kernel - Implement shared namecache locks

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

/*
 * Copyright (c) 2003,2004,2009 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.
 * 
 * Copyright (c) 1989, 1993, 1995
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Poul-Henning Kamp of the FreeBSD Project.
 *
 * 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. 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.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/mount.h>
#include <sys/vnode.h>
#include <sys/malloc.h>
#include <sys/sysproto.h>
#include <sys/spinlock.h>
#include <sys/proc.h>
#include <sys/namei.h>
#include <sys/nlookup.h>
#include <sys/filedesc.h>
#include <sys/fnv_hash.h>
#include <sys/globaldata.h>
#include <sys/kern_syscall.h>
#include <sys/dirent.h>
#include <ddb/ddb.h>

#include <sys/sysref2.h>
#include <sys/spinlock2.h>
#include <sys/mplock2.h>

#define MAX_RECURSION_DEPTH     64

/*
 * Random lookups in the cache are accomplished with a hash table using
 * a hash key of (nc_src_vp, name).  Each hash chain has its own spin lock.
 *
 * Negative entries may exist and correspond to resolved namecache
 * structures where nc_vp is NULL.  In a negative entry, NCF_WHITEOUT
 * will be set if the entry corresponds to a whited-out directory entry
 * (verses simply not finding the entry at all).   ncneglist is locked
 * with a global spinlock (ncspin).
 *
 * MPSAFE RULES:
 *
 * (1) A ncp must be referenced before it can be locked.
 *
 * (2) A ncp must be locked in order to modify it.
 *
 * (3) ncp locks are always ordered child -> parent.  That may seem
 *     backwards but forward scans use the hash table and thus can hold
 *     the parent unlocked when traversing downward.
 *
 *     This allows insert/rename/delete/dot-dot and other operations
 *     to use ncp->nc_parent links.
 *
 *     This also prevents a locked up e.g. NFS node from creating a
 *     chain reaction all the way back to the root vnode / namecache.
 *
 * (4) parent linkages require both the parent and child to be locked.
 */

/*
 * Structures associated with name cacheing.
 */
#define NCHHASH(hash)           (&nchashtbl[(hash) & nchash])
#define MINNEG                  1024
#define MINPOS                  1024
#define NCMOUNT_NUMCACHE        1009    /* prime number */

MALLOC_DEFINE(M_VFSCACHE, "vfscache", "VFS name cache entries");

LIST_HEAD(nchash_list, namecache);

struct nchash_head {
       struct nchash_list list;
       struct spinlock  spin;
};

struct ncmount_cache {
        struct spinlock spin;
        struct namecache *ncp;
        struct mount *mp;
        int isneg;              /* if != 0 mp is originator and not target */
};

static struct nchash_head       *nchashtbl;
static struct namecache_list    ncneglist;
static struct spinlock          ncspin;
static struct ncmount_cache     ncmount_cache[NCMOUNT_NUMCACHE];

/*
 * ncvp_debug - debug cache_fromvp().  This is used by the NFS server
 * to create the namecache infrastructure leading to a dangling vnode.
 *
 * 0    Only errors are reported
 * 1    Successes are reported
 * 2    Successes + the whole directory scan is reported
 * 3    Force the directory scan code run as if the parent vnode did not
 *      have a namecache record, even if it does have one.
 */
static int      ncvp_debug;
SYSCTL_INT(_debug, OID_AUTO, ncvp_debug, CTLFLAG_RW, &ncvp_debug, 0,
    "Namecache debug level (0-3)");

static u_long   nchash;                 /* size of hash table */
SYSCTL_ULONG(_debug, OID_AUTO, nchash, CTLFLAG_RD, &nchash, 0,
    "Size of namecache hash table");

static int      ncnegfactor = 16;       /* ratio of negative entries */
SYSCTL_INT(_debug, OID_AUTO, ncnegfactor, CTLFLAG_RW, &ncnegfactor, 0,
    "Ratio of namecache negative entries");

static int      nclockwarn;             /* warn on locked entries in ticks */
SYSCTL_INT(_debug, OID_AUTO, nclockwarn, CTLFLAG_RW, &nclockwarn, 0,
    "Warn on locked namecache entries in ticks");

static int      numdefered;             /* number of cache entries allocated */
SYSCTL_INT(_debug, OID_AUTO, numdefered, CTLFLAG_RD, &numdefered, 0,
    "Number of cache entries allocated");

static int      ncposlimit;             /* number of cache entries allocated */
SYSCTL_INT(_debug, OID_AUTO, ncposlimit, CTLFLAG_RW, &ncposlimit, 0,
    "Number of cache entries allocated");

static int      ncp_shared_lock_disable = 1;
SYSCTL_INT(_debug, OID_AUTO, ncp_shared_lock_disable, CTLFLAG_RW,
           &ncp_shared_lock_disable, 0, "Disable shared namecache locks");

SYSCTL_INT(_debug, OID_AUTO, vnsize, CTLFLAG_RD, 0, sizeof(struct vnode),
    "sizeof(struct vnode)");
SYSCTL_INT(_debug, OID_AUTO, ncsize, CTLFLAG_RD, 0, sizeof(struct namecache),
    "sizeof(struct namecache)");

static int      ncmount_cache_enable = 1;
SYSCTL_INT(_debug, OID_AUTO, ncmount_cache_enable, CTLFLAG_RW,
           &ncmount_cache_enable, 0, "mount point cache");
static long     ncmount_cache_hit;
SYSCTL_LONG(_debug, OID_AUTO, ncmount_cache_hit, CTLFLAG_RW,
            &ncmount_cache_hit, 0, "mpcache hits");
static long     ncmount_cache_miss;
SYSCTL_LONG(_debug, OID_AUTO, ncmount_cache_miss, CTLFLAG_RW,
            &ncmount_cache_miss, 0, "mpcache misses");
static long     ncmount_cache_overwrite;
SYSCTL_LONG(_debug, OID_AUTO, ncmount_cache_overwrite, CTLFLAG_RW,
            &ncmount_cache_overwrite, 0, "mpcache entry overwrites");

static int cache_resolve_mp(struct mount *mp);
static struct vnode *cache_dvpref(struct namecache *ncp);
static void _cache_lock(struct namecache *ncp);
static void _cache_setunresolved(struct namecache *ncp);
static void _cache_cleanneg(int count);
static void _cache_cleanpos(int count);
static void _cache_cleandefered(void);
static void _cache_unlink(struct namecache *ncp);

/*
 * The new name cache statistics
 */
SYSCTL_NODE(_vfs, OID_AUTO, cache, CTLFLAG_RW, 0, "Name cache statistics");
static int numneg;
SYSCTL_INT(_vfs_cache, OID_AUTO, numneg, CTLFLAG_RD, &numneg, 0,
    "Number of negative namecache entries");
static int numcache;
SYSCTL_INT(_vfs_cache, OID_AUTO, numcache, CTLFLAG_RD, &numcache, 0,
    "Number of namecaches entries");
static u_long numcalls;
SYSCTL_ULONG(_vfs_cache, OID_AUTO, numcalls, CTLFLAG_RD, &numcalls, 0,
    "Number of namecache lookups");
static u_long numchecks;
SYSCTL_ULONG(_vfs_cache, OID_AUTO, numchecks, CTLFLAG_RD, &numchecks, 0,
    "Number of checked entries in namecache lookups");

struct nchstats nchstats[SMP_MAXCPU];
/*
 * Export VFS cache effectiveness statistics to user-land.
 *
 * The statistics are left for aggregation to user-land so
 * neat things can be achieved, like observing per-CPU cache
 * distribution.
 */
static int
sysctl_nchstats(SYSCTL_HANDLER_ARGS)
{
        struct globaldata *gd;
        int i, error;

        error = 0;
        for (i = 0; i < ncpus; ++i) {
                gd = globaldata_find(i);
                if ((error = SYSCTL_OUT(req, (void *)&(*gd->gd_nchstats),
                        sizeof(struct nchstats))))
                        break;
        }

        return (error);
}
SYSCTL_PROC(_vfs_cache, OID_AUTO, nchstats, CTLTYPE_OPAQUE|CTLFLAG_RD,
  0, 0, sysctl_nchstats, "S,nchstats", "VFS cache effectiveness statistics");

static struct namecache *cache_zap(struct namecache *ncp, int nonblock);

/*
 * Namespace locking.  The caller must already hold a reference to the
 * namecache structure in order to lock/unlock it.  This function prevents
 * the namespace from being created or destroyed by accessors other then
 * the lock holder.
 *
 * Note that holding a locked namecache structure prevents other threads
 * from making namespace changes (e.g. deleting or creating), prevents
 * vnode association state changes by other threads, and prevents the
 * namecache entry from being resolved or unresolved by other threads.
 *
 * An exclusive lock owner has full authority to associate/disassociate
 * vnodes and resolve/unresolve the locked ncp.
 *
 * A shared lock owner only has authority to acquire the underlying vnode,
 * if any.
 *
 * The primary lock field is nc_lockstatus.  nc_locktd is set after the
 * fact (when locking) or cleared prior to unlocking.
 *
 * WARNING!  Holding a locked ncp will prevent a vnode from being destroyed
 *           or recycled, but it does NOT help you if the vnode had already
 *           initiated a recyclement.  If this is important, use cache_get()
 *           rather then cache_lock() (and deal with the differences in the
 *           way the refs counter is handled).  Or, alternatively, make an
 *           unconditional call to cache_validate() or cache_resolve()
 *           after cache_lock() returns.
 */
static
void
_cache_lock(struct namecache *ncp)
{
        thread_t td;
        int didwarn;
        int error;
        u_int count;

        KKASSERT(ncp->nc_refs != 0);
        didwarn = 0;
        td = curthread;

        for (;;) {
                count = ncp->nc_lockstatus;
                cpu_ccfence();

                if ((count & ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ)) == 0) {
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, count + 1)) {
                                /*
                                 * The vp associated with a locked ncp must
                                 * be held to prevent it from being recycled.
                                 *
                                 * WARNING!  If VRECLAIMED is set the vnode
                                 * could already be in the middle of a recycle.
                                 * Callers must use cache_vref() or
                                 * cache_vget() on the locked ncp to
                                 * validate the vp or set the cache entry
                                 * to unresolved.
                                 *
                                 * NOTE! vhold() is allowed if we hold a
                                 *       lock on the ncp (which we do).
                                 */
                                ncp->nc_locktd = td;
                                if (ncp->nc_vp)
                                        vhold(ncp->nc_vp);
                                break;
                        }
                        /* cmpset failed */
                        continue;
                }
                if (ncp->nc_locktd == td) {
                        KKASSERT((count & NC_SHLOCK_FLAG) == 0);
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, count + 1)) {
                                break;
                        }
                        /* cmpset failed */
                        continue;
                }
                tsleep_interlock(&ncp->nc_locktd, 0);
                if (atomic_cmpset_int(&ncp->nc_lockstatus, count,
                                      count | NC_EXLOCK_REQ) == 0) {
                        /* cmpset failed */
                        continue;
                }
                error = tsleep(&ncp->nc_locktd, PINTERLOCKED,
                               "clock", nclockwarn);
                if (error == EWOULDBLOCK) {
                        if (didwarn == 0) {
                                didwarn = ticks;
                                kprintf("[diagnostic] cache_lock: "
                                        "blocked on %p %08x",
                                        ncp, count);
                                kprintf(" \"%*.*s\"\n",
                                        ncp->nc_nlen, ncp->nc_nlen,
                                        ncp->nc_name);
                        }
                }
                /* loop */
        }
        if (didwarn) {
                kprintf("[diagnostic] cache_lock: unblocked %*.*s after "
                        "%d secs\n",
                        ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name,
                        (int)(ticks - didwarn) / hz);
        }
}

/*
 * The shared lock works similarly to the exclusive lock except
 * nc_locktd is left NULL and we need an interlock (VHOLD) to
 * prevent vhold() races, since the moment our cmpset_int succeeds
 * another cpu can come in and get its own shared lock.
 *
 * A critical section is needed to prevent interruption during the
 * VHOLD interlock.
 */
static
void
_cache_lock_shared(struct namecache *ncp)
{
        int didwarn;
        int error;
        u_int count;

        KKASSERT(ncp->nc_refs != 0);
        didwarn = 0;

        for (;;) {
                count = ncp->nc_lockstatus;
                cpu_ccfence();

                if ((count & ~NC_SHLOCK_REQ) == 0) {
                        crit_enter();
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                      count,
                                      (count + 1) | NC_SHLOCK_FLAG |
                                                    NC_SHLOCK_VHOLD)) {
                                /*
                                 * The vp associated with a locked ncp must
                                 * be held to prevent it from being recycled.
                                 *
                                 * WARNING!  If VRECLAIMED is set the vnode
                                 * could already be in the middle of a recycle.
                                 * Callers must use cache_vref() or
                                 * cache_vget() on the locked ncp to
                                 * validate the vp or set the cache entry
                                 * to unresolved.
                                 *
                                 * NOTE! vhold() is allowed if we hold a
                                 *       lock on the ncp (which we do).
                                 */
                                if (ncp->nc_vp)
                                        vhold(ncp->nc_vp);
                                atomic_clear_int(&ncp->nc_lockstatus,
                                                 NC_SHLOCK_VHOLD);
                                crit_exit();
                                break;
                        }
                        /* cmpset failed */
                        crit_exit();
                        continue;
                }

                /*
                 * If already held shared we can just bump the count, but
                 * only allow this if nobody is trying to get the lock
                 * exclusively.
                 *
                 * VHOLD is a bit of a hack.  Even though we successfully
                 * added another shared ref, the cpu that got the first
                 * shared ref might not yet have held the vnode.
                 */
                if ((count & (NC_EXLOCK_REQ|NC_SHLOCK_FLAG)) ==
                    NC_SHLOCK_FLAG) {
                        KKASSERT((count & ~(NC_EXLOCK_REQ |
                                            NC_SHLOCK_REQ |
                                            NC_SHLOCK_FLAG)) > 0);
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, count + 1)) {
                                while (ncp->nc_lockstatus & NC_SHLOCK_VHOLD)
                                        cpu_pause();
                                break;
                        }
                        continue;
                }
                tsleep_interlock(ncp, 0);
                if (atomic_cmpset_int(&ncp->nc_lockstatus, count,
                                      count | NC_SHLOCK_REQ) == 0) {
                        /* cmpset failed */
                        continue;
                }
                error = tsleep(ncp, PINTERLOCKED, "clocksh", nclockwarn);
                if (error == EWOULDBLOCK) {
                        if (didwarn == 0) {
                                didwarn = ticks;
                                kprintf("[diagnostic] cache_lock_shared: "
                                        "blocked on %p %08x",
                                        ncp, count);
                                kprintf(" \"%*.*s\"\n",
                                        ncp->nc_nlen, ncp->nc_nlen,
                                        ncp->nc_name);
                        }
                }
                /* loop */
        }
        if (didwarn) {
                kprintf("[diagnostic] cache_lock_shared: "
                        "unblocked %*.*s after %d secs\n",
                        ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name,
                        (int)(ticks - didwarn) / hz);
        }
}

/*
 * NOTE: nc_refs may be zero if the ncp is interlocked by circumstance,
 *       such as the case where one of its children is locked.
 */
static
int
_cache_lock_nonblock(struct namecache *ncp)
{
        thread_t td;
        u_int count;

        td = curthread;

        for (;;) {
                count = ncp->nc_lockstatus;

                if ((count & ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ)) == 0) {
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, count + 1)) {
                                /*
                                 * The vp associated with a locked ncp must
                                 * be held to prevent it from being recycled.
                                 *
                                 * WARNING!  If VRECLAIMED is set the vnode
                                 * could already be in the middle of a recycle.
                                 * Callers must use cache_vref() or
                                 * cache_vget() on the locked ncp to
                                 * validate the vp or set the cache entry
                                 * to unresolved.
                                 *
                                 * NOTE! vhold() is allowed if we hold a
                                 *       lock on the ncp (which we do).
                                 */
                                ncp->nc_locktd = td;
                                if (ncp->nc_vp)
                                        vhold(ncp->nc_vp);
                                break;
                        }
                        /* cmpset failed */
                        continue;
                }
                if (ncp->nc_locktd == td) {
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, count + 1)) {
                                break;
                        }
                        /* cmpset failed */
                        continue;
                }
                return(EWOULDBLOCK);
        }
        return(0);
}

/*
 * The shared lock works similarly to the exclusive lock except
 * nc_locktd is left NULL and we need an interlock (VHOLD) to
 * prevent vhold() races, since the moment our cmpset_int succeeds
 * another cpu can come in and get its own shared lock.
 *
 * A critical section is needed to prevent interruption during the
 * VHOLD interlock.
 */
static
int
_cache_lock_shared_nonblock(struct namecache *ncp)
{
        u_int count;

        for (;;) {
                count = ncp->nc_lockstatus;

                if ((count & ~NC_SHLOCK_REQ) == 0) {
                        crit_enter();
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                      count,
                                      (count + 1) | NC_SHLOCK_FLAG |
                                                    NC_SHLOCK_VHOLD)) {
                                /*
                                 * The vp associated with a locked ncp must
                                 * be held to prevent it from being recycled.
                                 *
                                 * WARNING!  If VRECLAIMED is set the vnode
                                 * could already be in the middle of a recycle.
                                 * Callers must use cache_vref() or
                                 * cache_vget() on the locked ncp to
                                 * validate the vp or set the cache entry
                                 * to unresolved.
                                 *
                                 * NOTE! vhold() is allowed if we hold a
                                 *       lock on the ncp (which we do).
                                 */
                                if (ncp->nc_vp)
                                        vhold(ncp->nc_vp);
                                atomic_clear_int(&ncp->nc_lockstatus,
                                                 NC_SHLOCK_VHOLD);
                                crit_exit();
                                break;
                        }
                        /* cmpset failed */
                        crit_exit();
                        continue;
                }

                /*
                 * If already held shared we can just bump the count, but
                 * only allow this if nobody is trying to get the lock
                 * exclusively.
                 *
                 * VHOLD is a bit of a hack.  Even though we successfully
                 * added another shared ref, the cpu that got the first
                 * shared ref might not yet have held the vnode.
                 */
                if ((count & (NC_EXLOCK_REQ|NC_SHLOCK_FLAG)) ==
                    NC_SHLOCK_FLAG) {
                        KKASSERT((count & ~(NC_EXLOCK_REQ |
                                            NC_SHLOCK_REQ |
                                            NC_SHLOCK_FLAG)) > 0);
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, count + 1)) {
                                while (ncp->nc_lockstatus & NC_SHLOCK_VHOLD)
                                        cpu_pause();
                                break;
                        }
                        continue;
                }
                return(EWOULDBLOCK);
        }
        return(0);
}

/*
 * Helper function
 *
 * NOTE: nc_refs can be 0 (degenerate case during _cache_drop).
 *
 *       nc_locktd must be NULLed out prior to nc_lockstatus getting cleared.
 */
static
void
_cache_unlock(struct namecache *ncp)
{
        thread_t td __debugvar = curthread;
        u_int count;
        u_int ncount;
        struct vnode *dropvp;

        KKASSERT(ncp->nc_refs >= 0);
        KKASSERT((ncp->nc_lockstatus & ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ)) > 0);
        KKASSERT((ncp->nc_lockstatus & NC_SHLOCK_FLAG) || ncp->nc_locktd == td);

        count = ncp->nc_lockstatus;
        cpu_ccfence();

        /*
         * Clear nc_locktd prior to the atomic op (excl lock only)
         */
        if ((count & ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ)) == 1)
                ncp->nc_locktd = NULL;
        dropvp = NULL;

        for (;;) {
                if ((count &
                     ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ|NC_SHLOCK_FLAG)) == 1) {
                        dropvp = ncp->nc_vp;
                        if (count & NC_EXLOCK_REQ)
                                ncount = count & NC_SHLOCK_REQ; /* cnt->0 */
                        else
                                ncount = 0;

                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, ncount)) {
                                if (count & NC_EXLOCK_REQ)
                                        wakeup(&ncp->nc_locktd);
                                else if (count & NC_SHLOCK_REQ)
                                        wakeup(ncp);
                                break;
                        }
                        dropvp = NULL;
                } else {
                        KKASSERT((count & NC_SHLOCK_VHOLD) == 0);
                        KKASSERT((count & ~(NC_EXLOCK_REQ |
                                            NC_SHLOCK_REQ |
                                            NC_SHLOCK_FLAG)) > 1);
                        if (atomic_cmpset_int(&ncp->nc_lockstatus,
                                              count, count - 1)) {
                                break;
                        }
                }
                count = ncp->nc_lockstatus;
                cpu_ccfence();
        }

        /*
         * Don't actually drop the vp until we successfully clean out
         * the lock, otherwise we may race another shared lock.
         */
        if (dropvp)
                vdrop(dropvp);
}

static
int
_cache_lockstatus(struct namecache *ncp)
{
        if (ncp->nc_locktd == curthread)
                return(LK_EXCLUSIVE);
        if (ncp->nc_lockstatus & NC_SHLOCK_FLAG)
                return(LK_SHARED);
        return(-1);
}

/*
 * cache_hold() and cache_drop() prevent the premature deletion of a
 * namecache entry but do not prevent operations (such as zapping) on
 * that namecache entry.
 *
 * This routine may only be called from outside this source module if
 * nc_refs is already at least 1.
 *
 * This is a rare case where callers are allowed to hold a spinlock,
 * so we can't ourselves.
 */
static __inline
struct namecache *
_cache_hold(struct namecache *ncp)
{
        atomic_add_int(&ncp->nc_refs, 1);
        return(ncp);
}

/*
 * Drop a cache entry, taking care to deal with races.
 *
 * For potential 1->0 transitions we must hold the ncp lock to safely
 * test its flags.  An unresolved entry with no children must be zapped
 * to avoid leaks.
 *
 * The call to cache_zap() itself will handle all remaining races and
 * will decrement the ncp's refs regardless.  If we are resolved or
 * have children nc_refs can safely be dropped to 0 without having to
 * zap the entry.
 *
 * NOTE: cache_zap() will re-check nc_refs and nc_list in a MPSAFE fashion.
 *
 * NOTE: cache_zap() may return a non-NULL referenced parent which must
 *       be dropped in a loop.
 */
static __inline
void
_cache_drop(struct namecache *ncp)
{
        int refs;

        while (ncp) {
                KKASSERT(ncp->nc_refs > 0);
                refs = ncp->nc_refs;

                if (refs == 1) {
                        if (_cache_lock_nonblock(ncp) == 0) {
                                ncp->nc_flag &= ~NCF_DEFEREDZAP;
                                if ((ncp->nc_flag & NCF_UNRESOLVED) &&
                                    TAILQ_EMPTY(&ncp->nc_list)) {
                                        ncp = cache_zap(ncp, 1);
                                        continue;
                                }
                                if (atomic_cmpset_int(&ncp->nc_refs, 1, 0)) {
                                        _cache_unlock(ncp);
                                        break;
                                }
                                _cache_unlock(ncp);
                        }
                } else {
                        if (atomic_cmpset_int(&ncp->nc_refs, refs, refs - 1))
                                break;
                }
                cpu_pause();
        }
}

/*
 * Link a new namecache entry to its parent and to the hash table.  Be
 * careful to avoid races if vhold() blocks in the future.
 *
 * Both ncp and par must be referenced and locked.
 *
 * NOTE: The hash table spinlock is likely held during this call, we
 *       can't do anything fancy.
 */
static void
_cache_link_parent(struct namecache *ncp, struct namecache *par,
                   struct nchash_head *nchpp)
{
        KKASSERT(ncp->nc_parent == NULL);
        ncp->nc_parent = par;
        ncp->nc_head = nchpp;

        /*
         * Set inheritance flags.  Note that the parent flags may be
         * stale due to getattr potentially not having been run yet
         * (it gets run during nlookup()'s).
         */
        ncp->nc_flag &= ~(NCF_SF_PNOCACHE | NCF_UF_PCACHE);
        if (par->nc_flag & (NCF_SF_NOCACHE | NCF_SF_PNOCACHE))
                ncp->nc_flag |= NCF_SF_PNOCACHE;
        if (par->nc_flag & (NCF_UF_CACHE | NCF_UF_PCACHE))
                ncp->nc_flag |= NCF_UF_PCACHE;

        LIST_INSERT_HEAD(&nchpp->list, ncp, nc_hash);

        if (TAILQ_EMPTY(&par->nc_list)) {
                TAILQ_INSERT_HEAD(&par->nc_list, ncp, nc_entry);
                /*
                 * Any vp associated with an ncp which has children must
                 * be held to prevent it from being recycled.
                 */
                if (par->nc_vp)
                        vhold(par->nc_vp);
        } else {
                TAILQ_INSERT_HEAD(&par->nc_list, ncp, nc_entry);
        }
}

/*
 * Remove the parent and hash associations from a namecache structure.
 * If this is the last child of the parent the cache_drop(par) will
 * attempt to recursively zap the parent.
 *
 * ncp must be locked.  This routine will acquire a temporary lock on
 * the parent as wlel as the appropriate hash chain.
 */
static void
_cache_unlink_parent(struct namecache *ncp)
{
        struct namecache *par;
        struct vnode *dropvp;

        if ((par = ncp->nc_parent) != NULL) {
                KKASSERT(ncp->nc_parent == par);
                _cache_hold(par);
                _cache_lock(par);
                spin_lock(&ncp->nc_head->spin);
                LIST_REMOVE(ncp, nc_hash);
                TAILQ_REMOVE(&par->nc_list, ncp, nc_entry);
                dropvp = NULL;
                if (par->nc_vp && TAILQ_EMPTY(&par->nc_list))
                        dropvp = par->nc_vp;
                spin_unlock(&ncp->nc_head->spin);
                ncp->nc_parent = NULL;
                ncp->nc_head = NULL;
                _cache_unlock(par);
                _cache_drop(par);

                /*
                 * We can only safely vdrop with no spinlocks held.
                 */
                if (dropvp)
                        vdrop(dropvp);
        }
}

/*
 * Allocate a new namecache structure.  Most of the code does not require
 * zero-termination of the string but it makes vop_compat_ncreate() easier.
 */
static struct namecache *
cache_alloc(int nlen)
{
        struct namecache *ncp;

        ncp = kmalloc(sizeof(*ncp), M_VFSCACHE, M_WAITOK|M_ZERO);
        if (nlen)
                ncp->nc_name = kmalloc(nlen + 1, M_VFSCACHE, M_WAITOK);
        ncp->nc_nlen = nlen;
        ncp->nc_flag = NCF_UNRESOLVED;
        ncp->nc_error = ENOTCONN;       /* needs to be resolved */
        ncp->nc_refs = 1;

        TAILQ_INIT(&ncp->nc_list);
        _cache_lock(ncp);
        return(ncp);
}

/*
 * Can only be called for the case where the ncp has never been
 * associated with anything (so no spinlocks are needed).
 */
static void
_cache_free(struct namecache *ncp)
{
        KKASSERT(ncp->nc_refs == 1 && ncp->nc_lockstatus == 1);
        if (ncp->nc_name)
                kfree(ncp->nc_name, M_VFSCACHE);
        kfree(ncp, M_VFSCACHE);
}

/*
 * [re]initialize a nchandle.
 */
void
cache_zero(struct nchandle *nch)
{
        nch->ncp = NULL;
        nch->mount = NULL;
}

/*
 * Ref and deref a namecache structure.
 *
 * The caller must specify a stable ncp pointer, typically meaning the
 * ncp is already referenced but this can also occur indirectly through
 * e.g. holding a lock on a direct child.
 *
 * WARNING: Caller may hold an unrelated read spinlock, which means we can't
 *          use read spinlocks here.
 *
 * MPSAFE if nch is
 */
struct nchandle *
cache_hold(struct nchandle *nch)
{
        _cache_hold(nch->ncp);
        atomic_add_int(&nch->mount->mnt_refs, 1);
        return(nch);
}

/*
 * Create a copy of a namecache handle for an already-referenced
 * entry.
 *
 * MPSAFE if nch is
 */
void
cache_copy(struct nchandle *nch, struct nchandle *target)
{
        *target = *nch;
        if (target->ncp)
                _cache_hold(target->ncp);
        atomic_add_int(&nch->mount->mnt_refs, 1);
}

/*
 * MPSAFE if nch is
 */
void
cache_changemount(struct nchandle *nch, struct mount *mp)
{
        atomic_add_int(&nch->mount->mnt_refs, -1);
        nch->mount = mp;
        atomic_add_int(&nch->mount->mnt_refs, 1);
}

void
cache_drop(struct nchandle *nch)
{
        atomic_add_int(&nch->mount->mnt_refs, -1);
        _cache_drop(nch->ncp);
        nch->ncp = NULL;
        nch->mount = NULL;
}

int
cache_lockstatus(struct nchandle *nch)
{
        return(_cache_lockstatus(nch->ncp));
}

void
cache_lock(struct nchandle *nch)
{
        _cache_lock(nch->ncp);
}

void
cache_lock_maybe_shared(struct nchandle *nch, int excl)
{
        struct namecache *ncp = nch->ncp;

        if (ncp_shared_lock_disable || excl ||
            (ncp->nc_flag & NCF_UNRESOLVED)) {
                _cache_lock(ncp);
        } else {
                _cache_lock_shared(ncp);
                if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                        if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED)) {
                                _cache_unlock(ncp);
                                _cache_lock(ncp);
                        }
                } else {
                        _cache_unlock(ncp);
                        _cache_lock(ncp);
                }
        }
}

/*
 * Relock nch1 given an unlocked nch1 and a locked nch2.  The caller
 * is responsible for checking both for validity on return as they
 * may have become invalid.
 *
 * We have to deal with potential deadlocks here, just ping pong
 * the lock until we get it (we will always block somewhere when
 * looping so this is not cpu-intensive).
 *
 * which = 0    nch1 not locked, nch2 is locked
 * which = 1    nch1 is locked, nch2 is not locked
 */
void
cache_relock(struct nchandle *nch1, struct ucred *cred1,
             struct nchandle *nch2, struct ucred *cred2)
{
        int which;

        which = 0;

        for (;;) {
                if (which == 0) {
                        if (cache_lock_nonblock(nch1) == 0) {
                                cache_resolve(nch1, cred1);
                                break;
                        }
                        cache_unlock(nch2);
                        cache_lock(nch1);
                        cache_resolve(nch1, cred1);
                        which = 1;
                } else {
                        if (cache_lock_nonblock(nch2) == 0) {
                                cache_resolve(nch2, cred2);
                                break;
                        }
                        cache_unlock(nch1);
                        cache_lock(nch2);
                        cache_resolve(nch2, cred2);
                        which = 0;
                }
        }
}

int
cache_lock_nonblock(struct nchandle *nch)
{
        return(_cache_lock_nonblock(nch->ncp));
}

void
cache_unlock(struct nchandle *nch)
{
        _cache_unlock(nch->ncp);
}

/*
 * ref-and-lock, unlock-and-deref functions.
 *
 * This function is primarily used by nlookup.  Even though cache_lock
 * holds the vnode, it is possible that the vnode may have already
 * initiated a recyclement.
 *
 * We want cache_get() to return a definitively usable vnode or a
 * definitively unresolved ncp.
 */
static
struct namecache *
_cache_get(struct namecache *ncp)
{
        _cache_hold(ncp);
        _cache_lock(ncp);
        if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                _cache_setunresolved(ncp);
        return(ncp);
}

/*
 * Attempt to obtain a shared lock on the ncp.  A shared lock will only
 * be obtained if the ncp is resolved and the vnode (if not ENOENT) is
 * valid.  Otherwise an exclusive lock will be acquired instead.
 */
static
struct namecache *
_cache_get_maybe_shared(struct namecache *ncp, int excl)
{
        if (ncp_shared_lock_disable || excl ||
            (ncp->nc_flag & NCF_UNRESOLVED)) {
                return(_cache_get(ncp));
        }
        _cache_hold(ncp);
        _cache_lock_shared(ncp);
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED)) {
                        _cache_unlock(ncp);
                        ncp = _cache_get(ncp);
                        _cache_drop(ncp);
                }
        } else {
                _cache_unlock(ncp);
                ncp = _cache_get(ncp);
                _cache_drop(ncp);
        }
        return(ncp);
}

/*
 * This is a special form of _cache_lock() which only succeeds if
 * it can get a pristine, non-recursive lock.  The caller must have
 * already ref'd the ncp.
 *
 * On success the ncp will be locked, on failure it will not.  The
 * ref count does not change either way.
 *
 * We want _cache_lock_special() (on success) to return a definitively
 * usable vnode or a definitively unresolved ncp.
 */
static int
_cache_lock_special(struct namecache *ncp)
{
        if (_cache_lock_nonblock(ncp) == 0) {
                if ((ncp->nc_lockstatus &
                     ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ)) == 1) {
                        if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                                _cache_setunresolved(ncp);
                        return(0);
                }
                _cache_unlock(ncp);
        }
        return(EWOULDBLOCK);
}

static int
_cache_lock_shared_special(struct namecache *ncp)
{
        if (_cache_lock_shared_nonblock(ncp) == 0) {
                if ((ncp->nc_lockstatus &
                     ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ)) == (NC_SHLOCK_FLAG | 1)) {
                        if (ncp->nc_vp == NULL ||
                            (ncp->nc_vp->v_flag & VRECLAIMED) == 0) {
                                return(0);
                        }
                }
                _cache_unlock(ncp);
        }
        return(EWOULDBLOCK);
}


/*
 * NOTE: The same nchandle can be passed for both arguments.
 */
void
cache_get(struct nchandle *nch, struct nchandle *target)
{
        KKASSERT(nch->ncp->nc_refs > 0);
        target->mount = nch->mount;
        target->ncp = _cache_get(nch->ncp);
        atomic_add_int(&target->mount->mnt_refs, 1);
}

void
cache_get_maybe_shared(struct nchandle *nch, struct nchandle *target, int excl)
{
        KKASSERT(nch->ncp->nc_refs > 0);
        target->mount = nch->mount;
        target->ncp = _cache_get_maybe_shared(nch->ncp, excl);
        atomic_add_int(&target->mount->mnt_refs, 1);
}

/*
 *
 */
static __inline
void
_cache_put(struct namecache *ncp)
{
        _cache_unlock(ncp);
        _cache_drop(ncp);
}

/*
 *
 */
void
cache_put(struct nchandle *nch)
{
        atomic_add_int(&nch->mount->mnt_refs, -1);
        _cache_put(nch->ncp);
        nch->ncp = NULL;
        nch->mount = NULL;
}

/*
 * Resolve an unresolved ncp by associating a vnode with it.  If the
 * vnode is NULL, a negative cache entry is created.
 *
 * The ncp should be locked on entry and will remain locked on return.
 */
static
void
_cache_setvp(struct mount *mp, struct namecache *ncp, struct vnode *vp)
{
        KKASSERT(ncp->nc_flag & NCF_UNRESOLVED);
        KKASSERT(_cache_lockstatus(ncp) == LK_EXCLUSIVE);

        if (vp != NULL) {
                /*
                 * Any vp associated with an ncp which has children must
                 * be held.  Any vp associated with a locked ncp must be held.
                 */
                if (!TAILQ_EMPTY(&ncp->nc_list))
                        vhold(vp);
                spin_lock(&vp->v_spin);
                ncp->nc_vp = vp;
                TAILQ_INSERT_HEAD(&vp->v_namecache, ncp, nc_vnode);
                spin_unlock(&vp->v_spin);
                if (ncp->nc_lockstatus & ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ))
                        vhold(vp);

                /*
                 * Set auxiliary flags
                 */
                switch(vp->v_type) {
                case VDIR:
                        ncp->nc_flag |= NCF_ISDIR;
                        break;
                case VLNK:
                        ncp->nc_flag |= NCF_ISSYMLINK;
                        /* XXX cache the contents of the symlink */
                        break;
                default:
                        break;
                }
                atomic_add_int(&numcache, 1);
                ncp->nc_error = 0;
                /* XXX: this is a hack to work-around the lack of a real pfs vfs
                 * implementation*/
                if (mp != NULL)
                        if (strncmp(mp->mnt_stat.f_fstypename, "null", 5) == 0)
                                vp->v_pfsmp = mp;
        } else {
                /*
                 * When creating a negative cache hit we set the
                 * namecache_gen.  A later resolve will clean out the
                 * negative cache hit if the mount point's namecache_gen
                 * has changed.  Used by devfs, could also be used by
                 * other remote FSs.
                 */
                ncp->nc_vp = NULL;
                spin_lock(&ncspin);
                TAILQ_INSERT_TAIL(&ncneglist, ncp, nc_vnode);
                ++numneg;
                spin_unlock(&ncspin);
                ncp->nc_error = ENOENT;
                if (mp)
                        VFS_NCPGEN_SET(mp, ncp);
        }
        ncp->nc_flag &= ~(NCF_UNRESOLVED | NCF_DEFEREDZAP);
}

/*
 *
 */
void
cache_setvp(struct nchandle *nch, struct vnode *vp)
{
        _cache_setvp(nch->mount, nch->ncp, vp);
}

/*
 *
 */
void
cache_settimeout(struct nchandle *nch, int nticks)
{
        struct namecache *ncp = nch->ncp;

        if ((ncp->nc_timeout = ticks + nticks) == 0)
                ncp->nc_timeout = 1;
}

/*
 * Disassociate the vnode or negative-cache association and mark a
 * namecache entry as unresolved again.  Note that the ncp is still
 * left in the hash table and still linked to its parent.
 *
 * The ncp should be locked and refd on entry and will remain locked and refd
 * on return.
 *
 * This routine is normally never called on a directory containing children.
 * However, NFS often does just that in its rename() code as a cop-out to
 * avoid complex namespace operations.  This disconnects a directory vnode
 * from its namecache and can cause the OLDAPI and NEWAPI to get out of
 * sync.
 *
 */
static
void
_cache_setunresolved(struct namecache *ncp)
{
        struct vnode *vp;

        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                ncp->nc_flag |= NCF_UNRESOLVED;
                ncp->nc_timeout = 0;
                ncp->nc_error = ENOTCONN;
                if ((vp = ncp->nc_vp) != NULL) {
                        atomic_add_int(&numcache, -1);
                        spin_lock(&vp->v_spin);
                        ncp->nc_vp = NULL;
                        TAILQ_REMOVE(&vp->v_namecache, ncp, nc_vnode);
                        spin_unlock(&vp->v_spin);

                        /*
                         * Any vp associated with an ncp with children is
                         * held by that ncp.  Any vp associated with a locked
                         * ncp is held by that ncp.  These conditions must be
                         * undone when the vp is cleared out from the ncp.
                         */
                        if (!TAILQ_EMPTY(&ncp->nc_list))
                                vdrop(vp);
                        if (ncp->nc_lockstatus & ~(NC_EXLOCK_REQ|NC_SHLOCK_REQ))
                                vdrop(vp);
                } else {
                        spin_lock(&ncspin);
                        TAILQ_REMOVE(&ncneglist, ncp, nc_vnode);
                        --numneg;
                        spin_unlock(&ncspin);
                }
                ncp->nc_flag &= ~(NCF_WHITEOUT|NCF_ISDIR|NCF_ISSYMLINK);
        }
}

/*
 * The cache_nresolve() code calls this function to automatically
 * set a resolved cache element to unresolved if it has timed out
 * or if it is a negative cache hit and the mount point namecache_gen
 * has changed.
 */
static __inline int
_cache_auto_unresolve_test(struct mount *mp, struct namecache *ncp)
{
        /*
         * Try to zap entries that have timed out.  We have
         * to be careful here because locked leafs may depend
         * on the vnode remaining intact in a parent, so only
         * do this under very specific conditions.
         */
        if (ncp->nc_timeout && (int)(ncp->nc_timeout - ticks) < 0 &&
            TAILQ_EMPTY(&ncp->nc_list)) {
                return 1;
        }

        /*
         * If a resolved negative cache hit is invalid due to
         * the mount's namecache generation being bumped, zap it.
         */
        if (ncp->nc_vp == NULL && VFS_NCPGEN_TEST(mp, ncp)) {
                return 1;
        }

        /*
         * Otherwise we are good
         */
        return 0;
}

static __inline void
_cache_auto_unresolve(struct mount *mp, struct namecache *ncp)
{
        /*
         * Already in an unresolved state, nothing to do.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                if (_cache_auto_unresolve_test(mp, ncp))
                        _cache_setunresolved(ncp);
        }
}

/*
 *
 */
void
cache_setunresolved(struct nchandle *nch)
{
        _cache_setunresolved(nch->ncp);
}

/*
 * Determine if we can clear NCF_ISMOUNTPT by scanning the mountlist
 * looking for matches.  This flag tells the lookup code when it must
 * check for a mount linkage and also prevents the directories in question
 * from being deleted or renamed.
 */
static
int
cache_clrmountpt_callback(struct mount *mp, void *data)
{
        struct nchandle *nch = data;

        if (mp->mnt_ncmounton.ncp == nch->ncp)
                return(1);
        if (mp->mnt_ncmountpt.ncp == nch->ncp)
                return(1);
        return(0);
}

/*
 *
 */
void
cache_clrmountpt(struct nchandle *nch)
{
        int count;

        count = mountlist_scan(cache_clrmountpt_callback, nch,
                               MNTSCAN_FORWARD|MNTSCAN_NOBUSY);
        if (count == 0)
                nch->ncp->nc_flag &= ~NCF_ISMOUNTPT;
}

/*
 * Invalidate portions of the namecache topology given a starting entry.
 * The passed ncp is set to an unresolved state and:
 *
 * The passed ncp must be referencxed and locked.  The routine may unlock
 * and relock ncp several times, and will recheck the children and loop
 * to catch races.  When done the passed ncp will be returned with the
 * reference and lock intact.
 *
 * CINV_DESTROY         - Set a flag in the passed ncp entry indicating
 *                        that the physical underlying nodes have been 
 *                        destroyed... as in deleted.  For example, when
 *                        a directory is removed.  This will cause record
 *                        lookups on the name to no longer be able to find
 *                        the record and tells the resolver to return failure
 *                        rather then trying to resolve through the parent.
 *
 *                        The topology itself, including ncp->nc_name,
 *                        remains intact.
 *
 *                        This only applies to the passed ncp, if CINV_CHILDREN
 *                        is specified the children are not flagged.
 *
 * CINV_CHILDREN        - Set all children (recursively) to an unresolved
 *                        state as well.
 *
 *                        Note that this will also have the side effect of
 *                        cleaning out any unreferenced nodes in the topology
 *                        from the leaves up as the recursion backs out.
 *
 * Note that the topology for any referenced nodes remains intact, but
 * the nodes will be marked as having been destroyed and will be set
 * to an unresolved state.
 *
 * It is possible for cache_inval() to race a cache_resolve(), meaning that
 * the namecache entry may not actually be invalidated on return if it was
 * revalidated while recursing down into its children.  This code guarentees
 * that the node(s) will go through an invalidation cycle, but does not 
 * guarentee that they will remain in an invalidated state. 
 *
 * Returns non-zero if a revalidation was detected during the invalidation
 * recursion, zero otherwise.  Note that since only the original ncp is
 * locked the revalidation ultimately can only indicate that the original ncp
 * *MIGHT* no have been reresolved.
 *
 * DEEP RECURSION HANDLING - If a recursive invalidation recurses deeply we
 * have to avoid blowing out the kernel stack.  We do this by saving the
 * deep namecache node and aborting the recursion, then re-recursing at that
 * node using a depth-first algorithm in order to allow multiple deep
 * recursions to chain through each other, then we restart the invalidation
 * from scratch.
 */

struct cinvtrack {
        struct namecache *resume_ncp;
        int depth;
};

static int _cache_inval_internal(struct namecache *, int, struct cinvtrack *);

static
int
_cache_inval(struct namecache *ncp, int flags)
{
        struct cinvtrack track;
        struct namecache *ncp2;
        int r;

        track.depth = 0;
        track.resume_ncp = NULL;

        for (;;) {
                r = _cache_inval_internal(ncp, flags, &track);
                if (track.resume_ncp == NULL)
                        break;
                kprintf("Warning: deep namecache recursion at %s\n",
                        ncp->nc_name);
                _cache_unlock(ncp);
                while ((ncp2 = track.resume_ncp) != NULL) {
                        track.resume_ncp = NULL;
                        _cache_lock(ncp2);
                        _cache_inval_internal(ncp2, flags & ~CINV_DESTROY,
                                             &track);
                        _cache_put(ncp2);
                }
                _cache_lock(ncp);
        }
        return(r);
}

int
cache_inval(struct nchandle *nch, int flags)
{
        return(_cache_inval(nch->ncp, flags));
}

/*
 * Helper for _cache_inval().  The passed ncp is refd and locked and
 * remains that way on return, but may be unlocked/relocked multiple
 * times by the routine.
 */
static int
_cache_inval_internal(struct namecache *ncp, int flags, struct cinvtrack *track)
{
        struct namecache *kid;
        struct namecache *nextkid;
        int rcnt = 0;

        KKASSERT(_cache_lockstatus(ncp) == LK_EXCLUSIVE);

        _cache_setunresolved(ncp);
        if (flags & CINV_DESTROY)
                ncp->nc_flag |= NCF_DESTROYED;
        if ((flags & CINV_CHILDREN) && 
            (kid = TAILQ_FIRST(&ncp->nc_list)) != NULL
        ) {
                _cache_hold(kid);
                if (++track->depth > MAX_RECURSION_DEPTH) {
                        track->resume_ncp = ncp;
                        _cache_hold(ncp);
                        ++rcnt;
                }
                _cache_unlock(ncp);
                while (kid) {
                        if (track->resume_ncp) {
                                _cache_drop(kid);
                                break;
                        }
                        if ((nextkid = TAILQ_NEXT(kid, nc_entry)) != NULL)
                                _cache_hold(nextkid);
                        if ((kid->nc_flag & NCF_UNRESOLVED) == 0 ||
                            TAILQ_FIRST(&kid->nc_list)
                        ) {
                                _cache_lock(kid);
                                rcnt += _cache_inval_internal(kid, flags & ~CINV_DESTROY, track);
                                _cache_unlock(kid);
                        }
                        _cache_drop(kid);
                        kid = nextkid;
                }
                --track->depth;
                _cache_lock(ncp);
        }

        /*
         * Someone could have gotten in there while ncp was unlocked,
         * retry if so.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0)
                ++rcnt;
        return (rcnt);
}

/*
 * Invalidate a vnode's namecache associations.  To avoid races against
 * the resolver we do not invalidate a node which we previously invalidated
 * but which was then re-resolved while we were in the invalidation loop.
 *
 * Returns non-zero if any namecache entries remain after the invalidation
 * loop completed.
 *
 * NOTE: Unlike the namecache topology which guarentees that ncp's will not
 *       be ripped out of the topology while held, the vnode's v_namecache
 *       list has no such restriction.  NCP's can be ripped out of the list
 *       at virtually any time if not locked, even if held.
 *
 *       In addition, the v_namecache list itself must be locked via
 *       the vnode's spinlock.
 */
int
cache_inval_vp(struct vnode *vp, int flags)
{
        struct namecache *ncp;
        struct namecache *next;

restart:
        spin_lock(&vp->v_spin);
        ncp = TAILQ_FIRST(&vp->v_namecache);
        if (ncp)
                _cache_hold(ncp);
        while (ncp) {
                /* loop entered with ncp held and vp spin-locked */
                if ((next = TAILQ_NEXT(ncp, nc_vnode)) != NULL)
                        _cache_hold(next);
                spin_unlock(&vp->v_spin);
                _cache_lock(ncp);
                if (ncp->nc_vp != vp) {
                        kprintf("Warning: cache_inval_vp: race-A detected on "
                                "%s\n", ncp->nc_name);
                        _cache_put(ncp);
                        if (next)
                                _cache_drop(next);
                        goto restart;
                }
                _cache_inval(ncp, flags);
                _cache_put(ncp);                /* also releases reference */
                ncp = next;
                spin_lock(&vp->v_spin);
                if (ncp && ncp->nc_vp != vp) {
                        spin_unlock(&vp->v_spin);
                        kprintf("Warning: cache_inval_vp: race-B detected on "
                                "%s\n", ncp->nc_name);
                        _cache_drop(ncp);
                        goto restart;
                }
        }
        spin_unlock(&vp->v_spin);
        return(TAILQ_FIRST(&vp->v_namecache) != NULL);
}

/*
 * This routine is used instead of the normal cache_inval_vp() when we
 * are trying to recycle otherwise good vnodes.
 *
 * Return 0 on success, non-zero if not all namecache records could be
 * disassociated from the vnode (for various reasons).
 */
int
cache_inval_vp_nonblock(struct vnode *vp)
{
        struct namecache *ncp;
        struct namecache *next;

        spin_lock(&vp->v_spin);
        ncp = TAILQ_FIRST(&vp->v_namecache);
        if (ncp)
                _cache_hold(ncp);
        while (ncp) {
                /* loop entered with ncp held */
                if ((next = TAILQ_NEXT(ncp, nc_vnode)) != NULL)
                        _cache_hold(next);
                spin_unlock(&vp->v_spin);
                if (_cache_lock_nonblock(ncp)) {
                        _cache_drop(ncp);
                        if (next)
                                _cache_drop(next);
                        goto done;
                }
                if (ncp->nc_vp != vp) {
                        kprintf("Warning: cache_inval_vp: race-A detected on "
                                "%s\n", ncp->nc_name);
                        _cache_put(ncp);
                        if (next)
                                _cache_drop(next);
                        goto done;
                }
                _cache_inval(ncp, 0);
                _cache_put(ncp);                /* also releases reference */
                ncp = next;
                spin_lock(&vp->v_spin);
                if (ncp && ncp->nc_vp != vp) {
                        spin_unlock(&vp->v_spin);
                        kprintf("Warning: cache_inval_vp: race-B detected on "
                                "%s\n", ncp->nc_name);
                        _cache_drop(ncp);
                        goto done;
                }
        }
        spin_unlock(&vp->v_spin);
done:
        return(TAILQ_FIRST(&vp->v_namecache) != NULL);
}

/*
 * The source ncp has been renamed to the target ncp.  Both fncp and tncp
 * must be locked.  The target ncp is destroyed (as a normal rename-over
 * would destroy the target file or directory).
 *
 * Because there may be references to the source ncp we cannot copy its
 * contents to the target.  Instead the source ncp is relinked as the target
 * and the target ncp is removed from the namecache topology.
 */
void
cache_rename(struct nchandle *fnch, struct nchandle *tnch)
{
        struct namecache *fncp = fnch->ncp;
        struct namecache *tncp = tnch->ncp;
        struct namecache *tncp_par;
        struct nchash_head *nchpp;
        u_int32_t hash;
        char *oname;
        char *nname;

        if (tncp->nc_nlen) {
                nname = kmalloc(tncp->nc_nlen + 1, M_VFSCACHE, M_WAITOK);
                bcopy(tncp->nc_name, nname, tncp->nc_nlen);
                nname[tncp->nc_nlen] = 0;
        } else {
                nname = NULL;
        }

        /*
         * Rename fncp (unlink)
         */
        _cache_unlink_parent(fncp);
        oname = fncp->nc_name;
        fncp->nc_name = nname;
        fncp->nc_nlen = tncp->nc_nlen;
        if (oname)
                kfree(oname, M_VFSCACHE);

        tncp_par = tncp->nc_parent;
        _cache_hold(tncp_par);
        _cache_lock(tncp_par);

        /*
         * Rename fncp (relink)
         */
        hash = fnv_32_buf(fncp->nc_name, fncp->nc_nlen, FNV1_32_INIT);
        hash = fnv_32_buf(&tncp_par, sizeof(tncp_par), hash);
        nchpp = NCHHASH(hash);

        spin_lock(&nchpp->spin);
        _cache_link_parent(fncp, tncp_par, nchpp);
        spin_unlock(&nchpp->spin);

        _cache_put(tncp_par);

        /*
         * Get rid of the overwritten tncp (unlink)
         */
        _cache_unlink(tncp);
}

/*
 * Perform actions consistent with unlinking a file.  The passed-in ncp
 * must be locked.
 *
 * The ncp is marked DESTROYED so it no longer shows up in searches,
 * and will be physically deleted when the vnode goes away.
 *
 * If the related vnode has no refs then we cycle it through vget()/vput()
 * to (possibly if we don't have a ref race) trigger a deactivation,
 * allowing the VFS to trivially detect and recycle the deleted vnode
 * via VOP_INACTIVE().
 *
 * NOTE: _cache_rename() will automatically call _cache_unlink() on the
 *       target ncp.
 */
void
cache_unlink(struct nchandle *nch)
{
        _cache_unlink(nch->ncp);
}

static void
_cache_unlink(struct namecache *ncp)
{
        struct vnode *vp;

        /*
         * Causes lookups to fail and allows another ncp with the same
         * name to be created under ncp->nc_parent.
         */
        ncp->nc_flag |= NCF_DESTROYED;

        /*
         * Attempt to trigger a deactivation.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0 &&
            (vp = ncp->nc_vp) != NULL &&
            !sysref_isactive(&vp->v_sysref)) {
                if (vget(vp, LK_SHARED) == 0)
                        vput(vp);
        }
}

/*
 * vget the vnode associated with the namecache entry.  Resolve the namecache
 * entry if necessary.  The passed ncp must be referenced and locked.
 *
 * lk_type may be LK_SHARED, LK_EXCLUSIVE.  A ref'd, possibly locked
 * (depending on the passed lk_type) will be returned in *vpp with an error
 * of 0, or NULL will be returned in *vpp with a non-0 error code.  The
 * most typical error is ENOENT, meaning that the ncp represents a negative
 * cache hit and there is no vnode to retrieve, but other errors can occur
 * too.
 *
 * The vget() can race a reclaim.  If this occurs we re-resolve the
 * namecache entry.
 *
 * There are numerous places in the kernel where vget() is called on a
 * vnode while one or more of its namecache entries is locked.  Releasing
 * a vnode never deadlocks against locked namecache entries (the vnode
 * will not get recycled while referenced ncp's exist).  This means we
 * can safely acquire the vnode.  In fact, we MUST NOT release the ncp
 * lock when acquiring the vp lock or we might cause a deadlock.
 *
 * NOTE: The passed-in ncp must be locked exclusively if it is initially
 *       unresolved.  If a reclaim race occurs the passed-in ncp will be
 *       relocked exclusively before being re-resolved.
 */
int
cache_vget(struct nchandle *nch, struct ucred *cred,
           int lk_type, struct vnode **vpp)
{
        struct namecache *ncp;
        struct vnode *vp;
        int error;

        ncp = nch->ncp;
again:
        vp = NULL;
        if (ncp->nc_flag & NCF_UNRESOLVED)
                error = cache_resolve(nch, cred);
        else
                error = 0;

        if (error == 0 && (vp = ncp->nc_vp) != NULL) {
                error = vget(vp, lk_type);
                if (error) {
                        /*
                         * VRECLAIM race
                         */
                        if (error == ENOENT) {
                                kprintf("Warning: vnode reclaim race detected "
                                        "in cache_vget on %p (%s)\n",
                                        vp, ncp->nc_name);
                                _cache_unlock(ncp);
                                _cache_lock(ncp);
                                _cache_setunresolved(ncp);
                                goto again;
                        }

                        /*
                         * Not a reclaim race, some other error.
                         */
                        KKASSERT(ncp->nc_vp == vp);
                        vp = NULL;
                } else {
                        KKASSERT(ncp->nc_vp == vp);
                        KKASSERT((vp->v_flag & VRECLAIMED) == 0);
                }
        }
        if (error == 0 && vp == NULL)
                error = ENOENT;
        *vpp = vp;
        return(error);
}

/*
 * Similar to cache_vget() but only acquires a ref on the vnode.
 *
 * NOTE: The passed-in ncp must be locked exclusively if it is initially
 *       unresolved.  If a reclaim race occurs the passed-in ncp will be
 *       relocked exclusively before being re-resolved.
 */
int
cache_vref(struct nchandle *nch, struct ucred *cred, struct vnode **vpp)
{
        struct namecache *ncp;
        struct vnode *vp;
        int error;

        ncp = nch->ncp;
again:
        vp = NULL;
        if (ncp->nc_flag & NCF_UNRESOLVED)
                error = cache_resolve(nch, cred);
        else
                error = 0;

        if (error == 0 && (vp = ncp->nc_vp) != NULL) {
                error = vget(vp, LK_SHARED);
                if (error) {
                        /*
                         * VRECLAIM race
                         */
                        if (error == ENOENT) {
                                kprintf("Warning: vnode reclaim race detected "
                                        "in cache_vget on %p (%s)\n",
                                        vp, ncp->nc_name);
                                _cache_unlock(ncp);
                                _cache_lock(ncp);
                                _cache_setunresolved(ncp);
                                goto again;
                        }

                        /*
                         * Not a reclaim race, some other error.
                         */
                        KKASSERT(ncp->nc_vp == vp);
                        vp = NULL;
                } else {
                        KKASSERT(ncp->nc_vp == vp);
                        KKASSERT((vp->v_flag & VRECLAIMED) == 0);
                        /* caller does not want a lock */
                        vn_unlock(vp);
                }
        }
        if (error == 0 && vp == NULL)
                error = ENOENT;
        *vpp = vp;
        return(error);
}

/*
 * Return a referenced vnode representing the parent directory of
 * ncp.
 *
 * Because the caller has locked the ncp it should not be possible for
 * the parent ncp to go away.  However, the parent can unresolve its
 * dvp at any time so we must be able to acquire a lock on the parent
 * to safely access nc_vp.
 *
 * We have to leave par unlocked when vget()ing dvp to avoid a deadlock,
 * so use vhold()/vdrop() while holding the lock to prevent dvp from
 * getting destroyed.
 *
 * NOTE: vhold() is allowed when dvp has 0 refs if we hold a
 *       lock on the ncp in question..
 */
static struct vnode *
cache_dvpref(struct namecache *ncp)
{
        struct namecache *par;
        struct vnode *dvp;

        dvp = NULL;
        if ((par = ncp->nc_parent) != NULL) {
                _cache_hold(par);
                _cache_lock(par);
                if ((par->nc_flag & NCF_UNRESOLVED) == 0) {
                        if ((dvp = par->nc_vp) != NULL)
                                vhold(dvp);
                }
                _cache_unlock(par);
                if (dvp) {
                        if (vget(dvp, LK_SHARED) == 0) {
                                vn_unlock(dvp);
                                vdrop(dvp);
                                /* return refd, unlocked dvp */
                        } else {
                                vdrop(dvp);
                                dvp = NULL;
                        }
                }
                _cache_drop(par);
        }
        return(dvp);
}

/*
 * Convert a directory vnode to a namecache record without any other 
 * knowledge of the topology.  This ONLY works with directory vnodes and
 * is ONLY used by the NFS server.  dvp must be refd but unlocked, and the
 * returned ncp (if not NULL) will be held and unlocked.
 *
 * If 'makeit' is 0 and dvp has no existing namecache record, NULL is returned.
 * If 'makeit' is 1 we attempt to track-down and create the namecache topology
 * for dvp.  This will fail only if the directory has been deleted out from
 * under the caller.  
 *
 * Callers must always check for a NULL return no matter the value of 'makeit'.
 *
 * To avoid underflowing the kernel stack each recursive call increments
 * the makeit variable.
 */

static int cache_inefficient_scan(struct nchandle *nch, struct ucred *cred,
                                  struct vnode *dvp, char *fakename);
static int cache_fromdvp_try(struct vnode *dvp, struct ucred *cred, 
                                  struct vnode **saved_dvp);

int
cache_fromdvp(struct vnode *dvp, struct ucred *cred, int makeit,
              struct nchandle *nch)
{
        struct vnode *saved_dvp;
        struct vnode *pvp;
        char *fakename;
        int error;

        nch->ncp = NULL;
        nch->mount = dvp->v_mount;
        saved_dvp = NULL;
        fakename = NULL;

        /*
         * Handle the makeit == 0 degenerate case
         */
        if (makeit == 0) {
                spin_lock(&dvp->v_spin);
                nch->ncp = TAILQ_FIRST(&dvp->v_namecache);
                if (nch->ncp)
                        cache_hold(nch);
                spin_unlock(&dvp->v_spin);
        }

        /*
         * Loop until resolution, inside code will break out on error.
         */
        while (makeit) {
                /*
                 * Break out if we successfully acquire a working ncp.
                 */
                spin_lock(&dvp->v_spin);
                nch->ncp = TAILQ_FIRST(&dvp->v_namecache);
                if (nch->ncp) {
                        cache_hold(nch);
                        spin_unlock(&dvp->v_spin);
                        break;
                }
                spin_unlock(&dvp->v_spin);

                /*
                 * If dvp is the root of its filesystem it should already
                 * have a namecache pointer associated with it as a side 
                 * effect of the mount, but it may have been disassociated.
                 */
                if (dvp->v_flag & VROOT) {
                        nch->ncp = _cache_get(nch->mount->mnt_ncmountpt.ncp);
                        error = cache_resolve_mp(nch->mount);
                        _cache_put(nch->ncp);
                        if (ncvp_debug) {
                                kprintf("cache_fromdvp: resolve root of mount %p error %d", 
                                        dvp->v_mount, error);
                        }
                        if (error) {
                                if (ncvp_debug)
                                        kprintf(" failed\n");
                                nch->ncp = NULL;
                                break;
                        }
                        if (ncvp_debug)
                                kprintf(" succeeded\n");
                        continue;
                }

                /*
                 * If we are recursed too deeply resort to an O(n^2)
                 * algorithm to resolve the namecache topology.  The
                 * resolved pvp is left referenced in saved_dvp to
                 * prevent the tree from being destroyed while we loop.
                 */
                if (makeit > 20) {
                        error = cache_fromdvp_try(dvp, cred, &saved_dvp);
                        if (error) {
                                kprintf("lookupdotdot(longpath) failed %d "
                                       "dvp %p\n", error, dvp);
                                nch->ncp = NULL;
                                break;
                        }
                        continue;
                }

                /*
                 * Get the parent directory and resolve its ncp.
                 */
                if (fakename) {
                        kfree(fakename, M_TEMP);
                        fakename = NULL;
                }
                error = vop_nlookupdotdot(*dvp->v_ops, dvp, &pvp, cred,
                                          &fakename);
                if (error) {
                        kprintf("lookupdotdot failed %d dvp %p\n", error, dvp);
                        break;
                }
                vn_unlock(pvp);

                /*
                 * Reuse makeit as a recursion depth counter.  On success
                 * nch will be fully referenced.
                 */
                cache_fromdvp(pvp, cred, makeit + 1, nch);
                vrele(pvp);
                if (nch->ncp == NULL)
                        break;

                /*
                 * Do an inefficient scan of pvp (embodied by ncp) to look
                 * for dvp.  This will create a namecache record for dvp on
                 * success.  We loop up to recheck on success.
                 *
                 * ncp and dvp are both held but not locked.
                 */
                error = cache_inefficient_scan(nch, cred, dvp, fakename);
                if (error) {
                        kprintf("cache_fromdvp: scan %p (%s) failed on dvp=%p\n",
                                pvp, nch->ncp->nc_name, dvp);
                        cache_drop(nch);
                        /* nch was NULLed out, reload mount */
                        nch->mount = dvp->v_mount;
                        break;
                }
                if (ncvp_debug) {
                        kprintf("cache_fromdvp: scan %p (%s) succeeded\n",
                                pvp, nch->ncp->nc_name);
                }
                cache_drop(nch);
                /* nch was NULLed out, reload mount */
                nch->mount = dvp->v_mount;
        }

        /*
         * If nch->ncp is non-NULL it will have been held already.
         */
        if (fakename)
                kfree(fakename, M_TEMP);
        if (saved_dvp)
                vrele(saved_dvp);
        if (nch->ncp)
                return (0);
        return (EINVAL);
}

/*
 * Go up the chain of parent directories until we find something
 * we can resolve into the namecache.  This is very inefficient.
 */
static
int
cache_fromdvp_try(struct vnode *dvp, struct ucred *cred,
                  struct vnode **saved_dvp)
{
        struct nchandle nch;
        struct vnode *pvp;
        int error;
        static time_t last_fromdvp_report;
        char *fakename;

        /*
         * Loop getting the parent directory vnode until we get something we
         * can resolve in the namecache.
         */
        vref(dvp);
        nch.mount = dvp->v_mount;
        nch.ncp = NULL;
        fakename = NULL;

        for (;;) {
                if (fakename) {
                        kfree(fakename, M_TEMP);
                        fakename = NULL;
                }
                error = vop_nlookupdotdot(*dvp->v_ops, dvp, &pvp, cred,
                                          &fakename);
                if (error) {
                        vrele(dvp);
                        break;
                }
                vn_unlock(pvp);
                spin_lock(&pvp->v_spin);
                if ((nch.ncp = TAILQ_FIRST(&pvp->v_namecache)) != NULL) {
                        _cache_hold(nch.ncp);
                        spin_unlock(&pvp->v_spin);
                        vrele(pvp);
                        break;
                }
                spin_unlock(&pvp->v_spin);
                if (pvp->v_flag & VROOT) {
                        nch.ncp = _cache_get(pvp->v_mount->mnt_ncmountpt.ncp);
                        error = cache_resolve_mp(nch.mount);
                        _cache_unlock(nch.ncp);
                        vrele(pvp);
                        if (error) {
                                _cache_drop(nch.ncp);
                                nch.ncp = NULL;
                                vrele(dvp);
                        }
                        break;
                }
                vrele(dvp);
                dvp = pvp;
        }
        if (error == 0) {
                if (last_fromdvp_report != time_second) {
                        last_fromdvp_report = time_second;
                        kprintf("Warning: extremely inefficient path "
                                "resolution on %s\n",
                                nch.ncp->nc_name);
                }
                error = cache_inefficient_scan(&nch, cred, dvp, fakename);

                /*
                 * Hopefully dvp now has a namecache record associated with
                 * it.  Leave it referenced to prevent the kernel from
                 * recycling the vnode.  Otherwise extremely long directory
                 * paths could result in endless recycling.
                 */
                if (*saved_dvp)
                    vrele(*saved_dvp);
                *saved_dvp = dvp;
                _cache_drop(nch.ncp);
        }
        if (fakename)
                kfree(fakename, M_TEMP);
        return (error);
}

/*
 * Do an inefficient scan of the directory represented by ncp looking for
 * the directory vnode dvp.  ncp must be held but not locked on entry and
 * will be held on return.  dvp must be refd but not locked on entry and
 * will remain refd on return.
 *
 * Why do this at all?  Well, due to its stateless nature the NFS server
 * converts file handles directly to vnodes without necessarily going through
 * the namecache ops that would otherwise create the namecache topology
 * leading to the vnode.  We could either (1) Change the namecache algorithms
 * to allow disconnect namecache records that are re-merged opportunistically,
 * or (2) Make the NFS server backtrack and scan to recover a connected
 * namecache topology in order to then be able to issue new API lookups.
 *
 * It turns out that (1) is a huge mess.  It takes a nice clean set of 
 * namecache algorithms and introduces a lot of complication in every subsystem
 * that calls into the namecache to deal with the re-merge case, especially
 * since we are using the namecache to placehold negative lookups and the
 * vnode might not be immediately assigned. (2) is certainly far less
 * efficient then (1), but since we are only talking about directories here
 * (which are likely to remain cached), the case does not actually run all
 * that often and has the supreme advantage of not polluting the namecache
 * algorithms.
 *
 * If a fakename is supplied just construct a namecache entry using the
 * fake name.
 */
static int
cache_inefficient_scan(struct nchandle *nch, struct ucred *cred, 
                       struct vnode *dvp, char *fakename)
{
        struct nlcomponent nlc;
        struct nchandle rncp;
        struct dirent *den;
        struct vnode *pvp;
        struct vattr vat;
        struct iovec iov;
        struct uio uio;
        int blksize;
        int eofflag;
        int bytes;
        char *rbuf;
        int error;

        vat.va_blocksize = 0;
        if ((error = VOP_GETATTR(dvp, &vat)) != 0)
                return (error);
        cache_lock(nch);
        error = cache_vref(nch, cred, &pvp);
        cache_unlock(nch);
        if (error)
                return (error);
        if (ncvp_debug) {
                kprintf("inefficient_scan: directory iosize %ld "
                        "vattr fileid = %lld\n",
                        vat.va_blocksize,
                        (long long)vat.va_fileid);
        }

        /*
         * Use the supplied fakename if not NULL.  Fake names are typically
         * not in the actual filesystem hierarchy.  This is used by HAMMER
         * to glue @@timestamp recursions together.
         */
        if (fakename) {
                nlc.nlc_nameptr = fakename;
                nlc.nlc_namelen = strlen(fakename);
                rncp = cache_nlookup(nch, &nlc);
                goto done;
        }

        if ((blksize = vat.va_blocksize) == 0)
                blksize = DEV_BSIZE;
        rbuf = kmalloc(blksize, M_TEMP, M_WAITOK);
        rncp.ncp = NULL;

        eofflag = 0;
        uio.uio_offset = 0;
again:
        iov.iov_base = rbuf;
        iov.iov_len = blksize;
        uio.uio_iov = &iov;
        uio.uio_iovcnt = 1;
        uio.uio_resid = blksize;
        uio.uio_segflg = UIO_SYSSPACE;
        uio.uio_rw = UIO_READ;
        uio.uio_td = curthread;

        if (ncvp_debug >= 2)
                kprintf("cache_inefficient_scan: readdir @ %08x\n", (int)uio.uio_offset);
        error = VOP_READDIR(pvp, &uio, cred, &eofflag, NULL, NULL);
        if (error == 0) {
                den = (struct dirent *)rbuf;
                bytes = blksize - uio.uio_resid;

                while (bytes > 0) {
                        if (ncvp_debug >= 2) {
                                kprintf("cache_inefficient_scan: %*.*s\n",
                                        den->d_namlen, den->d_namlen, 
                                        den->d_name);
                        }
                        if (den->d_type != DT_WHT &&
                            den->d_ino == vat.va_fileid) {
                                if (ncvp_debug) {
                                        kprintf("cache_inefficient_scan: "
                                               "MATCHED inode %lld path %s/%*.*s\n",
                                               (long long)vat.va_fileid,
                                               nch->ncp->nc_name,
                                               den->d_namlen, den->d_namlen,
                                               den->d_name);
                                }
                                nlc.nlc_nameptr = den->d_name;
                                nlc.nlc_namelen = den->d_namlen;
                                rncp = cache_nlookup(nch, &nlc);
                                KKASSERT(rncp.ncp != NULL);
                                break;
                        }
                        bytes -= _DIRENT_DIRSIZ(den);
                        den = _DIRENT_NEXT(den);
                }
                if (rncp.ncp == NULL && eofflag == 0 && uio.uio_resid != blksize)
                        goto again;
        }
        kfree(rbuf, M_TEMP);
done:
        vrele(pvp);
        if (rncp.ncp) {
                if (rncp.ncp->nc_flag & NCF_UNRESOLVED) {
                        _cache_setvp(rncp.mount, rncp.ncp, dvp);
                        if (ncvp_debug >= 2) {
                                kprintf("cache_inefficient_scan: setvp %s/%s = %p\n",
                                        nch->ncp->nc_name, rncp.ncp->nc_name, dvp);
                        }
                } else {
                        if (ncvp_debug >= 2) {
                                kprintf("cache_inefficient_scan: setvp %s/%s already set %p/%p\n", 
                                        nch->ncp->nc_name, rncp.ncp->nc_name, dvp,
                                        rncp.ncp->nc_vp);
                        }
                }
                if (rncp.ncp->nc_vp == NULL)
                        error = rncp.ncp->nc_error;
                /* 
                 * Release rncp after a successful nlookup.  rncp was fully
                 * referenced.
                 */
                cache_put(&rncp);
        } else {
                kprintf("cache_inefficient_scan: dvp %p NOT FOUND in %s\n",
                        dvp, nch->ncp->nc_name);
                error = ENOENT;
        }
        return (error);
}

/*
 * Zap a namecache entry.  The ncp is unconditionally set to an unresolved
 * state, which disassociates it from its vnode or ncneglist.
 *
 * Then, if there are no additional references to the ncp and no children,
 * the ncp is removed from the topology and destroyed.
 *
 * References and/or children may exist if the ncp is in the middle of the
 * topology, preventing the ncp from being destroyed.
 *
 * This function must be called with the ncp held and locked and will unlock
 * and drop it during zapping.
 *
 * If nonblock is non-zero and the parent ncp cannot be locked we give up.
 * This case can occur in the cache_drop() path.
 *
 * This function may returned a held (but NOT locked) parent node which the
 * caller must drop.  We do this so _cache_drop() can loop, to avoid
 * blowing out the kernel stack.
 *
 * WARNING!  For MPSAFE operation this routine must acquire up to three
 *           spin locks to be able to safely test nc_refs.  Lock order is
 *           very important.
 *
 *           hash spinlock if on hash list
 *           parent spinlock if child of parent
 *           (the ncp is unresolved so there is no vnode association)
 */
static struct namecache *
cache_zap(struct namecache *ncp, int nonblock)
{
        struct namecache *par;
        struct vnode *dropvp;
        int refs;

        /*
         * Disassociate the vnode or negative cache ref and set NCF_UNRESOLVED.
         */
        _cache_setunresolved(ncp);

        /*
         * Try to scrap the entry and possibly tail-recurse on its parent.
         * We only scrap unref'd (other then our ref) unresolved entries,
         * we do not scrap 'live' entries.
         *
         * Note that once the spinlocks are acquired if nc_refs == 1 no
         * other references are possible.  If it isn't, however, we have
         * to decrement but also be sure to avoid a 1->0 transition.
         */
        KKASSERT(ncp->nc_flag & NCF_UNRESOLVED);
        KKASSERT(ncp->nc_refs > 0);

        /*
         * Acquire locks.  Note that the parent can't go away while we hold
         * a child locked.
         */
        if ((par = ncp->nc_parent) != NULL) {
                if (nonblock) {
                        for (;;) {
                                if (_cache_lock_nonblock(par) == 0)
                                        break;
                                refs = ncp->nc_refs;
                                ncp->nc_flag |= NCF_DEFEREDZAP;
                                ++numdefered;   /* MP race ok */
                                if (atomic_cmpset_int(&ncp->nc_refs,
                                                      refs, refs - 1)) {
                                        _cache_unlock(ncp);
                                        return(NULL);
                                }
                                cpu_pause();
                        }
                        _cache_hold(par);
                } else {
                        _cache_hold(par);
                        _cache_lock(par);
                }
                spin_lock(&ncp->nc_head->spin);
        }

        /*
         * If someone other then us has a ref or we have children
         * we cannot zap the entry.  The 1->0 transition and any
         * further list operation is protected by the spinlocks
         * we have acquired but other transitions are not.
         */
        for (;;) {
                refs = ncp->nc_refs;
                if (refs == 1 && TAILQ_EMPTY(&ncp->nc_list))
                        break;
                if (atomic_cmpset_int(&ncp->nc_refs, refs, refs - 1)) {
                        if (par) {
                                spin_unlock(&ncp->nc_head->spin);
                                _cache_put(par);
                        }
                        _cache_unlock(ncp);
                        return(NULL);
                }
                cpu_pause();
        }

        /*
         * We are the only ref and with the spinlocks held no further
         * refs can be acquired by others.
         *
         * Remove us from the hash list and parent list.  We have to
         * drop a ref on the parent's vp if the parent's list becomes
         * empty.
         */
        dropvp = NULL;
        if (par) {
                struct nchash_head *nchpp = ncp->nc_head;

                KKASSERT(nchpp != NULL);
                LIST_REMOVE(ncp, nc_hash);
                TAILQ_REMOVE(&par->nc_list, ncp, nc_entry);
                if (par->nc_vp && TAILQ_EMPTY(&par->nc_list))
                        dropvp = par->nc_vp;
                ncp->nc_head = NULL;
                ncp->nc_parent = NULL;
                spin_unlock(&nchpp->spin);
                _cache_unlock(par);
        } else {
                KKASSERT(ncp->nc_head == NULL);
        }

        /*
         * ncp should not have picked up any refs.  Physically
         * destroy the ncp.
         */
        KKASSERT(ncp->nc_refs == 1);
        /* _cache_unlock(ncp) not required */
        ncp->nc_refs = -1;      /* safety */
        if (ncp->nc_name)
                kfree(ncp->nc_name, M_VFSCACHE);
        kfree(ncp, M_VFSCACHE);

        /*
         * Delayed drop (we had to release our spinlocks)
         *
         * The refed parent (if not  NULL) must be dropped.  The
         * caller is responsible for looping.
         */
        if (dropvp)
                vdrop(dropvp);
        return(par);
}

/*
 * Clean up dangling negative cache and defered-drop entries in the
 * namecache.
 */
typedef enum { CHI_LOW, CHI_HIGH } cache_hs_t;

static cache_hs_t neg_cache_hysteresis_state = CHI_LOW;
static cache_hs_t pos_cache_hysteresis_state = CHI_LOW;

void
cache_hysteresis(void)
{
        int poslimit;

        /*
         * Don't cache too many negative hits.  We use hysteresis to reduce
         * the impact on the critical path.
         */
        switch(neg_cache_hysteresis_state) {
        case CHI_LOW:
                if (numneg > MINNEG && numneg * ncnegfactor > numcache) {
                        _cache_cleanneg(10);
                        neg_cache_hysteresis_state = CHI_HIGH;
                }
                break;
        case CHI_HIGH:
                if (numneg > MINNEG * 9 / 10 && 
                    numneg * ncnegfactor * 9 / 10 > numcache
                ) {
                        _cache_cleanneg(10);
                } else {
                        neg_cache_hysteresis_state = CHI_LOW;
                }
                break;
        }

        /*
         * Don't cache too many positive hits.  We use hysteresis to reduce
         * the impact on the critical path.
         *
         * Excessive positive hits can accumulate due to large numbers of
         * hardlinks (the vnode cache will not prevent hl ncps from growing
         * into infinity).
         */
        if ((poslimit = ncposlimit) == 0)
                poslimit = desiredvnodes * 2;

        switch(pos_cache_hysteresis_state) {
        case CHI_LOW:
                if (numcache > poslimit && numcache > MINPOS) {
                        _cache_cleanpos(10);
                        pos_cache_hysteresis_state = CHI_HIGH;
                }
                break;
        case CHI_HIGH:
                if (numcache > poslimit * 5 / 6 && numcache > MINPOS) {
                        _cache_cleanpos(10);
                } else {
                        pos_cache_hysteresis_state = CHI_LOW;
                }
                break;
        }

        /*
         * Clean out dangling defered-zap ncps which could not
         * be cleanly dropped if too many build up.  Note
         * that numdefered is not an exact number as such ncps
         * can be reused and the counter is not handled in a MP
         * safe manner by design.
         */
        if (numdefered * ncnegfactor > numcache) {
                _cache_cleandefered();
        }
}

/*
 * NEW NAMECACHE LOOKUP API
 *
 * Lookup an entry in the namecache.  The passed par_nch must be referenced
 * and unlocked.  A referenced and locked nchandle with a non-NULL nch.ncp
 * is ALWAYS returned, eve if the supplied component is illegal.
 *
 * The resulting namecache entry should be returned to the system with
 * cache_put() or cache_unlock() + cache_drop().
 *
 * namecache locks are recursive but care must be taken to avoid lock order
 * reversals (hence why the passed par_nch must be unlocked).  Locking
 * rules are to order for parent traversals, not for child traversals.
 *
 * Nobody else will be able to manipulate the associated namespace (e.g.
 * create, delete, rename, rename-target) until the caller unlocks the
 * entry.
 *
 * The returned entry will be in one of three states:  positive hit (non-null
 * vnode), negative hit (null vnode), or unresolved (NCF_UNRESOLVED is set).
 * Unresolved entries must be resolved through the filesystem to associate the
 * vnode and/or determine whether a positive or negative hit has occured.
 *
 * It is not necessary to lock a directory in order to lock namespace under
 * that directory.  In fact, it is explicitly not allowed to do that.  A
 * directory is typically only locked when being created, renamed, or
 * destroyed.
 *
 * The directory (par) may be unresolved, in which case any returned child
 * will likely also be marked unresolved.  Likely but not guarenteed.  Since
 * the filesystem lookup requires a resolved directory vnode the caller is
 * responsible for resolving the namecache chain top-down.  This API 
 * specifically allows whole chains to be created in an unresolved state.
 */
struct nchandle
cache_nlookup(struct nchandle *par_nch, struct nlcomponent *nlc)
{
        struct nchandle nch;
        struct namecache *ncp;
        struct namecache *new_ncp;
        struct nchash_head *nchpp;
        struct mount *mp;
        u_int32_t hash;
        globaldata_t gd;
        int par_locked;

        numcalls++;
        gd = mycpu;
        mp = par_nch->mount;
        par_locked = 0;

        /*
         * This is a good time to call it, no ncp's are locked by
         * the caller or us.
         */
        cache_hysteresis();

        /*
         * Try to locate an existing entry
         */
        hash = fnv_32_buf(nlc->nlc_nameptr, nlc->nlc_namelen, FNV1_32_INIT);
        hash = fnv_32_buf(&par_nch->ncp, sizeof(par_nch->ncp), hash);
        new_ncp = NULL;
        nchpp = NCHHASH(hash);
restart:
        spin_lock(&nchpp->spin);
        LIST_FOREACH(ncp, &nchpp->list, nc_hash) {
                numchecks++;

                /*
                 * Break out if we find a matching entry.  Note that
                 * UNRESOLVED entries may match, but DESTROYED entries
                 * do not.
                 */
                if (ncp->nc_parent == par_nch->ncp &&
                    ncp->nc_nlen == nlc->nlc_namelen &&
                    bcmp(ncp->nc_name, nlc->nlc_nameptr, ncp->nc_nlen) == 0 &&
                    (ncp->nc_flag & NCF_DESTROYED) == 0
                ) {
                        _cache_hold(ncp);
                        spin_unlock(&nchpp->spin);
                        if (par_locked) {
                                _cache_unlock(par_nch->ncp);
                                par_locked = 0;
                        }
                        if (_cache_lock_special(ncp) == 0) {
                                _cache_auto_unresolve(mp, ncp);
                                if (new_ncp)
                                        _cache_free(new_ncp);
                                goto found;
                        }
                        _cache_get(ncp);
                        _cache_put(ncp);
                        _cache_drop(ncp);
                        goto restart;
                }
        }

        /*
         * We failed to locate an entry, create a new entry and add it to
         * the cache.  The parent ncp must also be locked so we
         * can link into it.
         *
         * We have to relookup after possibly blocking in kmalloc or
         * when locking par_nch.
         *
         * NOTE: nlc_namelen can be 0 and nlc_nameptr NULL as a special
         *       mount case, in which case nc_name will be NULL.
         */
        if (new_ncp == NULL) {
                spin_unlock(&nchpp->spin);
                new_ncp = cache_alloc(nlc->nlc_namelen);
                if (nlc->nlc_namelen) {
                        bcopy(nlc->nlc_nameptr, new_ncp->nc_name,
                              nlc->nlc_namelen);
                        new_ncp->nc_name[nlc->nlc_namelen] = 0;
                }
                goto restart;
        }
        if (par_locked == 0) {
                spin_unlock(&nchpp->spin);
                _cache_lock(par_nch->ncp);
                par_locked = 1;
                goto restart;
        }

        /*
         * WARNING!  We still hold the spinlock.  We have to set the hash
         *           table entry atomically.
         */
        ncp = new_ncp;
        _cache_link_parent(ncp, par_nch->ncp, nchpp);
        spin_unlock(&nchpp->spin);
        _cache_unlock(par_nch->ncp);
        /* par_locked = 0 - not used */
found:
        /*
         * stats and namecache size management
         */
        if (ncp->nc_flag & NCF_UNRESOLVED)
                ++gd->gd_nchstats->ncs_miss;
        else if (ncp->nc_vp)
                ++gd->gd_nchstats->ncs_goodhits;
        else
                ++gd->gd_nchstats->ncs_neghits;
        nch.mount = mp;
        nch.ncp = ncp;
        atomic_add_int(&nch.mount->mnt_refs, 1);
        return(nch);
}

/*
 * Attempt to lookup a namecache entry and return with a shared namecache
 * lock.
 */
int
cache_nlookup_maybe_shared(struct nchandle *par_nch, struct nlcomponent *nlc,
                           int excl, struct nchandle *res_nch)
{
        struct namecache *ncp;
        struct nchash_head *nchpp;
        struct mount *mp;
        u_int32_t hash;
        globaldata_t gd;

        /*
         * If exclusive requested or shared namecache locks are disabled,
         * return failure.
         */
        if (ncp_shared_lock_disable || excl)
                return(EWOULDBLOCK);

        numcalls++;
        gd = mycpu;
        mp = par_nch->mount;

        /*
         * This is a good time to call it, no ncp's are locked by
         * the caller or us.
         */
        cache_hysteresis();

        /*
         * Try to locate an existing entry
         */
        hash = fnv_32_buf(nlc->nlc_nameptr, nlc->nlc_namelen, FNV1_32_INIT);
        hash = fnv_32_buf(&par_nch->ncp, sizeof(par_nch->ncp), hash);
        nchpp = NCHHASH(hash);

        spin_lock(&nchpp->spin);

        LIST_FOREACH(ncp, &nchpp->list, nc_hash) {
                numchecks++;

                /*
                 * Break out if we find a matching entry.  Note that
                 * UNRESOLVED entries may match, but DESTROYED entries
                 * do not.
                 */
                if (ncp->nc_parent == par_nch->ncp &&
                    ncp->nc_nlen == nlc->nlc_namelen &&
                    bcmp(ncp->nc_name, nlc->nlc_nameptr, ncp->nc_nlen) == 0 &&
                    (ncp->nc_flag & NCF_DESTROYED) == 0
                ) {
                        _cache_hold(ncp);
                        spin_unlock(&nchpp->spin);
                        if (_cache_lock_shared_special(ncp) == 0) {
                                if ((ncp->nc_flag & NCF_UNRESOLVED) == 0 &&
                                    (ncp->nc_flag & NCF_DESTROYED) == 0 &&
                                    _cache_auto_unresolve_test(mp, ncp) == 0) {
                                        goto found;
                                }
                                _cache_unlock(ncp);
                        }
                        _cache_drop(ncp);
                        spin_lock(&nchpp->spin);
                        break;
                }
        }

        /*
         * Failure
         */
        spin_unlock(&nchpp->spin);
        return(EWOULDBLOCK);

        /*
         * Success
         *
         * Note that nc_error might be non-zero (e.g ENOENT).
         */
found:
        res_nch->mount = mp;
        res_nch->ncp = ncp;
        ++gd->gd_nchstats->ncs_goodhits;
        atomic_add_int(&res_nch->mount->mnt_refs, 1);

        KKASSERT(ncp->nc_error != EWOULDBLOCK);
        return(ncp->nc_error);
}

/*
 * This is a non-blocking verison of cache_nlookup() used by
 * nfs_readdirplusrpc_uio().  It can fail for any reason and
 * will return nch.ncp == NULL in that case.
 */
struct nchandle
cache_nlookup_nonblock(struct nchandle *par_nch, struct nlcomponent *nlc)
{
        struct nchandle nch;
        struct namecache *ncp;
        struct namecache *new_ncp;
        struct nchash_head *nchpp;
        struct mount *mp;
        u_int32_t hash;
        globaldata_t gd;
        int par_locked;

        numcalls++;
        gd = mycpu;
        mp = par_nch->mount;
        par_locked = 0;

        /*
         * Try to locate an existing entry
         */
        hash = fnv_32_buf(nlc->nlc_nameptr, nlc->nlc_namelen, FNV1_32_INIT);
        hash = fnv_32_buf(&par_nch->ncp, sizeof(par_nch->ncp), hash);
        new_ncp = NULL;
        nchpp = NCHHASH(hash);
restart:
        spin_lock(&nchpp->spin);
        LIST_FOREACH(ncp, &nchpp->list, nc_hash) {
                numchecks++;

                /*
                 * Break out if we find a matching entry.  Note that
                 * UNRESOLVED entries may match, but DESTROYED entries
                 * do not.
                 */
                if (ncp->nc_parent == par_nch->ncp &&
                    ncp->nc_nlen == nlc->nlc_namelen &&
                    bcmp(ncp->nc_name, nlc->nlc_nameptr, ncp->nc_nlen) == 0 &&
                    (ncp->nc_flag & NCF_DESTROYED) == 0
                ) {
                        _cache_hold(ncp);
                        spin_unlock(&nchpp->spin);
                        if (par_locked) {
                                _cache_unlock(par_nch->ncp);
                                par_locked = 0;
                        }
                        if (_cache_lock_special(ncp) == 0) {
                                _cache_auto_unresolve(mp, ncp);
                                if (new_ncp) {
                                        _cache_free(new_ncp);
                                        new_ncp = NULL;
                                }
                                goto found;
                        }
                        _cache_drop(ncp);
                        goto failed;
                }
        }

        /*
         * We failed to locate an entry, create a new entry and add it to
         * the cache.  The parent ncp must also be locked so we
         * can link into it.
         *
         * We have to relookup after possibly blocking in kmalloc or
         * when locking par_nch.
         *
         * NOTE: nlc_namelen can be 0 and nlc_nameptr NULL as a special
         *       mount case, in which case nc_name will be NULL.
         */
        if (new_ncp == NULL) {
                spin_unlock(&nchpp->spin);
                new_ncp = cache_alloc(nlc->nlc_namelen);
                if (nlc->nlc_namelen) {
                        bcopy(nlc->nlc_nameptr, new_ncp->nc_name,
                              nlc->nlc_namelen);
                        new_ncp->nc_name[nlc->nlc_namelen] = 0;
                }
                goto restart;
        }
        if (par_locked == 0) {
                spin_unlock(&nchpp->spin);
                if (_cache_lock_nonblock(par_nch->ncp) == 0) {
                        par_locked = 1;
                        goto restart;
                }
                goto failed;
        }

        /*
         * WARNING!  We still hold the spinlock.  We have to set the hash
         *           table entry atomically.
         */
        ncp = new_ncp;
        _cache_link_parent(ncp, par_nch->ncp, nchpp);
        spin_unlock(&nchpp->spin);
        _cache_unlock(par_nch->ncp);
        /* par_locked = 0 - not used */
found:
        /*
         * stats and namecache size management
         */
        if (ncp->nc_flag & NCF_UNRESOLVED)
                ++gd->gd_nchstats->ncs_miss;
        else if (ncp->nc_vp)
                ++gd->gd_nchstats->ncs_goodhits;
        else
                ++gd->gd_nchstats->ncs_neghits;
        nch.mount = mp;
        nch.ncp = ncp;
        atomic_add_int(&nch.mount->mnt_refs, 1);
        return(nch);
failed:
        if (new_ncp) {
                _cache_free(new_ncp);
                new_ncp = NULL;
        }
        nch.mount = NULL;
        nch.ncp = NULL;
        return(nch);
}

/*
 * The namecache entry is marked as being used as a mount point. 
 * Locate the mount if it is visible to the caller.  The DragonFly
 * mount system allows arbitrary loops in the topology and disentangles
 * those loops by matching against (mp, ncp) rather than just (ncp).
 * This means any given ncp can dive any number of mounts, depending
 * on the relative mount (e.g. nullfs) the caller is at in the topology.
 *
 * We use a very simple frontend cache to reduce SMP conflicts,
 * which we have to do because the mountlist scan needs an exclusive
 * lock around its ripout info list.  Not to mention that there might
 * be a lot of mounts.
 */
struct findmount_info {
        struct mount *result;
        struct mount *nch_mount;
        struct namecache *nch_ncp;
};

static
struct ncmount_cache *
ncmount_cache_lookup(struct mount *mp, struct namecache *ncp)
{
        int hash;

        hash = ((int)(intptr_t)mp / sizeof(*mp)) ^
               ((int)(intptr_t)ncp / sizeof(*ncp));
        hash = (hash & 0x7FFFFFFF) % NCMOUNT_NUMCACHE;
        return (&ncmount_cache[hash]);
}

static
int
cache_findmount_callback(struct mount *mp, void *data)
{
        struct findmount_info *info = data;

        /*
         * Check the mount's mounted-on point against the passed nch.
         */
        if (mp->mnt_ncmounton.mount == info->nch_mount &&
            mp->mnt_ncmounton.ncp == info->nch_ncp
        ) {
            info->result = mp;
            atomic_add_int(&mp->mnt_refs, 1);
            return(-1);
        }
        return(0);
}

struct mount *
cache_findmount(struct nchandle *nch)
{
        struct findmount_info info;
        struct ncmount_cache *ncc;
        struct mount *mp;

        /*
         * Fast
         */
        if (ncmount_cache_enable == 0) {
                ncc = NULL;
                goto skip;
        }
        ncc = ncmount_cache_lookup(nch->mount, nch->ncp);
        if (ncc->ncp == nch->ncp) {
                spin_lock_shared(&ncc->spin);
                if (ncc->isneg == 0 &&
                    ncc->ncp == nch->ncp && (mp = ncc->mp) != NULL) {
                        if (mp->mnt_ncmounton.mount == nch->mount &&
                            mp->mnt_ncmounton.ncp == nch->ncp) {
                                /*
                                 * Cache hit (positive)
                                 */
                                atomic_add_int(&mp->mnt_refs, 1);
                                spin_unlock_shared(&ncc->spin);
                                ++ncmount_cache_hit;
                                return(mp);
                        }
                        /* else cache miss */
                }
                if (ncc->isneg &&
                    ncc->ncp == nch->ncp && ncc->mp == nch->mount) {
                        /*
                         * Cache hit (negative)
                         */
                        spin_unlock_shared(&ncc->spin);
                        ++ncmount_cache_hit;
                        return(NULL);
                }
                spin_unlock_shared(&ncc->spin);
        }
skip:

        /*
         * Slow
         */
        info.result = NULL;
        info.nch_mount = nch->mount;
        info.nch_ncp = nch->ncp;
        mountlist_scan(cache_findmount_callback, &info,
                               MNTSCAN_FORWARD|MNTSCAN_NOBUSY);

        /*
         * Cache the result.
         *
         * Negative lookups: We cache the originating {ncp,mp}. (mp) is
         *                   only used for pointer comparisons and is not
         *                   referenced (otherwise there would be dangling
         *                   refs).
         *
         * Positive lookups: We cache the originating {ncp} and the target
         *                   (mp).  (mp) is referenced.
         *
         * Indeterminant:    If the match is undergoing an unmount we do
         *                   not cache it to avoid racing cache_unmounting(),
         *                   but still return the match.
         */
        if (ncc) {
                spin_lock(&ncc->spin);
                if (info.result == NULL) {
                        if (ncc->isneg == 0 && ncc->mp)
                                atomic_add_int(&ncc->mp->mnt_refs, -1);
                        ncc->ncp = nch->ncp;
                        ncc->mp = nch->mount;
                        ncc->isneg = 1;
                        spin_unlock(&ncc->spin);
                        ++ncmount_cache_overwrite;
                } else if ((info.result->mnt_kern_flag & MNTK_UNMOUNT) == 0) {
                        if (ncc->isneg == 0 && ncc->mp)
                                atomic_add_int(&ncc->mp->mnt_refs, -1);
                        atomic_add_int(&info.result->mnt_refs, 1);
                        ncc->ncp = nch->ncp;
                        ncc->mp = info.result;
                        ncc->isneg = 0;
                        spin_unlock(&ncc->spin);
                        ++ncmount_cache_overwrite;
                } else {
                        spin_unlock(&ncc->spin);
                }
                ++ncmount_cache_miss;
        }
        return(info.result);
}

void
cache_dropmount(struct mount *mp)
{
        atomic_add_int(&mp->mnt_refs, -1);
}

void
cache_ismounting(struct mount *mp)
{
        struct nchandle *nch = &mp->mnt_ncmounton;
        struct ncmount_cache *ncc;

        ncc = ncmount_cache_lookup(nch->mount, nch->ncp);
        if (ncc->isneg &&
            ncc->ncp == nch->ncp && ncc->mp == nch->mount) {
                spin_lock(&ncc->spin);
                if (ncc->isneg &&
                    ncc->ncp == nch->ncp && ncc->mp == nch->mount) {
                        ncc->ncp = NULL;
                        ncc->mp = NULL;
                }
                spin_unlock(&ncc->spin);
        }
}

void
cache_unmounting(struct mount *mp)
{
        struct nchandle *nch = &mp->mnt_ncmounton;
        struct ncmount_cache *ncc;

        ncc = ncmount_cache_lookup(nch->mount, nch->ncp);
        if (ncc->isneg == 0 &&
            ncc->ncp == nch->ncp && ncc->mp == mp) {
                spin_lock(&ncc->spin);
                if (ncc->isneg == 0 &&
                    ncc->ncp == nch->ncp && ncc->mp == mp) {
                        atomic_add_int(&mp->mnt_refs, -1);
                        ncc->ncp = NULL;
                        ncc->mp = NULL;
                }
                spin_unlock(&ncc->spin);
        }
}

/*
 * Resolve an unresolved namecache entry, generally by looking it up.
 * The passed ncp must be locked and refd. 
 *
 * Theoretically since a vnode cannot be recycled while held, and since
 * the nc_parent chain holds its vnode as long as children exist, the
 * direct parent of the cache entry we are trying to resolve should
 * have a valid vnode.  If not then generate an error that we can 
 * determine is related to a resolver bug.
 *
 * However, if a vnode was in the middle of a recyclement when the NCP
 * got locked, ncp->nc_vp might point to a vnode that is about to become
 * invalid.  cache_resolve() handles this case by unresolving the entry
 * and then re-resolving it.
 *
 * Note that successful resolution does not necessarily return an error
 * code of 0.  If the ncp resolves to a negative cache hit then ENOENT
 * will be returned.
 */
int
cache_resolve(struct nchandle *nch, struct ucred *cred)
{
        struct namecache *par_tmp;
        struct namecache *par;
        struct namecache *ncp;
        struct nchandle nctmp;
        struct mount *mp;
        struct vnode *dvp;
        int error;

        ncp = nch->ncp;
        mp = nch->mount;
        KKASSERT(_cache_lockstatus(ncp) == LK_EXCLUSIVE);
restart:
        /*
         * If the ncp is already resolved we have nothing to do.  However,
         * we do want to guarentee that a usable vnode is returned when
         * a vnode is present, so make sure it hasn't been reclaimed.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                        _cache_setunresolved(ncp);
                if ((ncp->nc_flag & NCF_UNRESOLVED) == 0)
                        return (ncp->nc_error);
        }

        /*
         * If the ncp was destroyed it will never resolve again.  This
         * can basically only happen when someone is chdir'd into an
         * empty directory which is then rmdir'd.  We want to catch this
         * here and not dive the VFS because the VFS might actually
         * have a way to re-resolve the disconnected ncp, which will
         * result in inconsistencies in the cdir/nch for proc->p_fd.
         */
        if (ncp->nc_flag & NCF_DESTROYED) {
                kprintf("Warning: cache_resolve: ncp '%s' was unlinked\n",
                        ncp->nc_name);
                return(EINVAL);
        }

        /*
         * Mount points need special handling because the parent does not
         * belong to the same filesystem as the ncp.
         */
        if (ncp == mp->mnt_ncmountpt.ncp)
                return (cache_resolve_mp(mp));

        /*
         * We expect an unbroken chain of ncps to at least the mount point,
         * and even all the way to root (but this code doesn't have to go
         * past the mount point).
         */
        if (ncp->nc_parent == NULL) {
                kprintf("EXDEV case 1 %p %*.*s\n", ncp,
                        ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name);
                ncp->nc_error = EXDEV;
                return(ncp->nc_error);
        }

        /*
         * The vp's of the parent directories in the chain are held via vhold()
         * due to the existance of the child, and should not disappear. 
         * However, there are cases where they can disappear:
         *
         *      - due to filesystem I/O errors.
         *      - due to NFS being stupid about tracking the namespace and
         *        destroys the namespace for entire directories quite often.
         *      - due to forced unmounts.
         *      - due to an rmdir (parent will be marked DESTROYED)
         *
         * When this occurs we have to track the chain backwards and resolve
         * it, looping until the resolver catches up to the current node.  We
         * could recurse here but we might run ourselves out of kernel stack
         * so we do it in a more painful manner.  This situation really should
         * not occur all that often, or if it does not have to go back too
         * many nodes to resolve the ncp.
         */
        while ((dvp = cache_dvpref(ncp)) == NULL) {
                /*
                 * This case can occur if a process is CD'd into a
                 * directory which is then rmdir'd.  If the parent is marked
                 * destroyed there is no point trying to resolve it.
                 */
                if (ncp->nc_parent->nc_flag & NCF_DESTROYED)
                        return(ENOENT);
                par = ncp->nc_parent;
                _cache_hold(par);
                _cache_lock(par);
                while ((par_tmp = par->nc_parent) != NULL &&
                       par_tmp->nc_vp == NULL) {
                        _cache_hold(par_tmp);
                        _cache_lock(par_tmp);
                        _cache_put(par);
                        par = par_tmp;
                }
                if (par->nc_parent == NULL) {
                        kprintf("EXDEV case 2 %*.*s\n",
                                par->nc_nlen, par->nc_nlen, par->nc_name);
                        _cache_put(par);
                        return (EXDEV);
                }
                kprintf("[diagnostic] cache_resolve: had to recurse on %*.*s\n",
                        par->nc_nlen, par->nc_nlen, par->nc_name);
                /*
                 * The parent is not set in stone, ref and lock it to prevent
                 * it from disappearing.  Also note that due to renames it
                 * is possible for our ncp to move and for par to no longer
                 * be one of its parents.  We resolve it anyway, the loop 
                 * will handle any moves.
                 */
                _cache_get(par);        /* additional hold/lock */
                _cache_put(par);        /* from earlier hold/lock */
                if (par == nch->mount->mnt_ncmountpt.ncp) {
                        cache_resolve_mp(nch->mount);
                } else if ((dvp = cache_dvpref(par)) == NULL) {
                        kprintf("[diagnostic] cache_resolve: raced on %*.*s\n", par->nc_nlen, par->nc_nlen, par->nc_name);
                        _cache_put(par);
                        continue;
                } else {
                        if (par->nc_flag & NCF_UNRESOLVED) {
                                nctmp.mount = mp;
                                nctmp.ncp = par;
                                par->nc_error = VOP_NRESOLVE(&nctmp, dvp, cred);
                        }
                        vrele(dvp);
                }
                if ((error = par->nc_error) != 0) {
                        if (par->nc_error != EAGAIN) {
                                kprintf("EXDEV case 3 %*.*s error %d\n",
                                    par->nc_nlen, par->nc_nlen, par->nc_name,
                                    par->nc_error);
                                _cache_put(par);
                                return(error);
                        }
                        kprintf("[diagnostic] cache_resolve: EAGAIN par %p %*.*s\n",
                                par, par->nc_nlen, par->nc_nlen, par->nc_name);
                }
                _cache_put(par);
                /* loop */
        }

        /*
         * Call VOP_NRESOLVE() to get the vp, then scan for any disconnected
         * ncp's and reattach them.  If this occurs the original ncp is marked
         * EAGAIN to force a relookup.
         *
         * NOTE: in order to call VOP_NRESOLVE(), the parent of the passed
         * ncp must already be resolved.
         */
        if (dvp) {
                nctmp.mount = mp;
                nctmp.ncp = ncp;
                ncp->nc_error = VOP_NRESOLVE(&nctmp, dvp, cred);
                vrele(dvp);
        } else {
                ncp->nc_error = EPERM;
        }
        if (ncp->nc_error == EAGAIN) {
                kprintf("[diagnostic] cache_resolve: EAGAIN ncp %p %*.*s\n",
                        ncp, ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name);
                goto restart;
        }
        return(ncp->nc_error);
}

/*
 * Resolve the ncp associated with a mount point.  Such ncp's almost always
 * remain resolved and this routine is rarely called.  NFS MPs tends to force
 * re-resolution more often due to its mac-truck-smash-the-namecache
 * method of tracking namespace changes.
 *
 * The semantics for this call is that the passed ncp must be locked on
 * entry and will be locked on return.  However, if we actually have to
 * resolve the mount point we temporarily unlock the entry in order to
 * avoid race-to-root deadlocks due to e.g. dead NFS mounts.  Because of
 * the unlock we have to recheck the flags after we relock.
 */
static int
cache_resolve_mp(struct mount *mp)
{
        struct namecache *ncp = mp->mnt_ncmountpt.ncp;
        struct vnode *vp;
        int error;

        KKASSERT(mp != NULL);

        /*
         * If the ncp is already resolved we have nothing to do.  However,
         * we do want to guarentee that a usable vnode is returned when
         * a vnode is present, so make sure it hasn't been reclaimed.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                        _cache_setunresolved(ncp);
        }

        if (ncp->nc_flag & NCF_UNRESOLVED) {
                _cache_unlock(ncp);
                while (vfs_busy(mp, 0))
                        ;
                error = VFS_ROOT(mp, &vp);
                _cache_lock(ncp);

                /*
                 * recheck the ncp state after relocking.
                 */
                if (ncp->nc_flag & NCF_UNRESOLVED) {
                        ncp->nc_error = error;
                        if (error == 0) {
                                _cache_setvp(mp, ncp, vp);
                                vput(vp);
                        } else {
                                kprintf("[diagnostic] cache_resolve_mp: failed"
                                        " to resolve mount %p err=%d ncp=%p\n",
                                        mp, error, ncp);
                                _cache_setvp(mp, ncp, NULL);
                        }
                } else if (error == 0) {
                        vput(vp);
                }
                vfs_unbusy(mp);
        }
        return(ncp->nc_error);
}

/*
 * Clean out negative cache entries when too many have accumulated.
 */
static void
_cache_cleanneg(int count)
{
        struct namecache *ncp;

        /*
         * Attempt to clean out the specified number of negative cache
         * entries.
         */
        while (count) {
                spin_lock(&ncspin);
                ncp = TAILQ_FIRST(&ncneglist);
                if (ncp == NULL) {
                        spin_unlock(&ncspin);
                        break;
                }
                TAILQ_REMOVE(&ncneglist, ncp, nc_vnode);
                TAILQ_INSERT_TAIL(&ncneglist, ncp, nc_vnode);
                _cache_hold(ncp);
                spin_unlock(&ncspin);

                /*
                 * This can race, so we must re-check that the ncp
                 * is on the ncneglist after successfully locking it.
                 */
                if (_cache_lock_special(ncp) == 0) {
                        if (ncp->nc_vp == NULL &&
                            (ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                                ncp = cache_zap(ncp, 1);
                                if (ncp)
                                        _cache_drop(ncp);
                        } else {
                                kprintf("cache_cleanneg: race avoided\n");
                                _cache_unlock(ncp);
                        }
                } else {
                        _cache_drop(ncp);
                }
                --count;
        }
}

/*
 * Clean out positive cache entries when too many have accumulated.
 */
static void
_cache_cleanpos(int count)
{
        static volatile int rover;
        struct nchash_head *nchpp;
        struct namecache *ncp;
        int rover_copy;

        /*
         * Attempt to clean out the specified number of negative cache
         * entries.
         */
        while (count) {
                rover_copy = ++rover;   /* MPSAFEENOUGH */
                cpu_ccfence();
                nchpp = NCHHASH(rover_copy);

                spin_lock(&nchpp->spin);
                ncp = LIST_FIRST(&nchpp->list);
                if (ncp)
                        _cache_hold(ncp);
                spin_unlock(&nchpp->spin);

                if (ncp) {
                        if (_cache_lock_special(ncp) == 0) {
                                ncp = cache_zap(ncp, 1);
                                if (ncp)
                                        _cache_drop(ncp);
                        } else {
                                _cache_drop(ncp);
                        }
                }
                --count;
        }
}

/*
 * This is a kitchen sink function to clean out ncps which we
 * tried to zap from cache_drop() but failed because we were
 * unable to acquire the parent lock.
 *
 * Such entries can also be removed via cache_inval_vp(), such
 * as when unmounting.
 */
static void
_cache_cleandefered(void)
{
        struct nchash_head *nchpp;
        struct namecache *ncp;
        struct namecache dummy;
        int i;

        numdefered = 0;
        bzero(&dummy, sizeof(dummy));
        dummy.nc_flag = NCF_DESTROYED;

        for (i = 0; i <= nchash; ++i) {
                nchpp = &nchashtbl[i];

                spin_lock(&nchpp->spin);
                LIST_INSERT_HEAD(&nchpp->list, &dummy, nc_hash);
                ncp = &dummy;
                while ((ncp = LIST_NEXT(ncp, nc_hash)) != NULL) {
                        if ((ncp->nc_flag & NCF_DEFEREDZAP) == 0)
                                continue;
                        LIST_REMOVE(&dummy, nc_hash);
                        LIST_INSERT_AFTER(ncp, &dummy, nc_hash);
                        _cache_hold(ncp);
                        spin_unlock(&nchpp->spin);
                        if (_cache_lock_nonblock(ncp) == 0) {
                                ncp->nc_flag &= ~NCF_DEFEREDZAP;
                                _cache_unlock(ncp);
                        }
                        _cache_drop(ncp);
                        spin_lock(&nchpp->spin);
                        ncp = &dummy;
                }
                LIST_REMOVE(&dummy, nc_hash);
                spin_unlock(&nchpp->spin);
        }
}

/*
 * Name cache initialization, from vfsinit() when we are booting
 */
void
nchinit(void)
{
        int i;
        globaldata_t gd;

        /* initialise per-cpu namecache effectiveness statistics. */
        for (i = 0; i < ncpus; ++i) {
                gd = globaldata_find(i);
                gd->gd_nchstats = &nchstats[i];
        }
        TAILQ_INIT(&ncneglist);
        spin_init(&ncspin);
        nchashtbl = hashinit_ext(desiredvnodes / 2,
                                 sizeof(struct nchash_head),
                                 M_VFSCACHE, &nchash);
        for (i = 0; i <= (int)nchash; ++i) {
                LIST_INIT(&nchashtbl[i].list);
                spin_init(&nchashtbl[i].spin);
        }
        for (i = 0; i < NCMOUNT_NUMCACHE; ++i)
                spin_init(&ncmount_cache[i].spin);
        nclockwarn = 5 * hz;
}

/*
 * Called from start_init() to bootstrap the root filesystem.  Returns
 * a referenced, unlocked namecache record.
 */
void
cache_allocroot(struct nchandle *nch, struct mount *mp, struct vnode *vp)
{
        nch->ncp = cache_alloc(0);
        nch->mount = mp;
        atomic_add_int(&mp->mnt_refs, 1);
        if (vp)
                _cache_setvp(nch->mount, nch->ncp, vp);
}

/*
 * vfs_cache_setroot()
 *
 *      Create an association between the root of our namecache and
 *      the root vnode.  This routine may be called several times during
 *      booting.
 *
 *      If the caller intends to save the returned namecache pointer somewhere
 *      it must cache_hold() it.
 */
void
vfs_cache_setroot(struct vnode *nvp, struct nchandle *nch)
{
        struct vnode *ovp;
        struct nchandle onch;

        ovp = rootvnode;
        onch = rootnch;
        rootvnode = nvp;
        if (nch)
                rootnch = *nch;
        else
                cache_zero(&rootnch);
        if (ovp)
                vrele(ovp);
        if (onch.ncp)
                cache_drop(&onch);
}

/*
 * XXX OLD API COMPAT FUNCTION.  This really messes up the new namecache
 * topology and is being removed as quickly as possible.  The new VOP_N*()
 * API calls are required to make specific adjustments using the supplied
 * ncp pointers rather then just bogusly purging random vnodes.
 *
 * Invalidate all namecache entries to a particular vnode as well as 
 * any direct children of that vnode in the namecache.  This is a 
 * 'catch all' purge used by filesystems that do not know any better.
 *
 * Note that the linkage between the vnode and its namecache entries will
 * be removed, but the namecache entries themselves might stay put due to
 * active references from elsewhere in the system or due to the existance of
 * the children.   The namecache topology is left intact even if we do not
 * know what the vnode association is.  Such entries will be marked
 * NCF_UNRESOLVED.
 */
void
cache_purge(struct vnode *vp)
{
        cache_inval_vp(vp, CINV_DESTROY | CINV_CHILDREN);
}

/*
 * Flush all entries referencing a particular filesystem.
 *
 * Since we need to check it anyway, we will flush all the invalid
 * entries at the same time.
 */
#if 0

void
cache_purgevfs(struct mount *mp)
{
        struct nchash_head *nchpp;
        struct namecache *ncp, *nnp;

        /*
         * Scan hash tables for applicable entries.
         */
        for (nchpp = &nchashtbl[nchash]; nchpp >= nchashtbl; nchpp--) {
                spin_lock_wr(&nchpp->spin); XXX
                ncp = LIST_FIRST(&nchpp->list);
                if (ncp)
                        _cache_hold(ncp);
                while (ncp) {
                        nnp = LIST_NEXT(ncp, nc_hash);
                        if (nnp)
                                _cache_hold(nnp);
                        if (ncp->nc_mount == mp) {
                                _cache_lock(ncp);
                                ncp = cache_zap(ncp, 0);
                                if (ncp)
                                        _cache_drop(ncp);
                        } else {
                                _cache_drop(ncp);
                        }
                        ncp = nnp;
                }
                spin_unlock_wr(&nchpp->spin); XXX
        }
}

#endif

static int disablecwd;
SYSCTL_INT(_debug, OID_AUTO, disablecwd, CTLFLAG_RW, &disablecwd, 0,
    "Disable getcwd");

static u_long numcwdcalls;
SYSCTL_ULONG(_vfs_cache, OID_AUTO, numcwdcalls, CTLFLAG_RD, &numcwdcalls, 0,
    "Number of current directory resolution calls");
static u_long numcwdfailnf;
SYSCTL_ULONG(_vfs_cache, OID_AUTO, numcwdfailnf, CTLFLAG_RD, &numcwdfailnf, 0,
    "Number of current directory failures due to lack of file");
static u_long numcwdfailsz;
SYSCTL_ULONG(_vfs_cache, OID_AUTO, numcwdfailsz, CTLFLAG_RD, &numcwdfailsz, 0,
    "Number of current directory failures due to large result");
static u_long numcwdfound;
SYSCTL_ULONG(_vfs_cache, OID_AUTO, numcwdfound, CTLFLAG_RD, &numcwdfound, 0,
    "Number of current directory resolution successes");

/*
 * MPALMOSTSAFE
 */
int
sys___getcwd(struct __getcwd_args *uap)
{
        u_int buflen;
        int error;
        char *buf;
        char *bp;

        if (disablecwd)
                return (ENODEV);

        buflen = uap->buflen;
        if (buflen == 0)
                return (EINVAL);
        if (buflen > MAXPATHLEN)
                buflen = MAXPATHLEN;

        buf = kmalloc(buflen, M_TEMP, M_WAITOK);
        bp = kern_getcwd(buf, buflen, &error);
        if (error == 0)
                error = copyout(bp, uap->buf, strlen(bp) + 1);
        kfree(buf, M_TEMP);
        return (error);
}

char *
kern_getcwd(char *buf, size_t buflen, int *error)
{
        struct proc *p = curproc;
        char *bp;
        int i, slash_prefixed;
        struct filedesc *fdp;
        struct nchandle nch;
        struct namecache *ncp;

        numcwdcalls++;
        bp = buf;
        bp += buflen - 1;
        *bp = '\0';
        fdp = p->p_fd;
        slash_prefixed = 0;

        nch = fdp->fd_ncdir;
        ncp = nch.ncp;
        if (ncp)
                _cache_hold(ncp);

        while (ncp && (ncp != fdp->fd_nrdir.ncp ||
               nch.mount != fdp->fd_nrdir.mount)
        ) {
                /*
                 * While traversing upwards if we encounter the root
                 * of the current mount we have to skip to the mount point
                 * in the underlying filesystem.
                 */
                if (ncp == nch.mount->mnt_ncmountpt.ncp) {
                        nch = nch.mount->mnt_ncmounton;
                        _cache_drop(ncp);
                        ncp = nch.ncp;
                        if (ncp)
                                _cache_hold(ncp);
                        continue;
                }

                /*
                 * Prepend the path segment
                 */
                for (i = ncp->nc_nlen - 1; i >= 0; i--) {
                        if (bp == buf) {
                                numcwdfailsz++;
                                *error = ERANGE;
                                bp = NULL;
                                goto done;
                        }
                        *--bp = ncp->nc_name[i];
                }
                if (bp == buf) {
                        numcwdfailsz++;
                        *error = ERANGE;
                        bp = NULL;
                        goto done;
                }
                *--bp = '/';
                slash_prefixed = 1;

                /*
                 * Go up a directory.  This isn't a mount point so we don't
                 * have to check again.
                 */
                while ((nch.ncp = ncp->nc_parent) != NULL) {
                        if (ncp_shared_lock_disable)
                                _cache_lock(ncp);
                        else
                                _cache_lock_shared(ncp);
                        if (nch.ncp != ncp->nc_parent) {
                                _cache_unlock(ncp);
                                continue;
                        }
                        _cache_hold(nch.ncp);
                        _cache_unlock(ncp);
                        break;
                }
                _cache_drop(ncp);
                ncp = nch.ncp;
        }
        if (ncp == NULL) {
                numcwdfailnf++;
                *error = ENOENT;
                bp = NULL;
                goto done;
        }
        if (!slash_prefixed) {
                if (bp == buf) {
                        numcwdfailsz++;
                        *error = ERANGE;
                        bp = NULL;
                        goto done;
                }
                *--bp = '/';
        }
        numcwdfound++;
        *error = 0;
done:
        if (ncp)
                _cache_drop(ncp);
        return (bp);
}

/*
 * Thus begins the fullpath magic.
 *
 * The passed nchp is referenced but not locked.
 */
static int disablefullpath;
SYSCTL_INT(_debug, OID_AUTO, disablefullpath, CTLFLAG_RW,
    &disablefullpath, 0,
    "Disable fullpath lookups");

static u_int numfullpathcalls;
SYSCTL_UINT(_vfs_cache, OID_AUTO, numfullpathcalls, CTLFLAG_RD,
    &numfullpathcalls, 0,
    "Number of full path resolutions in progress");
static u_int numfullpathfailnf;
SYSCTL_UINT(_vfs_cache, OID_AUTO, numfullpathfailnf, CTLFLAG_RD,
    &numfullpathfailnf, 0,
    "Number of full path resolution failures due to lack of file");
static u_int numfullpathfailsz;
SYSCTL_UINT(_vfs_cache, OID_AUTO, numfullpathfailsz, CTLFLAG_RD,
    &numfullpathfailsz, 0,
    "Number of full path resolution failures due to insufficient memory");
static u_int numfullpathfound;
SYSCTL_UINT(_vfs_cache, OID_AUTO, numfullpathfound, CTLFLAG_RD,
    &numfullpathfound, 0,
    "Number of full path resolution successes");

int
cache_fullpath(struct proc *p, struct nchandle *nchp, struct nchandle *nchbase,
               char **retbuf, char **freebuf, int guess)
{
        struct nchandle fd_nrdir;
        struct nchandle nch;
        struct namecache *ncp;
        struct mount *mp, *new_mp;
        char *bp, *buf;
        int slash_prefixed;
        int error = 0;
        int i;

        atomic_add_int(&numfullpathcalls, -1);

        *retbuf = NULL; 
        *freebuf = NULL;

        buf = kmalloc(MAXPATHLEN, M_TEMP, M_WAITOK);
        bp = buf + MAXPATHLEN - 1;
        *bp = '\0';
        if (nchbase)
                fd_nrdir = *nchbase;
        else if (p != NULL)
                fd_nrdir = p->p_fd->fd_nrdir;
        else
                fd_nrdir = rootnch;
        slash_prefixed = 0;
        nch = *nchp;
        ncp = nch.ncp;
        if (ncp)
                _cache_hold(ncp);
        mp = nch.mount;

        while (ncp && (ncp != fd_nrdir.ncp || mp != fd_nrdir.mount)) {
                new_mp = NULL;

                /*
                 * If we are asked to guess the upwards path, we do so whenever
                 * we encounter an ncp marked as a mountpoint. We try to find
                 * the actual mountpoint by finding the mountpoint with this
                 * ncp.
                 */
                if (guess && (ncp->nc_flag & NCF_ISMOUNTPT)) {
                        new_mp = mount_get_by_nc(ncp);
                }
                /*
                 * While traversing upwards if we encounter the root
                 * of the current mount we have to skip to the mount point.
                 */
                if (ncp == mp->mnt_ncmountpt.ncp) {
                        new_mp = mp;
                }
                if (new_mp) {
                        nch = new_mp->mnt_ncmounton;
                        _cache_drop(ncp);
                        ncp = nch.ncp;
                        if (ncp)
                                _cache_hold(ncp);
                        mp = nch.mount;
                        continue;
                }

                /*
                 * Prepend the path segment
                 */
                for (i = ncp->nc_nlen - 1; i >= 0; i--) {
                        if (bp == buf) {
                                numfullpathfailsz++;
                                kfree(buf, M_TEMP);
                                error = ENOMEM;
                                goto done;
                        }
                        *--bp = ncp->nc_name[i];
                }
                if (bp == buf) {
                        numfullpathfailsz++;
                        kfree(buf, M_TEMP);
                        error = ENOMEM;
                        goto done;
                }
                *--bp = '/';
                slash_prefixed = 1;

                /*
                 * Go up a directory.  This isn't a mount point so we don't
                 * have to check again.
                 *
                 * We can only safely access nc_parent with ncp held locked.
                 */
                while ((nch.ncp = ncp->nc_parent) != NULL) {
                        _cache_lock(ncp);
                        if (nch.ncp != ncp->nc_parent) {
                                _cache_unlock(ncp);
                                continue;
                        }
                        _cache_hold(nch.ncp);
                        _cache_unlock(ncp);
                        break;
                }
                _cache_drop(ncp);
                ncp = nch.ncp;
        }
        if (ncp == NULL) {
                numfullpathfailnf++;
                kfree(buf, M_TEMP);
                error = ENOENT;
                goto done;
        }

        if (!slash_prefixed) {
                if (bp == buf) {
                        numfullpathfailsz++;
                        kfree(buf, M_TEMP);
                        error = ENOMEM;
                        goto done;
                }
                *--bp = '/';
        }
        numfullpathfound++;
        *retbuf = bp; 
        *freebuf = buf;
        error = 0;
done:
        if (ncp)
                _cache_drop(ncp);
        return(error);
}

int
vn_fullpath(struct proc *p, struct vnode *vn, char **retbuf, char **freebuf,
    int guess)
{
        struct namecache *ncp;
        struct nchandle nch;
        int error;

        *freebuf = NULL;
        atomic_add_int(&numfullpathcalls, 1);
        if (disablefullpath)
                return (ENODEV);

        if (p == NULL)
                return (EINVAL);

        /* vn is NULL, client wants us to use p->p_textvp */
        if (vn == NULL) {
                if ((vn = p->p_textvp) == NULL)
                        return (EINVAL);
        }
        spin_lock(&vn->v_spin);
        TAILQ_FOREACH(ncp, &vn->v_namecache, nc_vnode) {
                if (ncp->nc_nlen)
                        break;
        }
        if (ncp == NULL) {
                spin_unlock(&vn->v_spin);
                return (EINVAL);
        }
        _cache_hold(ncp);
        spin_unlock(&vn->v_spin);

        atomic_add_int(&numfullpathcalls, -1);
        nch.ncp = ncp;
        nch.mount = vn->v_mount;
        error = cache_fullpath(p, &nch, NULL, retbuf, freebuf, guess);
        _cache_drop(ncp);
        return (error);
}