cache: avoid smr in cache_neg_evict in favoro of the already held bucket lock

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

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * 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. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)vfs_cache.c 8.5 (Berkeley) 3/22/95
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#include "opt_ddb.h"
#include "opt_ktrace.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/capsicum.h>
#include <sys/counter.h>
#include <sys/filedesc.h>
#include <sys/fnv_hash.h>
#include <sys/kernel.h>
#include <sys/ktr.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/fcntl.h>
#include <sys/jail.h>
#include <sys/mount.h>
#include <sys/namei.h>
#include <sys/proc.h>
#include <sys/seqc.h>
#include <sys/sdt.h>
#include <sys/smr.h>
#include <sys/smp.h>
#include <sys/syscallsubr.h>
#include <sys/sysctl.h>
#include <sys/sysproto.h>
#include <sys/vnode.h>
#include <ck_queue.h>
#ifdef KTRACE
#include <sys/ktrace.h>
#endif

#include <sys/capsicum.h>

#include <security/audit/audit.h>
#include <security/mac/mac_framework.h>

#ifdef DDB
#include <ddb/ddb.h>
#endif

#include <vm/uma.h>

SDT_PROVIDER_DECLARE(vfs);
SDT_PROBE_DEFINE3(vfs, namecache, enter, done, "struct vnode *", "char *",
    "struct vnode *");
SDT_PROBE_DEFINE3(vfs, namecache, enter, duplicate, "struct vnode *", "char *",
    "struct vnode *");
SDT_PROBE_DEFINE2(vfs, namecache, enter_negative, done, "struct vnode *",
    "char *");
SDT_PROBE_DEFINE2(vfs, namecache, fullpath_smr, hit, "struct vnode *",
    "const char *");
SDT_PROBE_DEFINE4(vfs, namecache, fullpath_smr, miss, "struct vnode *",
    "struct namecache *", "int", "int");
SDT_PROBE_DEFINE1(vfs, namecache, fullpath, entry, "struct vnode *");
SDT_PROBE_DEFINE3(vfs, namecache, fullpath, hit, "struct vnode *",
    "char *", "struct vnode *");
SDT_PROBE_DEFINE1(vfs, namecache, fullpath, miss, "struct vnode *");
SDT_PROBE_DEFINE3(vfs, namecache, fullpath, return, "int",
    "struct vnode *", "char *");
SDT_PROBE_DEFINE3(vfs, namecache, lookup, hit, "struct vnode *", "char *",
    "struct vnode *");
SDT_PROBE_DEFINE2(vfs, namecache, lookup, hit__negative,
    "struct vnode *", "char *");
SDT_PROBE_DEFINE2(vfs, namecache, lookup, miss, "struct vnode *",
    "char *");
SDT_PROBE_DEFINE2(vfs, namecache, removecnp, hit, "struct vnode *",
    "struct componentname *");
SDT_PROBE_DEFINE2(vfs, namecache, removecnp, miss, "struct vnode *",
    "struct componentname *");
SDT_PROBE_DEFINE1(vfs, namecache, purge, done, "struct vnode *");
SDT_PROBE_DEFINE1(vfs, namecache, purge_negative, done, "struct vnode *");
SDT_PROBE_DEFINE1(vfs, namecache, purgevfs, done, "struct mount *");
SDT_PROBE_DEFINE3(vfs, namecache, zap, done, "struct vnode *", "char *",
    "struct vnode *");
SDT_PROBE_DEFINE2(vfs, namecache, zap_negative, done, "struct vnode *",
    "char *");
SDT_PROBE_DEFINE2(vfs, namecache, evict_negative, done, "struct vnode *",
    "char *");

SDT_PROBE_DEFINE3(vfs, fplookup, lookup, done, "struct nameidata", "int", "bool");
SDT_PROBE_DECLARE(vfs, namei, lookup, entry);
SDT_PROBE_DECLARE(vfs, namei, lookup, return);

/*
 * This structure describes the elements in the cache of recent
 * names looked up by namei.
 */
struct negstate {
        u_char neg_flag;
};
_Static_assert(sizeof(struct negstate) <= sizeof(struct vnode *),
    "the state must fit in a union with a pointer without growing it");

struct  namecache {
        LIST_ENTRY(namecache) nc_src;   /* source vnode list */
        TAILQ_ENTRY(namecache) nc_dst;  /* destination vnode list */
        CK_SLIST_ENTRY(namecache) nc_hash;/* hash chain */
        struct  vnode *nc_dvp;          /* vnode of parent of name */
        union {
                struct  vnode *nu_vp;   /* vnode the name refers to */
                struct  negstate nu_neg;/* negative entry state */
        } n_un;
        u_char  nc_flag;                /* flag bits */
        u_char  nc_nlen;                /* length of name */
        char    nc_name[0];             /* segment name + nul */
};

/*
 * struct namecache_ts repeats struct namecache layout up to the
 * nc_nlen member.
 * struct namecache_ts is used in place of struct namecache when time(s) need
 * to be stored.  The nc_dotdottime field is used when a cache entry is mapping
 * both a non-dotdot directory name plus dotdot for the directory's
 * parent.
 *
 * See below for alignment requirement.
 */
struct  namecache_ts {
        struct  timespec nc_time;       /* timespec provided by fs */
        struct  timespec nc_dotdottime; /* dotdot timespec provided by fs */
        int     nc_ticks;               /* ticks value when entry was added */
        struct namecache nc_nc;
};

/*
 * At least mips n32 performs 64-bit accesses to timespec as found
 * in namecache_ts and requires them to be aligned. Since others
 * may be in the same spot suffer a little bit and enforce the
 * alignment for everyone. Note this is a nop for 64-bit platforms.
 */
#define CACHE_ZONE_ALIGNMENT    UMA_ALIGNOF(time_t)
#define CACHE_PATH_CUTOFF       39

#define CACHE_ZONE_SMALL_SIZE           (sizeof(struct namecache) + CACHE_PATH_CUTOFF + 1)
#define CACHE_ZONE_SMALL_TS_SIZE        (sizeof(struct namecache_ts) + CACHE_PATH_CUTOFF + 1)
#define CACHE_ZONE_LARGE_SIZE           (sizeof(struct namecache) + NAME_MAX + 1)
#define CACHE_ZONE_LARGE_TS_SIZE        (sizeof(struct namecache_ts) + NAME_MAX + 1)

_Static_assert((CACHE_ZONE_SMALL_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");
_Static_assert((CACHE_ZONE_SMALL_TS_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");
_Static_assert((CACHE_ZONE_LARGE_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");
_Static_assert((CACHE_ZONE_LARGE_TS_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");

#define nc_vp           n_un.nu_vp
#define nc_neg          n_un.nu_neg

/*
 * Flags in namecache.nc_flag
 */
#define NCF_WHITE       0x01
#define NCF_ISDOTDOT    0x02
#define NCF_TS          0x04
#define NCF_DTS         0x08
#define NCF_DVDROP      0x10
#define NCF_NEGATIVE    0x20
#define NCF_INVALID     0x40
#define NCF_WIP         0x80

/*
 * Flags in negstate.neg_flag
 */
#define NEG_HOT         0x01

/*
 * Mark an entry as invalid.
 *
 * This is called before it starts getting deconstructed.
 */
static void
cache_ncp_invalidate(struct namecache *ncp)
{

        KASSERT((ncp->nc_flag & NCF_INVALID) == 0,
            ("%s: entry %p already invalid", __func__, ncp));
        atomic_store_char(&ncp->nc_flag, ncp->nc_flag | NCF_INVALID);
        atomic_thread_fence_rel();
}

/*
 * Check whether the entry can be safely used.
 *
 * All places which elide locks are supposed to call this after they are
 * done with reading from an entry.
 */
static bool
cache_ncp_canuse(struct namecache *ncp)
{

        atomic_thread_fence_acq();
        return ((atomic_load_char(&ncp->nc_flag) & (NCF_INVALID | NCF_WIP)) == 0);
}

/*
 * Name caching works as follows:
 *
 * Names found by directory scans are retained in a cache
 * for future reference.  It is managed LRU, so frequently
 * used names will hang around.  Cache is indexed by hash value
 * obtained from (dvp, name) where dvp refers to the directory
 * containing name.
 *
 * If it is a "negative" entry, (i.e. for a name that is known NOT to
 * exist) the vnode pointer will be NULL.
 *
 * Upon reaching the last segment of a path, if the reference
 * is for DELETE, or NOCACHE is set (rewrite), and the
 * name is located in the cache, it will be dropped.
 *
 * These locks are used (in the order in which they can be taken):
 * NAME         TYPE    ROLE
 * vnodelock    mtx     vnode lists and v_cache_dd field protection
 * bucketlock   mtx     for access to given set of hash buckets
 * neglist      mtx     negative entry LRU management
 *
 * It is legal to take multiple vnodelock and bucketlock locks. The locking
 * order is lower address first. Both are recursive.
 *
 * "." lookups are lockless.
 *
 * ".." and vnode -> name lookups require vnodelock.
 *
 * name -> vnode lookup requires the relevant bucketlock to be held for reading.
 *
 * Insertions and removals of entries require involved vnodes and bucketlocks
 * to be locked to provide safe operation against other threads modifying the
 * cache.
 *
 * Some lookups result in removal of the found entry (e.g. getting rid of a
 * negative entry with the intent to create a positive one), which poses a
 * problem when multiple threads reach the state. Similarly, two different
 * threads can purge two different vnodes and try to remove the same name.
 *
 * If the already held vnode lock is lower than the second required lock, we
 * can just take the other lock. However, in the opposite case, this could
 * deadlock. As such, this is resolved by trylocking and if that fails unlocking
 * the first node, locking everything in order and revalidating the state.
 */

VFS_SMR_DECLARE;

/*
 * Structures associated with name caching.
 */
#define NCHHASH(hash) \
        (&nchashtbl[(hash) & nchash])
static __read_mostly CK_SLIST_HEAD(nchashhead, namecache) *nchashtbl;/* Hash Table */
static u_long __read_mostly     nchash;                 /* size of hash table */
SYSCTL_ULONG(_debug, OID_AUTO, nchash, CTLFLAG_RD, &nchash, 0,
    "Size of namecache hash table");
static u_long __read_mostly     ncnegfactor = 5; /* ratio of negative entries */
SYSCTL_ULONG(_vfs, OID_AUTO, ncnegfactor, CTLFLAG_RW, &ncnegfactor, 0,
    "Ratio of negative namecache entries");
static u_long __exclusive_cache_line    numneg; /* number of negative entries allocated */
static u_long __exclusive_cache_line    numcache;/* number of cache entries allocated */
u_int ncsizefactor = 2;
SYSCTL_UINT(_vfs, OID_AUTO, ncsizefactor, CTLFLAG_RW, &ncsizefactor, 0,
    "Size factor for namecache");
static u_int __read_mostly      ncsize; /* the size as computed on creation or resizing */

struct nchstats nchstats;               /* cache effectiveness statistics */

static bool __read_frequently cache_fast_revlookup = true;
SYSCTL_BOOL(_vfs, OID_AUTO, cache_fast_revlookup, CTLFLAG_RW,
    &cache_fast_revlookup, 0, "");

static u_int __exclusive_cache_line neg_cycle;

#define ncneghash       3
#define numneglists     (ncneghash + 1)

struct neglist {
        struct mtx              nl_evict_lock;
        struct mtx              nl_lock __aligned(CACHE_LINE_SIZE);
        TAILQ_HEAD(, namecache) nl_list;
        TAILQ_HEAD(, namecache) nl_hotlist;
        u_long                  nl_hotnum;
} __aligned(CACHE_LINE_SIZE);

static struct neglist neglists[numneglists];

static inline struct neglist *
NCP2NEGLIST(struct namecache *ncp)
{

        return (&neglists[(((uintptr_t)(ncp) >> 8) & ncneghash)]);
}

static inline struct negstate *
NCP2NEGSTATE(struct namecache *ncp)
{

        MPASS(ncp->nc_flag & NCF_NEGATIVE);
        return (&ncp->nc_neg);
}

#define numbucketlocks (ncbuckethash + 1)
static u_int __read_mostly  ncbuckethash;
static struct mtx_padalign __read_mostly  *bucketlocks;
#define HASH2BUCKETLOCK(hash) \
        ((struct mtx *)(&bucketlocks[((hash) & ncbuckethash)]))

#define numvnodelocks (ncvnodehash + 1)
static u_int __read_mostly  ncvnodehash;
static struct mtx __read_mostly *vnodelocks;
static inline struct mtx *
VP2VNODELOCK(struct vnode *vp)
{

        return (&vnodelocks[(((uintptr_t)(vp) >> 8) & ncvnodehash)]);
}

/*
 * UMA zones for the VFS cache.
 *
 * The small cache is used for entries with short names, which are the
 * most common.  The large cache is used for entries which are too big to
 * fit in the small cache.
 */
static uma_zone_t __read_mostly cache_zone_small;
static uma_zone_t __read_mostly cache_zone_small_ts;
static uma_zone_t __read_mostly cache_zone_large;
static uma_zone_t __read_mostly cache_zone_large_ts;

static struct namecache *
cache_alloc(int len, int ts)
{
        struct namecache_ts *ncp_ts;
        struct namecache *ncp;

        if (__predict_false(ts)) {
                if (len <= CACHE_PATH_CUTOFF)
                        ncp_ts = uma_zalloc_smr(cache_zone_small_ts, M_WAITOK);
                else
                        ncp_ts = uma_zalloc_smr(cache_zone_large_ts, M_WAITOK);
                ncp = &ncp_ts->nc_nc;
        } else {
                if (len <= CACHE_PATH_CUTOFF)
                        ncp = uma_zalloc_smr(cache_zone_small, M_WAITOK);
                else
                        ncp = uma_zalloc_smr(cache_zone_large, M_WAITOK);
        }
        return (ncp);
}

static void
cache_free(struct namecache *ncp)
{
        struct namecache_ts *ncp_ts;

        MPASS(ncp != NULL);
        if ((ncp->nc_flag & NCF_DVDROP) != 0)
                vdrop(ncp->nc_dvp);
        if (__predict_false(ncp->nc_flag & NCF_TS)) {
                ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
                if (ncp->nc_nlen <= CACHE_PATH_CUTOFF)
                        uma_zfree_smr(cache_zone_small_ts, ncp_ts);
                else
                        uma_zfree_smr(cache_zone_large_ts, ncp_ts);
        } else {
                if (ncp->nc_nlen <= CACHE_PATH_CUTOFF)
                        uma_zfree_smr(cache_zone_small, ncp);
                else
                        uma_zfree_smr(cache_zone_large, ncp);
        }
}

static void
cache_out_ts(struct namecache *ncp, struct timespec *tsp, int *ticksp)
{
        struct namecache_ts *ncp_ts;

        KASSERT((ncp->nc_flag & NCF_TS) != 0 ||
            (tsp == NULL && ticksp == NULL),
            ("No NCF_TS"));

        if (tsp == NULL)
                return;

        ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
        *tsp = ncp_ts->nc_time;
        *ticksp = ncp_ts->nc_ticks;
}

#ifdef DEBUG_CACHE
static int __read_mostly        doingcache = 1; /* 1 => enable the cache */
SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
    "VFS namecache enabled");
#endif

/* Export size information to userland */
SYSCTL_INT(_debug_sizeof, OID_AUTO, namecache, CTLFLAG_RD, SYSCTL_NULL_INT_PTR,
    sizeof(struct namecache), "sizeof(struct namecache)");

/*
 * The new name cache statistics
 */
static SYSCTL_NODE(_vfs, OID_AUTO, cache, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
    "Name cache statistics");
#define STATNODE_ULONG(name, descr)                                     \
        SYSCTL_ULONG(_vfs_cache, OID_AUTO, name, CTLFLAG_RD, &name, 0, descr);
#define STATNODE_COUNTER(name, descr)                                   \
        static COUNTER_U64_DEFINE_EARLY(name);                          \
        SYSCTL_COUNTER_U64(_vfs_cache, OID_AUTO, name, CTLFLAG_RD, &name, \
            descr);
STATNODE_ULONG(numneg, "Number of negative cache entries");
STATNODE_ULONG(numcache, "Number of cache entries");
STATNODE_COUNTER(numcachehv, "Number of namecache entries with vnodes held");
STATNODE_COUNTER(numdrops, "Number of dropped entries due to reaching the limit");
STATNODE_COUNTER(dothits, "Number of '.' hits");
STATNODE_COUNTER(dotdothits, "Number of '..' hits");
STATNODE_COUNTER(nummiss, "Number of cache misses");
STATNODE_COUNTER(nummisszap, "Number of cache misses we do not want to cache");
STATNODE_COUNTER(numposzaps,
    "Number of cache hits (positive) we do not want to cache");
STATNODE_COUNTER(numposhits, "Number of cache hits (positive)");
STATNODE_COUNTER(numnegzaps,
    "Number of cache hits (negative) we do not want to cache");
STATNODE_COUNTER(numneghits, "Number of cache hits (negative)");
/* These count for vn_getcwd(), too. */
STATNODE_COUNTER(numfullpathcalls, "Number of fullpath search calls");
STATNODE_COUNTER(numfullpathfail1, "Number of fullpath search errors (ENOTDIR)");
STATNODE_COUNTER(numfullpathfail2,
    "Number of fullpath search errors (VOP_VPTOCNP failures)");
STATNODE_COUNTER(numfullpathfail4, "Number of fullpath search errors (ENOMEM)");
STATNODE_COUNTER(numfullpathfound, "Number of successful fullpath calls");
STATNODE_COUNTER(zap_and_exit_bucket_relock_success,
    "Number of successful removals after relocking");
static long zap_and_exit_bucket_fail; STATNODE_ULONG(zap_and_exit_bucket_fail,
    "Number of times zap_and_exit failed to lock");
static long zap_and_exit_bucket_fail2; STATNODE_ULONG(zap_and_exit_bucket_fail2,
    "Number of times zap_and_exit failed to lock");
static long cache_lock_vnodes_cel_3_failures;
STATNODE_ULONG(cache_lock_vnodes_cel_3_failures,
    "Number of times 3-way vnode locking failed");

static void cache_zap_locked(struct namecache *ncp);
static int vn_fullpath_hardlink(struct nameidata *ndp, char **retbuf,
    char **freebuf, size_t *buflen);
static int vn_fullpath_any_smr(struct vnode *vp, struct vnode *rdir, char *buf,
    char **retbuf, size_t *buflen, bool slash_prefixed, size_t addend);
static int vn_fullpath_any(struct vnode *vp, struct vnode *rdir, char *buf,
    char **retbuf, size_t *buflen);
static int vn_fullpath_dir(struct vnode *vp, struct vnode *rdir, char *buf,
    char **retbuf, size_t *len, bool slash_prefixed, size_t addend);

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

static inline void
cache_assert_vlp_locked(struct mtx *vlp)
{

        if (vlp != NULL)
                mtx_assert(vlp, MA_OWNED);
}

static inline void
cache_assert_vnode_locked(struct vnode *vp)
{
        struct mtx *vlp;

        vlp = VP2VNODELOCK(vp);
        cache_assert_vlp_locked(vlp);
}

/*
 * TODO: With the value stored we can do better than computing the hash based
 * on the address. The choice of FNV should also be revisited.
 */
static void
cache_prehash(struct vnode *vp)
{

        vp->v_nchash = fnv_32_buf(&vp, sizeof(vp), FNV1_32_INIT);
}

static uint32_t
cache_get_hash(char *name, u_char len, struct vnode *dvp)
{

        return (fnv_32_buf(name, len, dvp->v_nchash));
}

static inline struct nchashhead *
NCP2BUCKET(struct namecache *ncp)
{
        uint32_t hash;

        hash = cache_get_hash(ncp->nc_name, ncp->nc_nlen, ncp->nc_dvp);
        return (NCHHASH(hash));
}

static inline struct mtx *
NCP2BUCKETLOCK(struct namecache *ncp)
{
        uint32_t hash;

        hash = cache_get_hash(ncp->nc_name, ncp->nc_nlen, ncp->nc_dvp);
        return (HASH2BUCKETLOCK(hash));
}

#ifdef INVARIANTS
static void
cache_assert_bucket_locked(struct namecache *ncp)
{
        struct mtx *blp;

        blp = NCP2BUCKETLOCK(ncp);
        mtx_assert(blp, MA_OWNED);
}

static void
cache_assert_bucket_unlocked(struct namecache *ncp)
{
        struct mtx *blp;

        blp = NCP2BUCKETLOCK(ncp);
        mtx_assert(blp, MA_NOTOWNED);
}
#else
#define cache_assert_bucket_locked(x) do { } while (0)
#define cache_assert_bucket_unlocked(x) do { } while (0)
#endif

#define cache_sort_vnodes(x, y) _cache_sort_vnodes((void **)(x), (void **)(y))
static void
_cache_sort_vnodes(void **p1, void **p2)
{
        void *tmp;

        MPASS(*p1 != NULL || *p2 != NULL);

        if (*p1 > *p2) {
                tmp = *p2;
                *p2 = *p1;
                *p1 = tmp;
        }
}

static void
cache_lock_all_buckets(void)
{
        u_int i;

        for (i = 0; i < numbucketlocks; i++)
                mtx_lock(&bucketlocks[i]);
}

static void
cache_unlock_all_buckets(void)
{
        u_int i;

        for (i = 0; i < numbucketlocks; i++)
                mtx_unlock(&bucketlocks[i]);
}

static void
cache_lock_all_vnodes(void)
{
        u_int i;

        for (i = 0; i < numvnodelocks; i++)
                mtx_lock(&vnodelocks[i]);
}

static void
cache_unlock_all_vnodes(void)
{
        u_int i;

        for (i = 0; i < numvnodelocks; i++)
                mtx_unlock(&vnodelocks[i]);
}

static int
cache_trylock_vnodes(struct mtx *vlp1, struct mtx *vlp2)
{

        cache_sort_vnodes(&vlp1, &vlp2);

        if (vlp1 != NULL) {
                if (!mtx_trylock(vlp1))
                        return (EAGAIN);
        }
        if (!mtx_trylock(vlp2)) {
                if (vlp1 != NULL)
                        mtx_unlock(vlp1);
                return (EAGAIN);
        }

        return (0);
}

static void
cache_lock_vnodes(struct mtx *vlp1, struct mtx *vlp2)
{

        MPASS(vlp1 != NULL || vlp2 != NULL);
        MPASS(vlp1 <= vlp2);

        if (vlp1 != NULL)
                mtx_lock(vlp1);
        if (vlp2 != NULL)
                mtx_lock(vlp2);
}

static void
cache_unlock_vnodes(struct mtx *vlp1, struct mtx *vlp2)
{

        MPASS(vlp1 != NULL || vlp2 != NULL);

        if (vlp1 != NULL)
                mtx_unlock(vlp1);
        if (vlp2 != NULL)
                mtx_unlock(vlp2);
}

static int
sysctl_nchstats(SYSCTL_HANDLER_ARGS)
{
        struct nchstats snap;

        if (req->oldptr == NULL)
                return (SYSCTL_OUT(req, 0, sizeof(snap)));

        snap = nchstats;
        snap.ncs_goodhits = counter_u64_fetch(numposhits);
        snap.ncs_neghits = counter_u64_fetch(numneghits);
        snap.ncs_badhits = counter_u64_fetch(numposzaps) +
            counter_u64_fetch(numnegzaps);
        snap.ncs_miss = counter_u64_fetch(nummisszap) +
            counter_u64_fetch(nummiss);

        return (SYSCTL_OUT(req, &snap, sizeof(snap)));
}
SYSCTL_PROC(_vfs_cache, OID_AUTO, nchstats, CTLTYPE_OPAQUE | CTLFLAG_RD |
    CTLFLAG_MPSAFE, 0, 0, sysctl_nchstats, "LU",
    "VFS cache effectiveness statistics");

#ifdef DIAGNOSTIC
/*
 * Grab an atomic snapshot of the name cache hash chain lengths
 */
static SYSCTL_NODE(_debug, OID_AUTO, hashstat,
    CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
    "hash table stats");

static int
sysctl_debug_hashstat_rawnchash(SYSCTL_HANDLER_ARGS)
{
        struct nchashhead *ncpp;
        struct namecache *ncp;
        int i, error, n_nchash, *cntbuf;

retry:
        n_nchash = nchash + 1;  /* nchash is max index, not count */
        if (req->oldptr == NULL)
                return SYSCTL_OUT(req, 0, n_nchash * sizeof(int));
        cntbuf = malloc(n_nchash * sizeof(int), M_TEMP, M_ZERO | M_WAITOK);
        cache_lock_all_buckets();
        if (n_nchash != nchash + 1) {
                cache_unlock_all_buckets();
                free(cntbuf, M_TEMP);
                goto retry;
        }
        /* Scan hash tables counting entries */
        for (ncpp = nchashtbl, i = 0; i < n_nchash; ncpp++, i++)
                CK_SLIST_FOREACH(ncp, ncpp, nc_hash)
                        cntbuf[i]++;
        cache_unlock_all_buckets();
        for (error = 0, i = 0; i < n_nchash; i++)
                if ((error = SYSCTL_OUT(req, &cntbuf[i], sizeof(int))) != 0)
                        break;
        free(cntbuf, M_TEMP);
        return (error);
}
SYSCTL_PROC(_debug_hashstat, OID_AUTO, rawnchash, CTLTYPE_INT|CTLFLAG_RD|
    CTLFLAG_MPSAFE, 0, 0, sysctl_debug_hashstat_rawnchash, "S,int",
    "nchash chain lengths");

static int
sysctl_debug_hashstat_nchash(SYSCTL_HANDLER_ARGS)
{
        int error;
        struct nchashhead *ncpp;
        struct namecache *ncp;
        int n_nchash;
        int count, maxlength, used, pct;

        if (!req->oldptr)
                return SYSCTL_OUT(req, 0, 4 * sizeof(int));

        cache_lock_all_buckets();
        n_nchash = nchash + 1;  /* nchash is max index, not count */
        used = 0;
        maxlength = 0;

        /* Scan hash tables for applicable entries */
        for (ncpp = nchashtbl; n_nchash > 0; n_nchash--, ncpp++) {
                count = 0;
                CK_SLIST_FOREACH(ncp, ncpp, nc_hash) {
                        count++;
                }
                if (count)
                        used++;
                if (maxlength < count)
                        maxlength = count;
        }
        n_nchash = nchash + 1;
        cache_unlock_all_buckets();
        pct = (used * 100) / (n_nchash / 100);
        error = SYSCTL_OUT(req, &n_nchash, sizeof(n_nchash));
        if (error)
                return (error);
        error = SYSCTL_OUT(req, &used, sizeof(used));
        if (error)
                return (error);
        error = SYSCTL_OUT(req, &maxlength, sizeof(maxlength));
        if (error)
                return (error);
        error = SYSCTL_OUT(req, &pct, sizeof(pct));
        if (error)
                return (error);
        return (0);
}
SYSCTL_PROC(_debug_hashstat, OID_AUTO, nchash, CTLTYPE_INT|CTLFLAG_RD|
    CTLFLAG_MPSAFE, 0, 0, sysctl_debug_hashstat_nchash, "I",
    "nchash statistics (number of total/used buckets, maximum chain length, usage percentage)");
#endif

/*
 * Negative entries management
 *
 * Various workloads create plenty of negative entries and barely use them
 * afterwards. Moreover malicious users can keep performing bogus lookups
 * adding even more entries. For example "make tinderbox" as of writing this
 * comment ends up with 2.6M namecache entries in total, 1.2M of which are
 * negative.
 *
 * As such, a rather aggressive eviction method is needed. The currently
 * employed method is a placeholder.
 *
 * Entries are split over numneglists separate lists, each of which is further
 * split into hot and cold entries. Entries get promoted after getting a hit.
 * Eviction happens on addition of new entry.
 */
static SYSCTL_NODE(_vfs_cache, OID_AUTO, neg, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
    "Name cache negative entry statistics");

SYSCTL_ULONG(_vfs_cache_neg, OID_AUTO, count, CTLFLAG_RD, &numneg, 0,
    "Number of negative cache entries");

static COUNTER_U64_DEFINE_EARLY(neg_created);
SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, created, CTLFLAG_RD, &neg_created,
    "Number of created negative entries");

static COUNTER_U64_DEFINE_EARLY(neg_evicted);
SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evicted, CTLFLAG_RD, &neg_evicted,
    "Number of evicted negative entries");

static COUNTER_U64_DEFINE_EARLY(neg_evict_skipped_empty);
SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evict_skipped_empty, CTLFLAG_RD,
    &neg_evict_skipped_empty,
    "Number of times evicting failed due to lack of entries");

static COUNTER_U64_DEFINE_EARLY(neg_evict_skipped_missed);
SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evict_skipped_missed, CTLFLAG_RD,
    &neg_evict_skipped_missed,
    "Number of times evicting failed due to target entry disappearing");

static COUNTER_U64_DEFINE_EARLY(neg_evict_skipped_contended);
SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evict_skipped_contended, CTLFLAG_RD,
    &neg_evict_skipped_contended,
    "Number of times evicting failed due to contention");

SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, hits, CTLFLAG_RD, &numneghits,
    "Number of cache hits (negative)");

static int
sysctl_neg_hot(SYSCTL_HANDLER_ARGS)
{
        int i, out;

        out = 0;
        for (i = 0; i < numneglists; i++)
                out += neglists[i].nl_hotnum;

        return (SYSCTL_OUT(req, &out, sizeof(out)));
}
SYSCTL_PROC(_vfs_cache_neg, OID_AUTO, hot, CTLTYPE_INT | CTLFLAG_RD |
    CTLFLAG_MPSAFE, 0, 0, sysctl_neg_hot, "I",
    "Number of hot negative entries");

static void
cache_neg_init(struct namecache *ncp)
{
        struct negstate *ns;

        ncp->nc_flag |= NCF_NEGATIVE;
        ns = NCP2NEGSTATE(ncp);
        ns->neg_flag = 0;
        counter_u64_add(neg_created, 1);
}

/*
 * Move a negative entry to the hot list.
 */
static void
cache_neg_promote(struct namecache *ncp)
{
        struct neglist *nl;
        struct negstate *ns;

        ns = NCP2NEGSTATE(ncp);
        nl = NCP2NEGLIST(ncp);
        mtx_assert(&nl->nl_lock, MA_OWNED);
        if ((ns->neg_flag & NEG_HOT) == 0) {
                TAILQ_REMOVE(&nl->nl_list, ncp, nc_dst);
                TAILQ_INSERT_TAIL(&nl->nl_hotlist, ncp, nc_dst);
                nl->nl_hotnum++;
                ns->neg_flag |= NEG_HOT;
        }
}

/*
 * Move a negative entry to the hot list if it matches the lookup.
 *
 * We have to take locks, but they may be contended and in the worst
 * case we may need to go off CPU. We don't want to spin within the
 * smr section and we can't block with it. Exiting the section means
 * the found entry could have been evicted. We are going to look it
 * up again.
 */
static bool
cache_neg_promote_cond(struct vnode *dvp, struct componentname *cnp,
    struct namecache *oncp, uint32_t hash)
{
        struct namecache *ncp;
        struct neglist *nl;
        u_char nc_flag;

        nl = NCP2NEGLIST(oncp);

        mtx_lock(&nl->nl_lock);
        /*
         * For hash iteration.
         */
        vfs_smr_enter();

        /*
         * Avoid all surprises by only succeeding if we got the same entry and
         * bailing completely otherwise.
         * XXX There are no provisions to keep the vnode around, meaning we may
         * end up promoting a negative entry for a *new* vnode and returning
         * ENOENT on its account. This is the error we want to return anyway
         * and promotion is harmless.
         *
         * In particular at this point there can be a new ncp which matches the
         * search but hashes to a different neglist.
         */
        CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                if (ncp == oncp)
                        break;
        }

        /*
         * No match to begin with.
         */
        if (__predict_false(ncp == NULL)) {
                goto out_abort;
        }

        /*
         * The newly found entry may be something different...
         */
        if (!(ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
            !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))) {
                goto out_abort;
        }

        /*
         * ... and not even negative.
         */
        nc_flag = atomic_load_char(&ncp->nc_flag);
        if ((nc_flag & NCF_NEGATIVE) == 0) {
                goto out_abort;
        }

        if (__predict_false(!cache_ncp_canuse(ncp))) {
                goto out_abort;
        }

        cache_neg_promote(ncp);

        SDT_PROBE2(vfs, namecache, lookup, hit__negative, dvp, ncp->nc_name);
        counter_u64_add(numneghits, 1);
        vfs_smr_exit();
        mtx_unlock(&nl->nl_lock);
        return (true);
out_abort:
        vfs_smr_exit();
        mtx_unlock(&nl->nl_lock);
        return (false);
}

static void
cache_neg_hit(struct namecache *ncp)
{
        struct neglist *nl;
        struct negstate *ns;

        ns = NCP2NEGSTATE(ncp);
        if ((ns->neg_flag & NEG_HOT) != 0)
                return;
        nl = NCP2NEGLIST(ncp);
        mtx_lock(&nl->nl_lock);
        cache_neg_promote(ncp);
        mtx_unlock(&nl->nl_lock);
}

static void
cache_neg_insert(struct namecache *ncp)
{
        struct neglist *nl;

        MPASS(ncp->nc_flag & NCF_NEGATIVE);
        cache_assert_bucket_locked(ncp);
        nl = NCP2NEGLIST(ncp);
        mtx_lock(&nl->nl_lock);
        TAILQ_INSERT_TAIL(&nl->nl_list, ncp, nc_dst);
        mtx_unlock(&nl->nl_lock);
        atomic_add_long(&numneg, 1);
}

static void
cache_neg_remove(struct namecache *ncp)
{
        struct neglist *nl;
        struct negstate *ns;

        cache_assert_bucket_locked(ncp);
        nl = NCP2NEGLIST(ncp);
        ns = NCP2NEGSTATE(ncp);
        mtx_lock(&nl->nl_lock);
        if ((ns->neg_flag & NEG_HOT) != 0) {
                TAILQ_REMOVE(&nl->nl_hotlist, ncp, nc_dst);
                nl->nl_hotnum--;
        } else {
                TAILQ_REMOVE(&nl->nl_list, ncp, nc_dst);
        }
        mtx_unlock(&nl->nl_lock);
        atomic_subtract_long(&numneg, 1);
}

static struct neglist *
cache_neg_evict_select(void)
{
        struct neglist *nl;
        u_int c;

        c = atomic_fetchadd_int(&neg_cycle, 1) + 1;
        nl = &neglists[c % numneglists];
        if (!mtx_trylock(&nl->nl_evict_lock)) {
                counter_u64_add(neg_evict_skipped_contended, 1);
                return (NULL);
        }
        return (nl);
}

static void
cache_neg_evict(void)
{
        struct namecache *ncp, *ncp2;
        struct neglist *nl;
        struct negstate *ns;
        struct vnode *dvp;
        struct mtx *dvlp;
        struct mtx *blp;
        uint32_t hash;
        u_char nlen;

        nl = cache_neg_evict_select();
        if (nl == NULL) {
                return;
        }

        mtx_lock(&nl->nl_lock);
        ncp = TAILQ_FIRST(&nl->nl_hotlist);
        if (ncp != NULL) {
                ns = NCP2NEGSTATE(ncp);
                TAILQ_REMOVE(&nl->nl_hotlist, ncp, nc_dst);
                TAILQ_INSERT_TAIL(&nl->nl_list, ncp, nc_dst);
                nl->nl_hotnum--;
                ns->neg_flag &= ~NEG_HOT;
        }
        ncp = TAILQ_FIRST(&nl->nl_list);
        if (ncp == NULL) {
                counter_u64_add(neg_evict_skipped_empty, 1);
                mtx_unlock(&nl->nl_lock);
                mtx_unlock(&nl->nl_evict_lock);
                return;
        }
        ns = NCP2NEGSTATE(ncp);
        nlen = ncp->nc_nlen;
        dvp = ncp->nc_dvp;
        hash = cache_get_hash(ncp->nc_name, nlen, dvp);
        dvlp = VP2VNODELOCK(dvp);
        blp = HASH2BUCKETLOCK(hash);
        mtx_unlock(&nl->nl_lock);
        mtx_unlock(&nl->nl_evict_lock);
        mtx_lock(dvlp);
        mtx_lock(blp);
        /*
         * Note that since all locks were dropped above, the entry may be
         * gone or reallocated to be something else.
         */
        CK_SLIST_FOREACH(ncp2, (NCHHASH(hash)), nc_hash) {
                if (ncp2 == ncp && ncp2->nc_dvp == dvp &&
                    ncp2->nc_nlen == nlen && (ncp2->nc_flag & NCF_NEGATIVE) != 0)
                        break;
        }
        if (ncp2 == NULL) {
                counter_u64_add(neg_evict_skipped_missed, 1);
                ncp = NULL;
        } else {
                MPASS(dvlp == VP2VNODELOCK(ncp->nc_dvp));
                MPASS(blp == NCP2BUCKETLOCK(ncp));
                SDT_PROBE2(vfs, namecache, evict_negative, done, ncp->nc_dvp,
                    ncp->nc_name);
                cache_zap_locked(ncp);
                counter_u64_add(neg_evicted, 1);
        }
        mtx_unlock(blp);
        mtx_unlock(dvlp);
        if (ncp != NULL)
                cache_free(ncp);
}

/*
 * cache_zap_locked():
 *
 *   Removes a namecache entry from cache, whether it contains an actual
 *   pointer to a vnode or if it is just a negative cache entry.
 */
static void
cache_zap_locked(struct namecache *ncp)
{
        struct nchashhead *ncpp;

        if (!(ncp->nc_flag & NCF_NEGATIVE))
                cache_assert_vnode_locked(ncp->nc_vp);
        cache_assert_vnode_locked(ncp->nc_dvp);
        cache_assert_bucket_locked(ncp);

        cache_ncp_invalidate(ncp);

        ncpp = NCP2BUCKET(ncp);
        CK_SLIST_REMOVE(ncpp, ncp, namecache, nc_hash);
        if (!(ncp->nc_flag & NCF_NEGATIVE)) {
                SDT_PROBE3(vfs, namecache, zap, done, ncp->nc_dvp,
                    ncp->nc_name, ncp->nc_vp);
                TAILQ_REMOVE(&ncp->nc_vp->v_cache_dst, ncp, nc_dst);
                if (ncp == ncp->nc_vp->v_cache_dd) {
                        vn_seqc_write_begin_unheld(ncp->nc_vp);
                        ncp->nc_vp->v_cache_dd = NULL;
                        vn_seqc_write_end(ncp->nc_vp);
                }
        } else {
                SDT_PROBE2(vfs, namecache, zap_negative, done, ncp->nc_dvp,
                    ncp->nc_name);
                cache_neg_remove(ncp);
        }
        if (ncp->nc_flag & NCF_ISDOTDOT) {
                if (ncp == ncp->nc_dvp->v_cache_dd) {
                        vn_seqc_write_begin_unheld(ncp->nc_dvp);
                        ncp->nc_dvp->v_cache_dd = NULL;
                        vn_seqc_write_end(ncp->nc_dvp);
                }
        } else {
                LIST_REMOVE(ncp, nc_src);
                if (LIST_EMPTY(&ncp->nc_dvp->v_cache_src)) {
                        ncp->nc_flag |= NCF_DVDROP;
                        counter_u64_add(numcachehv, -1);
                }
        }
        atomic_subtract_long(&numcache, 1);
}

static void
cache_zap_negative_locked_vnode_kl(struct namecache *ncp, struct vnode *vp)
{
        struct mtx *blp;

        MPASS(ncp->nc_dvp == vp);
        MPASS(ncp->nc_flag & NCF_NEGATIVE);
        cache_assert_vnode_locked(vp);

        blp = NCP2BUCKETLOCK(ncp);
        mtx_lock(blp);
        cache_zap_locked(ncp);
        mtx_unlock(blp);
}

static bool
cache_zap_locked_vnode_kl2(struct namecache *ncp, struct vnode *vp,
    struct mtx **vlpp)
{
        struct mtx *pvlp, *vlp1, *vlp2, *to_unlock;
        struct mtx *blp;

        MPASS(vp == ncp->nc_dvp || vp == ncp->nc_vp);
        cache_assert_vnode_locked(vp);

        if (ncp->nc_flag & NCF_NEGATIVE) {
                if (*vlpp != NULL) {
                        mtx_unlock(*vlpp);
                        *vlpp = NULL;
                }
                cache_zap_negative_locked_vnode_kl(ncp, vp);
                return (true);
        }

        pvlp = VP2VNODELOCK(vp);
        blp = NCP2BUCKETLOCK(ncp);
        vlp1 = VP2VNODELOCK(ncp->nc_dvp);
        vlp2 = VP2VNODELOCK(ncp->nc_vp);

        if (*vlpp == vlp1 || *vlpp == vlp2) {
                to_unlock = *vlpp;
                *vlpp = NULL;
        } else {
                if (*vlpp != NULL) {
                        mtx_unlock(*vlpp);
                        *vlpp = NULL;
                }
                cache_sort_vnodes(&vlp1, &vlp2);
                if (vlp1 == pvlp) {
                        mtx_lock(vlp2);
                        to_unlock = vlp2;
                } else {
                        if (!mtx_trylock(vlp1))
                                goto out_relock;
                        to_unlock = vlp1;
                }
        }
        mtx_lock(blp);
        cache_zap_locked(ncp);
        mtx_unlock(blp);
        if (to_unlock != NULL)
                mtx_unlock(to_unlock);
        return (true);

out_relock:
        mtx_unlock(vlp2);
        mtx_lock(vlp1);
        mtx_lock(vlp2);
        MPASS(*vlpp == NULL);
        *vlpp = vlp1;
        return (false);
}

/*
 * If trylocking failed we can get here. We know enough to take all needed locks
 * in the right order and re-lookup the entry.
 */
static int
cache_zap_unlocked_bucket(struct namecache *ncp, struct componentname *cnp,
    struct vnode *dvp, struct mtx *dvlp, struct mtx *vlp, uint32_t hash,
    struct mtx *blp)
{
        struct namecache *rncp;

        cache_assert_bucket_unlocked(ncp);

        cache_sort_vnodes(&dvlp, &vlp);
        cache_lock_vnodes(dvlp, vlp);
        mtx_lock(blp);
        CK_SLIST_FOREACH(rncp, (NCHHASH(hash)), nc_hash) {
                if (rncp == ncp && rncp->nc_dvp == dvp &&
                    rncp->nc_nlen == cnp->cn_namelen &&
                    !bcmp(rncp->nc_name, cnp->cn_nameptr, rncp->nc_nlen))
                        break;
        }
        if (rncp != NULL) {
                cache_zap_locked(rncp);
                mtx_unlock(blp);
                cache_unlock_vnodes(dvlp, vlp);
                counter_u64_add(zap_and_exit_bucket_relock_success, 1);
                return (0);
        }

        mtx_unlock(blp);
        cache_unlock_vnodes(dvlp, vlp);
        return (EAGAIN);
}

static int __noinline
cache_zap_locked_bucket(struct namecache *ncp, struct componentname *cnp,
    uint32_t hash, struct mtx *blp)
{
        struct mtx *dvlp, *vlp;
        struct vnode *dvp;

        cache_assert_bucket_locked(ncp);

        dvlp = VP2VNODELOCK(ncp->nc_dvp);
        vlp = NULL;
        if (!(ncp->nc_flag & NCF_NEGATIVE))
                vlp = VP2VNODELOCK(ncp->nc_vp);
        if (cache_trylock_vnodes(dvlp, vlp) == 0) {
                cache_zap_locked(ncp);
                mtx_unlock(blp);
                cache_unlock_vnodes(dvlp, vlp);
                return (0);
        }

        dvp = ncp->nc_dvp;
        mtx_unlock(blp);
        return (cache_zap_unlocked_bucket(ncp, cnp, dvp, dvlp, vlp, hash, blp));
}

static __noinline int
cache_remove_cnp(struct vnode *dvp, struct componentname *cnp)
{
        struct namecache *ncp;
        struct mtx *blp;
        struct mtx *dvlp, *dvlp2;
        uint32_t hash;
        int error;

        if (cnp->cn_namelen == 2 &&
            cnp->cn_nameptr[0] == '.' && cnp->cn_nameptr[1] == '.') {
                dvlp = VP2VNODELOCK(dvp);
                dvlp2 = NULL;
                mtx_lock(dvlp);
retry_dotdot:
                ncp = dvp->v_cache_dd;
                if (ncp == NULL) {
                        mtx_unlock(dvlp);
                        if (dvlp2 != NULL)
                                mtx_unlock(dvlp2);
                        SDT_PROBE2(vfs, namecache, removecnp, miss, dvp, cnp);
                        return (0);
                }
                if ((ncp->nc_flag & NCF_ISDOTDOT) != 0) {
                        if (!cache_zap_locked_vnode_kl2(ncp, dvp, &dvlp2))
                                goto retry_dotdot;
                        MPASS(dvp->v_cache_dd == NULL);
                        mtx_unlock(dvlp);
                        if (dvlp2 != NULL)
                                mtx_unlock(dvlp2);
                        cache_free(ncp);
                } else {
                        vn_seqc_write_begin(dvp);
                        dvp->v_cache_dd = NULL;
                        vn_seqc_write_end(dvp);
                        mtx_unlock(dvlp);
                        if (dvlp2 != NULL)
                                mtx_unlock(dvlp2);
                }
                SDT_PROBE2(vfs, namecache, removecnp, hit, dvp, cnp);
                return (1);
        }

        hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);
        blp = HASH2BUCKETLOCK(hash);
retry:
        if (CK_SLIST_EMPTY(NCHHASH(hash)))
                goto out_no_entry;

        mtx_lock(blp);

        CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                    !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                        break;
        }

        if (ncp == NULL) {
                mtx_unlock(blp);
                goto out_no_entry;
        }

        error = cache_zap_locked_bucket(ncp, cnp, hash, blp);
        if (__predict_false(error != 0)) {
                zap_and_exit_bucket_fail++;
                goto retry;
        }
        counter_u64_add(numposzaps, 1);
        SDT_PROBE2(vfs, namecache, removecnp, hit, dvp, cnp);
        cache_free(ncp);
        return (1);
out_no_entry:
        counter_u64_add(nummisszap, 1);
        SDT_PROBE2(vfs, namecache, removecnp, miss, dvp, cnp);
        return (0);
}

static int __noinline
cache_lookup_dot(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
    struct timespec *tsp, int *ticksp)
{
        int ltype;

        *vpp = dvp;
        counter_u64_add(dothits, 1);
        SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ".", *vpp);
        if (tsp != NULL)
                timespecclear(tsp);
        if (ticksp != NULL)
                *ticksp = ticks;
        vrefact(*vpp);
        /*
         * When we lookup "." we still can be asked to lock it
         * differently...
         */
        ltype = cnp->cn_lkflags & LK_TYPE_MASK;
        if (ltype != VOP_ISLOCKED(*vpp)) {
                if (ltype == LK_EXCLUSIVE) {
                        vn_lock(*vpp, LK_UPGRADE | LK_RETRY);
                        if (VN_IS_DOOMED((*vpp))) {
                                /* forced unmount */
                                vrele(*vpp);
                                *vpp = NULL;
                                return (ENOENT);
                        }
                } else
                        vn_lock(*vpp, LK_DOWNGRADE | LK_RETRY);
        }
        return (-1);
}

static int __noinline
cache_lookup_dotdot(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
    struct timespec *tsp, int *ticksp)
{
        struct namecache_ts *ncp_ts;
        struct namecache *ncp;
        struct mtx *dvlp;
        enum vgetstate vs;
        int error, ltype;
        bool whiteout;

        MPASS((cnp->cn_flags & ISDOTDOT) != 0);

        if ((cnp->cn_flags & MAKEENTRY) == 0) {
                cache_remove_cnp(dvp, cnp);
                return (0);
        }

        counter_u64_add(dotdothits, 1);
retry:
        dvlp = VP2VNODELOCK(dvp);
        mtx_lock(dvlp);
        ncp = dvp->v_cache_dd;
        if (ncp == NULL) {
                SDT_PROBE3(vfs, namecache, lookup, miss, dvp, "..", NULL);
                mtx_unlock(dvlp);
                return (0);
        }
        if ((ncp->nc_flag & NCF_ISDOTDOT) != 0) {
                if (ncp->nc_flag & NCF_NEGATIVE)
                        *vpp = NULL;
                else
                        *vpp = ncp->nc_vp;
        } else
                *vpp = ncp->nc_dvp;
        if (*vpp == NULL)
                goto negative_success;
        SDT_PROBE3(vfs, namecache, lookup, hit, dvp, "..", *vpp);
        cache_out_ts(ncp, tsp, ticksp);
        if ((ncp->nc_flag & (NCF_ISDOTDOT | NCF_DTS)) ==
            NCF_DTS && tsp != NULL) {
                ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
                *tsp = ncp_ts->nc_dotdottime;
        }

        MPASS(dvp != *vpp);
        ltype = VOP_ISLOCKED(dvp);
        VOP_UNLOCK(dvp);
        vs = vget_prep(*vpp);
        mtx_unlock(dvlp);
        error = vget_finish(*vpp, cnp->cn_lkflags, vs);
        vn_lock(dvp, ltype | LK_RETRY);
        if (VN_IS_DOOMED(dvp)) {
                if (error == 0)
                        vput(*vpp);
                *vpp = NULL;
                return (ENOENT);
        }
        if (error) {
                *vpp = NULL;
                goto retry;
        }
        return (-1);
negative_success:
        if (__predict_false(cnp->cn_nameiop == CREATE)) {
                if (cnp->cn_flags & ISLASTCN) {
                        counter_u64_add(numnegzaps, 1);
                        cache_zap_negative_locked_vnode_kl(ncp, dvp);
                        mtx_unlock(dvlp);
                        cache_free(ncp);
                        return (0);
                }
        }

        SDT_PROBE2(vfs, namecache, lookup, hit__negative, dvp, ncp->nc_name);
        cache_out_ts(ncp, tsp, ticksp);
        counter_u64_add(numneghits, 1);
        whiteout = (ncp->nc_flag & NCF_WHITE);
        cache_neg_hit(ncp);
        mtx_unlock(dvlp);
        if (whiteout)
                cnp->cn_flags |= ISWHITEOUT;
        return (ENOENT);
}

/**
 * Lookup a name in the name cache
 *
 * # Arguments
 *
 * - dvp:       Parent directory in which to search.
 * - vpp:       Return argument.  Will contain desired vnode on cache hit.
 * - cnp:       Parameters of the name search.  The most interesting bits of
 *              the cn_flags field have the following meanings:
 *      - MAKEENTRY:    If clear, free an entry from the cache rather than look
 *                      it up.
 *      - ISDOTDOT:     Must be set if and only if cn_nameptr == ".."
 * - tsp:       Return storage for cache timestamp.  On a successful (positive
 *              or negative) lookup, tsp will be filled with any timespec that
 *              was stored when this cache entry was created.  However, it will
 *              be clear for "." entries.
 * - ticks:     Return storage for alternate cache timestamp.  On a successful
 *              (positive or negative) lookup, it will contain the ticks value
 *              that was current when the cache entry was created, unless cnp
 *              was ".".
 *
 * Either both tsp and ticks have to be provided or neither of them.
 *
 * # Returns
 *
 * - -1:        A positive cache hit.  vpp will contain the desired vnode.
 * - ENOENT:    A negative cache hit, or dvp was recycled out from under us due
 *              to a forced unmount.  vpp will not be modified.  If the entry
 *              is a whiteout, then the ISWHITEOUT flag will be set in
 *              cnp->cn_flags.
 * - 0:         A cache miss.  vpp will not be modified.
 *
 * # Locking
 *
 * On a cache hit, vpp will be returned locked and ref'd.  If we're looking up
 * .., dvp is unlocked.  If we're looking up . an extra ref is taken, but the
 * lock is not recursively acquired.
 */
static int __noinline
cache_lookup_fallback(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
    struct timespec *tsp, int *ticksp)
{
        struct namecache *ncp;
        struct mtx *blp;
        uint32_t hash;
        enum vgetstate vs;
        int error;
        bool whiteout;

        MPASS((cnp->cn_flags & (MAKEENTRY | ISDOTDOT)) == MAKEENTRY);

retry:
        hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);
        blp = HASH2BUCKETLOCK(hash);
        mtx_lock(blp);

        CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                    !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                        break;
        }

        if (__predict_false(ncp == NULL)) {
                mtx_unlock(blp);
                SDT_PROBE3(vfs, namecache, lookup, miss, dvp, cnp->cn_nameptr,
                    NULL);
                counter_u64_add(nummiss, 1);
                return (0);
        }

        if (ncp->nc_flag & NCF_NEGATIVE)
                goto negative_success;

        counter_u64_add(numposhits, 1);
        *vpp = ncp->nc_vp;
        SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ncp->nc_name, *vpp);
        cache_out_ts(ncp, tsp, ticksp);
        MPASS(dvp != *vpp);
        vs = vget_prep(*vpp);
        mtx_unlock(blp);
        error = vget_finish(*vpp, cnp->cn_lkflags, vs);
        if (error) {
                *vpp = NULL;
                goto retry;
        }
        return (-1);
negative_success:
        if (__predict_false(cnp->cn_nameiop == CREATE)) {
                if (cnp->cn_flags & ISLASTCN) {
                        counter_u64_add(numnegzaps, 1);
                        error = cache_zap_locked_bucket(ncp, cnp, hash, blp);
                        if (__predict_false(error != 0)) {
                                zap_and_exit_bucket_fail2++;
                                goto retry;
                        }
                        cache_free(ncp);
                        return (0);
                }
        }

        SDT_PROBE2(vfs, namecache, lookup, hit__negative, dvp, ncp->nc_name);
        cache_out_ts(ncp, tsp, ticksp);
        counter_u64_add(numneghits, 1);
        whiteout = (ncp->nc_flag & NCF_WHITE);
        cache_neg_hit(ncp);
        mtx_unlock(blp);
        if (whiteout)
                cnp->cn_flags |= ISWHITEOUT;
        return (ENOENT);
}

int
cache_lookup(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
    struct timespec *tsp, int *ticksp)
{
        struct namecache *ncp;
        struct negstate *ns;
        uint32_t hash;
        enum vgetstate vs;
        int error;
        bool whiteout, neg_hot;
        u_short nc_flag;

        MPASS((tsp == NULL && ticksp == NULL) || (tsp != NULL && ticksp != NULL));

#ifdef DEBUG_CACHE
        if (__predict_false(!doingcache)) {
                cnp->cn_flags &= ~MAKEENTRY;
                return (0);
        }
#endif

        if (__predict_false(cnp->cn_nameptr[0] == '.')) {
                if (cnp->cn_namelen == 1)
                        return (cache_lookup_dot(dvp, vpp, cnp, tsp, ticksp));
                if (cnp->cn_namelen == 2 && cnp->cn_nameptr[1] == '.')
                        return (cache_lookup_dotdot(dvp, vpp, cnp, tsp, ticksp));
        }

        MPASS((cnp->cn_flags & ISDOTDOT) == 0);

        if ((cnp->cn_flags & MAKEENTRY) == 0) {
                cache_remove_cnp(dvp, cnp);
                return (0);
        }

        hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);
        vfs_smr_enter();

        CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                    !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                        break;
        }

        if (__predict_false(ncp == NULL)) {
                vfs_smr_exit();
                SDT_PROBE3(vfs, namecache, lookup, miss, dvp, cnp->cn_nameptr,
                    NULL);
                counter_u64_add(nummiss, 1);
                return (0);
        }

        nc_flag = atomic_load_char(&ncp->nc_flag);
        if (nc_flag & NCF_NEGATIVE)
                goto negative_success;

        counter_u64_add(numposhits, 1);
        *vpp = ncp->nc_vp;
        SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ncp->nc_name, *vpp);
        cache_out_ts(ncp, tsp, ticksp);
        MPASS(dvp != *vpp);
        if (!cache_ncp_canuse(ncp)) {
                vfs_smr_exit();
                *vpp = NULL;
                goto out_fallback;
        }
        vs = vget_prep_smr(*vpp);
        vfs_smr_exit();
        if (__predict_false(vs == VGET_NONE)) {
                *vpp = NULL;
                goto out_fallback;
        }
        error = vget_finish(*vpp, cnp->cn_lkflags, vs);
        if (error) {
                *vpp = NULL;
                goto out_fallback;
        }
        return (-1);
negative_success:
        if (__predict_false(cnp->cn_nameiop == CREATE)) {
                if (cnp->cn_flags & ISLASTCN) {
                        vfs_smr_exit();
                        goto out_fallback;
                }
        }

        cache_out_ts(ncp, tsp, ticksp);
        whiteout = (ncp->nc_flag & NCF_WHITE);
        ns = NCP2NEGSTATE(ncp);
        neg_hot = ((ns->neg_flag & NEG_HOT) != 0);
        if (__predict_false(!cache_ncp_canuse(ncp))) {
                vfs_smr_exit();
                goto out_fallback;
        }
        if (!neg_hot) {
                vfs_smr_exit();
                if (!cache_neg_promote_cond(dvp, cnp, ncp, hash))
                        goto out_fallback;
        } else {
                SDT_PROBE2(vfs, namecache, lookup, hit__negative, dvp, ncp->nc_name);
                counter_u64_add(numneghits, 1);
                vfs_smr_exit();
        }
        if (whiteout)
                cnp->cn_flags |= ISWHITEOUT;
        return (ENOENT);
out_fallback:
        return (cache_lookup_fallback(dvp, vpp, cnp, tsp, ticksp));
}

struct celockstate {
        struct mtx *vlp[3];
        struct mtx *blp[2];
};
CTASSERT((nitems(((struct celockstate *)0)->vlp) == 3));
CTASSERT((nitems(((struct celockstate *)0)->blp) == 2));

static inline void
cache_celockstate_init(struct celockstate *cel)
{

        bzero(cel, sizeof(*cel));
}

static void
cache_lock_vnodes_cel(struct celockstate *cel, struct vnode *vp,
    struct vnode *dvp)
{
        struct mtx *vlp1, *vlp2;

        MPASS(cel->vlp[0] == NULL);
        MPASS(cel->vlp[1] == NULL);
        MPASS(cel->vlp[2] == NULL);

        MPASS(vp != NULL || dvp != NULL);

        vlp1 = VP2VNODELOCK(vp);
        vlp2 = VP2VNODELOCK(dvp);
        cache_sort_vnodes(&vlp1, &vlp2);

        if (vlp1 != NULL) {
                mtx_lock(vlp1);
                cel->vlp[0] = vlp1;
        }
        mtx_lock(vlp2);
        cel->vlp[1] = vlp2;
}

static void
cache_unlock_vnodes_cel(struct celockstate *cel)
{

        MPASS(cel->vlp[0] != NULL || cel->vlp[1] != NULL);

        if (cel->vlp[0] != NULL)
                mtx_unlock(cel->vlp[0]);
        if (cel->vlp[1] != NULL)
                mtx_unlock(cel->vlp[1]);
        if (cel->vlp[2] != NULL)
                mtx_unlock(cel->vlp[2]);
}

static bool
cache_lock_vnodes_cel_3(struct celockstate *cel, struct vnode *vp)
{
        struct mtx *vlp;
        bool ret;

        cache_assert_vlp_locked(cel->vlp[0]);
        cache_assert_vlp_locked(cel->vlp[1]);
        MPASS(cel->vlp[2] == NULL);

        MPASS(vp != NULL);
        vlp = VP2VNODELOCK(vp);

        ret = true;
        if (vlp >= cel->vlp[1]) {
                mtx_lock(vlp);
        } else {
                if (mtx_trylock(vlp))
                        goto out;
                cache_lock_vnodes_cel_3_failures++;
                cache_unlock_vnodes_cel(cel);
                if (vlp < cel->vlp[0]) {
                        mtx_lock(vlp);
                        mtx_lock(cel->vlp[0]);
                        mtx_lock(cel->vlp[1]);
                } else {
                        if (cel->vlp[0] != NULL)
                                mtx_lock(cel->vlp[0]);
                        mtx_lock(vlp);
                        mtx_lock(cel->vlp[1]);
                }
                ret = false;
        }
out:
        cel->vlp[2] = vlp;
        return (ret);
}

static void
cache_lock_buckets_cel(struct celockstate *cel, struct mtx *blp1,
    struct mtx *blp2)
{

        MPASS(cel->blp[0] == NULL);
        MPASS(cel->blp[1] == NULL);

        cache_sort_vnodes(&blp1, &blp2);

        if (blp1 != NULL) {
                mtx_lock(blp1);
                cel->blp[0] = blp1;
        }
        mtx_lock(blp2);
        cel->blp[1] = blp2;
}

static void
cache_unlock_buckets_cel(struct celockstate *cel)
{

        if (cel->blp[0] != NULL)
                mtx_unlock(cel->blp[0]);
        mtx_unlock(cel->blp[1]);
}

/*
 * Lock part of the cache affected by the insertion.
 *
 * This means vnodelocks for dvp, vp and the relevant bucketlock.
 * However, insertion can result in removal of an old entry. In this
 * case we have an additional vnode and bucketlock pair to lock.
 *
 * That is, in the worst case we have to lock 3 vnodes and 2 bucketlocks, while
 * preserving the locking order (smaller address first).
 */
static void
cache_enter_lock(struct celockstate *cel, struct vnode *dvp, struct vnode *vp,
    uint32_t hash)
{
        struct namecache *ncp;
        struct mtx *blps[2];

        blps[0] = HASH2BUCKETLOCK(hash);
        for (;;) {
                blps[1] = NULL;
                cache_lock_vnodes_cel(cel, dvp, vp);
                if (vp == NULL || vp->v_type != VDIR)
                        break;
                ncp = vp->v_cache_dd;
                if (ncp == NULL)
                        break;
                if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
                        break;
                MPASS(ncp->nc_dvp == vp);
                blps[1] = NCP2BUCKETLOCK(ncp);
                if (ncp->nc_flag & NCF_NEGATIVE)
                        break;
                if (cache_lock_vnodes_cel_3(cel, ncp->nc_vp))
                        break;
                /*
                 * All vnodes got re-locked. Re-validate the state and if
                 * nothing changed we are done. Otherwise restart.
                 */
                if (ncp == vp->v_cache_dd &&
                    (ncp->nc_flag & NCF_ISDOTDOT) != 0 &&
                    blps[1] == NCP2BUCKETLOCK(ncp) &&
                    VP2VNODELOCK(ncp->nc_vp) == cel->vlp[2])
                        break;
                cache_unlock_vnodes_cel(cel);
                cel->vlp[0] = NULL;
                cel->vlp[1] = NULL;
                cel->vlp[2] = NULL;
        }
        cache_lock_buckets_cel(cel, blps[0], blps[1]);
}

static void
cache_enter_lock_dd(struct celockstate *cel, struct vnode *dvp, struct vnode *vp,
    uint32_t hash)
{
        struct namecache *ncp;
        struct mtx *blps[2];

        blps[0] = HASH2BUCKETLOCK(hash);
        for (;;) {
                blps[1] = NULL;
                cache_lock_vnodes_cel(cel, dvp, vp);
                ncp = dvp->v_cache_dd;
                if (ncp == NULL)
                        break;
                if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
                        break;
                MPASS(ncp->nc_dvp == dvp);
                blps[1] = NCP2BUCKETLOCK(ncp);
                if (ncp->nc_flag & NCF_NEGATIVE)
                        break;
                if (cache_lock_vnodes_cel_3(cel, ncp->nc_vp))
                        break;
                if (ncp == dvp->v_cache_dd &&
                    (ncp->nc_flag & NCF_ISDOTDOT) != 0 &&
                    blps[1] == NCP2BUCKETLOCK(ncp) &&
                    VP2VNODELOCK(ncp->nc_vp) == cel->vlp[2])
                        break;
                cache_unlock_vnodes_cel(cel);
                cel->vlp[0] = NULL;
                cel->vlp[1] = NULL;
                cel->vlp[2] = NULL;
        }
        cache_lock_buckets_cel(cel, blps[0], blps[1]);
}

static void
cache_enter_unlock(struct celockstate *cel)
{

        cache_unlock_buckets_cel(cel);
        cache_unlock_vnodes_cel(cel);
}

static void __noinline
cache_enter_dotdot_prep(struct vnode *dvp, struct vnode *vp,
    struct componentname *cnp)
{
        struct celockstate cel;
        struct namecache *ncp;
        uint32_t hash;
        int len;

        if (dvp->v_cache_dd == NULL)
                return;
        len = cnp->cn_namelen;
        cache_celockstate_init(&cel);
        hash = cache_get_hash(cnp->cn_nameptr, len, dvp);
        cache_enter_lock_dd(&cel, dvp, vp, hash);
        vn_seqc_write_begin(dvp);
        ncp = dvp->v_cache_dd;
        if (ncp != NULL && (ncp->nc_flag & NCF_ISDOTDOT)) {
                KASSERT(ncp->nc_dvp == dvp, ("wrong isdotdot parent"));
                cache_zap_locked(ncp);
        } else {
                ncp = NULL;
        }
        dvp->v_cache_dd = NULL;
        vn_seqc_write_end(dvp);
        cache_enter_unlock(&cel);
        if (ncp != NULL)
                cache_free(ncp);
}

/*
 * Add an entry to the cache.
 */
void
cache_enter_time(struct vnode *dvp, struct vnode *vp, struct componentname *cnp,
    struct timespec *tsp, struct timespec *dtsp)
{
        struct celockstate cel;
        struct namecache *ncp, *n2, *ndd;
        struct namecache_ts *ncp_ts;
        struct nchashhead *ncpp;
        uint32_t hash;
        int flag;
        int len;
        u_long lnumcache;

        VNPASS(!VN_IS_DOOMED(dvp), dvp);
        VNPASS(dvp->v_type != VNON, dvp);
        if (vp != NULL) {
                VNPASS(!VN_IS_DOOMED(vp), vp);
                VNPASS(vp->v_type != VNON, vp);
        }

#ifdef DEBUG_CACHE
        if (__predict_false(!doingcache))
                return;
#endif

        flag = 0;
        if (__predict_false(cnp->cn_nameptr[0] == '.')) {
                if (cnp->cn_namelen == 1)
                        return;
                if (cnp->cn_namelen == 2 && cnp->cn_nameptr[1] == '.') {
                        cache_enter_dotdot_prep(dvp, vp, cnp);
                        flag = NCF_ISDOTDOT;
                }
        }

        /*
         * Avoid blowout in namecache entries.
         *
         * Bugs:
         * 1. filesystems may end up tryng to add an already existing entry
         * (for example this can happen after a cache miss during concurrent
         * lookup), in which case we will call cache_neg_evict despite not
         * adding anything.
         * 2. the routine may fail to free anything and no provisions are made
         * to make it try harder (see the inside for failure modes)
         * 3. it only ever looks at negative entries.
         */
        lnumcache = atomic_fetchadd_long(&numcache, 1) + 1;
        if (numneg * ncnegfactor > lnumcache) {
                cache_neg_evict();
                lnumcache = atomic_load_long(&numcache);
        }
        if (__predict_false(lnumcache >= ncsize)) {
                atomic_subtract_long(&numcache, 1);
                counter_u64_add(numdrops, 1);
                return;
        }

        cache_celockstate_init(&cel);
        ndd = NULL;
        ncp_ts = NULL;

        /*
         * Calculate the hash key and setup as much of the new
         * namecache entry as possible before acquiring the lock.
         */
        ncp = cache_alloc(cnp->cn_namelen, tsp != NULL);
        ncp->nc_flag = flag | NCF_WIP;
        ncp->nc_vp = vp;
        if (vp == NULL)
                cache_neg_init(ncp);
        ncp->nc_dvp = dvp;
        if (tsp != NULL) {
                ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
                ncp_ts->nc_time = *tsp;
                ncp_ts->nc_ticks = ticks;
                ncp_ts->nc_nc.nc_flag |= NCF_TS;
                if (dtsp != NULL) {
                        ncp_ts->nc_dotdottime = *dtsp;
                        ncp_ts->nc_nc.nc_flag |= NCF_DTS;
                }
        }
        len = ncp->nc_nlen = cnp->cn_namelen;
        hash = cache_get_hash(cnp->cn_nameptr, len, dvp);
        memcpy(ncp->nc_name, cnp->cn_nameptr, len);
        ncp->nc_name[len] = '\0';
        cache_enter_lock(&cel, dvp, vp, hash);

        /*
         * See if this vnode or negative entry is already in the cache
         * with this name.  This can happen with concurrent lookups of
         * the same path name.
         */
        ncpp = NCHHASH(hash);
        CK_SLIST_FOREACH(n2, ncpp, nc_hash) {
                if (n2->nc_dvp == dvp &&
                    n2->nc_nlen == cnp->cn_namelen &&
                    !bcmp(n2->nc_name, cnp->cn_nameptr, n2->nc_nlen)) {
                        MPASS(cache_ncp_canuse(n2));
                        if ((n2->nc_flag & NCF_NEGATIVE) != 0)
                                KASSERT(vp == NULL,
                                    ("%s: found entry pointing to a different vnode (%p != %p)",
                                    __func__, NULL, vp));
                        else
                                KASSERT(n2->nc_vp == vp,
                                    ("%s: found entry pointing to a different vnode (%p != %p)",
                                    __func__, n2->nc_vp, vp));
                        /*
                         * Entries are supposed to be immutable unless in the
                         * process of getting destroyed. Accommodating for
                         * changing timestamps is possible but not worth it.
                         * This should be harmless in terms of correctness, in
                         * the worst case resulting in an earlier expiration.
                         * Alternatively, the found entry can be replaced
                         * altogether.
                         */
                        MPASS((n2->nc_flag & (NCF_TS | NCF_DTS)) == (ncp->nc_flag & (NCF_TS | NCF_DTS)));
#if 0
                        if (tsp != NULL) {
                                KASSERT((n2->nc_flag & NCF_TS) != 0,
                                    ("no NCF_TS"));
                                n2_ts = __containerof(n2, struct namecache_ts, nc_nc);
                                n2_ts->nc_time = ncp_ts->nc_time;
                                n2_ts->nc_ticks = ncp_ts->nc_ticks;
                                if (dtsp != NULL) {
                                        n2_ts->nc_dotdottime = ncp_ts->nc_dotdottime;
                                        n2_ts->nc_nc.nc_flag |= NCF_DTS;
                                }
                        }
#endif
                        SDT_PROBE3(vfs, namecache, enter, duplicate, dvp, ncp->nc_name,
                            vp);
                        goto out_unlock_free;
                }
        }

        if (flag == NCF_ISDOTDOT) {
                /*
                 * See if we are trying to add .. entry, but some other lookup
                 * has populated v_cache_dd pointer already.
                 */
                if (dvp->v_cache_dd != NULL)
                        goto out_unlock_free;
                KASSERT(vp == NULL || vp->v_type == VDIR,
                    ("wrong vnode type %p", vp));
                vn_seqc_write_begin(dvp);
                dvp->v_cache_dd = ncp;
                vn_seqc_write_end(dvp);
        }

        if (vp != NULL) {
                if (flag != NCF_ISDOTDOT) {
                        /*
                         * For this case, the cache entry maps both the
                         * directory name in it and the name ".." for the
                         * directory's parent.
                         */
                        vn_seqc_write_begin(vp);
                        if ((ndd = vp->v_cache_dd) != NULL) {
                                if ((ndd->nc_flag & NCF_ISDOTDOT) != 0)
                                        cache_zap_locked(ndd);
                                else
                                        ndd = NULL;
                        }
                        vp->v_cache_dd = ncp;
                        vn_seqc_write_end(vp);
                } else if (vp->v_type != VDIR) {
                        if (vp->v_cache_dd != NULL) {
                                vn_seqc_write_begin(vp);
                                vp->v_cache_dd = NULL;
                                vn_seqc_write_end(vp);
                        }
                }
        }

        if (flag != NCF_ISDOTDOT) {
                if (LIST_EMPTY(&dvp->v_cache_src)) {
                        vhold(dvp);
                        counter_u64_add(numcachehv, 1);
                }
                LIST_INSERT_HEAD(&dvp->v_cache_src, ncp, nc_src);
        }

        /*
         * If the entry is "negative", we place it into the
         * "negative" cache queue, otherwise, we place it into the
         * destination vnode's cache entries queue.
         */
        if (vp != NULL) {
                TAILQ_INSERT_HEAD(&vp->v_cache_dst, ncp, nc_dst);
                SDT_PROBE3(vfs, namecache, enter, done, dvp, ncp->nc_name,
                    vp);
        } else {
                if (cnp->cn_flags & ISWHITEOUT)
                        ncp->nc_flag |= NCF_WHITE;
                cache_neg_insert(ncp);
                SDT_PROBE2(vfs, namecache, enter_negative, done, dvp,
                    ncp->nc_name);
        }

        /*
         * Insert the new namecache entry into the appropriate chain
         * within the cache entries table.
         */
        CK_SLIST_INSERT_HEAD(ncpp, ncp, nc_hash);

        atomic_thread_fence_rel();
        /*
         * Mark the entry as fully constructed.
         * It is immutable past this point until its removal.
         */
        atomic_store_char(&ncp->nc_flag, ncp->nc_flag & ~NCF_WIP);

        cache_enter_unlock(&cel);
        if (ndd != NULL)
                cache_free(ndd);
        return;
out_unlock_free:
        cache_enter_unlock(&cel);
        atomic_subtract_long(&numcache, 1);
        cache_free(ncp);
        return;
}

static u_int
cache_roundup_2(u_int val)
{
        u_int res;

        for (res = 1; res <= val; res <<= 1)
                continue;

        return (res);
}

static struct nchashhead *
nchinittbl(u_long elements, u_long *hashmask)
{
        struct nchashhead *hashtbl;
        u_long hashsize, i;

        hashsize = cache_roundup_2(elements) / 2;

        hashtbl = malloc((u_long)hashsize * sizeof(*hashtbl), M_VFSCACHE, M_WAITOK);
        for (i = 0; i < hashsize; i++)
                CK_SLIST_INIT(&hashtbl[i]);
        *hashmask = hashsize - 1;
        return (hashtbl);
}

static void
ncfreetbl(struct nchashhead *hashtbl)
{

        free(hashtbl, M_VFSCACHE);
}

/*
 * Name cache initialization, from vfs_init() when we are booting
 */
static void
nchinit(void *dummy __unused)
{
        u_int i;

        cache_zone_small = uma_zcreate("S VFS Cache", CACHE_ZONE_SMALL_SIZE,
            NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);
        cache_zone_small_ts = uma_zcreate("STS VFS Cache", CACHE_ZONE_SMALL_TS_SIZE,
            NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);
        cache_zone_large = uma_zcreate("L VFS Cache", CACHE_ZONE_LARGE_SIZE,
            NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);
        cache_zone_large_ts = uma_zcreate("LTS VFS Cache", CACHE_ZONE_LARGE_TS_SIZE,
            NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);

        VFS_SMR_ZONE_SET(cache_zone_small);
        VFS_SMR_ZONE_SET(cache_zone_small_ts);
        VFS_SMR_ZONE_SET(cache_zone_large);
        VFS_SMR_ZONE_SET(cache_zone_large_ts);

        ncsize = desiredvnodes * ncsizefactor;
        nchashtbl = nchinittbl(desiredvnodes * 2, &nchash);
        ncbuckethash = cache_roundup_2(mp_ncpus * mp_ncpus) - 1;
        if (ncbuckethash < 7) /* arbitrarily chosen to avoid having one lock */
                ncbuckethash = 7;
        if (ncbuckethash > nchash)
                ncbuckethash = nchash;
        bucketlocks = malloc(sizeof(*bucketlocks) * numbucketlocks, M_VFSCACHE,
            M_WAITOK | M_ZERO);
        for (i = 0; i < numbucketlocks; i++)
                mtx_init(&bucketlocks[i], "ncbuc", NULL, MTX_DUPOK | MTX_RECURSE);
        ncvnodehash = ncbuckethash;
        vnodelocks = malloc(sizeof(*vnodelocks) * numvnodelocks, M_VFSCACHE,
            M_WAITOK | M_ZERO);
        for (i = 0; i < numvnodelocks; i++)
                mtx_init(&vnodelocks[i], "ncvn", NULL, MTX_DUPOK | MTX_RECURSE);

        for (i = 0; i < numneglists; i++) {
                mtx_init(&neglists[i].nl_evict_lock, "ncnege", NULL, MTX_DEF);
                mtx_init(&neglists[i].nl_lock, "ncnegl", NULL, MTX_DEF);
                TAILQ_INIT(&neglists[i].nl_list);
                TAILQ_INIT(&neglists[i].nl_hotlist);
        }
}
SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_SECOND, nchinit, NULL);

void
cache_vnode_init(struct vnode *vp)
{

        LIST_INIT(&vp->v_cache_src);
        TAILQ_INIT(&vp->v_cache_dst);
        vp->v_cache_dd = NULL;
        cache_prehash(vp);
}

void
cache_changesize(u_long newmaxvnodes)
{
        struct nchashhead *new_nchashtbl, *old_nchashtbl;
        u_long new_nchash, old_nchash;
        struct namecache *ncp;
        uint32_t hash;
        u_long newncsize;
        int i;

        newncsize = newmaxvnodes * ncsizefactor;
        newmaxvnodes = cache_roundup_2(newmaxvnodes * 2);
        if (newmaxvnodes < numbucketlocks)
                newmaxvnodes = numbucketlocks;

        new_nchashtbl = nchinittbl(newmaxvnodes, &new_nchash);
        /* If same hash table size, nothing to do */
        if (nchash == new_nchash) {
                ncfreetbl(new_nchashtbl);
                return;
        }
        /*
         * Move everything from the old hash table to the new table.
         * None of the namecache entries in the table can be removed
         * because to do so, they have to be removed from the hash table.
         */
        cache_lock_all_vnodes();
        cache_lock_all_buckets();
        old_nchashtbl = nchashtbl;
        old_nchash = nchash;
        nchashtbl = new_nchashtbl;
        nchash = new_nchash;
        for (i = 0; i <= old_nchash; i++) {
                while ((ncp = CK_SLIST_FIRST(&old_nchashtbl[i])) != NULL) {
                        hash = cache_get_hash(ncp->nc_name, ncp->nc_nlen,
                            ncp->nc_dvp);
                        CK_SLIST_REMOVE(&old_nchashtbl[i], ncp, namecache, nc_hash);
                        CK_SLIST_INSERT_HEAD(NCHHASH(hash), ncp, nc_hash);
                }
        }
        ncsize = newncsize;
        cache_unlock_all_buckets();
        cache_unlock_all_vnodes();
        ncfreetbl(old_nchashtbl);
}

/*
 * Invalidate all entries from and to a particular vnode.
 */
static void
cache_purge_impl(struct vnode *vp)
{
        TAILQ_HEAD(, namecache) ncps;
        struct namecache *ncp, *nnp;
        struct mtx *vlp, *vlp2;

        TAILQ_INIT(&ncps);
        vlp = VP2VNODELOCK(vp);
        vlp2 = NULL;
        mtx_lock(vlp);
retry:
        while (!LIST_EMPTY(&vp->v_cache_src)) {
                ncp = LIST_FIRST(&vp->v_cache_src);
                if (!cache_zap_locked_vnode_kl2(ncp, vp, &vlp2))
                        goto retry;
                TAILQ_INSERT_TAIL(&ncps, ncp, nc_dst);
        }
        while (!TAILQ_EMPTY(&vp->v_cache_dst)) {
                ncp = TAILQ_FIRST(&vp->v_cache_dst);
                if (!cache_zap_locked_vnode_kl2(ncp, vp, &vlp2))
                        goto retry;
                TAILQ_INSERT_TAIL(&ncps, ncp, nc_dst);
        }
        ncp = vp->v_cache_dd;
        if (ncp != NULL) {
                KASSERT(ncp->nc_flag & NCF_ISDOTDOT,
                   ("lost dotdot link"));
                if (!cache_zap_locked_vnode_kl2(ncp, vp, &vlp2))
                        goto retry;
                TAILQ_INSERT_TAIL(&ncps, ncp, nc_dst);
        }
        KASSERT(vp->v_cache_dd == NULL, ("incomplete purge"));
        mtx_unlock(vlp);
        if (vlp2 != NULL)
                mtx_unlock(vlp2);
        TAILQ_FOREACH_SAFE(ncp, &ncps, nc_dst, nnp) {
                cache_free(ncp);
        }
}

/*
 * Opportunistic check to see if there is anything to do.
 */
static bool
cache_has_entries(struct vnode *vp)
{

        if (LIST_EMPTY(&vp->v_cache_src) && TAILQ_EMPTY(&vp->v_cache_dst) &&
            vp->v_cache_dd == NULL)
                return (false);
        return (true);
}

void
cache_purge(struct vnode *vp)
{

        SDT_PROBE1(vfs, namecache, purge, done, vp);
        if (!cache_has_entries(vp))
                return;
        cache_purge_impl(vp);
}

/*
 * Only to be used by vgone.
 */
void
cache_purge_vgone(struct vnode *vp)
{
        struct mtx *vlp;

        VNPASS(VN_IS_DOOMED(vp), vp);
        if (cache_has_entries(vp)) {
                cache_purge_impl(vp);
                return;
        }

        /*
         * Serialize against a potential thread doing cache_purge.
         */
        vlp = VP2VNODELOCK(vp);
        mtx_wait_unlocked(vlp);
        if (cache_has_entries(vp)) {
                cache_purge_impl(vp);
                return;
        }
        return;
}

/*
 * Invalidate all negative entries for a particular directory vnode.
 */
void
cache_purge_negative(struct vnode *vp)
{
        TAILQ_HEAD(, namecache) ncps;
        struct namecache *ncp, *nnp;
        struct mtx *vlp;

        SDT_PROBE1(vfs, namecache, purge_negative, done, vp);
        if (LIST_EMPTY(&vp->v_cache_src))
                return;
        TAILQ_INIT(&ncps);
        vlp = VP2VNODELOCK(vp);
        mtx_lock(vlp);
        LIST_FOREACH_SAFE(ncp, &vp->v_cache_src, nc_src, nnp) {
                if (!(ncp->nc_flag & NCF_NEGATIVE))
                        continue;
                cache_zap_negative_locked_vnode_kl(ncp, vp);
                TAILQ_INSERT_TAIL(&ncps, ncp, nc_dst);
        }
        mtx_unlock(vlp);
        TAILQ_FOREACH_SAFE(ncp, &ncps, nc_dst, nnp) {
                cache_free(ncp);
        }
}

void
cache_rename(struct vnode *fdvp, struct vnode *fvp, struct vnode *tdvp,
    struct vnode *tvp, struct componentname *fcnp, struct componentname *tcnp)
{

        ASSERT_VOP_IN_SEQC(fdvp);
        ASSERT_VOP_IN_SEQC(fvp);
        ASSERT_VOP_IN_SEQC(tdvp);
        if (tvp != NULL)
                ASSERT_VOP_IN_SEQC(tvp);

        cache_purge(fvp);
        if (tvp != NULL) {
                cache_purge(tvp);
                KASSERT(!cache_remove_cnp(tdvp, tcnp),
                    ("%s: lingering negative entry", __func__));
        } else {
                cache_remove_cnp(tdvp, tcnp);
        }
}

/*
 * Flush all entries referencing a particular filesystem.
 */
void
cache_purgevfs(struct mount *mp)
{
        struct vnode *vp, *mvp;

        SDT_PROBE1(vfs, namecache, purgevfs, done, mp);
        /*
         * Somewhat wasteful iteration over all vnodes. Would be better to
         * support filtering and avoid the interlock to begin with.
         */
        MNT_VNODE_FOREACH_ALL(vp, mp, mvp) {
                if (!cache_has_entries(vp)) {
                        VI_UNLOCK(vp);
                        continue;
                }
                vholdl(vp);
                VI_UNLOCK(vp);
                cache_purge(vp);
                vdrop(vp);
        }
}

/*
 * Perform canonical checks and cache lookup and pass on to filesystem
 * through the vop_cachedlookup only if needed.
 */

int
vfs_cache_lookup(struct vop_lookup_args *ap)
{
        struct vnode *dvp;
        int error;
        struct vnode **vpp = ap->a_vpp;
        struct componentname *cnp = ap->a_cnp;
        int flags = cnp->cn_flags;

        *vpp = NULL;
        dvp = ap->a_dvp;

        if (dvp->v_type != VDIR)
                return (ENOTDIR);

        if ((flags & ISLASTCN) && (dvp->v_mount->mnt_flag & MNT_RDONLY) &&
            (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME))
                return (EROFS);

        error = vn_dir_check_exec(dvp, cnp);
        if (error != 0)
                return (error);

        error = cache_lookup(dvp, vpp, cnp, NULL, NULL);
        if (error == 0)
                return (VOP_CACHEDLOOKUP(dvp, vpp, cnp));
        if (error == -1)
                return (0);
        return (error);
}

/* Implementation of the getcwd syscall. */
int
sys___getcwd(struct thread *td, struct __getcwd_args *uap)
{
        char *buf, *retbuf;
        size_t buflen;
        int error;

        buflen = uap->buflen;
        if (__predict_false(buflen < 2))
                return (EINVAL);
        if (buflen > MAXPATHLEN)
                buflen = MAXPATHLEN;

        buf = uma_zalloc(namei_zone, M_WAITOK);
        error = vn_getcwd(buf, &retbuf, &buflen);
        if (error == 0)
                error = copyout(retbuf, uap->buf, buflen);
        uma_zfree(namei_zone, buf);
        return (error);
}

int
vn_getcwd(char *buf, char **retbuf, size_t *buflen)
{
        struct pwd *pwd;
        int error;

        vfs_smr_enter();
        pwd = pwd_get_smr();
        error = vn_fullpath_any_smr(pwd->pwd_cdir, pwd->pwd_rdir, buf, retbuf,
            buflen, false, 0);
        VFS_SMR_ASSERT_NOT_ENTERED();
        if (error < 0) {
                pwd = pwd_hold(curthread);
                error = vn_fullpath_any(pwd->pwd_cdir, pwd->pwd_rdir, buf,
                    retbuf, buflen);
                pwd_drop(pwd);
        }

#ifdef KTRACE
        if (KTRPOINT(curthread, KTR_NAMEI) && error == 0)
                ktrnamei(*retbuf);
#endif
        return (error);
}

static int
kern___realpathat(struct thread *td, int fd, const char *path, char *buf,
    size_t size, int flags, enum uio_seg pathseg)
{
        struct nameidata nd;
        char *retbuf, *freebuf;
        int error;

        if (flags != 0)
                return (EINVAL);
        NDINIT_ATRIGHTS(&nd, LOOKUP, FOLLOW | SAVENAME | WANTPARENT | AUDITVNODE1,
            pathseg, path, fd, &cap_fstat_rights, td);
        if ((error = namei(&nd)) != 0)
                return (error);
        error = vn_fullpath_hardlink(&nd, &retbuf, &freebuf, &size);
        if (error == 0) {
                error = copyout(retbuf, buf, size);
                free(freebuf, M_TEMP);
        }
        NDFREE(&nd, 0);
        return (error);
}

int
sys___realpathat(struct thread *td, struct __realpathat_args *uap)
{

        return (kern___realpathat(td, uap->fd, uap->path, uap->buf, uap->size,
            uap->flags, UIO_USERSPACE));
}

/*
 * Retrieve the full filesystem path that correspond to a vnode from the name
 * cache (if available)
 */
int
vn_fullpath(struct vnode *vp, char **retbuf, char **freebuf)
{
        struct pwd *pwd;
        char *buf;
        size_t buflen;
        int error;

        if (__predict_false(vp == NULL))
                return (EINVAL);

        buflen = MAXPATHLEN;
        buf = malloc(buflen, M_TEMP, M_WAITOK);
        vfs_smr_enter();
        pwd = pwd_get_smr();
        error = vn_fullpath_any_smr(vp, pwd->pwd_rdir, buf, retbuf, &buflen, false, 0);
        VFS_SMR_ASSERT_NOT_ENTERED();
        if (error < 0) {
                pwd = pwd_hold(curthread);
                error = vn_fullpath_any(vp, pwd->pwd_rdir, buf, retbuf, &buflen);
                pwd_drop(pwd);
        }
        if (error == 0)
                *freebuf = buf;
        else
                free(buf, M_TEMP);
        return (error);
}

/*
 * This function is similar to vn_fullpath, but it attempts to lookup the
 * pathname relative to the global root mount point.  This is required for the
 * auditing sub-system, as audited pathnames must be absolute, relative to the
 * global root mount point.
 */
int
vn_fullpath_global(struct vnode *vp, char **retbuf, char **freebuf)
{
        char *buf;
        size_t buflen;
        int error;

        if (__predict_false(vp == NULL))
                return (EINVAL);
        buflen = MAXPATHLEN;
        buf = malloc(buflen, M_TEMP, M_WAITOK);
        vfs_smr_enter();
        error = vn_fullpath_any_smr(vp, rootvnode, buf, retbuf, &buflen, false, 0);
        VFS_SMR_ASSERT_NOT_ENTERED();
        if (error < 0) {
                error = vn_fullpath_any(vp, rootvnode, buf, retbuf, &buflen);
        }
        if (error == 0)
                *freebuf = buf;
        else
                free(buf, M_TEMP);
        return (error);
}

static struct namecache *
vn_dd_from_dst(struct vnode *vp)
{
        struct namecache *ncp;

        cache_assert_vnode_locked(vp);
        TAILQ_FOREACH(ncp, &vp->v_cache_dst, nc_dst) {
                if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
                        return (ncp);
        }
        return (NULL);
}

int
vn_vptocnp(struct vnode **vp, struct ucred *cred, char *buf, size_t *buflen)
{
        struct vnode *dvp;
        struct namecache *ncp;
        struct mtx *vlp;
        int error;

        vlp = VP2VNODELOCK(*vp);
        mtx_lock(vlp);
        ncp = (*vp)->v_cache_dd;
        if (ncp != NULL && (ncp->nc_flag & NCF_ISDOTDOT) == 0) {
                KASSERT(ncp == vn_dd_from_dst(*vp),
                    ("%s: mismatch for dd entry (%p != %p)", __func__,
                    ncp, vn_dd_from_dst(*vp)));
        } else {
                ncp = vn_dd_from_dst(*vp);
        }
        if (ncp != NULL) {
                if (*buflen < ncp->nc_nlen) {
                        mtx_unlock(vlp);
                        vrele(*vp);
                        counter_u64_add(numfullpathfail4, 1);
                        error = ENOMEM;
                        SDT_PROBE3(vfs, namecache, fullpath, return, error,
                            vp, NULL);
                        return (error);
                }
                *buflen -= ncp->nc_nlen;
                memcpy(buf + *buflen, ncp->nc_name, ncp->nc_nlen);
                SDT_PROBE3(vfs, namecache, fullpath, hit, ncp->nc_dvp,
                    ncp->nc_name, vp);
                dvp = *vp;
                *vp = ncp->nc_dvp;
                vref(*vp);
                mtx_unlock(vlp);
                vrele(dvp);
                return (0);
        }
        SDT_PROBE1(vfs, namecache, fullpath, miss, vp);

        mtx_unlock(vlp);
        vn_lock(*vp, LK_SHARED | LK_RETRY);
        error = VOP_VPTOCNP(*vp, &dvp, cred, buf, buflen);
        vput(*vp);
        if (error) {
                counter_u64_add(numfullpathfail2, 1);
                SDT_PROBE3(vfs, namecache, fullpath, return,  error, vp, NULL);
                return (error);
        }

        *vp = dvp;
        if (VN_IS_DOOMED(dvp)) {
                /* forced unmount */
                vrele(dvp);
                error = ENOENT;
                SDT_PROBE3(vfs, namecache, fullpath, return, error, vp, NULL);
                return (error);
        }
        /*
         * *vp has its use count incremented still.
         */

        return (0);
}

/*
 * Resolve a directory to a pathname.
 *
 * The name of the directory can always be found in the namecache or fetched
 * from the filesystem. There is also guaranteed to be only one parent, meaning
 * we can just follow vnodes up until we find the root.
 *
 * The vnode must be referenced.
 */
static int
vn_fullpath_dir(struct vnode *vp, struct vnode *rdir, char *buf, char **retbuf,
    size_t *len, bool slash_prefixed, size_t addend)
{
#ifdef KDTRACE_HOOKS
        struct vnode *startvp = vp;
#endif
        struct vnode *vp1;
        size_t buflen;
        int error;

        VNPASS(vp->v_type == VDIR || VN_IS_DOOMED(vp), vp);
        VNPASS(vp->v_usecount > 0, vp);

        buflen = *len;

        if (!slash_prefixed) {
                MPASS(*len >= 2);
                buflen--;
                buf[buflen] = '\0';
        }

        error = 0;

        SDT_PROBE1(vfs, namecache, fullpath, entry, vp);
        counter_u64_add(numfullpathcalls, 1);
        while (vp != rdir && vp != rootvnode) {
                /*
                 * The vp vnode must be already fully constructed,
                 * since it is either found in namecache or obtained
                 * from VOP_VPTOCNP().  We may test for VV_ROOT safely
                 * without obtaining the vnode lock.
                 */
                if ((vp->v_vflag & VV_ROOT) != 0) {
                        vn_lock(vp, LK_RETRY | LK_SHARED);

                        /*
                         * With the vnode locked, check for races with
                         * unmount, forced or not.  Note that we
                         * already verified that vp is not equal to
                         * the root vnode, which means that
                         * mnt_vnodecovered can be NULL only for the
                         * case of unmount.
                         */
                        if (VN_IS_DOOMED(vp) ||
                            (vp1 = vp->v_mount->mnt_vnodecovered) == NULL ||
                            vp1->v_mountedhere != vp->v_mount) {
                                vput(vp);
                                error = ENOENT;
                                SDT_PROBE3(vfs, namecache, fullpath, return,
                                    error, vp, NULL);
                                break;
                        }

                        vref(vp1);
                        vput(vp);
                        vp = vp1;
                        continue;
                }
                if (vp->v_type != VDIR) {
                        vrele(vp);
                        counter_u64_add(numfullpathfail1, 1);
                        error = ENOTDIR;
                        SDT_PROBE3(vfs, namecache, fullpath, return,
                            error, vp, NULL);
                        break;
                }
                error = vn_vptocnp(&vp, curthread->td_ucred, buf, &buflen);
                if (error)
                        break;
                if (buflen == 0) {
                        vrele(vp);
                        error = ENOMEM;
                        SDT_PROBE3(vfs, namecache, fullpath, return, error,
                            startvp, NULL);
                        break;
                }
                buf[--buflen] = '/';
                slash_prefixed = true;
        }
        if (error)
                return (error);
        if (!slash_prefixed) {
                if (buflen == 0) {
                        vrele(vp);
                        counter_u64_add(numfullpathfail4, 1);
                        SDT_PROBE3(vfs, namecache, fullpath, return, ENOMEM,
                            startvp, NULL);
                        return (ENOMEM);
                }
                buf[--buflen] = '/';
        }
        counter_u64_add(numfullpathfound, 1);
        vrele(vp);

        *retbuf = buf + buflen;
        SDT_PROBE3(vfs, namecache, fullpath, return, 0, startvp, *retbuf);
        *len -= buflen;
        *len += addend;
        return (0);
}

/*
 * Resolve an arbitrary vnode to a pathname.
 *
 * Note 2 caveats:
 * - hardlinks are not tracked, thus if the vnode is not a directory this can
 *   resolve to a different path than the one used to find it
 * - namecache is not mandatory, meaning names are not guaranteed to be added
 *   (in which case resolving fails)
 */
static void __inline
cache_rev_failed_impl(int *reason, int line)
{

        *reason = line;
}
#define cache_rev_failed(var)   cache_rev_failed_impl((var), __LINE__)

static int
vn_fullpath_any_smr(struct vnode *vp, struct vnode *rdir, char *buf,
    char **retbuf, size_t *buflen, bool slash_prefixed, size_t addend)
{
#ifdef KDTRACE_HOOKS
        struct vnode *startvp = vp;
#endif
        struct vnode *tvp;
        struct mount *mp;
        struct namecache *ncp;
        size_t orig_buflen;
        int reason;
        int error;
#ifdef KDTRACE_HOOKS
        int i;
#endif
        seqc_t vp_seqc, tvp_seqc;
        u_char nc_flag;

        VFS_SMR_ASSERT_ENTERED();

        if (!cache_fast_revlookup) {
                vfs_smr_exit();
                return (-1);
        }

        orig_buflen = *buflen;

        if (!slash_prefixed) {
                MPASS(*buflen >= 2);
                *buflen -= 1;
                buf[*buflen] = '\0';
        }

        if (vp == rdir || vp == rootvnode) {
                if (!slash_prefixed) {
                        *buflen -= 1;
                        buf[*buflen] = '/';
                }
                goto out_ok;
        }

#ifdef KDTRACE_HOOKS
        i = 0;
#endif
        error = -1;
        ncp = NULL; /* for sdt probe down below */
        vp_seqc = vn_seqc_read_any(vp);
        if (seqc_in_modify(vp_seqc)) {
                cache_rev_failed(&reason);
                goto out_abort;
        }

        for (;;) {
#ifdef KDTRACE_HOOKS
                i++;
#endif
                if ((vp->v_vflag & VV_ROOT) != 0) {
                        mp = atomic_load_ptr(&vp->v_mount);
                        if (mp == NULL) {
                                cache_rev_failed(&reason);
                                goto out_abort;
                        }
                        tvp = atomic_load_ptr(&mp->mnt_vnodecovered);
                        tvp_seqc = vn_seqc_read_any(tvp);
                        if (seqc_in_modify(tvp_seqc)) {
                                cache_rev_failed(&reason);
                                goto out_abort;
                        }
                        if (!vn_seqc_consistent(vp, vp_seqc)) {
                                cache_rev_failed(&reason);
                                goto out_abort;
                        }
                        vp = tvp;
                        vp_seqc = tvp_seqc;
                        continue;
                }
                ncp = atomic_load_ptr(&vp->v_cache_dd);
                if (ncp == NULL) {
                        cache_rev_failed(&reason);
                        goto out_abort;
                }
                nc_flag = atomic_load_char(&ncp->nc_flag);
                if ((nc_flag & NCF_ISDOTDOT) != 0) {
                        cache_rev_failed(&reason);
                        goto out_abort;
                }
                if (!cache_ncp_canuse(ncp)) {
                        cache_rev_failed(&reason);
                        goto out_abort;
                }
                if (ncp->nc_nlen >= *buflen) {
                        cache_rev_failed(&reason);
                        error = ENOMEM;
                        goto out_abort;
                }
                *buflen -= ncp->nc_nlen;
                memcpy(buf + *buflen, ncp->nc_name, ncp->nc_nlen);
                *buflen -= 1;
                buf[*buflen] = '/';
                tvp = ncp->nc_dvp;
                tvp_seqc = vn_seqc_read_any(tvp);
                if (seqc_in_modify(tvp_seqc)) {
                        cache_rev_failed(&reason);
                        goto out_abort;
                }
                if (!vn_seqc_consistent(vp, vp_seqc)) {
                        cache_rev_failed(&reason);
                        goto out_abort;
                }
                vp = tvp;
                vp_seqc = tvp_seqc;
                if (vp == rdir || vp == rootvnode)
                        break;
        }
out_ok:
        vfs_smr_exit();
        *retbuf = buf + *buflen;
        *buflen = orig_buflen - *buflen + addend;
        SDT_PROBE2(vfs, namecache, fullpath_smr, hit, startvp, *retbuf);
        return (0);

out_abort:
        *buflen = orig_buflen;
        SDT_PROBE4(vfs, namecache, fullpath_smr, miss, startvp, ncp, reason, i);
        vfs_smr_exit();
        return (error);
}

static int
vn_fullpath_any(struct vnode *vp, struct vnode *rdir, char *buf, char **retbuf,
    size_t *buflen)
{
        size_t orig_buflen;
        bool slash_prefixed;
        int error;

        if (*buflen < 2)
                return (EINVAL);

        orig_buflen = *buflen;

        vref(vp);
        slash_prefixed = false;
        if (vp->v_type != VDIR) {
                *buflen -= 1;
                buf[*buflen] = '\0';
                error = vn_vptocnp(&vp, curthread->td_ucred, buf, buflen);
                if (error)
                        return (error);
                if (*buflen == 0) {
                        vrele(vp);
                        return (ENOMEM);
                }
                *buflen -= 1;
                buf[*buflen] = '/';
                slash_prefixed = true;
        }

        return (vn_fullpath_dir(vp, rdir, buf, retbuf, buflen, slash_prefixed,
            orig_buflen - *buflen));
}

/*
 * Resolve an arbitrary vnode to a pathname (taking care of hardlinks).
 *
 * Since the namecache does not track handlings, the caller is expected to first
 * look up the target vnode with SAVENAME | WANTPARENT flags passed to namei.
 *
 * Then we have 2 cases:
 * - if the found vnode is a directory, the path can be constructed just by
 *   fullowing names up the chain
 * - otherwise we populate the buffer with the saved name and start resolving
 *   from the parent
 */
static int
vn_fullpath_hardlink(struct nameidata *ndp, char **retbuf, char **freebuf,
    size_t *buflen)
{
        char *buf, *tmpbuf;
        struct pwd *pwd;
        struct componentname *cnp;
        struct vnode *vp;
        size_t addend;
        int error;
        bool slash_prefixed;
        enum vtype type;

        if (*buflen < 2)
                return (EINVAL);
        if (*buflen > MAXPATHLEN)
                *buflen = MAXPATHLEN;

        slash_prefixed = false;

        buf = malloc(*buflen, M_TEMP, M_WAITOK);

        addend = 0;
        vp = ndp->ni_vp;
        /*
         * Check for VBAD to work around the vp_crossmp bug in lookup().
         *
         * For example consider tmpfs on /tmp and realpath /tmp. ni_vp will be
         * set to mount point's root vnode while ni_dvp will be vp_crossmp.
         * If the type is VDIR (like in this very case) we can skip looking
         * at ni_dvp in the first place. However, since vnodes get passed here
         * unlocked the target may transition to doomed state (type == VBAD)
         * before we get to evaluate the condition. If this happens, we will
         * populate part of the buffer and descend to vn_fullpath_dir with
         * vp == vp_crossmp. Prevent the problem by checking for VBAD.
         *
         * This should be atomic_load(&vp->v_type) but it is ilegal to take
         * an address of a bit field, even if said field is sized to char.
         * Work around the problem by reading the value into a full-sized enum
         * and then re-reading it with atomic_load which will still prevent
         * the compiler from re-reading down the road.
         */
        type = vp->v_type;
        type = atomic_load_int(&type);
        if (type == VBAD) {
                error = ENOENT;
                goto out_bad;
        }
        if (type != VDIR) {
                cnp = &ndp->ni_cnd;
                addend = cnp->cn_namelen + 2;
                if (*buflen < addend) {
                        error = ENOMEM;
                        goto out_bad;
                }
                *buflen -= addend;
                tmpbuf = buf + *buflen;
                tmpbuf[0] = '/';
                memcpy(&tmpbuf[1], cnp->cn_nameptr, cnp->cn_namelen);
                tmpbuf[addend - 1] = '\0';
                slash_prefixed = true;
                vp = ndp->ni_dvp;
        }

        vfs_smr_enter();
        pwd = pwd_get_smr();
        error = vn_fullpath_any_smr(vp, pwd->pwd_rdir, buf, retbuf, buflen,
            slash_prefixed, addend);
        VFS_SMR_ASSERT_NOT_ENTERED();
        if (error < 0) {
                pwd = pwd_hold(curthread);
                vref(vp);
                error = vn_fullpath_dir(vp, pwd->pwd_rdir, buf, retbuf, buflen,
                    slash_prefixed, addend);
                pwd_drop(pwd);
                if (error != 0)
                        goto out_bad;
        }

        *freebuf = buf;

        return (0);
out_bad:
        free(buf, M_TEMP);
        return (error);
}

struct vnode *
vn_dir_dd_ino(struct vnode *vp)
{
        struct namecache *ncp;
        struct vnode *ddvp;
        struct mtx *vlp;
        enum vgetstate vs;

        ASSERT_VOP_LOCKED(vp, "vn_dir_dd_ino");
        vlp = VP2VNODELOCK(vp);
        mtx_lock(vlp);
        TAILQ_FOREACH(ncp, &(vp->v_cache_dst), nc_dst) {
                if ((ncp->nc_flag & NCF_ISDOTDOT) != 0)
                        continue;
                ddvp = ncp->nc_dvp;
                vs = vget_prep(ddvp);
                mtx_unlock(vlp);
                if (vget_finish(ddvp, LK_SHARED | LK_NOWAIT, vs))
                        return (NULL);
                return (ddvp);
        }
        mtx_unlock(vlp);
        return (NULL);
}

int
vn_commname(struct vnode *vp, char *buf, u_int buflen)
{
        struct namecache *ncp;
        struct mtx *vlp;
        int l;

        vlp = VP2VNODELOCK(vp);
        mtx_lock(vlp);
        TAILQ_FOREACH(ncp, &vp->v_cache_dst, nc_dst)
                if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
                        break;
        if (ncp == NULL) {
                mtx_unlock(vlp);
                return (ENOENT);
        }
        l = min(ncp->nc_nlen, buflen - 1);
        memcpy(buf, ncp->nc_name, l);
        mtx_unlock(vlp);
        buf[l] = '\0';
        return (0);
}

/*
 * This function updates path string to vnode's full global path
 * and checks the size of the new path string against the pathlen argument.
 *
 * Requires a locked, referenced vnode.
 * Vnode is re-locked on success or ENODEV, otherwise unlocked.
 *
 * If vp is a directory, the call to vn_fullpath_global() always succeeds
 * because it falls back to the ".." lookup if the namecache lookup fails.
 */
int
vn_path_to_global_path(struct thread *td, struct vnode *vp, char *path,
    u_int pathlen)
{
        struct nameidata nd;
        struct vnode *vp1;
        char *rpath, *fbuf;
        int error;

        ASSERT_VOP_ELOCKED(vp, __func__);

        /* Construct global filesystem path from vp. */
        VOP_UNLOCK(vp);
        error = vn_fullpath_global(vp, &rpath, &fbuf);

        if (error != 0) {
                vrele(vp);
                return (error);
        }

        if (strlen(rpath) >= pathlen) {
                vrele(vp);
                error = ENAMETOOLONG;
                goto out;
        }

        /*
         * Re-lookup the vnode by path to detect a possible rename.
         * As a side effect, the vnode is relocked.
         * If vnode was renamed, return ENOENT.
         */
        NDINIT(&nd, LOOKUP, FOLLOW | LOCKLEAF | AUDITVNODE1,
            UIO_SYSSPACE, path, td);
        error = namei(&nd);
        if (error != 0) {
                vrele(vp);
                goto out;
        }
        NDFREE(&nd, NDF_ONLY_PNBUF);
        vp1 = nd.ni_vp;
        vrele(vp);
        if (vp1 == vp)
                strcpy(path, rpath);
        else {
                vput(vp1);
                error = ENOENT;
        }

out:
        free(fbuf, M_TEMP);
        return (error);
}

#ifdef DDB
static void
db_print_vpath(struct vnode *vp)
{

        while (vp != NULL) {
                db_printf("%p: ", vp);
                if (vp == rootvnode) {
                        db_printf("/");
                        vp = NULL;
                } else {
                        if (vp->v_vflag & VV_ROOT) {
                                db_printf("<mount point>");
                                vp = vp->v_mount->mnt_vnodecovered;
                        } else {
                                struct namecache *ncp;
                                char *ncn;
                                int i;

                                ncp = TAILQ_FIRST(&vp->v_cache_dst);
                                if (ncp != NULL) {
                                        ncn = ncp->nc_name;
                                        for (i = 0; i < ncp->nc_nlen; i++)
                                                db_printf("%c", *ncn++);
                                        vp = ncp->nc_dvp;
                                } else {
                                        vp = NULL;
                                }
                        }
                }
                db_printf("\n");
        }

        return;
}

DB_SHOW_COMMAND(vpath, db_show_vpath)
{
        struct vnode *vp;

        if (!have_addr) {
                db_printf("usage: show vpath <struct vnode *>\n");
                return;
        }

        vp = (struct vnode *)addr;
        db_print_vpath(vp);
}

#endif

static bool __read_frequently cache_fast_lookup = true;
SYSCTL_BOOL(_vfs, OID_AUTO, cache_fast_lookup, CTLFLAG_RW,
    &cache_fast_lookup, 0, "");

#define CACHE_FPL_FAILED        -2020

static void
cache_fpl_cleanup_cnp(struct componentname *cnp)
{

        uma_zfree(namei_zone, cnp->cn_pnbuf);
#ifdef DIAGNOSTIC
        cnp->cn_pnbuf = NULL;
        cnp->cn_nameptr = NULL;
#endif
}

static void
cache_fpl_handle_root(struct nameidata *ndp, struct vnode **dpp)
{
        struct componentname *cnp;

        cnp = &ndp->ni_cnd;
        while (*(cnp->cn_nameptr) == '/') {
                cnp->cn_nameptr++;
                ndp->ni_pathlen--;
        }

        *dpp = ndp->ni_rootdir;
}

/*
 * Components of nameidata (or objects it can point to) which may
 * need restoring in case fast path lookup fails.
 */
struct nameidata_saved {
        long cn_namelen;
        char *cn_nameptr;
        size_t ni_pathlen;
        int cn_flags;
};

struct cache_fpl {
        struct nameidata *ndp;
        struct componentname *cnp;
        struct pwd *pwd;
        struct vnode *dvp;
        struct vnode *tvp;
        seqc_t dvp_seqc;
        seqc_t tvp_seqc;
        struct nameidata_saved snd;
        int line;
        enum cache_fpl_status status:8;
        bool in_smr;
        bool fsearch;
};

static void
cache_fpl_checkpoint(struct cache_fpl *fpl, struct nameidata_saved *snd)
{

        snd->cn_flags = fpl->ndp->ni_cnd.cn_flags;
        snd->cn_namelen = fpl->ndp->ni_cnd.cn_namelen;
        snd->cn_nameptr = fpl->ndp->ni_cnd.cn_nameptr;
        snd->ni_pathlen = fpl->ndp->ni_pathlen;
}

static void
cache_fpl_restore(struct cache_fpl *fpl, struct nameidata_saved *snd)
{

        fpl->ndp->ni_cnd.cn_flags = snd->cn_flags;
        fpl->ndp->ni_cnd.cn_namelen = snd->cn_namelen;
        fpl->ndp->ni_cnd.cn_nameptr = snd->cn_nameptr;
        fpl->ndp->ni_pathlen = snd->ni_pathlen;
}

#ifdef INVARIANTS
#define cache_fpl_smr_assert_entered(fpl) ({                    \
        struct cache_fpl *_fpl = (fpl);                         \
        MPASS(_fpl->in_smr == true);                            \
        VFS_SMR_ASSERT_ENTERED();                               \
})
#define cache_fpl_smr_assert_not_entered(fpl) ({                \
        struct cache_fpl *_fpl = (fpl);                         \
        MPASS(_fpl->in_smr == false);                           \
        VFS_SMR_ASSERT_NOT_ENTERED();                           \
})
#else
#define cache_fpl_smr_assert_entered(fpl) do { } while (0)
#define cache_fpl_smr_assert_not_entered(fpl) do { } while (0)
#endif

#define cache_fpl_smr_enter_initial(fpl) ({                     \
        struct cache_fpl *_fpl = (fpl);                         \
        vfs_smr_enter();                                        \
        _fpl->in_smr = true;                                    \
})

#define cache_fpl_smr_enter(fpl) ({                             \
        struct cache_fpl *_fpl = (fpl);                         \
        MPASS(_fpl->in_smr == false);                           \
        vfs_smr_enter();                                        \
        _fpl->in_smr = true;                                    \
})

#define cache_fpl_smr_exit(fpl) ({                              \
        struct cache_fpl *_fpl = (fpl);                         \
        MPASS(_fpl->in_smr == true);                            \
        vfs_smr_exit();                                         \
        _fpl->in_smr = false;                                   \
})

static int
cache_fpl_aborted_impl(struct cache_fpl *fpl, int line)
{

        if (fpl->status != CACHE_FPL_STATUS_UNSET) {
                KASSERT(fpl->status == CACHE_FPL_STATUS_PARTIAL,
                    ("%s: converting to abort from %d at %d, set at %d\n",
                    __func__, fpl->status, line, fpl->line));
        }
        fpl->status = CACHE_FPL_STATUS_ABORTED;
        fpl->line = line;
        return (CACHE_FPL_FAILED);
}

#define cache_fpl_aborted(x)    cache_fpl_aborted_impl((x), __LINE__)

static int
cache_fpl_partial_impl(struct cache_fpl *fpl, int line)
{

        KASSERT(fpl->status == CACHE_FPL_STATUS_UNSET,
            ("%s: setting to partial at %d, but already set to %d at %d\n",
            __func__, line, fpl->status, fpl->line));
        cache_fpl_smr_assert_entered(fpl);
        fpl->status = CACHE_FPL_STATUS_PARTIAL;
        fpl->line = line;
        return (CACHE_FPL_FAILED);
}

#define cache_fpl_partial(x)    cache_fpl_partial_impl((x), __LINE__)

static int
cache_fpl_handled_impl(struct cache_fpl *fpl, int error, int line)
{

        KASSERT(fpl->status == CACHE_FPL_STATUS_UNSET,
            ("%s: setting to handled at %d, but already set to %d at %d\n",
            __func__, line, fpl->status, fpl->line));
        cache_fpl_smr_assert_not_entered(fpl);
        MPASS(error != CACHE_FPL_FAILED);
        fpl->status = CACHE_FPL_STATUS_HANDLED;
        fpl->line = line;
        return (error);
}

#define cache_fpl_handled(x, e) cache_fpl_handled_impl((x), (e), __LINE__)

#define CACHE_FPL_SUPPORTED_CN_FLAGS \
        (LOCKLEAF | LOCKPARENT | WANTPARENT | NOCACHE | FOLLOW | LOCKSHARED | SAVENAME | \
         SAVESTART | WILLBEDIR | ISOPEN | NOMACCHECK | AUDITVNODE1 | AUDITVNODE2 | NOCAPCHECK)

#define CACHE_FPL_INTERNAL_CN_FLAGS \
        (ISDOTDOT | MAKEENTRY | ISLASTCN)

_Static_assert((CACHE_FPL_SUPPORTED_CN_FLAGS & CACHE_FPL_INTERNAL_CN_FLAGS) == 0,
    "supported and internal flags overlap");

static bool
cache_fpl_islastcn(struct nameidata *ndp)
{

        return (*ndp->ni_next == 0);
}

static bool
cache_fpl_isdotdot(struct componentname *cnp)
{

        if (cnp->cn_namelen == 2 &&
            cnp->cn_nameptr[1] == '.' && cnp->cn_nameptr[0] == '.')
                return (true);
        return (false);
}

static bool
cache_can_fplookup(struct cache_fpl *fpl)
{
        struct nameidata *ndp;
        struct componentname *cnp;
        struct thread *td;

        ndp = fpl->ndp;
        cnp = fpl->cnp;
        td = cnp->cn_thread;

        if (!cache_fast_lookup) {
                cache_fpl_aborted(fpl);
                return (false);
        }
#ifdef MAC
        if (mac_vnode_check_lookup_enabled()) {
                cache_fpl_aborted(fpl);
                return (false);
        }
#endif
        if ((cnp->cn_flags & ~CACHE_FPL_SUPPORTED_CN_FLAGS) != 0) {
                cache_fpl_aborted(fpl);
                return (false);
        }
        if (IN_CAPABILITY_MODE(td)) {
                cache_fpl_aborted(fpl);
                return (false);
        }
        if (AUDITING_TD(td)) {
                cache_fpl_aborted(fpl);
                return (false);
        }
        if (ndp->ni_startdir != NULL) {
                cache_fpl_aborted(fpl);
                return (false);
        }
        return (true);
}

static int
cache_fplookup_dirfd(struct cache_fpl *fpl, struct vnode **vpp)
{
        struct nameidata *ndp;
        int error;
        bool fsearch;

        ndp = fpl->ndp;
        error = fgetvp_lookup_smr(ndp->ni_dirfd, ndp, vpp, &fsearch);
        if (__predict_false(error != 0)) {
                cache_fpl_smr_exit(fpl);
                return (cache_fpl_aborted(fpl));
        }
        fpl->fsearch = fsearch;
        return (0);
}

static bool
cache_fplookup_vnode_supported(struct vnode *vp)
{

        return (vp->v_type != VLNK);
}

static int __noinline
cache_fplookup_negative_promote(struct cache_fpl *fpl, struct namecache *oncp,
    uint32_t hash)
{
        struct componentname *cnp;
        struct vnode *dvp;

        cnp = fpl->cnp;
        dvp = fpl->dvp;

        cache_fpl_smr_exit(fpl);
        if (cache_neg_promote_cond(dvp, cnp, oncp, hash))
                return (cache_fpl_handled(fpl, ENOENT));
        else
                return (cache_fpl_aborted(fpl));
}

/*
 * The target vnode is not supported, prepare for the slow path to take over.
 */
static int __noinline
cache_fplookup_partial_setup(struct cache_fpl *fpl)
{
        struct nameidata *ndp;
        struct componentname *cnp;
        enum vgetstate dvs;
        struct vnode *dvp;
        struct pwd *pwd;
        seqc_t dvp_seqc;

        ndp = fpl->ndp;
        cnp = fpl->cnp;
        pwd = fpl->pwd;
        dvp = fpl->dvp;
        dvp_seqc = fpl->dvp_seqc;

        if (!pwd_hold_smr(pwd)) {
                cache_fpl_smr_exit(fpl);
                return (cache_fpl_aborted(fpl));
        }

        dvs = vget_prep_smr(dvp);
        cache_fpl_smr_exit(fpl);
        if (__predict_false(dvs == VGET_NONE)) {
                pwd_drop(pwd);
                return (cache_fpl_aborted(fpl));
        }

        vget_finish_ref(dvp, dvs);
        if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                vrele(dvp);
                pwd_drop(pwd);
                return (cache_fpl_aborted(fpl));
        }

        cache_fpl_restore(fpl, &fpl->snd);

        ndp->ni_startdir = dvp;
        cnp->cn_flags |= MAKEENTRY;
        if (cache_fpl_islastcn(ndp))
                cnp->cn_flags |= ISLASTCN;
        if (cache_fpl_isdotdot(cnp))
                cnp->cn_flags |= ISDOTDOT;

        return (0);
}

static int
cache_fplookup_final_child(struct cache_fpl *fpl, enum vgetstate tvs)
{
        struct componentname *cnp;
        struct vnode *tvp;
        seqc_t tvp_seqc;
        int error, lkflags;

        cnp = fpl->cnp;
        tvp = fpl->tvp;
        tvp_seqc = fpl->tvp_seqc;

        if ((cnp->cn_flags & LOCKLEAF) != 0) {
                lkflags = LK_SHARED;
                if ((cnp->cn_flags & LOCKSHARED) == 0)
                        lkflags = LK_EXCLUSIVE;
                error = vget_finish(tvp, lkflags, tvs);
                if (__predict_false(error != 0)) {
                        return (cache_fpl_aborted(fpl));
                }
        } else {
                vget_finish_ref(tvp, tvs);
        }

        if (!vn_seqc_consistent(tvp, tvp_seqc)) {
                if ((cnp->cn_flags & LOCKLEAF) != 0)
                        vput(tvp);
                else
                        vrele(tvp);
                return (cache_fpl_aborted(fpl));
        }

        return (cache_fpl_handled(fpl, 0));
}

/*
 * They want to possibly modify the state of the namecache.
 *
 * Don't try to match the API contract, just leave.
 * TODO: this leaves scalability on the table
 */
static int
cache_fplookup_final_modifying(struct cache_fpl *fpl)
{
        struct componentname *cnp;

        cnp = fpl->cnp;
        MPASS(cnp->cn_nameiop != LOOKUP);
        return (cache_fpl_partial(fpl));
}

static int __noinline
cache_fplookup_final_withparent(struct cache_fpl *fpl)
{
        struct componentname *cnp;
        enum vgetstate dvs, tvs;
        struct vnode *dvp, *tvp;
        seqc_t dvp_seqc;
        int error;

        cnp = fpl->cnp;
        dvp = fpl->dvp;
        dvp_seqc = fpl->dvp_seqc;
        tvp = fpl->tvp;

        MPASS((cnp->cn_flags & (LOCKPARENT|WANTPARENT)) != 0);

        /*
         * This is less efficient than it can be for simplicity.
         */
        dvs = vget_prep_smr(dvp);
        if (__predict_false(dvs == VGET_NONE)) {
                return (cache_fpl_aborted(fpl));
        }
        tvs = vget_prep_smr(tvp);
        if (__predict_false(tvs == VGET_NONE)) {
                cache_fpl_smr_exit(fpl);
                vget_abort(dvp, dvs);
                return (cache_fpl_aborted(fpl));
        }

        cache_fpl_smr_exit(fpl);

        if ((cnp->cn_flags & LOCKPARENT) != 0) {
                error = vget_finish(dvp, LK_EXCLUSIVE, dvs);
                if (__predict_false(error != 0)) {
                        vget_abort(tvp, tvs);
                        return (cache_fpl_aborted(fpl));
                }
        } else {
                vget_finish_ref(dvp, dvs);
        }

        if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                vget_abort(tvp, tvs);
                if ((cnp->cn_flags & LOCKPARENT) != 0)
                        vput(dvp);
                else
                        vrele(dvp);
                return (cache_fpl_aborted(fpl));
        }

        error = cache_fplookup_final_child(fpl, tvs);
        if (__predict_false(error != 0)) {
                MPASS(fpl->status == CACHE_FPL_STATUS_ABORTED);
                if ((cnp->cn_flags & LOCKPARENT) != 0)
                        vput(dvp);
                else
                        vrele(dvp);
                return (error);
        }

        MPASS(fpl->status == CACHE_FPL_STATUS_HANDLED);
        return (0);
}

static int
cache_fplookup_final(struct cache_fpl *fpl)
{
        struct componentname *cnp;
        enum vgetstate tvs;
        struct vnode *dvp, *tvp;
        seqc_t dvp_seqc;

        cnp = fpl->cnp;
        dvp = fpl->dvp;
        dvp_seqc = fpl->dvp_seqc;
        tvp = fpl->tvp;

        VNPASS(cache_fplookup_vnode_supported(dvp), dvp);

        if (cnp->cn_nameiop != LOOKUP) {
                return (cache_fplookup_final_modifying(fpl));
        }

        if ((cnp->cn_flags & (LOCKPARENT|WANTPARENT)) != 0)
                return (cache_fplookup_final_withparent(fpl));

        tvs = vget_prep_smr(tvp);
        if (__predict_false(tvs == VGET_NONE)) {
                return (cache_fpl_partial(fpl));
        }

        if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                cache_fpl_smr_exit(fpl);
                vget_abort(tvp, tvs);
                return (cache_fpl_aborted(fpl));
        }

        cache_fpl_smr_exit(fpl);
        return (cache_fplookup_final_child(fpl, tvs));
}

static int __noinline
cache_fplookup_dot(struct cache_fpl *fpl)
{
        struct vnode *dvp;

        dvp = fpl->dvp;

        fpl->tvp = dvp;
        fpl->tvp_seqc = vn_seqc_read_any(dvp);
        if (seqc_in_modify(fpl->tvp_seqc)) {
                return (cache_fpl_aborted(fpl));
        }

        counter_u64_add(dothits, 1);
        SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ".", dvp);

        return (0);
}

static int __noinline
cache_fplookup_dotdot(struct cache_fpl *fpl)
{
        struct nameidata *ndp;
        struct componentname *cnp;
        struct namecache *ncp;
        struct vnode *dvp;
        struct prison *pr;
        u_char nc_flag;

        ndp = fpl->ndp;
        cnp = fpl->cnp;
        dvp = fpl->dvp;

        /*
         * XXX this is racy the same way regular lookup is
         */
        for (pr = cnp->cn_cred->cr_prison; pr != NULL;
            pr = pr->pr_parent)
                if (dvp == pr->pr_root)
                        break;

        if (dvp == ndp->ni_rootdir ||
            dvp == ndp->ni_topdir ||
            dvp == rootvnode ||
            pr != NULL) {
                fpl->tvp = dvp;
                fpl->tvp_seqc = vn_seqc_read_any(dvp);
                if (seqc_in_modify(fpl->tvp_seqc)) {
                        return (cache_fpl_aborted(fpl));
                }
                return (0);
        }

        if ((dvp->v_vflag & VV_ROOT) != 0) {
                /*
                 * TODO
                 * The opposite of climb mount is needed here.
                 */
                return (cache_fpl_aborted(fpl));
        }

        ncp = atomic_load_ptr(&dvp->v_cache_dd);
        if (ncp == NULL) {
                return (cache_fpl_aborted(fpl));
        }

        nc_flag = atomic_load_char(&ncp->nc_flag);
        if ((nc_flag & NCF_ISDOTDOT) != 0) {
                if ((nc_flag & NCF_NEGATIVE) != 0)
                        return (cache_fpl_aborted(fpl));
                fpl->tvp = ncp->nc_vp;
        } else {
                fpl->tvp = ncp->nc_dvp;
        }

        if (__predict_false(!cache_ncp_canuse(ncp))) {
                return (cache_fpl_aborted(fpl));
        }

        fpl->tvp_seqc = vn_seqc_read_any(fpl->tvp);
        if (seqc_in_modify(fpl->tvp_seqc)) {
                return (cache_fpl_partial(fpl));
        }

        counter_u64_add(dotdothits, 1);
        return (0);
}

static int
cache_fplookup_next(struct cache_fpl *fpl)
{
        struct componentname *cnp;
        struct namecache *ncp;
        struct negstate *ns;
        struct vnode *dvp, *tvp;
        u_char nc_flag;
        uint32_t hash;
        bool neg_hot;

        cnp = fpl->cnp;
        dvp = fpl->dvp;

        if (__predict_false(cnp->cn_namelen == 1 && cnp->cn_nameptr[0] == '.')) {
                return (cache_fplookup_dot(fpl));
        }

        hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);

        CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                    !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                        break;
        }

        /*
         * If there is no entry we have to punt to the slow path to perform
         * actual lookup. Should there be nothing with this name a negative
         * entry will be created.
         */
        if (__predict_false(ncp == NULL)) {
                return (cache_fpl_partial(fpl));
        }

        tvp = atomic_load_ptr(&ncp->nc_vp);
        nc_flag = atomic_load_char(&ncp->nc_flag);
        if ((nc_flag & NCF_NEGATIVE) != 0) {
                /*
                 * If they want to create an entry we need to replace this one.
                 */
                if (__predict_false(fpl->cnp->cn_nameiop != LOOKUP)) {
                        return (cache_fpl_partial(fpl));
                }
                ns = NCP2NEGSTATE(ncp);
                neg_hot = ((ns->neg_flag & NEG_HOT) != 0);
                if (__predict_false(!cache_ncp_canuse(ncp))) {
                        return (cache_fpl_partial(fpl));
                }
                if (__predict_false((nc_flag & NCF_WHITE) != 0)) {
                        return (cache_fpl_partial(fpl));
                }
                if (!neg_hot) {
                        return (cache_fplookup_negative_promote(fpl, ncp, hash));
                }
                SDT_PROBE2(vfs, namecache, lookup, hit__negative, dvp,
                    ncp->nc_name);
                counter_u64_add(numneghits, 1);
                cache_fpl_smr_exit(fpl);
                return (cache_fpl_handled(fpl, ENOENT));
        }

        if (__predict_false(!cache_ncp_canuse(ncp))) {
                return (cache_fpl_partial(fpl));
        }

        fpl->tvp = tvp;
        fpl->tvp_seqc = vn_seqc_read_any(tvp);
        if (seqc_in_modify(fpl->tvp_seqc)) {
                return (cache_fpl_partial(fpl));
        }

        if (!cache_fplookup_vnode_supported(tvp)) {
                return (cache_fpl_partial(fpl));
        }

        counter_u64_add(numposhits, 1);
        SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ncp->nc_name, tvp);
        return (0);
}

static bool
cache_fplookup_mp_supported(struct mount *mp)
{

        if (mp == NULL)
                return (false);
        if ((mp->mnt_kern_flag & MNTK_FPLOOKUP) == 0)
                return (false);
        return (true);
}

/*
 * Walk up the mount stack (if any).
 *
 * Correctness is provided in the following ways:
 * - all vnodes are protected from freeing with SMR
 * - struct mount objects are type stable making them always safe to access
 * - stability of the particular mount is provided by busying it
 * - relationship between the vnode which is mounted on and the mount is
 *   verified with the vnode sequence counter after busying
 * - association between root vnode of the mount and the mount is protected
 *   by busy
 *
 * From that point on we can read the sequence counter of the root vnode
 * and get the next mount on the stack (if any) using the same protection.
 *
 * By the end of successful walk we are guaranteed the reached state was
 * indeed present at least at some point which matches the regular lookup.
 */
static int __noinline
cache_fplookup_climb_mount(struct cache_fpl *fpl)
{
        struct mount *mp, *prev_mp;
        struct vnode *vp;
        seqc_t vp_seqc;

        vp = fpl->tvp;
        vp_seqc = fpl->tvp_seqc;

        VNPASS(vp->v_type == VDIR || vp->v_type == VBAD, vp);
        mp = atomic_load_ptr(&vp->v_mountedhere);
        if (mp == NULL)
                return (0);

        prev_mp = NULL;
        for (;;) {
                if (!vfs_op_thread_enter_crit(mp)) {
                        if (prev_mp != NULL)
                                vfs_op_thread_exit_crit(prev_mp);
                        return (cache_fpl_partial(fpl));
                }
                if (prev_mp != NULL)
                        vfs_op_thread_exit_crit(prev_mp);
                if (!vn_seqc_consistent(vp, vp_seqc)) {
                        vfs_op_thread_exit_crit(mp);
                        return (cache_fpl_partial(fpl));
                }
                if (!cache_fplookup_mp_supported(mp)) {
                        vfs_op_thread_exit_crit(mp);
                        return (cache_fpl_partial(fpl));
                }
                vp = atomic_load_ptr(&mp->mnt_rootvnode);
                if (vp == NULL || VN_IS_DOOMED(vp)) {
                        vfs_op_thread_exit_crit(mp);
                        return (cache_fpl_partial(fpl));
                }
                vp_seqc = vn_seqc_read_any(vp);
                if (seqc_in_modify(vp_seqc)) {
                        vfs_op_thread_exit_crit(mp);
                        return (cache_fpl_partial(fpl));
                }
                prev_mp = mp;
                mp = atomic_load_ptr(&vp->v_mountedhere);
                if (mp == NULL)
                        break;
        }

        vfs_op_thread_exit_crit(prev_mp);
        fpl->tvp = vp;
        fpl->tvp_seqc = vp_seqc;
        return (0);
}

static bool
cache_fplookup_need_climb_mount(struct cache_fpl *fpl)
{
        struct mount *mp;
        struct vnode *vp;

        vp = fpl->tvp;

        /*
         * Hack: while this is a union, the pointer tends to be NULL so save on
         * a branch.
         */
        mp = atomic_load_ptr(&vp->v_mountedhere);
        if (mp == NULL)
                return (false);
        if (vp->v_type == VDIR)
                return (true);
        return (false);
}

/*
 * Parse the path.
 *
 * The code was originally copy-pasted from regular lookup and despite
 * clean ups leaves performance on the table. Any modifications here
 * must take into account that in case off fallback the resulting
 * nameidata state has to be compatible with the original.
 */
static int
cache_fplookup_parse(struct cache_fpl *fpl)
{
        struct nameidata *ndp;
        struct componentname *cnp;
        char *cp;

        ndp = fpl->ndp;
        cnp = fpl->cnp;

        /*
         * Search a new directory.
         *
         * The last component of the filename is left accessible via
         * cnp->cn_nameptr for callers that need the name. Callers needing
         * the name set the SAVENAME flag. When done, they assume
         * responsibility for freeing the pathname buffer.
         */
        for (cp = cnp->cn_nameptr; *cp != 0 && *cp != '/'; cp++)
                continue;
        cnp->cn_namelen = cp - cnp->cn_nameptr;
        if (__predict_false(cnp->cn_namelen > NAME_MAX)) {
                cache_fpl_smr_exit(fpl);
                return (cache_fpl_handled(fpl, ENAMETOOLONG));
        }
        ndp->ni_pathlen -= cnp->cn_namelen;
        KASSERT(ndp->ni_pathlen <= PATH_MAX,
            ("%s: ni_pathlen underflow to %zd\n", __func__, ndp->ni_pathlen));
        ndp->ni_next = cp;

        /*
         * Replace multiple slashes by a single slash and trailing slashes
         * by a null.  This must be done before VOP_LOOKUP() because some
         * fs's don't know about trailing slashes.  Remember if there were
         * trailing slashes to handle symlinks, existing non-directories
         * and non-existing files that won't be directories specially later.
         */
        while (*cp == '/' && (cp[1] == '/' || cp[1] == '\0')) {
                cp++;
                ndp->ni_pathlen--;
                if (*cp == '\0') {
                        /*
                         * TODO
                         * Regular lookup performs the following:
                         * *ndp->ni_next = '\0';
                         * cnp->cn_flags |= TRAILINGSLASH;
                         *
                         * Which is problematic since it modifies data read
                         * from userspace. Then if fast path lookup was to
                         * abort we would have to either restore it or convey
                         * the flag. Since this is a corner case just ignore
                         * it for simplicity.
                         */
                        return (cache_fpl_partial(fpl));
                }
        }
        ndp->ni_next = cp;

        /*
         * Check for degenerate name (e.g. / or "")
         * which is a way of talking about a directory,
         * e.g. like "/." or ".".
         *
         * TODO
         * Another corner case handled by the regular lookup
         */
        if (__predict_false(cnp->cn_nameptr[0] == '\0')) {
                return (cache_fpl_partial(fpl));
        }
        return (0);
}

static void
cache_fplookup_parse_advance(struct cache_fpl *fpl)
{
        struct nameidata *ndp;
        struct componentname *cnp;

        ndp = fpl->ndp;
        cnp = fpl->cnp;

        cnp->cn_nameptr = ndp->ni_next;
        while (*cnp->cn_nameptr == '/') {
                cnp->cn_nameptr++;
                ndp->ni_pathlen--;
        }
}

/*
 * See the API contract for VOP_FPLOOKUP_VEXEC.
 */
static int __noinline
cache_fplookup_failed_vexec(struct cache_fpl *fpl, int error)
{
        struct componentname *cnp;
        struct vnode *dvp;
        seqc_t dvp_seqc;

        cnp = fpl->cnp;
        dvp = fpl->dvp;
        dvp_seqc = fpl->dvp_seqc;

        /*
         * Hack: they may be looking up foo/bar, where foo is a
         * regular file. In such a case we need to turn ENOTDIR,
         * but we may happen to get here with a different error.
         */
        if (dvp->v_type != VDIR) {
                /*
                 * The check here is predominantly to catch
                 * EOPNOTSUPP from dead_vnodeops. If the vnode
                 * gets doomed past this point it is going to
                 * fail seqc verification.
                 */
                if (VN_IS_DOOMED(dvp)) {
                        return (cache_fpl_aborted(fpl));
                }
                error = ENOTDIR;
        }

        /*
         * Hack: handle O_SEARCH.
         *
         * Open Group Base Specifications Issue 7, 2018 edition states:
         * If the access mode of the open file description associated with the
         * file descriptor is not O_SEARCH, the function shall check whether
         * directory searches are permitted using the current permissions of
         * the directory underlying the file descriptor. If the access mode is
         * O_SEARCH, the function shall not perform the check.
         *
         * Regular lookup tests for the NOEXECCHECK flag for every path
         * component to decide whether to do the permission check. However,
         * since most lookups never have the flag (and when they do it is only
         * present for the first path component), lockless lookup only acts on
         * it if there is a permission problem. Here the flag is represented
         * with a boolean so that we don't have to clear it on the way out.
         *
         * For simplicity this always aborts.
         * TODO: check if this is the first lookup and ignore the permission
         * problem. Note the flag has to survive fallback (if it happens to be
         * performed).
         */
        if (fpl->fsearch) {
                return (cache_fpl_aborted(fpl));
        }

        switch (error) {
        case EAGAIN:
                if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                        error = cache_fpl_aborted(fpl);
                } else {
                        cache_fpl_partial(fpl);
                }
                break;
        default:
                if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                        error = cache_fpl_aborted(fpl);
                } else {
                        cache_fpl_smr_exit(fpl);
                        cache_fpl_handled(fpl, error);
                }
                break;
        }
        return (error);
}

static int
cache_fplookup_impl(struct vnode *dvp, struct cache_fpl *fpl)
{
        struct nameidata *ndp;
        struct componentname *cnp;
        struct mount *mp;
        int error;

        error = CACHE_FPL_FAILED;
        ndp = fpl->ndp;
        cnp = fpl->cnp;

        cache_fpl_checkpoint(fpl, &fpl->snd);

        fpl->dvp = dvp;
        fpl->dvp_seqc = vn_seqc_read_any(fpl->dvp);
        if (seqc_in_modify(fpl->dvp_seqc)) {
                cache_fpl_aborted(fpl);
                goto out;
        }
        mp = atomic_load_ptr(&fpl->dvp->v_mount);
        if (!cache_fplookup_mp_supported(mp)) {
                cache_fpl_aborted(fpl);
                goto out;
        }

        VNPASS(cache_fplookup_vnode_supported(fpl->dvp), fpl->dvp);

        for (;;) {
                error = cache_fplookup_parse(fpl);
                if (__predict_false(error != 0)) {
                        break;
                }

                VNPASS(cache_fplookup_vnode_supported(fpl->dvp), fpl->dvp);

                error = VOP_FPLOOKUP_VEXEC(fpl->dvp, cnp->cn_cred);
                if (__predict_false(error != 0)) {
                        error = cache_fplookup_failed_vexec(fpl, error);
                        break;
                }

                if (__predict_false(cache_fpl_isdotdot(cnp))) {
                        error = cache_fplookup_dotdot(fpl);
                        if (__predict_false(error != 0)) {
                                break;
                        }
                } else {
                        error = cache_fplookup_next(fpl);
                        if (__predict_false(error != 0)) {
                                break;
                        }

                        VNPASS(!seqc_in_modify(fpl->tvp_seqc), fpl->tvp);

                        if (cache_fplookup_need_climb_mount(fpl)) {
                                error = cache_fplookup_climb_mount(fpl);
                                if (__predict_false(error != 0)) {
                                        break;
                                }
                        }
                }

                VNPASS(!seqc_in_modify(fpl->tvp_seqc), fpl->tvp);

                if (cache_fpl_islastcn(ndp)) {
                        error = cache_fplookup_final(fpl);
                        break;
                }

                if (!vn_seqc_consistent(fpl->dvp, fpl->dvp_seqc)) {
                        error = cache_fpl_aborted(fpl);
                        break;
                }

                fpl->dvp = fpl->tvp;
                fpl->dvp_seqc = fpl->tvp_seqc;

                cache_fplookup_parse_advance(fpl);
                cache_fpl_checkpoint(fpl, &fpl->snd);
        }
out:
        switch (fpl->status) {
        case CACHE_FPL_STATUS_UNSET:
                __assert_unreachable();
                break;
        case CACHE_FPL_STATUS_PARTIAL:
                cache_fpl_smr_assert_entered(fpl);
                return (cache_fplookup_partial_setup(fpl));
        case CACHE_FPL_STATUS_ABORTED:
                if (fpl->in_smr)
                        cache_fpl_smr_exit(fpl);
                return (CACHE_FPL_FAILED);
        case CACHE_FPL_STATUS_HANDLED:
                MPASS(error != CACHE_FPL_FAILED);
                cache_fpl_smr_assert_not_entered(fpl);
                if (__predict_false(error != 0)) {
                        ndp->ni_dvp = NULL;
                        ndp->ni_vp = NULL;
                        cache_fpl_cleanup_cnp(cnp);
                        return (error);
                }
                ndp->ni_dvp = fpl->dvp;
                ndp->ni_vp = fpl->tvp;
                if (cnp->cn_flags & SAVENAME)
                        cnp->cn_flags |= HASBUF;
                else
                        cache_fpl_cleanup_cnp(cnp);
                return (error);
        }
}

/*
 * Fast path lookup protected with SMR and sequence counters.
 *
 * Note: all VOP_FPLOOKUP_VEXEC routines have a comment referencing this one.
 *
 * Filesystems can opt in by setting the MNTK_FPLOOKUP flag and meeting criteria
 * outlined below.
 *
 * Traditional vnode lookup conceptually looks like this:
 *
 * vn_lock(current);
 * for (;;) {
 *      next = find();
 *      vn_lock(next);
 *      vn_unlock(current);
 *      current = next;
 *      if (last)
 *          break;
 * }
 * return (current);
 *
 * Each jump to the next vnode is safe memory-wise and atomic with respect to
 * any modifications thanks to holding respective locks.
 *
 * The same guarantee can be provided with a combination of safe memory
 * reclamation and sequence counters instead. If all operations which affect
 * the relationship between the current vnode and the one we are looking for
 * also modify the counter, we can verify whether all the conditions held as
 * we made the jump. This includes things like permissions, mount points etc.
 * Counter modification is provided by enclosing relevant places in
 * vn_seqc_write_begin()/end() calls.
 *
 * Thus this translates to:
 *
 * vfs_smr_enter();
 * dvp_seqc = seqc_read_any(dvp);
 * if (seqc_in_modify(dvp_seqc)) // someone is altering the vnode
 *     abort();
 * for (;;) {
 *      tvp = find();
 *      tvp_seqc = seqc_read_any(tvp);
 *      if (seqc_in_modify(tvp_seqc)) // someone is altering the target vnode
 *          abort();
 *      if (!seqc_consistent(dvp, dvp_seqc) // someone is altering the vnode
 *          abort();
 *      dvp = tvp; // we know nothing of importance has changed
 *      dvp_seqc = tvp_seqc; // store the counter for the tvp iteration
 *      if (last)
 *          break;
 * }
 * vget(); // secure the vnode
 * if (!seqc_consistent(tvp, tvp_seqc) // final check
 *          abort();
 * // at this point we know nothing has changed for any parent<->child pair
 * // as they were crossed during the lookup, meaning we matched the guarantee
 * // of the locked variant
 * return (tvp);
 *
 * The API contract for VOP_FPLOOKUP_VEXEC routines is as follows:
 * - they are called while within vfs_smr protection which they must never exit
 * - EAGAIN can be returned to denote checking could not be performed, it is
 *   always valid to return it
 * - if the sequence counter has not changed the result must be valid
 * - if the sequence counter has changed both false positives and false negatives
 *   are permitted (since the result will be rejected later)
 * - for simple cases of unix permission checks vaccess_vexec_smr can be used
 *
 * Caveats to watch out for:
 * - vnodes are passed unlocked and unreferenced with nothing stopping
 *   VOP_RECLAIM, in turn meaning that ->v_data can become NULL. It is advised
 *   to use atomic_load_ptr to fetch it.
 * - the aforementioned object can also get freed, meaning absent other means it
 *   should be protected with vfs_smr
 * - either safely checking permissions as they are modified or guaranteeing
 *   their stability is left to the routine
 */
int
cache_fplookup(struct nameidata *ndp, enum cache_fpl_status *status,
    struct pwd **pwdp)
{
        struct cache_fpl fpl;
        struct pwd *pwd;
        struct vnode *dvp;
        struct componentname *cnp;
        struct nameidata_saved orig;
        int error;

        MPASS(ndp->ni_lcf == 0);

        fpl.status = CACHE_FPL_STATUS_UNSET;
        fpl.ndp = ndp;
        fpl.cnp = &ndp->ni_cnd;
        MPASS(curthread == fpl.cnp->cn_thread);

        if ((fpl.cnp->cn_flags & SAVESTART) != 0)
                MPASS(fpl.cnp->cn_nameiop != LOOKUP);

        if (!cache_can_fplookup(&fpl)) {
                SDT_PROBE3(vfs, fplookup, lookup, done, ndp, fpl.line, fpl.status);
                *status = fpl.status;
                return (EOPNOTSUPP);
        }

        cache_fpl_checkpoint(&fpl, &orig);

        cache_fpl_smr_enter_initial(&fpl);
        fpl.fsearch = false;
        pwd = pwd_get_smr();
        fpl.pwd = pwd;
        ndp->ni_rootdir = pwd->pwd_rdir;
        ndp->ni_topdir = pwd->pwd_jdir;

        cnp = fpl.cnp;
        cnp->cn_nameptr = cnp->cn_pnbuf;
        if (cnp->cn_pnbuf[0] == '/') {
                cache_fpl_handle_root(ndp, &dvp);
        } else {
                if (ndp->ni_dirfd == AT_FDCWD) {
                        dvp = pwd->pwd_cdir;
                } else {
                        error = cache_fplookup_dirfd(&fpl, &dvp);
                        if (__predict_false(error != 0)) {
                                goto out;
                        }
                }
        }

        SDT_PROBE4(vfs, namei, lookup, entry, dvp, cnp->cn_pnbuf, cnp->cn_flags, true);

        error = cache_fplookup_impl(dvp, &fpl);
out:
        cache_fpl_smr_assert_not_entered(&fpl);
        SDT_PROBE3(vfs, fplookup, lookup, done, ndp, fpl.line, fpl.status);

        *status = fpl.status;
        switch (fpl.status) {
        case CACHE_FPL_STATUS_UNSET:
                __assert_unreachable();
                break;
        case CACHE_FPL_STATUS_HANDLED:
                SDT_PROBE3(vfs, namei, lookup, return, error,
                    (error == 0 ? ndp->ni_vp : NULL), true);
                break;
        case CACHE_FPL_STATUS_PARTIAL:
                *pwdp = fpl.pwd;
                /*
                 * Status restored by cache_fplookup_partial_setup.
                 */
                break;
        case CACHE_FPL_STATUS_ABORTED:
                cache_fpl_restore(&fpl, &orig);
                break;
        }
        return (error);
}