kernel - Fix not-quite-nonblocking VX lock in allocfreevnode()

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

/*
 * Copyright (c) 2004 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $DragonFly: src/sys/kern/vfs_lock.c,v 1.30 2008/06/30 03:57:41 dillon Exp $
 */

/*
 * External virtual filesystem routines
 */
#include "opt_ddb.h"

#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>
#include <sys/sysref2.h>

static void vnode_terminate(struct vnode *vp);
static boolean_t vnode_ctor(void *obj, void *private, int ocflags);
static void vnode_dtor(void *obj, void *private);

static MALLOC_DEFINE(M_VNODE, "vnodes", "vnode structures");
static struct sysref_class vnode_sysref_class = {
        .name =         "vnode",
        .mtype =        M_VNODE,
        .proto =        SYSREF_PROTO_VNODE,
        .offset =       offsetof(struct vnode, v_sysref),
        .objsize =      sizeof(struct vnode),
        .mag_capacity = 256,
        .flags =        SRC_MANAGEDINIT,
        .ctor =         vnode_ctor,
        .dtor =         vnode_dtor,
        .ops = {
                .terminate = (sysref_terminate_func_t)vnode_terminate
        }
};

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

int  freevnodes = 0;
SYSCTL_INT(_debug, OID_AUTO, freevnodes, CTLFLAG_RD,
                &freevnodes, 0, "");
static int wantfreevnodes = 25;
SYSCTL_INT(_debug, OID_AUTO, wantfreevnodes, CTLFLAG_RW,
                &wantfreevnodes, 0, "");
#ifdef TRACKVNODE
static ulong trackvnode;
SYSCTL_ULONG(_debug, OID_AUTO, trackvnode, CTLFLAG_RW,
                &trackvnode, 0, "");
#endif

/*
 * Called from vfsinit()
 */
void
vfs_lock_init(void)
{
        TAILQ_INIT(&vnode_free_list);
        TAILQ_INSERT_HEAD(&vnode_free_list, &vnode_free_mid, v_freelist);
        spin_init(&vfs_spin);
}

/*
 * 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);
}

/*
 * Inline helper functions.
 *
 * WARNING: vbusy() may only be called while the vnode lock or VX lock
 *          is held.  The vnode spinlock need not be held.
 *
 * MPSAFE
 */
static __inline
void
__vbusy_interlocked(struct vnode *vp)
{
        TAILQ_REMOVE(&vnode_free_list, vp, v_freelist);
        freevnodes--;
        _vclrflags(vp, VFREE);
}

static __inline 
void
__vbusy(struct vnode *vp)
{
#ifdef TRACKVNODE
        if ((ulong)vp == trackvnode)
                kprintf("__vbusy %p %08x\n", vp, vp->v_flag);
#endif
        spin_lock_wr(&vfs_spin);
        __vbusy_interlocked(vp);
        spin_unlock_wr(&vfs_spin);
}

/*
 * Put a vnode on the free list.  The caller has cleared VCACHED or owns the
 * implied sysref related to having removed the vnode from the freelist
 * (and VCACHED is already clear in that case).
 *
 * MPSAFE
 */
static __inline
void
__vfree(struct vnode *vp)
{
#ifdef TRACKVNODE
        if ((ulong)vp == trackvnode) {
                kprintf("__vfree %p %08x\n", vp, vp->v_flag);
                print_backtrace();
        }
#endif
        spin_lock_wr(&vfs_spin);
        if (vp->v_flag & VRECLAIMED)
                TAILQ_INSERT_HEAD(&vnode_free_list, vp, v_freelist);
        else if (vp->v_flag & (VAGE0 | VAGE1))
                TAILQ_INSERT_BEFORE(&vnode_free_mid, vp, v_freelist);
        else
                TAILQ_INSERT_TAIL(&vnode_free_list, vp, v_freelist);
        freevnodes++;
        _vsetflags(vp, VFREE);
        spin_unlock_wr(&vfs_spin);
}

/*
 * Put a vnode on the free list.  The caller has cleared VCACHED or owns the
 * implied sysref related to having removed the vnode from the freelist
 * (and VCACHED is already clear in that case).
 *
 * MPSAFE
 */
static __inline
void
__vfreetail(struct vnode *vp)
{
#ifdef TRACKVNODE
        if ((ulong)vp == trackvnode)
                kprintf("__vfreetail %p %08x\n", vp, vp->v_flag);
#endif
        spin_lock_wr(&vfs_spin);
        TAILQ_INSERT_TAIL(&vnode_free_list, vp, v_freelist);
        freevnodes++;
        _vsetflags(vp, VFREE);
        spin_unlock_wr(&vfs_spin);
}

/*
 * Return a C boolean if we should put the vnode on the freelist (VFREE),
 * or leave it / mark it as VCACHED.
 *
 * This routine is only valid if the vnode is already either VFREE or
 * VCACHED, or if it can become VFREE or VCACHED via vnode_terminate().
 *
 * WARNING!  This functions is typically called with v_spinlock held.
 *
 * MPSAFE
 */
static __inline boolean_t
vshouldfree(struct vnode *vp)
{
        return (vp->v_auxrefs == 0 &&
            (vp->v_object == NULL || vp->v_object->resident_page_count == 0));
}

/*
 * Add a ref to an active vnode.  This function should never be called
 * with an inactive vnode (use vget() instead).
 *
 * MPSAFE
 */
void
vref(struct vnode *vp)
{
        KKASSERT(vp->v_sysref.refcnt > 0 && 
                 (vp->v_flag & (VFREE|VINACTIVE)) == 0);
        sysref_get(&vp->v_sysref);
}

/*
 * Release a ref on an active or inactive vnode.  The sysref termination
 * function will be called when the active last active reference is released,
 * and the vnode is returned to the objcache when the last inactive
 * reference is released.
 */
void
vrele(struct vnode *vp)
{
        sysref_put(&vp->v_sysref);
}

/*
 * Add an auxiliary data structure reference to the vnode.  Auxiliary
 * references do not change the state of the vnode or prevent them
 * from being deactivated, reclaimed, or placed on or removed from
 * the free list.
 *
 * An auxiliary reference DOES prevent the vnode from being destroyed,
 * allowing you to vx_lock() it, test state, etc.
 *
 * An auxiliary reference DOES NOT move a vnode out of the VFREE state
 * once it has entered it.
 *
 * WARNING!  vhold() and vhold_interlocked() must not acquire v_spinlock.
 *           The spinlock may or may not already be held by the caller.
 *           vdrop() will clean up the free list state.
 *
 * MPSAFE
 */
void
vhold(struct vnode *vp)
{
        KKASSERT(vp->v_sysref.refcnt != 0);
        atomic_add_int(&vp->v_auxrefs, 1);
}

void
vhold_interlocked(struct vnode *vp)
{
        atomic_add_int(&vp->v_auxrefs, 1);
}

/*
 * Remove an auxiliary reference from the vnode.
 *
 * vdrop needs to check for a VCACHE->VFREE transition to catch cases
 * where a vnode is held past its reclamation.  We use v_spinlock to
 * interlock VCACHED -> !VCACHED transitions.
 *
 * MPSAFE
 */
void
vdrop(struct vnode *vp)
{
        KKASSERT(vp->v_sysref.refcnt != 0 && vp->v_auxrefs > 0);
        spin_lock_wr(&vp->v_spinlock);
        atomic_subtract_int(&vp->v_auxrefs, 1);
        if ((vp->v_flag & VCACHED) && vshouldfree(vp)) {
                _vclrflags(vp, VCACHED);
                __vfree(vp);
        }
        spin_unlock_wr(&vp->v_spinlock);
}

/*
 * This function is called when the last active reference on the vnode
 * is released, typically via vrele().  SYSREF will give the vnode a
 * negative ref count, indicating that it is undergoing termination or
 * is being set aside for the cache, and one final sysref_put() is
 * required to actually return it to the memory subsystem.
 *
 * However, because vnodes may have auxiliary structural references via
 * v_auxrefs, we must interlock auxiliary references against termination
 * via the VX lock mechanism.  It is possible for a vnode to be reactivated
 * while we were blocked on the lock.
 *
 * MPSAFE
 */
void
vnode_terminate(struct vnode *vp)
{
        vx_lock(vp);
        if (sysref_isinactive(&vp->v_sysref)) {
                /*
                 * Deactivate the vnode by marking it VFREE or VCACHED.
                 * The vnode can be reactivated from either state until
                 * reclaimed.  These states inherit the 'last' sysref on the
                 * vnode.
                 *
                 * NOTE: There may be additional inactive references from
                 * other entities blocking on the VX lock while we hold it,
                 * but this does not prevent us from changing the vnode's
                 * state.
                 *
                 * NOTE: The vnode could already be marked inactive.  XXX
                 *       how?
                 *
                 * NOTE: v_mount may be NULL due to assignment 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: VCACHED should not be set on entry.  We lose control
                 *       of the sysref the instant the vnode is placed on the
                 *       free list or when VCACHED is set.
                 *
                 *       The VX lock is sufficient when transitioning
                 *       to +VCACHED but not sufficient for the vshouldfree()
                 *       interlocked test.
                 */
                if ((vp->v_flag & VINACTIVE) == 0) {
                        _vsetflags(vp, VINACTIVE);
                        if (vp->v_mount)
                                VOP_INACTIVE(vp);
                }
                spin_lock_wr(&vp->v_spinlock);
                KKASSERT((vp->v_flag & (VFREE|VCACHED)) == 0);
                if (vshouldfree(vp))
                        __vfree(vp);
                else
                        _vsetflags(vp, VCACHED); /* inactive but not yet free*/
                spin_unlock_wr(&vp->v_spinlock);
                vx_unlock(vp);
        } else {
                /*
                 * Someone reactivated the vnode while were blocked on the
                 * VX lock.  Release the VX lock and release the (now active)
                 * last reference which is no longer last.
                 */
                vx_unlock(vp);
                vrele(vp);
        }
}

/*
 * Physical vnode constructor / destructor.  These are only executed on
 * the backend of the objcache.  They are NOT executed on every vnode
 * allocation or deallocation.
 *
 * MPSAFE
 */
boolean_t
vnode_ctor(void *obj, void *private, int ocflags)
{
        struct vnode *vp = obj;

        lwkt_token_init(&vp->v_token);
        lockinit(&vp->v_lock, "vnode", 0, 0);
        ccms_dataspace_init(&vp->v_ccms);
        TAILQ_INIT(&vp->v_namecache);
        RB_INIT(&vp->v_rbclean_tree);
        RB_INIT(&vp->v_rbdirty_tree);
        RB_INIT(&vp->v_rbhash_tree);
        return(TRUE);
}

/*
 * MPSAFE
 */
void
vnode_dtor(void *obj, void *private)
{
        struct vnode *vp = obj;

        ccms_dataspace_destroy(&vp->v_ccms);
}

/****************************************************************
 *                      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.
 *
 * MPSAFE
 */
void
vx_lock(struct vnode *vp)
{
        lockmgr(&vp->v_lock, LK_EXCLUSIVE);
}

static int
vx_lock_nonblock(struct vnode *vp)
{
        return(lockmgr(&vp->v_lock, LK_EXCLUSIVE | LK_NOWAIT | LK_NOSPINWAIT));
}

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

/****************************************************************
 *                      VNODE ACQUISITION FUNCTIONS             *
 ****************************************************************
 *
 * These functions must be used when accessing a vnode via an auxiliary
 * reference such as the namecache or free list, or when you wish to
 * do a combo ref+lock sequence.
 *
 * 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.  0->1
         * transitions and refs during termination are allowed here so
         * call sysref directly.
         *
         * NOTE: The requested lock might be a shared lock and does
         *       not protect our access to the refcnt or other fields.
         */
        sysref_get(&vp->v_sysref);
        if ((error = vn_lock(vp, flags)) != 0) {
                /*
                 * The lock failed, undo and return an error.
                 */
                sysref_put(&vp->v_sysref);
        } 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 the vnode is marked VFREE or VCACHED it needs to be
                 * reactivated, otherwise it had better already be active.
                 * VINACTIVE must also be cleared.
                 *
                 * In the VFREE/VCACHED case we have to throw away the
                 * sysref that was earmarking those cases and preventing
                 * the vnode from being destroyed.  Our sysref is still held.
                 *
                 * The spinlock is our only real protection here.
                 */
                spin_lock_wr(&vp->v_spinlock);
                if (vp->v_flag & VFREE) {
                        __vbusy(vp);
                        sysref_activate(&vp->v_sysref);
                        spin_unlock_wr(&vp->v_spinlock);
                        sysref_put(&vp->v_sysref);
                } else if (vp->v_flag & VCACHED) {
                        _vclrflags(vp, VCACHED);
                        sysref_activate(&vp->v_sysref);
                        spin_unlock_wr(&vp->v_spinlock);
                        sysref_put(&vp->v_sysref);
                } else {
                        if (sysref_isinactive(&vp->v_sysref)) {
                                sysref_activate(&vp->v_sysref);
                                kprintf("Warning vp %p reactivation race\n",
                                        vp);
                        }
                        spin_unlock_wr(&vp->v_spinlock);
                }
                _vclrflags(vp, VINACTIVE);
                error = 0;
        }
        return(error);
}

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

/*
 * XXX The vx_*() locks should use auxrefs, not the main reference counter.
 *
 * MPSAFE
 */
void
vx_get(struct vnode *vp)
{
        sysref_get(&vp->v_sysref);
        lockmgr(&vp->v_lock, LK_EXCLUSIVE);
}

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

        sysref_get(&vp->v_sysref);
        error = lockmgr(&vp->v_lock, LK_EXCLUSIVE | LK_NOWAIT);
        if (error)
                sysref_put(&vp->v_sysref);
        return(error);
}

/*
 * Relase a VX lock that also held a ref on the vnode.
 *
 * vx_put needs to check for a VCACHE->VFREE transition to catch the
 * case where e.g. vnlru issues a vgone*().
 *
 * MPSAFE
 */
void
vx_put(struct vnode *vp)
{
        spin_lock_wr(&vp->v_spinlock);
        if ((vp->v_flag & VCACHED) && vshouldfree(vp)) {
                _vclrflags(vp, VCACHED);
                __vfree(vp);
        }
        spin_unlock_wr(&vp->v_spinlock);
        lockmgr(&vp->v_lock, LK_RELEASE);
        sysref_put(&vp->v_sysref);
}

/*
 * Try to reuse a vnode from the free list.
 *
 * NOTE: The returned vnode is not completely initialized.
 *
 * WARNING: The freevnodes count can race, NULL can be returned even if
 *          freevnodes != 0.
 *
 * MPSAFE
 */
static
struct vnode *
allocfreevnode(void)
{
        struct vnode *vp;
        int count;

        for (count = 0; count < freevnodes; count++) {
                /*
                 * Try to lock the first vnode on the free list.
                 * Cycle if we can't.
                 *
                 * We use a bad hack in vx_lock_nonblock() which avoids
                 * the lock order reversal between vfs_spin and v_spinlock.
                 * This is very fragile code and I don't want to use
                 * vhold here.
                 */
                spin_lock_wr(&vfs_spin);
                vp = TAILQ_FIRST(&vnode_free_list);
                if (vp == &vnode_free_mid)
                        vp = TAILQ_NEXT(vp, v_freelist);
                if (vx_lock_nonblock(vp)) {
                        KKASSERT(vp->v_flag & VFREE);
                        TAILQ_REMOVE(&vnode_free_list, vp, v_freelist);
                        TAILQ_INSERT_TAIL(&vnode_free_list,
                                          vp, v_freelist);
                        spin_unlock_wr(&vfs_spin);
                        continue;
                }

                /*
                 * We inherit the sysref associated the vnode on the free
                 * list.  Because VCACHED is clear the vnode will not
                 * be placed back on the free list.  We own the sysref
                 * free and clear and thus control the disposition of
                 * the vnode.
                 */
                __vbusy_interlocked(vp);
                spin_unlock_wr(&vfs_spin);
#ifdef TRACKVNODE
                if ((ulong)vp == trackvnode)
                        kprintf("allocfreevnode %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.
                 *
                 * We will make this test several times as auxrefs can
                 * get incremented on us without any spinlocks being held
                 * until we have removed all namecache and inode references
                 * to the vnode.
                 *
                 * Because VCACHED is already in the correct state (cleared)
                 * we cannot race other vdrop()s occuring at the same time
                 * and can safely place vp on the free list.
                 *
                 * The free list association reinherits the sysref.
                 */
                if (vp->v_auxrefs) {
                        __vfreetail(vp);
                        vx_unlock(vp);
                        continue;
                }

                /*
                 * We inherit the reference that was previously associated
                 * with the vnode being on the free list.  VCACHED had better
                 * not be set because the reference and VX lock prevents
                 * the sysref from transitioning to an active state.
                 */
                KKASSERT((vp->v_flag & (VINACTIVE|VCACHED)) == VINACTIVE);
                KKASSERT(sysref_isinactive(&vp->v_sysref));

                /*
                 * Holding the VX lock on an inactive vnode prevents it
                 * from being reactivated or reused.  New namecache
                 * associations can only be made using active vnodes.
                 *
                 * Another thread may be blocked on our vnode lock while
                 * holding a namecache lock.  We can only reuse this vnode
                 * if we can clear all namecache associations without
                 * blocking.
                 *
                 * Because VCACHED is already in the correct state (cleared)
                 * we cannot race other vdrop()s occuring at the same time
                 * and can safely place vp on the free list.
                 */
                if ((vp->v_flag & VRECLAIMED) == 0) {
                        if (cache_inval_vp_nonblock(vp)) {
                                __vfreetail(vp);
                                vx_unlock(vp);
                                continue;
                        }
                        vgone_vxlocked(vp);
                        /* vnode is still VX locked */
                }

                /*
                 * We can reuse the vnode if no primary or auxiliary
                 * references remain other then ours, else put it
                 * back on the free list and keep looking.
                 *
                 * Either the free list inherits the last reference
                 * or we fall through and sysref_activate() the last
                 * reference.
                 *
                 * Since the vnode is in a VRECLAIMED state, no new
                 * namecache associations could have been made.
                 */
                KKASSERT(TAILQ_EMPTY(&vp->v_namecache));
                if (vp->v_auxrefs ||
                    !sysref_islastdeactivation(&vp->v_sysref)) {
                        __vfreetail(vp);
                        vx_unlock(vp);
                        continue;
                }

                /*
                 * Return a VX locked vnode suitable for reuse.  The caller
                 * inherits the sysref.
                 */
                return(vp);
        }
        return(NULL);
}

/*
 * Obtain a new vnode from the freelist, allocating more if necessary.
 * The returned vnode is VX locked & refd.
 *
 * 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.
 *
 * MPSAFE
 */
struct vnode *
allocvnode(int lktimeout, int lkflags)
{
        struct vnode *vp;

        /*
         * Try to reuse vnodes if we hit the max.  This situation only
         * occurs in certain large-memory (2G+) situations.  We cannot
         * attempt to directly reclaim vnodes due to nasty recursion
         * problems.
         */
        while (numvnodes - freevnodes > desiredvnodes)
                vnlru_proc_wait();

        /*
         * Try to build up as many vnodes as we can before reallocating
         * from the free list.  A vnode on the free list simply means
         * that it is inactive with no resident pages.  It may or may not
         * have been reclaimed and could have valuable information associated 
         * with it that we shouldn't throw away unless we really need to.
         *
         * HAMMER NOTE: Re-establishing a vnode is a fairly expensive
         * operation for HAMMER but this should benefit UFS as well.
         */
        if (freevnodes >= wantfreevnodes && numvnodes >= desiredvnodes)
                vp = allocfreevnode();
        else
                vp = NULL;
        if (vp == NULL) {
                vp = sysref_alloc(&vnode_sysref_class);
                lockmgr(&vp->v_lock, LK_EXCLUSIVE);
                numvnodes++;
        }

        /*
         * We are using a managed sysref class, vnode fields are only
         * zerod on initial allocation from the backing store, not
         * on reallocation.  Thus we have to clear these fields for both
         * reallocation and reuse.
         */
#ifdef INVARIANTS
        if (vp->v_data)
                panic("cleaned vnode isn't");
        if (bio_track_active(&vp->v_track_read) ||
            bio_track_active(&vp->v_track_write)) {
                panic("Clean vnode has pending I/O's");
        }
        if (vp->v_flag & VONWORKLST)
                panic("Clean vnode still pending on syncer worklist!");
        if (!RB_EMPTY(&vp->v_rbdirty_tree))
                panic("Clean vnode still has dirty buffers!");
        if (!RB_EMPTY(&vp->v_rbclean_tree))
                panic("Clean vnode still has clean buffers!");
        if (!RB_EMPTY(&vp->v_rbhash_tree))
                panic("Clean vnode still on hash tree!");
        KKASSERT(vp->v_mount == NULL);
#endif
        vp->v_flag = VAGE0 | VAGE1;
        vp->v_lastw = 0;
        vp->v_lasta = 0;
        vp->v_cstart = 0;
        vp->v_clen = 0;
        vp->v_socket = 0;
        vp->v_opencount = 0;
        vp->v_writecount = 0;   /* XXX */

        /*
         * 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;
        lockreinit(&vp->v_lock, "vnode", lktimeout, lkflags);
        KKASSERT(TAILQ_EMPTY(&vp->v_namecache));
        /* exclusive lock still held */

        /*
         * Note: sysref needs to be activated to convert -0x40000000 to +1.
         * The -0x40000000 comes from the last ref on reuse, and from
         * sysref_init() on allocate.
         */
        sysref_activate(&vp->v_sysref);
        vp->v_filesize = NOOFFSET;
        vp->v_type = VNON;
        vp->v_tag = 0;
        vp->v_ops = NULL;
        vp->v_data = NULL;
        KKASSERT(vp->v_mount == NULL);

        return (vp);
}

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

        while (n) {
                --n;
                if ((vp = allocfreevnode()) == NULL)
                        break;
                vx_put(vp);
                --numvnodes;
        }
        return(count);
}