                if (vp == NULL) {
                        spin_unlock(&vfs_spin);
                        break;
                }

                /*
                 * non-blocking vx_get will also ref the vnode on success.
                 */
                if (vx_get_nonblock(vp)) {
                        KKASSERT(vp->v_state == VS_INACTIVE);
                        TAILQ_REMOVE(&vnode_inactive_list, vp, v_list);
                        TAILQ_INSERT_TAIL(&vnode_inactive_list, vp, v_list);
                        spin_unlock(&vfs_spin);
                        continue;
                }

                /*
                 * Because we are holding vfs_spin the vnode should currently
                 * be inactive and VREF_TERMINATE should still be set.
                 *
                 * Once vfs_spin is released the vnode's state should remain
                 * unmodified due to both the lock and ref on it.
                 */
                KKASSERT(vp->v_state == VS_INACTIVE);
                spin_unlock(&vfs_spin);
#ifdef TRACKVNODE
                if ((u_long)vp == trackvnode)
                        kprintf("cleanfreevnode %p %08x\n", vp, vp->v_flag);
#endif

                /*
                 * Do not reclaim/reuse a vnode while auxillary refs exists.
                 * This includes namecache refs due to a related ncp being
                 * locked or having children, a VM object association, or
                 * other hold users.
                 *
                 * Do not reclaim/reuse a vnode if someone else has a real
                 * ref on it.  This can occur if a filesystem temporarily
                 * releases the vnode lock during VOP_RECLAIM.
                 */
                if (vp->v_auxrefs ||
                    (vp->v_refcnt & ~VREF_FINALIZE) != VREF_TERMINATE + 1) {
failed:
                        if (vp->v_state == VS_INACTIVE) {
                                spin_lock(&vfs_spin);
                                if (vp->v_state == VS_INACTIVE) {
                                        TAILQ_REMOVE(&vnode_inactive_list,
                                                     vp, v_list);
                                        TAILQ_INSERT_TAIL(&vnode_inactive_list,
                                                          vp, v_list);
                                }
                                spin_unlock(&vfs_spin);
                        }
                        vx_put(vp);
                        continue;
                }

                /*
                 * VINACTIVE and VREF_TERMINATE are expected to both be set
                 * for vnodes pulled from the inactive list, and cannot be
                 * changed while we hold the vx lock.
                 *
                 * Try to reclaim the vnode.
                 */
                KKASSERT(vp->v_flag & VINACTIVE);
                KKASSERT(vp->v_refcnt & VREF_TERMINATE);

                if ((vp->v_flag & VRECLAIMED) == 0) {
                        if (cache_inval_vp_nonblock(vp))
                                goto failed;
                        vgone_vxlocked(vp);
                        /* vnode is still VX locked */
                }

                /*
                 * At this point if there are no other refs or auxrefs on
                 * the vnode with the inactive list locked, and we remove
                 * the vnode from the inactive list, it should not be
                 * possible for anyone else to access the vnode any more.
                 *
                 * Since the vnode is in a VRECLAIMED state, no new
                 * namecache associations could have been made and the
                 * vnode should have already been removed from its mountlist.
                 *
                 * Since we hold a VX lock on the vnode it cannot have been
                 * reactivated (moved out of the inactive list).
                 */
                KKASSERT(TAILQ_EMPTY(&vp->v_namecache));
                spin_lock(&vfs_spin);
                if (vp->v_auxrefs ||
                    (vp->v_refcnt & ~VREF_FINALIZE) != VREF_TERMINATE + 1) {
                        spin_unlock(&vfs_spin);
                        goto failed;
                }
                KKASSERT(vp->v_state == VS_INACTIVE);
                TAILQ_REMOVE(&vnode_inactive_list, vp, v_list);
                --inactivevnodes;
                vp->v_state = VS_DYING;
                spin_unlock(&vfs_spin);

                /*
                 * Nothing should have been able to access this vp.  Only
                 * our ref should remain now.
                 */
                atomic_clear_int(&vp->v_refcnt, VREF_TERMINATE|VREF_FINALIZE);
                KASSERT(vp->v_refcnt == 1,
                        ("vp %p badrefs %08x", vp, vp->v_refcnt));

                /*
                 * Return a VX locked vnode suitable for reuse.
                 */
                return(vp);
        }
        return(NULL);
}

/*
 * Obtain a new vnode.  The returned vnode is VX locked & vrefd.
 *
 * All new vnodes set the VAGE flags.  An open() of the vnode will
 * decrement the (2-bit) flags.  Vnodes which are opened several times
 * are thus retained in the cache over vnodes which are merely stat()d.
 *
 * We always allocate the vnode.  Attempting to recycle existing vnodes
 * here can lead to numerous deadlocks, particularly with softupdates.
 */
struct vnode *
allocvnode(int lktimeout, int lkflags)
{
        struct vnode *vp;

        /*
         * Do not flag for synchronous recyclement unless there are enough
         * freeable vnodes to recycle and the number of vnodes has
         * significantly exceeded our target.  We want the normal vnlru
         * process to handle the cleaning (at 9/10's) before we are forced
         * to flag it here at 11/10's for userexit path processing.
         */
        if (numvnodes >= maxvnodes * 11 / 10 &&
            cachedvnodes + inactivevnodes >= maxvnodes * 5 / 10) {
                struct thread *td = curthread;
                if (td->td_lwp)
                        atomic_set_int(&td->td_lwp->lwp_mpflags, LWP_MP_VNLRU);
        }

        /*
         * lktimeout only applies when LK_TIMELOCK is used, and only
         * the pageout daemon uses it.  The timeout may not be zero
         * or the pageout daemon can deadlock in low-VM situations.
         */
        if (lktimeout == 0)
                lktimeout = hz / 10;

        vp = kmalloc(sizeof(*vp), M_VNODE, M_ZERO | M_WAITOK);

        lwkt_token_init(&vp->v_token, "vnode");
        lockinit(&vp->v_lock, "vnode", lktimeout, lkflags);
        TAILQ_INIT(&vp->v_namecache);
        RB_INIT(&vp->v_rbclean_tree);
        RB_INIT(&vp->v_rbdirty_tree);
        RB_INIT(&vp->v_rbhash_tree);
        spin_init(&vp->v_spin, "allocvnode");

        lockmgr(&vp->v_lock, LK_EXCLUSIVE);
        atomic_add_int(&numvnodes, 1);
        vp->v_refcnt = 1;
        vp->v_flag = VAGE0 | VAGE1;
        vp->v_pbuf_count = nswbuf_kva / NSWBUF_SPLIT;

        KKASSERT(TAILQ_EMPTY(&vp->v_namecache));
        /* exclusive lock still held */

        vp->v_filesize = NOOFFSET;
        vp->v_type = VNON;
        vp->v_tag = 0;
        vp->v_state = VS_CACHED;
        _vactivate(vp);

        return (vp);
}

/*
 * Called after a process has allocated a vnode via allocvnode()
 * and we detected that too many vnodes were present.
 *
 * This function is called just prior to a return to userland if the
 * process at some point had to allocate a new vnode during the last
 * system call and the vnode count was found to be excessive.
 *
 * This is a synchronous path that we do not normally want to execute.
 *
 * Flagged at >= 11/10's, runs if >= 10/10, vnlru runs at 9/10.
 *
 * WARNING: Sometimes numvnodes can blow out due to children being
 *          present under directory vnodes in the namecache.  For the
 *          moment use an if() instead of a while() and note that if
 *          we were to use a while() we would still have to break out
 *          if freesomevnodes() returned 0.  vnlru will also be trying
 *          hard to free vnodes at the same time (with a lower trigger
 *          pointer).
 */
void
allocvnode_gc(void)
{
        if (numvnodes >= maxvnodes &&
            cachedvnodes + inactivevnodes >= maxvnodes * 5 / 10) {
                freesomevnodes(batchfreevnodes);
        }
}

int
freesomevnodes(int n)
{
        struct vnode *vp;
        int count = 0;

        while (n) {
                if ((vp = cleanfreevnode(n)) == NULL)
                        break;
                vx_unlock(vp);
                --n;
                ++count;
                kfree(vp, M_VNODE);
                atomic_add_int(&numvnodes, -1);
        }
        return(count);
}