bhnd(9): Fix a few mandoc related issues

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

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 2008 Isilon Systems, Inc.
 * Copyright (c) 2008 Ilya Maykov <ivmaykov@gmail.com>
 * Copyright (c) 1998 Berkeley Software Design, Inc.
 * All rights reserved.
 *
 * 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. Berkeley Software Design Inc's name may not be used to endorse or
 *    promote products derived from this software without specific prior
 *    written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY BERKELEY SOFTWARE DESIGN INC ``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 BERKELEY SOFTWARE DESIGN INC 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.
 *
 *      from BSDI $Id: mutex_witness.c,v 1.1.2.20 2000/04/27 03:10:27 cp Exp $
 *      and BSDI $Id: synch_machdep.c,v 2.3.2.39 2000/04/27 03:10:25 cp Exp $
 */

/*
 * Implementation of the `witness' lock verifier.  Originally implemented for
 * mutexes in BSD/OS.  Extended to handle generic lock objects and lock
 * classes in FreeBSD.
 */

/*
 *      Main Entry: witness
 *      Pronunciation: 'wit-n&s
 *      Function: noun
 *      Etymology: Middle English witnesse, from Old English witnes knowledge,
 *          testimony, witness, from 2wit
 *      Date: before 12th century
 *      1 : attestation of a fact or event : TESTIMONY
 *      2 : one that gives evidence; specifically : one who testifies in
 *          a cause or before a judicial tribunal
 *      3 : one asked to be present at a transaction so as to be able to
 *          testify to its having taken place
 *      4 : one who has personal knowledge of something
 *      5 a : something serving as evidence or proof : SIGN
 *        b : public affirmation by word or example of usually
 *            religious faith or conviction <the heroic witness to divine
 *            life -- Pilot>
 *      6 capitalized : a member of the Jehovah's Witnesses 
 */

/*
 * Special rules concerning Giant and lock orders:
 *
 * 1) Giant must be acquired before any other mutexes.  Stated another way,
 *    no other mutex may be held when Giant is acquired.
 *
 * 2) Giant must be released when blocking on a sleepable lock.
 *
 * This rule is less obvious, but is a result of Giant providing the same
 * semantics as spl().  Basically, when a thread sleeps, it must release
 * Giant.  When a thread blocks on a sleepable lock, it sleeps.  Hence rule
 * 2).
 *
 * 3) Giant may be acquired before or after sleepable locks.
 *
 * This rule is also not quite as obvious.  Giant may be acquired after
 * a sleepable lock because it is a non-sleepable lock and non-sleepable
 * locks may always be acquired while holding a sleepable lock.  The second
 * case, Giant before a sleepable lock, follows from rule 2) above.  Suppose
 * you have two threads T1 and T2 and a sleepable lock X.  Suppose that T1
 * acquires X and blocks on Giant.  Then suppose that T2 acquires Giant and
 * blocks on X.  When T2 blocks on X, T2 will release Giant allowing T1 to
 * execute.  Thus, acquiring Giant both before and after a sleepable lock
 * will not result in a lock order reversal.
 */

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

#include "opt_ddb.h"
#include "opt_hwpmc_hooks.h"
#include "opt_stack.h"
#include "opt_witness.h"

#include <sys/param.h>
#include <sys/bus.h>
#include <sys/kdb.h>
#include <sys/kernel.h>
#include <sys/ktr.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mutex.h>
#include <sys/priv.h>
#include <sys/proc.h>
#include <sys/sbuf.h>
#include <sys/sched.h>
#include <sys/stack.h>
#include <sys/sysctl.h>
#include <sys/syslog.h>
#include <sys/systm.h>

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

#include <machine/stdarg.h>

#if !defined(DDB) && !defined(STACK)
#error "DDB or STACK options are required for WITNESS"
#endif

/* Note that these traces do not work with KTR_ALQ. */
#if 0
#define KTR_WITNESS     KTR_SUBSYS
#else
#define KTR_WITNESS     0
#endif

#define LI_RECURSEMASK  0x0000ffff      /* Recursion depth of lock instance. */
#define LI_EXCLUSIVE    0x00010000      /* Exclusive lock instance. */
#define LI_NORELEASE    0x00020000      /* Lock not allowed to be released. */
#define LI_SLEEPABLE    0x00040000      /* Lock may be held while sleeping. */

#ifndef WITNESS_COUNT
#define WITNESS_COUNT           1536
#endif
#define WITNESS_HASH_SIZE       251     /* Prime, gives load factor < 2 */
#define WITNESS_PENDLIST        (512 + (MAXCPU * 4))

/* Allocate 256 KB of stack data space */
#define WITNESS_LO_DATA_COUNT   2048

/* Prime, gives load factor of ~2 at full load */
#define WITNESS_LO_HASH_SIZE    1021

/*
 * XXX: This is somewhat bogus, as we assume here that at most 2048 threads
 * will hold LOCK_NCHILDREN locks.  We handle failure ok, and we should
 * probably be safe for the most part, but it's still a SWAG.
 */
#define LOCK_NCHILDREN  5
#define LOCK_CHILDCOUNT 2048

#define MAX_W_NAME      64

#define FULLGRAPH_SBUF_SIZE     512

/*
 * These flags go in the witness relationship matrix and describe the
 * relationship between any two struct witness objects.
 */
#define WITNESS_UNRELATED        0x00    /* No lock order relation. */
#define WITNESS_PARENT           0x01    /* Parent, aka direct ancestor. */
#define WITNESS_ANCESTOR         0x02    /* Direct or indirect ancestor. */
#define WITNESS_CHILD            0x04    /* Child, aka direct descendant. */
#define WITNESS_DESCENDANT       0x08    /* Direct or indirect descendant. */
#define WITNESS_ANCESTOR_MASK    (WITNESS_PARENT | WITNESS_ANCESTOR)
#define WITNESS_DESCENDANT_MASK  (WITNESS_CHILD | WITNESS_DESCENDANT)
#define WITNESS_RELATED_MASK                                            \
        (WITNESS_ANCESTOR_MASK | WITNESS_DESCENDANT_MASK)
#define WITNESS_REVERSAL         0x10    /* A lock order reversal has been
                                          * observed. */
#define WITNESS_RESERVED1        0x20    /* Unused flag, reserved. */
#define WITNESS_RESERVED2        0x40    /* Unused flag, reserved. */
#define WITNESS_LOCK_ORDER_KNOWN 0x80    /* This lock order is known. */

/* Descendant to ancestor flags */
#define WITNESS_DTOA(x) (((x) & WITNESS_RELATED_MASK) >> 2)

/* Ancestor to descendant flags */
#define WITNESS_ATOD(x) (((x) & WITNESS_RELATED_MASK) << 2)

#define WITNESS_INDEX_ASSERT(i)                                         \
        MPASS((i) > 0 && (i) <= w_max_used_index && (i) < witness_count)

static MALLOC_DEFINE(M_WITNESS, "Witness", "Witness");

/*
 * Lock instances.  A lock instance is the data associated with a lock while
 * it is held by witness.  For example, a lock instance will hold the
 * recursion count of a lock.  Lock instances are held in lists.  Spin locks
 * are held in a per-cpu list while sleep locks are held in per-thread list.
 */
struct lock_instance {
        struct lock_object      *li_lock;
        const char              *li_file;
        int                     li_line;
        u_int                   li_flags;
};

/*
 * A simple list type used to build the list of locks held by a thread
 * or CPU.  We can't simply embed the list in struct lock_object since a
 * lock may be held by more than one thread if it is a shared lock.  Locks
 * are added to the head of the list, so we fill up each list entry from
 * "the back" logically.  To ease some of the arithmetic, we actually fill
 * in each list entry the normal way (children[0] then children[1], etc.) but
 * when we traverse the list we read children[count-1] as the first entry
 * down to children[0] as the final entry.
 */
struct lock_list_entry {
        struct lock_list_entry  *ll_next;
        struct lock_instance    ll_children[LOCK_NCHILDREN];
        u_int                   ll_count;
};

/*
 * The main witness structure. One of these per named lock type in the system
 * (for example, "vnode interlock").
 */
struct witness {
        char                    w_name[MAX_W_NAME];
        uint32_t                w_index;  /* Index in the relationship matrix */
        struct lock_class       *w_class;
        STAILQ_ENTRY(witness)   w_list;         /* List of all witnesses. */
        STAILQ_ENTRY(witness)   w_typelist;     /* Witnesses of a type. */
        struct witness          *w_hash_next; /* Linked list in hash buckets. */
        const char              *w_file; /* File where last acquired */
        uint32_t                w_line; /* Line where last acquired */
        uint32_t                w_refcount;
        uint16_t                w_num_ancestors; /* direct/indirect
                                                  * ancestor count */
        uint16_t                w_num_descendants; /* direct/indirect
                                                    * descendant count */
        int16_t                 w_ddb_level;
        unsigned                w_displayed:1;
        unsigned                w_reversed:1;
};

STAILQ_HEAD(witness_list, witness);

/*
 * The witness hash table. Keys are witness names (const char *), elements are
 * witness objects (struct witness *).
 */
struct witness_hash {
        struct witness  *wh_array[WITNESS_HASH_SIZE];
        uint32_t        wh_size;
        uint32_t        wh_count;
};

/*
 * Key type for the lock order data hash table.
 */
struct witness_lock_order_key {
        uint16_t        from;
        uint16_t        to;
};

struct witness_lock_order_data {
        struct stack                    wlod_stack;
        struct witness_lock_order_key   wlod_key;
        struct witness_lock_order_data  *wlod_next;
};

/*
 * The witness lock order data hash table. Keys are witness index tuples
 * (struct witness_lock_order_key), elements are lock order data objects
 * (struct witness_lock_order_data). 
 */
struct witness_lock_order_hash {
        struct witness_lock_order_data  *wloh_array[WITNESS_LO_HASH_SIZE];
        u_int   wloh_size;
        u_int   wloh_count;
};

struct witness_blessed {
        const char      *b_lock1;
        const char      *b_lock2;
};

struct witness_pendhelp {
        const char              *wh_type;
        struct lock_object      *wh_lock;
};

struct witness_order_list_entry {
        const char              *w_name;
        struct lock_class       *w_class;
};

/*
 * Returns 0 if one of the locks is a spin lock and the other is not.
 * Returns 1 otherwise.
 */
static __inline int
witness_lock_type_equal(struct witness *w1, struct witness *w2)
{

        return ((w1->w_class->lc_flags & (LC_SLEEPLOCK | LC_SPINLOCK)) ==
                (w2->w_class->lc_flags & (LC_SLEEPLOCK | LC_SPINLOCK)));
}

static __inline int
witness_lock_order_key_equal(const struct witness_lock_order_key *a,
    const struct witness_lock_order_key *b)
{

        return (a->from == b->from && a->to == b->to);
}

static int      _isitmyx(struct witness *w1, struct witness *w2, int rmask,
                    const char *fname);
static void     adopt(struct witness *parent, struct witness *child);
static int      blessed(struct witness *, struct witness *);
static void     depart(struct witness *w);
static struct witness   *enroll(const char *description,
                            struct lock_class *lock_class);
static struct lock_instance     *find_instance(struct lock_list_entry *list,
                                    const struct lock_object *lock);
static int      isitmychild(struct witness *parent, struct witness *child);
static int      isitmydescendant(struct witness *parent, struct witness *child);
static void     itismychild(struct witness *parent, struct witness *child);
static int      sysctl_debug_witness_badstacks(SYSCTL_HANDLER_ARGS);
static int      sysctl_debug_witness_watch(SYSCTL_HANDLER_ARGS);
static int      sysctl_debug_witness_fullgraph(SYSCTL_HANDLER_ARGS);
static int      sysctl_debug_witness_channel(SYSCTL_HANDLER_ARGS);
static void     witness_add_fullgraph(struct sbuf *sb, struct witness *parent);
#ifdef DDB
static void     witness_ddb_compute_levels(void);
static void     witness_ddb_display(int(*)(const char *fmt, ...));
static void     witness_ddb_display_descendants(int(*)(const char *fmt, ...),
                    struct witness *, int indent);
static void     witness_ddb_display_list(int(*prnt)(const char *fmt, ...),
                    struct witness_list *list);
static void     witness_ddb_level_descendants(struct witness *parent, int l);
static void     witness_ddb_list(struct thread *td);
#endif
static void     witness_enter_debugger(const char *msg);
static void     witness_debugger(int cond, const char *msg);
static void     witness_free(struct witness *m);
static struct witness   *witness_get(void);
static uint32_t witness_hash_djb2(const uint8_t *key, uint32_t size);
static struct witness   *witness_hash_get(const char *key);
static void     witness_hash_put(struct witness *w);
static void     witness_init_hash_tables(void);
static void     witness_increment_graph_generation(void);
static void     witness_lock_list_free(struct lock_list_entry *lle);
static struct lock_list_entry   *witness_lock_list_get(void);
static int      witness_lock_order_add(struct witness *parent,
                    struct witness *child);
static int      witness_lock_order_check(struct witness *parent,
                    struct witness *child);
static struct witness_lock_order_data   *witness_lock_order_get(
                                            struct witness *parent,
                                            struct witness *child);
static void     witness_list_lock(struct lock_instance *instance,
                    int (*prnt)(const char *fmt, ...));
static int      witness_output(const char *fmt, ...) __printflike(1, 2);
static int      witness_output_drain(void *arg __unused, const char *data,
                    int len);
static int      witness_voutput(const char *fmt, va_list ap) __printflike(1, 0);
static void     witness_setflag(struct lock_object *lock, int flag, int set);

FEATURE(witness, "kernel has witness(9) support");

static SYSCTL_NODE(_debug, OID_AUTO, witness, CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
    "Witness Locking");

/*
 * If set to 0, lock order checking is disabled.  If set to -1,
 * witness is completely disabled.  Otherwise witness performs full
 * lock order checking for all locks.  At runtime, lock order checking
 * may be toggled.  However, witness cannot be reenabled once it is
 * completely disabled.
 */
static int witness_watch = 1;
SYSCTL_PROC(_debug_witness, OID_AUTO, watch,
    CTLFLAG_RWTUN | CTLTYPE_INT | CTLFLAG_MPSAFE, NULL, 0,
    sysctl_debug_witness_watch, "I",
    "witness is watching lock operations");

#ifdef KDB
/*
 * When KDB is enabled and witness_kdb is 1, it will cause the system
 * to drop into kdebug() when:
 *      - a lock hierarchy violation occurs
 *      - locks are held when going to sleep.
 */
#ifdef WITNESS_KDB
int     witness_kdb = 1;
#else
int     witness_kdb = 0;
#endif
SYSCTL_INT(_debug_witness, OID_AUTO, kdb, CTLFLAG_RWTUN, &witness_kdb, 0, "");
#endif /* KDB */

#if defined(DDB) || defined(KDB)
/*
 * When DDB or KDB is enabled and witness_trace is 1, it will cause the system
 * to print a stack trace:
 *      - a lock hierarchy violation occurs
 *      - locks are held when going to sleep.
 */
int     witness_trace = 1;
SYSCTL_INT(_debug_witness, OID_AUTO, trace, CTLFLAG_RWTUN, &witness_trace, 0, "");
#endif /* DDB || KDB */

#ifdef WITNESS_SKIPSPIN
int     witness_skipspin = 1;
#else
int     witness_skipspin = 0;
#endif
SYSCTL_INT(_debug_witness, OID_AUTO, skipspin, CTLFLAG_RDTUN, &witness_skipspin, 0, "");

int badstack_sbuf_size;

int witness_count = WITNESS_COUNT;
SYSCTL_INT(_debug_witness, OID_AUTO, witness_count, CTLFLAG_RDTUN, 
    &witness_count, 0, "");

/*
 * Output channel for witness messages.  By default we print to the console.
 */
enum witness_channel {
        WITNESS_CONSOLE,
        WITNESS_LOG,
        WITNESS_NONE,
};

static enum witness_channel witness_channel = WITNESS_CONSOLE;
SYSCTL_PROC(_debug_witness, OID_AUTO, output_channel,
    CTLTYPE_STRING | CTLFLAG_RWTUN | CTLFLAG_MPSAFE, NULL, 0,
    sysctl_debug_witness_channel, "A",
    "Output channel for warnings");

/*
 * Call this to print out the relations between locks.
 */
SYSCTL_PROC(_debug_witness, OID_AUTO, fullgraph,
    CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_MPSAFE, NULL, 0,
    sysctl_debug_witness_fullgraph, "A",
    "Show locks relation graphs");

/*
 * Call this to print out the witness faulty stacks.
 */
SYSCTL_PROC(_debug_witness, OID_AUTO, badstacks,
    CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_MPSAFE, NULL, 0,
    sysctl_debug_witness_badstacks, "A",
    "Show bad witness stacks");

static struct mtx w_mtx;

/* w_list */
static struct witness_list w_free = STAILQ_HEAD_INITIALIZER(w_free);
static struct witness_list w_all = STAILQ_HEAD_INITIALIZER(w_all);

/* w_typelist */
static struct witness_list w_spin = STAILQ_HEAD_INITIALIZER(w_spin);
static struct witness_list w_sleep = STAILQ_HEAD_INITIALIZER(w_sleep);

/* lock list */
static struct lock_list_entry *w_lock_list_free = NULL;
static struct witness_pendhelp pending_locks[WITNESS_PENDLIST];
static u_int pending_cnt;

static int w_free_cnt, w_spin_cnt, w_sleep_cnt;
SYSCTL_INT(_debug_witness, OID_AUTO, free_cnt, CTLFLAG_RD, &w_free_cnt, 0, "");
SYSCTL_INT(_debug_witness, OID_AUTO, spin_cnt, CTLFLAG_RD, &w_spin_cnt, 0, "");
SYSCTL_INT(_debug_witness, OID_AUTO, sleep_cnt, CTLFLAG_RD, &w_sleep_cnt, 0,
    "");

static struct witness *w_data;
static uint8_t **w_rmatrix;
static struct lock_list_entry w_locklistdata[LOCK_CHILDCOUNT];
static struct witness_hash w_hash;      /* The witness hash table. */

/* The lock order data hash */
static struct witness_lock_order_data w_lodata[WITNESS_LO_DATA_COUNT];
static struct witness_lock_order_data *w_lofree = NULL;
static struct witness_lock_order_hash w_lohash;
static int w_max_used_index = 0;
static unsigned int w_generation = 0;
static const char w_notrunning[] = "Witness not running\n";
static const char w_stillcold[] = "Witness is still cold\n";
#ifdef __i386__
static const char w_notallowed[] = "The sysctl is disabled on the arch\n";
#endif

static struct witness_order_list_entry order_lists[] = {
        /*
         * sx locks
         */
        { "proctree", &lock_class_sx },
        { "allproc", &lock_class_sx },
        { "allprison", &lock_class_sx },
        { NULL, NULL },
        /*
         * Various mutexes
         */
        { "Giant", &lock_class_mtx_sleep },
        { "pipe mutex", &lock_class_mtx_sleep },
        { "sigio lock", &lock_class_mtx_sleep },
        { "process group", &lock_class_mtx_sleep },
#ifdef  HWPMC_HOOKS
        { "pmc-sleep", &lock_class_mtx_sleep },
#endif
        { "process lock", &lock_class_mtx_sleep },
        { "session", &lock_class_mtx_sleep },
        { "uidinfo hash", &lock_class_rw },
        { "time lock", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * umtx
         */
        { "umtx lock", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * Sockets
         */
        { "accept", &lock_class_mtx_sleep },
        { "so_snd", &lock_class_mtx_sleep },
        { "so_rcv", &lock_class_mtx_sleep },
        { "sellck", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * Routing
         */
        { "so_rcv", &lock_class_mtx_sleep },
        { "radix node head", &lock_class_rm },
        { "ifaddr", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * IPv4 multicast:
         * protocol locks before interface locks, after UDP locks.
         */
        { "in_multi_sx", &lock_class_sx },
        { "udpinp", &lock_class_rw },
        { "in_multi_list_mtx", &lock_class_mtx_sleep },
        { "igmp_mtx", &lock_class_mtx_sleep },
        { "ifnet_rw", &lock_class_rw },
        { "if_addr_lock", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * IPv6 multicast:
         * protocol locks before interface locks, after UDP locks.
         */
        { "in6_multi_sx", &lock_class_sx },
        { "udpinp", &lock_class_rw },
        { "in6_multi_list_mtx", &lock_class_mtx_sleep },
        { "mld_mtx", &lock_class_mtx_sleep },
        { "ifnet_rw", &lock_class_rw },
        { "if_addr_lock", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * UNIX Domain Sockets
         */
        { "unp_link_rwlock", &lock_class_rw },
        { "unp_list_lock", &lock_class_mtx_sleep },
        { "unp", &lock_class_mtx_sleep },
        { "so_snd", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * UDP/IP
         */
        { "udp", &lock_class_mtx_sleep },
        { "udpinp", &lock_class_rw },
        { "so_snd", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * TCP/IP
         */
        { "tcp", &lock_class_mtx_sleep },
        { "tcpinp", &lock_class_rw },
        { "so_snd", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * BPF
         */
        { "bpf global lock", &lock_class_sx },
        { "bpf cdev lock", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * NFS server
         */
        { "nfsd_mtx", &lock_class_mtx_sleep },
        { "so_snd", &lock_class_mtx_sleep },
        { NULL, NULL },

        /*
         * IEEE 802.11
         */
        { "802.11 com lock", &lock_class_mtx_sleep},
        { NULL, NULL },
        /*
         * Network drivers
         */
        { "network driver", &lock_class_mtx_sleep},
        { NULL, NULL },

        /*
         * Netgraph
         */
        { "ng_node", &lock_class_mtx_sleep },
        { "ng_worklist", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * CDEV
         */
        { "vm map (system)", &lock_class_mtx_sleep },
        { "vnode interlock", &lock_class_mtx_sleep },
        { "cdev", &lock_class_mtx_sleep },
        { "devthrd", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * VM
         */
        { "vm map (user)", &lock_class_sx },
        { "vm object", &lock_class_rw },
        { "vm page", &lock_class_mtx_sleep },
        { "pmap pv global", &lock_class_rw },
        { "pmap", &lock_class_mtx_sleep },
        { "pmap pv list", &lock_class_rw },
        { "vm page free queue", &lock_class_mtx_sleep },
        { "vm pagequeue", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * kqueue/VFS interaction
         */
        { "kqueue", &lock_class_mtx_sleep },
        { "struct mount mtx", &lock_class_mtx_sleep },
        { "vnode interlock", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * VFS namecache
         */
        { "ncvn", &lock_class_mtx_sleep },
        { "ncbuc", &lock_class_mtx_sleep },
        { "vnode interlock", &lock_class_mtx_sleep },
        { "ncneg", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * ZFS locking
         */
        { "dn->dn_mtx", &lock_class_sx },
        { "dr->dt.di.dr_mtx", &lock_class_sx },
        { "db->db_mtx", &lock_class_sx },
        { NULL, NULL },
        /*
         * TCP log locks
         */
        { "TCP ID tree", &lock_class_rw },
        { "tcp log id bucket", &lock_class_mtx_sleep },
        { "tcpinp", &lock_class_rw },
        { "TCP log expireq", &lock_class_mtx_sleep },
        { NULL, NULL },
        /*
         * spin locks
         */
#ifdef SMP
        { "ap boot", &lock_class_mtx_spin },
#endif
        { "rm.mutex_mtx", &lock_class_mtx_spin },
        { "sio", &lock_class_mtx_spin },
#ifdef __i386__
        { "cy", &lock_class_mtx_spin },
#endif
        { "scc_hwmtx", &lock_class_mtx_spin },
        { "uart_hwmtx", &lock_class_mtx_spin },
        { "fast_taskqueue", &lock_class_mtx_spin },
        { "intr table", &lock_class_mtx_spin },
        { "process slock", &lock_class_mtx_spin },
        { "syscons video lock", &lock_class_mtx_spin },
        { "sleepq chain", &lock_class_mtx_spin },
        { "rm_spinlock", &lock_class_mtx_spin },
        { "turnstile chain", &lock_class_mtx_spin },
        { "turnstile lock", &lock_class_mtx_spin },
        { "sched lock", &lock_class_mtx_spin },
        { "td_contested", &lock_class_mtx_spin },
        { "callout", &lock_class_mtx_spin },
        { "entropy harvest mutex", &lock_class_mtx_spin },
#ifdef SMP
        { "smp rendezvous", &lock_class_mtx_spin },
#endif
#ifdef __powerpc__
        { "tlb0", &lock_class_mtx_spin },
#endif
        { NULL, NULL },
        { "sched lock", &lock_class_mtx_spin },
#ifdef  HWPMC_HOOKS
        { "pmc-per-proc", &lock_class_mtx_spin },
#endif
        { NULL, NULL },
        /*
         * leaf locks
         */
        { "intrcnt", &lock_class_mtx_spin },
        { "icu", &lock_class_mtx_spin },
#ifdef __i386__
        { "allpmaps", &lock_class_mtx_spin },
        { "descriptor tables", &lock_class_mtx_spin },
#endif
        { "clk", &lock_class_mtx_spin },
        { "cpuset", &lock_class_mtx_spin },
        { "mprof lock", &lock_class_mtx_spin },
        { "zombie lock", &lock_class_mtx_spin },
        { "ALD Queue", &lock_class_mtx_spin },
#if defined(__i386__) || defined(__amd64__)
        { "pcicfg", &lock_class_mtx_spin },
        { "NDIS thread lock", &lock_class_mtx_spin },
#endif
        { "tw_osl_io_lock", &lock_class_mtx_spin },
        { "tw_osl_q_lock", &lock_class_mtx_spin },
        { "tw_cl_io_lock", &lock_class_mtx_spin },
        { "tw_cl_intr_lock", &lock_class_mtx_spin },
        { "tw_cl_gen_lock", &lock_class_mtx_spin },
#ifdef  HWPMC_HOOKS
        { "pmc-leaf", &lock_class_mtx_spin },
#endif
        { "blocked lock", &lock_class_mtx_spin },
        { NULL, NULL },
        { NULL, NULL }
};

/*
 * Pairs of locks which have been blessed.  Witness does not complain about
 * order problems with blessed lock pairs.  Please do not add an entry to the
 * table without an explanatory comment.
 */
static struct witness_blessed blessed_list[] = {
        /*
         * See the comment in ufs_dirhash.c.  Basically, a vnode lock serializes
         * both lock orders, so a deadlock cannot happen as a result of this
         * LOR.
         */
        { "dirhash",    "bufwait" },

        /*
         * A UFS vnode may be locked in vget() while a buffer belonging to the
         * parent directory vnode is locked.
         */
        { "ufs",        "bufwait" },
};

/*
 * This global is set to 0 once it becomes safe to use the witness code.
 */
static int witness_cold = 1;

/*
 * This global is set to 1 once the static lock orders have been enrolled
 * so that a warning can be issued for any spin locks enrolled later.
 */
static int witness_spin_warn = 0;

/* Trim useless garbage from filenames. */
static const char *
fixup_filename(const char *file)
{

        if (file == NULL)
                return (NULL);
        while (strncmp(file, "../", 3) == 0)
                file += 3;
        return (file);
}

/*
 * Calculate the size of early witness structures.
 */
int
witness_startup_count(void)
{
        int sz;

        sz = sizeof(struct witness) * witness_count;
        sz += sizeof(*w_rmatrix) * (witness_count + 1);
        sz += sizeof(*w_rmatrix[0]) * (witness_count + 1) *
            (witness_count + 1);

        return (sz);
}

/*
 * The WITNESS-enabled diagnostic code.  Note that the witness code does
 * assume that the early boot is single-threaded at least until after this
 * routine is completed.
 */
void
witness_startup(void *mem)
{
        struct lock_object *lock;
        struct witness_order_list_entry *order;
        struct witness *w, *w1;
        uintptr_t p;
        int i;

        p = (uintptr_t)mem;
        w_data = (void *)p;
        p += sizeof(struct witness) * witness_count;

        w_rmatrix = (void *)p;
        p += sizeof(*w_rmatrix) * (witness_count + 1);

        for (i = 0; i < witness_count + 1; i++) {
                w_rmatrix[i] = (void *)p;
                p += sizeof(*w_rmatrix[i]) * (witness_count + 1);
        }
        badstack_sbuf_size = witness_count * 256;

        /*
         * We have to release Giant before initializing its witness
         * structure so that WITNESS doesn't get confused.
         */
        mtx_unlock(&Giant);
        mtx_assert(&Giant, MA_NOTOWNED);

        CTR1(KTR_WITNESS, "%s: initializing witness", __func__);
        mtx_init(&w_mtx, "witness lock", NULL, MTX_SPIN | MTX_QUIET |
            MTX_NOWITNESS | MTX_NOPROFILE);
        for (i = witness_count - 1; i >= 0; i--) {
                w = &w_data[i];
                memset(w, 0, sizeof(*w));
                w_data[i].w_index = i;  /* Witness index never changes. */
                witness_free(w);
        }
        KASSERT(STAILQ_FIRST(&w_free)->w_index == 0,
            ("%s: Invalid list of free witness objects", __func__));

        /* Witness with index 0 is not used to aid in debugging. */
        STAILQ_REMOVE_HEAD(&w_free, w_list);
        w_free_cnt--;

        for (i = 0; i < witness_count; i++) {
                memset(w_rmatrix[i], 0, sizeof(*w_rmatrix[i]) * 
                    (witness_count + 1));
        }

        for (i = 0; i < LOCK_CHILDCOUNT; i++)
                witness_lock_list_free(&w_locklistdata[i]);
        witness_init_hash_tables();

        /* First add in all the specified order lists. */
        for (order = order_lists; order->w_name != NULL; order++) {
                w = enroll(order->w_name, order->w_class);
                if (w == NULL)
                        continue;
                w->w_file = "order list";
                for (order++; order->w_name != NULL; order++) {
                        w1 = enroll(order->w_name, order->w_class);
                        if (w1 == NULL)
                                continue;
                        w1->w_file = "order list";
                        itismychild(w, w1);
                        w = w1;
                }
        }
        witness_spin_warn = 1;

        /* Iterate through all locks and add them to witness. */
        for (i = 0; pending_locks[i].wh_lock != NULL; i++) {
                lock = pending_locks[i].wh_lock;
                KASSERT(lock->lo_flags & LO_WITNESS,
                    ("%s: lock %s is on pending list but not LO_WITNESS",
                    __func__, lock->lo_name));
                lock->lo_witness = enroll(pending_locks[i].wh_type,
                    LOCK_CLASS(lock));
        }

        /* Mark the witness code as being ready for use. */
        witness_cold = 0;

        mtx_lock(&Giant);
}

void
witness_init(struct lock_object *lock, const char *type)
{
        struct lock_class *class;

        /* Various sanity checks. */
        class = LOCK_CLASS(lock);
        if ((lock->lo_flags & LO_RECURSABLE) != 0 &&
            (class->lc_flags & LC_RECURSABLE) == 0)
                kassert_panic("%s: lock (%s) %s can not be recursable",
                    __func__, class->lc_name, lock->lo_name);
        if ((lock->lo_flags & LO_SLEEPABLE) != 0 &&
            (class->lc_flags & LC_SLEEPABLE) == 0)
                kassert_panic("%s: lock (%s) %s can not be sleepable",
                    __func__, class->lc_name, lock->lo_name);
        if ((lock->lo_flags & LO_UPGRADABLE) != 0 &&
            (class->lc_flags & LC_UPGRADABLE) == 0)
                kassert_panic("%s: lock (%s) %s can not be upgradable",
                    __func__, class->lc_name, lock->lo_name);

        /*
         * If we shouldn't watch this lock, then just clear lo_witness.
         * Otherwise, if witness_cold is set, then it is too early to
         * enroll this lock, so defer it to witness_initialize() by adding
         * it to the pending_locks list.  If it is not too early, then enroll
         * the lock now.
         */
        if (witness_watch < 1 || KERNEL_PANICKED() ||
            (lock->lo_flags & LO_WITNESS) == 0)
                lock->lo_witness = NULL;
        else if (witness_cold) {
                pending_locks[pending_cnt].wh_lock = lock;
                pending_locks[pending_cnt++].wh_type = type;
                if (pending_cnt > WITNESS_PENDLIST)
                        panic("%s: pending locks list is too small, "
                            "increase WITNESS_PENDLIST\n",
                            __func__);
        } else
                lock->lo_witness = enroll(type, class);
}

void
witness_destroy(struct lock_object *lock)
{
        struct lock_class *class;
        struct witness *w;

        class = LOCK_CLASS(lock);

        if (witness_cold)
                panic("lock (%s) %s destroyed while witness_cold",
                    class->lc_name, lock->lo_name);

        /* XXX: need to verify that no one holds the lock */
        if ((lock->lo_flags & LO_WITNESS) == 0 || lock->lo_witness == NULL)
                return;
        w = lock->lo_witness;

        mtx_lock_spin(&w_mtx);
        MPASS(w->w_refcount > 0);
        w->w_refcount--;

        if (w->w_refcount == 0)
                depart(w);
        mtx_unlock_spin(&w_mtx);
}

#ifdef DDB
static void
witness_ddb_compute_levels(void)
{
        struct witness *w;

        /*
         * First clear all levels.
         */
        STAILQ_FOREACH(w, &w_all, w_list)
                w->w_ddb_level = -1;

        /*
         * Look for locks with no parents and level all their descendants.
         */
        STAILQ_FOREACH(w, &w_all, w_list) {
                /* If the witness has ancestors (is not a root), skip it. */
                if (w->w_num_ancestors > 0)
                        continue;
                witness_ddb_level_descendants(w, 0);
        }
}

static void
witness_ddb_level_descendants(struct witness *w, int l)
{
        int i;

        if (w->w_ddb_level >= l)
                return;

        w->w_ddb_level = l;
        l++;

        for (i = 1; i <= w_max_used_index; i++) {
                if (w_rmatrix[w->w_index][i] & WITNESS_PARENT)
                        witness_ddb_level_descendants(&w_data[i], l);
        }
}

static void
witness_ddb_display_descendants(int(*prnt)(const char *fmt, ...),
    struct witness *w, int indent)
{
        int i;

        for (i = 0; i < indent; i++)
                prnt(" ");
        prnt("%s (type: %s, depth: %d, active refs: %d)",
             w->w_name, w->w_class->lc_name,
             w->w_ddb_level, w->w_refcount);
        if (w->w_displayed) {
                prnt(" -- (already displayed)\n");
                return;
        }
        w->w_displayed = 1;
        if (w->w_file != NULL && w->w_line != 0)
                prnt(" -- last acquired @ %s:%d\n", fixup_filename(w->w_file),
                    w->w_line);
        else
                prnt(" -- never acquired\n");
        indent++;
        WITNESS_INDEX_ASSERT(w->w_index);
        for (i = 1; i <= w_max_used_index; i++) {
                if (db_pager_quit)
                        return;
                if (w_rmatrix[w->w_index][i] & WITNESS_PARENT)
                        witness_ddb_display_descendants(prnt, &w_data[i],
                            indent);
        }
}

static void
witness_ddb_display_list(int(*prnt)(const char *fmt, ...),
    struct witness_list *list)
{
        struct witness *w;

        STAILQ_FOREACH(w, list, w_typelist) {
                if (w->w_file == NULL || w->w_ddb_level > 0)
                        continue;

                /* This lock has no anscestors - display its descendants. */
                witness_ddb_display_descendants(prnt, w, 0);
                if (db_pager_quit)
                        return;
        }
}

static void
witness_ddb_display(int(*prnt)(const char *fmt, ...))
{
        struct witness *w;

        KASSERT(witness_cold == 0, ("%s: witness_cold", __func__));
        witness_ddb_compute_levels();

        /* Clear all the displayed flags. */
        STAILQ_FOREACH(w, &w_all, w_list)
                w->w_displayed = 0;

        /*
         * First, handle sleep locks which have been acquired at least
         * once.
         */
        prnt("Sleep locks:\n");
        witness_ddb_display_list(prnt, &w_sleep);
        if (db_pager_quit)
                return;

        /*
         * Now do spin locks which have been acquired at least once.
         */
        prnt("\nSpin locks:\n");
        witness_ddb_display_list(prnt, &w_spin);
        if (db_pager_quit)
                return;

        /*
         * Finally, any locks which have not been acquired yet.
         */
        prnt("\nLocks which were never acquired:\n");
        STAILQ_FOREACH(w, &w_all, w_list) {
                if (w->w_file != NULL || w->w_refcount == 0)
                        continue;
                prnt("%s (type: %s, depth: %d)\n", w->w_name,
                    w->w_class->lc_name, w->w_ddb_level);
                if (db_pager_quit)
                        return;
        }
}
#endif /* DDB */

int
witness_defineorder(struct lock_object *lock1, struct lock_object *lock2)
{

        if (witness_watch == -1 || KERNEL_PANICKED())
                return (0);

        /* Require locks that witness knows about. */
        if (lock1 == NULL || lock1->lo_witness == NULL || lock2 == NULL ||
            lock2->lo_witness == NULL)
                return (EINVAL);

        mtx_assert(&w_mtx, MA_NOTOWNED);
        mtx_lock_spin(&w_mtx);

        /*
         * If we already have either an explicit or implied lock order that
         * is the other way around, then return an error.
         */
        if (witness_watch &&
            isitmydescendant(lock2->lo_witness, lock1->lo_witness)) {
                mtx_unlock_spin(&w_mtx);
                return (EDOOFUS);
        }

        /* Try to add the new order. */
        CTR3(KTR_WITNESS, "%s: adding %s as a child of %s", __func__,
            lock2->lo_witness->w_name, lock1->lo_witness->w_name);
        itismychild(lock1->lo_witness, lock2->lo_witness);
        mtx_unlock_spin(&w_mtx);
        return (0);
}

void
witness_checkorder(struct lock_object *lock, int flags, const char *file,
    int line, struct lock_object *interlock)
{
        struct lock_list_entry *lock_list, *lle;
        struct lock_instance *lock1, *lock2, *plock;
        struct lock_class *class, *iclass;
        struct witness *w, *w1;
        struct thread *td;
        int i, j;

        if (witness_cold || witness_watch < 1 || lock->lo_witness == NULL ||
            KERNEL_PANICKED())
                return;

        w = lock->lo_witness;
        class = LOCK_CLASS(lock);
        td = curthread;

        if (class->lc_flags & LC_SLEEPLOCK) {
                /*
                 * Since spin locks include a critical section, this check
                 * implicitly enforces a lock order of all sleep locks before
                 * all spin locks.
                 */
                if (td->td_critnest != 0 && !kdb_active)
                        kassert_panic("acquiring blockable sleep lock with "
                            "spinlock or critical section held (%s) %s @ %s:%d",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);

                /*
                 * If this is the first lock acquired then just return as
                 * no order checking is needed.
                 */
                lock_list = td->td_sleeplocks;
                if (lock_list == NULL || lock_list->ll_count == 0)
                        return;
        } else {
                /*
                 * If this is the first lock, just return as no order
                 * checking is needed.  Avoid problems with thread
                 * migration pinning the thread while checking if
                 * spinlocks are held.  If at least one spinlock is held
                 * the thread is in a safe path and it is allowed to
                 * unpin it.
                 */
                sched_pin();
                lock_list = PCPU_GET(spinlocks);
                if (lock_list == NULL || lock_list->ll_count == 0) {
                        sched_unpin();
                        return;
                }
                sched_unpin();
        }

        /*
         * Check to see if we are recursing on a lock we already own.  If
         * so, make sure that we don't mismatch exclusive and shared lock
         * acquires.
         */
        lock1 = find_instance(lock_list, lock);
        if (lock1 != NULL) {
                if ((lock1->li_flags & LI_EXCLUSIVE) != 0 &&
                    (flags & LOP_EXCLUSIVE) == 0) {
                        witness_output("shared lock of (%s) %s @ %s:%d\n",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                        witness_output("while exclusively locked from %s:%d\n",
                            fixup_filename(lock1->li_file), lock1->li_line);
                        kassert_panic("excl->share");
                }
                if ((lock1->li_flags & LI_EXCLUSIVE) == 0 &&
                    (flags & LOP_EXCLUSIVE) != 0) {
                        witness_output("exclusive lock of (%s) %s @ %s:%d\n",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                        witness_output("while share locked from %s:%d\n",
                            fixup_filename(lock1->li_file), lock1->li_line);
                        kassert_panic("share->excl");
                }
                return;
        }

        /* Warn if the interlock is not locked exactly once. */
        if (interlock != NULL) {
                iclass = LOCK_CLASS(interlock);
                lock1 = find_instance(lock_list, interlock);
                if (lock1 == NULL)
                        kassert_panic("interlock (%s) %s not locked @ %s:%d",
                            iclass->lc_name, interlock->lo_name,
                            fixup_filename(file), line);
                else if ((lock1->li_flags & LI_RECURSEMASK) != 0)
                        kassert_panic("interlock (%s) %s recursed @ %s:%d",
                            iclass->lc_name, interlock->lo_name,
                            fixup_filename(file), line);
        }

        /*
         * Find the previously acquired lock, but ignore interlocks.
         */
        plock = &lock_list->ll_children[lock_list->ll_count - 1];
        if (interlock != NULL && plock->li_lock == interlock) {
                if (lock_list->ll_count > 1)
                        plock =
                            &lock_list->ll_children[lock_list->ll_count - 2];
                else {
                        lle = lock_list->ll_next;

                        /*
                         * The interlock is the only lock we hold, so
                         * simply return.
                         */
                        if (lle == NULL)
                                return;
                        plock = &lle->ll_children[lle->ll_count - 1];
                }
        }

        /*
         * Try to perform most checks without a lock.  If this succeeds we
         * can skip acquiring the lock and return success.  Otherwise we redo
         * the check with the lock held to handle races with concurrent updates.
         */
        w1 = plock->li_lock->lo_witness;
        if (witness_lock_order_check(w1, w))
                return;

        mtx_lock_spin(&w_mtx);
        if (witness_lock_order_check(w1, w)) {
                mtx_unlock_spin(&w_mtx);
                return;
        }
        witness_lock_order_add(w1, w);

        /*
         * Check for duplicate locks of the same type.  Note that we only
         * have to check for this on the last lock we just acquired.  Any
         * other cases will be caught as lock order violations.
         */
        if (w1 == w) {
                i = w->w_index;
                if (!(lock->lo_flags & LO_DUPOK) && !(flags & LOP_DUPOK) &&
                    !(w_rmatrix[i][i] & WITNESS_REVERSAL)) {
                    w_rmatrix[i][i] |= WITNESS_REVERSAL;
                        w->w_reversed = 1;
                        mtx_unlock_spin(&w_mtx);
                        witness_output(
                            "acquiring duplicate lock of same type: \"%s\"\n", 
                            w->w_name);
                        witness_output(" 1st %s @ %s:%d\n", plock->li_lock->lo_name,
                            fixup_filename(plock->li_file), plock->li_line);
                        witness_output(" 2nd %s @ %s:%d\n", lock->lo_name,
                            fixup_filename(file), line);
                        witness_debugger(1, __func__);
                } else
                        mtx_unlock_spin(&w_mtx);
                return;
        }
        mtx_assert(&w_mtx, MA_OWNED);

        /*
         * If we know that the lock we are acquiring comes after
         * the lock we most recently acquired in the lock order tree,
         * then there is no need for any further checks.
         */
        if (isitmychild(w1, w))
                goto out;

        for (j = 0, lle = lock_list; lle != NULL; lle = lle->ll_next) {
                for (i = lle->ll_count - 1; i >= 0; i--, j++) {
                        struct stack pstack;
                        bool pstackv, trace;

                        MPASS(j < LOCK_CHILDCOUNT * LOCK_NCHILDREN);
                        lock1 = &lle->ll_children[i];

                        /*
                         * Ignore the interlock.
                         */
                        if (interlock == lock1->li_lock)
                                continue;

                        /*
                         * If this lock doesn't undergo witness checking,
                         * then skip it.
                         */
                        w1 = lock1->li_lock->lo_witness;
                        if (w1 == NULL) {
                                KASSERT((lock1->li_lock->lo_flags & LO_WITNESS) == 0,
                                    ("lock missing witness structure"));
                                continue;
                        }

                        /*
                         * If we are locking Giant and this is a sleepable
                         * lock, then skip it.
                         */
                        if ((lock1->li_flags & LI_SLEEPABLE) != 0 &&
                            lock == &Giant.lock_object)
                                continue;

                        /*
                         * If we are locking a sleepable lock and this lock
                         * is Giant, then skip it.
                         */
                        if ((lock->lo_flags & LO_SLEEPABLE) != 0 &&
                            (flags & LOP_NOSLEEP) == 0 &&
                            lock1->li_lock == &Giant.lock_object)
                                continue;

                        /*
                         * If we are locking a sleepable lock and this lock
                         * isn't sleepable, we want to treat it as a lock
                         * order violation to enfore a general lock order of
                         * sleepable locks before non-sleepable locks.
                         */
                        if ((lock->lo_flags & LO_SLEEPABLE) != 0 &&
                            (flags & LOP_NOSLEEP) == 0 &&
                            (lock1->li_flags & LI_SLEEPABLE) == 0)
                                goto reversal;

                        /*
                         * If we are locking Giant and this is a non-sleepable
                         * lock, then treat it as a reversal.
                         */
                        if ((lock1->li_flags & LI_SLEEPABLE) == 0 &&
                            lock == &Giant.lock_object)
                                goto reversal;

                        /*
                         * Check the lock order hierarchy for a reveresal.
                         */
                        if (!isitmydescendant(w, w1))
                                continue;
                reversal:

                        /*
                         * We have a lock order violation, check to see if it
                         * is allowed or has already been yelled about.
                         */

                        /* Bail if this violation is known */
                        if (w_rmatrix[w1->w_index][w->w_index] & WITNESS_REVERSAL)
                                goto out;

                        /* Record this as a violation */
                        w_rmatrix[w1->w_index][w->w_index] |= WITNESS_REVERSAL;
                        w_rmatrix[w->w_index][w1->w_index] |= WITNESS_REVERSAL;
                        w->w_reversed = w1->w_reversed = 1;
                        witness_increment_graph_generation();

                        /*
                         * If the lock order is blessed, bail before logging
                         * anything.  We don't look for other lock order
                         * violations though, which may be a bug.
                         */
                        if (blessed(w, w1))
                                goto out;

                        trace = atomic_load_int(&witness_trace);
                        if (trace) {
                                struct witness_lock_order_data *data;

                                pstackv = false;
                                data = witness_lock_order_get(w, w1);
                                if (data != NULL) {
                                        stack_copy(&data->wlod_stack,
                                            &pstack);
                                        pstackv = true;
                                }
                        }
                        mtx_unlock_spin(&w_mtx);

#ifdef WITNESS_NO_VNODE
                        /*
                         * There are known LORs between VNODE locks. They are
                         * not an indication of a bug. VNODE locks are flagged
                         * as such (LO_IS_VNODE) and we don't yell if the LOR
                         * is between 2 VNODE locks.
                         */
                        if ((lock->lo_flags & LO_IS_VNODE) != 0 &&
                            (lock1->li_lock->lo_flags & LO_IS_VNODE) != 0)
                                return;
#endif

                        /*
                         * Ok, yell about it.
                         */
                        if ((lock->lo_flags & LO_SLEEPABLE) != 0 &&
                            (flags & LOP_NOSLEEP) == 0 &&
                            (lock1->li_flags & LI_SLEEPABLE) == 0)
                                witness_output(
                "lock order reversal: (sleepable after non-sleepable)\n");
                        else if ((lock1->li_flags & LI_SLEEPABLE) == 0
                            && lock == &Giant.lock_object)
                                witness_output(
                "lock order reversal: (Giant after non-sleepable)\n");
                        else
                                witness_output("lock order reversal:\n");

                        /*
                         * Try to locate an earlier lock with
                         * witness w in our list.
                         */
                        do {
                                lock2 = &lle->ll_children[i];
                                MPASS(lock2->li_lock != NULL);
                                if (lock2->li_lock->lo_witness == w)
                                        break;
                                if (i == 0 && lle->ll_next != NULL) {
                                        lle = lle->ll_next;
                                        i = lle->ll_count - 1;
                                        MPASS(i >= 0 && i < LOCK_NCHILDREN);
                                } else
                                        i--;
                        } while (i >= 0);
                        if (i < 0) {
                                witness_output(" 1st %p %s (%s, %s) @ %s:%d\n",
                                    lock1->li_lock, lock1->li_lock->lo_name,
                                    w1->w_name, w1->w_class->lc_name,
                                    fixup_filename(lock1->li_file),
                                    lock1->li_line);
                                witness_output(" 2nd %p %s (%s, %s) @ %s:%d\n",
                                    lock, lock->lo_name, w->w_name,
                                    w->w_class->lc_name, fixup_filename(file),
                                    line);
                        } else {
                                struct witness *w2 = lock2->li_lock->lo_witness;

                                witness_output(" 1st %p %s (%s, %s) @ %s:%d\n",
                                    lock2->li_lock, lock2->li_lock->lo_name,
                                    w2->w_name, w2->w_class->lc_name,
                                    fixup_filename(lock2->li_file),
                                    lock2->li_line);
                                witness_output(" 2nd %p %s (%s, %s) @ %s:%d\n",
                                    lock1->li_lock, lock1->li_lock->lo_name,
                                    w1->w_name, w1->w_class->lc_name,
                                    fixup_filename(lock1->li_file),
                                    lock1->li_line);
                                witness_output(" 3rd %p %s (%s, %s) @ %s:%d\n", lock,
                                    lock->lo_name, w->w_name,
                                    w->w_class->lc_name, fixup_filename(file),
                                    line);
                        }
                        if (trace) {
                                char buf[64];
                                struct sbuf sb;

                                sbuf_new(&sb, buf, sizeof(buf), SBUF_FIXEDLEN);
                                sbuf_set_drain(&sb, witness_output_drain,
                                    NULL);

                                if (pstackv) {
                                        sbuf_printf(&sb,
                                    "lock order %s -> %s established at:\n",
                                            w->w_name, w1->w_name);
                                        stack_sbuf_print_flags(&sb, &pstack,
                                            M_NOWAIT, STACK_SBUF_FMT_LONG);
                                }

                                sbuf_printf(&sb,
                                    "lock order %s -> %s attempted at:\n",
                                    w1->w_name, w->w_name);
                                stack_save(&pstack);
                                stack_sbuf_print_flags(&sb, &pstack, M_NOWAIT,
                                    STACK_SBUF_FMT_LONG);

                                sbuf_finish(&sb);
                                sbuf_delete(&sb);
                        }
                        witness_enter_debugger(__func__);
                        return;
                }
        }

        /*
         * If requested, build a new lock order.  However, don't build a new
         * relationship between a sleepable lock and Giant if it is in the
         * wrong direction.  The correct lock order is that sleepable locks
         * always come before Giant.
         */
        if (flags & LOP_NEWORDER &&
            !(plock->li_lock == &Giant.lock_object &&
            (lock->lo_flags & LO_SLEEPABLE) != 0 &&
            (flags & LOP_NOSLEEP) == 0)) {
                CTR3(KTR_WITNESS, "%s: adding %s as a child of %s", __func__,
                    w->w_name, plock->li_lock->lo_witness->w_name);
                itismychild(plock->li_lock->lo_witness, w);
        }
out:
        mtx_unlock_spin(&w_mtx);
}

void
witness_lock(struct lock_object *lock, int flags, const char *file, int line)
{
        struct lock_list_entry **lock_list, *lle;
        struct lock_instance *instance;
        struct witness *w;
        struct thread *td;

        if (witness_cold || witness_watch == -1 || lock->lo_witness == NULL ||
            KERNEL_PANICKED())
                return;
        w = lock->lo_witness;
        td = curthread;

        /* Determine lock list for this lock. */
        if (LOCK_CLASS(lock)->lc_flags & LC_SLEEPLOCK)
                lock_list = &td->td_sleeplocks;
        else
                lock_list = PCPU_PTR(spinlocks);

        /* Check to see if we are recursing on a lock we already own. */
        instance = find_instance(*lock_list, lock);
        if (instance != NULL) {
                instance->li_flags++;
                CTR4(KTR_WITNESS, "%s: pid %d recursed on %s r=%d", __func__,
                    td->td_proc->p_pid, lock->lo_name,
                    instance->li_flags & LI_RECURSEMASK);
                instance->li_file = file;
                instance->li_line = line;
                return;
        }

        /* Update per-witness last file and line acquire. */
        w->w_file = file;
        w->w_line = line;

        /* Find the next open lock instance in the list and fill it. */
        lle = *lock_list;
        if (lle == NULL || lle->ll_count == LOCK_NCHILDREN) {
                lle = witness_lock_list_get();
                if (lle == NULL)
                        return;
                lle->ll_next = *lock_list;
                CTR3(KTR_WITNESS, "%s: pid %d added lle %p", __func__,
                    td->td_proc->p_pid, lle);
                *lock_list = lle;
        }
        instance = &lle->ll_children[lle->ll_count++];
        instance->li_lock = lock;
        instance->li_line = line;
        instance->li_file = file;
        instance->li_flags = 0;
        if ((flags & LOP_EXCLUSIVE) != 0)
                instance->li_flags |= LI_EXCLUSIVE;
        if ((lock->lo_flags & LO_SLEEPABLE) != 0 && (flags & LOP_NOSLEEP) == 0)
                instance->li_flags |= LI_SLEEPABLE;
        CTR4(KTR_WITNESS, "%s: pid %d added %s as lle[%d]", __func__,
            td->td_proc->p_pid, lock->lo_name, lle->ll_count - 1);
}

void
witness_upgrade(struct lock_object *lock, int flags, const char *file, int line)
{
        struct lock_instance *instance;
        struct lock_class *class;

        KASSERT(witness_cold == 0, ("%s: witness_cold", __func__));
        if (lock->lo_witness == NULL || witness_watch == -1 || KERNEL_PANICKED())
                return;
        class = LOCK_CLASS(lock);
        if (witness_watch) {
                if ((lock->lo_flags & LO_UPGRADABLE) == 0)
                        kassert_panic(
                            "upgrade of non-upgradable lock (%s) %s @ %s:%d",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                if ((class->lc_flags & LC_SLEEPLOCK) == 0)
                        kassert_panic(
                            "upgrade of non-sleep lock (%s) %s @ %s:%d",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
        }
        instance = find_instance(curthread->td_sleeplocks, lock);
        if (instance == NULL) {
                kassert_panic("upgrade of unlocked lock (%s) %s @ %s:%d",
                    class->lc_name, lock->lo_name,
                    fixup_filename(file), line);
                return;
        }
        if (witness_watch) {
                if ((instance->li_flags & LI_EXCLUSIVE) != 0)
                        kassert_panic(
                            "upgrade of exclusive lock (%s) %s @ %s:%d",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                if ((instance->li_flags & LI_RECURSEMASK) != 0)
                        kassert_panic(
                            "upgrade of recursed lock (%s) %s r=%d @ %s:%d",
                            class->lc_name, lock->lo_name,
                            instance->li_flags & LI_RECURSEMASK,
                            fixup_filename(file), line);
        }
        instance->li_flags |= LI_EXCLUSIVE;
}

void
witness_downgrade(struct lock_object *lock, int flags, const char *file,
    int line)
{
        struct lock_instance *instance;
        struct lock_class *class;

        KASSERT(witness_cold == 0, ("%s: witness_cold", __func__));
        if (lock->lo_witness == NULL || witness_watch == -1 || KERNEL_PANICKED())
                return;
        class = LOCK_CLASS(lock);
        if (witness_watch) {
                if ((lock->lo_flags & LO_UPGRADABLE) == 0)
                        kassert_panic(
                            "downgrade of non-upgradable lock (%s) %s @ %s:%d",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                if ((class->lc_flags & LC_SLEEPLOCK) == 0)
                        kassert_panic(
                            "downgrade of non-sleep lock (%s) %s @ %s:%d",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
        }
        instance = find_instance(curthread->td_sleeplocks, lock);
        if (instance == NULL) {
                kassert_panic("downgrade of unlocked lock (%s) %s @ %s:%d",
                    class->lc_name, lock->lo_name,
                    fixup_filename(file), line);
                return;
        }
        if (witness_watch) {
                if ((instance->li_flags & LI_EXCLUSIVE) == 0)
                        kassert_panic(
                            "downgrade of shared lock (%s) %s @ %s:%d",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                if ((instance->li_flags & LI_RECURSEMASK) != 0)
                        kassert_panic(
                            "downgrade of recursed lock (%s) %s r=%d @ %s:%d",
                            class->lc_name, lock->lo_name,
                            instance->li_flags & LI_RECURSEMASK,
                            fixup_filename(file), line);
        }
        instance->li_flags &= ~LI_EXCLUSIVE;
}

void
witness_unlock(struct lock_object *lock, int flags, const char *file, int line)
{
        struct lock_list_entry **lock_list, *lle;
        struct lock_instance *instance;
        struct lock_class *class;
        struct thread *td;
        register_t s;
        int i, j;

        if (witness_cold || lock->lo_witness == NULL || KERNEL_PANICKED())
                return;
        td = curthread;
        class = LOCK_CLASS(lock);

        /* Find lock instance associated with this lock. */
        if (class->lc_flags & LC_SLEEPLOCK)
                lock_list = &td->td_sleeplocks;
        else
                lock_list = PCPU_PTR(spinlocks);
        lle = *lock_list;
        for (; *lock_list != NULL; lock_list = &(*lock_list)->ll_next)
                for (i = 0; i < (*lock_list)->ll_count; i++) {
                        instance = &(*lock_list)->ll_children[i];
                        if (instance->li_lock == lock)
                                goto found;
                }

        /*
         * When disabling WITNESS through witness_watch we could end up in
         * having registered locks in the td_sleeplocks queue.
         * We have to make sure we flush these queues, so just search for
         * eventual register locks and remove them.
         */
        if (witness_watch > 0) {
                kassert_panic("lock (%s) %s not locked @ %s:%d", class->lc_name,
                    lock->lo_name, fixup_filename(file), line);
                return;
        } else {
                return;
        }
found:

        /* First, check for shared/exclusive mismatches. */
        if ((instance->li_flags & LI_EXCLUSIVE) != 0 && witness_watch > 0 &&
            (flags & LOP_EXCLUSIVE) == 0) {
                witness_output("shared unlock of (%s) %s @ %s:%d\n",
                    class->lc_name, lock->lo_name, fixup_filename(file), line);
                witness_output("while exclusively locked from %s:%d\n",
                    fixup_filename(instance->li_file), instance->li_line);
                kassert_panic("excl->ushare");
        }
        if ((instance->li_flags & LI_EXCLUSIVE) == 0 && witness_watch > 0 &&
            (flags & LOP_EXCLUSIVE) != 0) {
                witness_output("exclusive unlock of (%s) %s @ %s:%d\n",
                    class->lc_name, lock->lo_name, fixup_filename(file), line);
                witness_output("while share locked from %s:%d\n",
                    fixup_filename(instance->li_file),
                    instance->li_line);
                kassert_panic("share->uexcl");
        }
        /* If we are recursed, unrecurse. */
        if ((instance->li_flags & LI_RECURSEMASK) > 0) {
                CTR4(KTR_WITNESS, "%s: pid %d unrecursed on %s r=%d", __func__,
                    td->td_proc->p_pid, instance->li_lock->lo_name,
                    instance->li_flags);
                instance->li_flags--;
                return;
        }
        /* The lock is now being dropped, check for NORELEASE flag */
        if ((instance->li_flags & LI_NORELEASE) != 0 && witness_watch > 0) {
                witness_output("forbidden unlock of (%s) %s @ %s:%d\n",
                    class->lc_name, lock->lo_name, fixup_filename(file), line);
                kassert_panic("lock marked norelease");
        }

        /* Otherwise, remove this item from the list. */
        s = intr_disable();
        CTR4(KTR_WITNESS, "%s: pid %d removed %s from lle[%d]", __func__,
            td->td_proc->p_pid, instance->li_lock->lo_name,
            (*lock_list)->ll_count - 1);
        for (j = i; j < (*lock_list)->ll_count - 1; j++)
                (*lock_list)->ll_children[j] =
                    (*lock_list)->ll_children[j + 1];
        (*lock_list)->ll_count--;
        intr_restore(s);

        /*
         * In order to reduce contention on w_mtx, we want to keep always an
         * head object into lists so that frequent allocation from the 
         * free witness pool (and subsequent locking) is avoided.
         * In order to maintain the current code simple, when the head
         * object is totally unloaded it means also that we do not have
         * further objects in the list, so the list ownership needs to be
         * hand over to another object if the current head needs to be freed.
         */
        if ((*lock_list)->ll_count == 0) {
                if (*lock_list == lle) {
                        if (lle->ll_next == NULL)
                                return;
                } else
                        lle = *lock_list;
                *lock_list = lle->ll_next;
                CTR3(KTR_WITNESS, "%s: pid %d removed lle %p", __func__,
                    td->td_proc->p_pid, lle);
                witness_lock_list_free(lle);
        }
}

void
witness_thread_exit(struct thread *td)
{
        struct lock_list_entry *lle;
        int i, n;

        lle = td->td_sleeplocks;
        if (lle == NULL || KERNEL_PANICKED())
                return;
        if (lle->ll_count != 0) {
                for (n = 0; lle != NULL; lle = lle->ll_next)
                        for (i = lle->ll_count - 1; i >= 0; i--) {
                                if (n == 0)
                                        witness_output(
                    "Thread %p exiting with the following locks held:\n", td);
                                n++;
                                witness_list_lock(&lle->ll_children[i],
                                    witness_output);
                                
                        }
                kassert_panic(
                    "Thread %p cannot exit while holding sleeplocks\n", td);
        }
        witness_lock_list_free(lle);
}

/*
 * Warn if any locks other than 'lock' are held.  Flags can be passed in to
 * exempt Giant and sleepable locks from the checks as well.  If any
 * non-exempt locks are held, then a supplied message is printed to the
 * output channel along with a list of the offending locks.  If indicated in the
 * flags then a failure results in a panic as well.
 */
int
witness_warn(int flags, struct lock_object *lock, const char *fmt, ...)
{
        struct lock_list_entry *lock_list, *lle;
        struct lock_instance *lock1;
        struct thread *td;
        va_list ap;
        int i, n;

        if (witness_cold || witness_watch < 1 || KERNEL_PANICKED())
                return (0);
        n = 0;
        td = curthread;
        for (lle = td->td_sleeplocks; lle != NULL; lle = lle->ll_next)
                for (i = lle->ll_count - 1; i >= 0; i--) {
                        lock1 = &lle->ll_children[i];
                        if (lock1->li_lock == lock)
                                continue;
                        if (flags & WARN_GIANTOK &&
                            lock1->li_lock == &Giant.lock_object)
                                continue;
                        if (flags & WARN_SLEEPOK &&
                            (lock1->li_flags & LI_SLEEPABLE) != 0)
                                continue;
                        if (n == 0) {
                                va_start(ap, fmt);
                                vprintf(fmt, ap);
                                va_end(ap);
                                printf(" with the following %slocks held:\n",
                                    (flags & WARN_SLEEPOK) != 0 ?
                                    "non-sleepable " : "");
                        }
                        n++;
                        witness_list_lock(lock1, printf);
                }

        /*
         * Pin the thread in order to avoid problems with thread migration.
         * Once that all verifies are passed about spinlocks ownership,
         * the thread is in a safe path and it can be unpinned.
         */
        sched_pin();
        lock_list = PCPU_GET(spinlocks);
        if (lock_list != NULL && lock_list->ll_count != 0) {
                sched_unpin();

                /*
                 * We should only have one spinlock and as long as
                 * the flags cannot match for this locks class,
                 * check if the first spinlock is the one curthread
                 * should hold.
                 */
                lock1 = &lock_list->ll_children[lock_list->ll_count - 1];
                if (lock_list->ll_count == 1 && lock_list->ll_next == NULL &&
                    lock1->li_lock == lock && n == 0)
                        return (0);

                va_start(ap, fmt);
                vprintf(fmt, ap);
                va_end(ap);
                printf(" with the following %slocks held:\n",
                    (flags & WARN_SLEEPOK) != 0 ?  "non-sleepable " : "");
                n += witness_list_locks(&lock_list, printf);
        } else
                sched_unpin();
        if (flags & WARN_PANIC && n)
                kassert_panic("%s", __func__);
        else
                witness_debugger(n, __func__);
        return (n);
}

const char *
witness_file(struct lock_object *lock)
{
        struct witness *w;

        if (witness_cold || witness_watch < 1 || lock->lo_witness == NULL)
                return ("?");
        w = lock->lo_witness;
        return (w->w_file);
}

int
witness_line(struct lock_object *lock)
{
        struct witness *w;

        if (witness_cold || witness_watch < 1 || lock->lo_witness == NULL)
                return (0);
        w = lock->lo_witness;
        return (w->w_line);
}

static struct witness *
enroll(const char *description, struct lock_class *lock_class)
{
        struct witness *w;

        MPASS(description != NULL);

        if (witness_watch == -1 || KERNEL_PANICKED())
                return (NULL);
        if ((lock_class->lc_flags & LC_SPINLOCK)) {
                if (witness_skipspin)
                        return (NULL);
        } else if ((lock_class->lc_flags & LC_SLEEPLOCK) == 0) {
                kassert_panic("lock class %s is not sleep or spin",
                    lock_class->lc_name);
                return (NULL);
        }

        mtx_lock_spin(&w_mtx);
        w = witness_hash_get(description);
        if (w)
                goto found;
        if ((w = witness_get()) == NULL)
                return (NULL);
        MPASS(strlen(description) < MAX_W_NAME);
        strcpy(w->w_name, description);
        w->w_class = lock_class;
        w->w_refcount = 1;
        STAILQ_INSERT_HEAD(&w_all, w, w_list);
        if (lock_class->lc_flags & LC_SPINLOCK) {
                STAILQ_INSERT_HEAD(&w_spin, w, w_typelist);
                w_spin_cnt++;
        } else if (lock_class->lc_flags & LC_SLEEPLOCK) {
                STAILQ_INSERT_HEAD(&w_sleep, w, w_typelist);
                w_sleep_cnt++;
        }

        /* Insert new witness into the hash */
        witness_hash_put(w);
        witness_increment_graph_generation();
        mtx_unlock_spin(&w_mtx);
        return (w);
found:
        w->w_refcount++;
        if (w->w_refcount == 1)
                w->w_class = lock_class;
        mtx_unlock_spin(&w_mtx);
        if (lock_class != w->w_class)
                kassert_panic(
                    "lock (%s) %s does not match earlier (%s) lock",
                    description, lock_class->lc_name,
                    w->w_class->lc_name);
        return (w);
}

static void
depart(struct witness *w)
{

        MPASS(w->w_refcount == 0);
        if (w->w_class->lc_flags & LC_SLEEPLOCK) {
                w_sleep_cnt--;
        } else {
                w_spin_cnt--;
        }
        /*
         * Set file to NULL as it may point into a loadable module.
         */
        w->w_file = NULL;
        w->w_line = 0;
        witness_increment_graph_generation();
}

static void
adopt(struct witness *parent, struct witness *child)
{
        int pi, ci, i, j;

        if (witness_cold == 0)
                mtx_assert(&w_mtx, MA_OWNED);

        /* If the relationship is already known, there's no work to be done. */
        if (isitmychild(parent, child))
                return;

        /* When the structure of the graph changes, bump up the generation. */
        witness_increment_graph_generation();

        /*
         * The hard part ... create the direct relationship, then propagate all
         * indirect relationships.
         */
        pi = parent->w_index;
        ci = child->w_index;
        WITNESS_INDEX_ASSERT(pi);
        WITNESS_INDEX_ASSERT(ci);
        MPASS(pi != ci);
        w_rmatrix[pi][ci] |= WITNESS_PARENT;
        w_rmatrix[ci][pi] |= WITNESS_CHILD;

        /*
         * If parent was not already an ancestor of child,
         * then we increment the descendant and ancestor counters.
         */
        if ((w_rmatrix[pi][ci] & WITNESS_ANCESTOR) == 0) {
                parent->w_num_descendants++;
                child->w_num_ancestors++;
        }

        /* 
         * Find each ancestor of 'pi'. Note that 'pi' itself is counted as 
         * an ancestor of 'pi' during this loop.
         */
        for (i = 1; i <= w_max_used_index; i++) {
                if ((w_rmatrix[i][pi] & WITNESS_ANCESTOR_MASK) == 0 && 
                    (i != pi))
                        continue;

                /* Find each descendant of 'i' and mark it as a descendant. */
                for (j = 1; j <= w_max_used_index; j++) {
                        /* 
                         * Skip children that are already marked as
                         * descendants of 'i'.
                         */
                        if (w_rmatrix[i][j] & WITNESS_ANCESTOR_MASK)
                                continue;

                        /*
                         * We are only interested in descendants of 'ci'. Note
                         * that 'ci' itself is counted as a descendant of 'ci'.
                         */
                        if ((w_rmatrix[ci][j] & WITNESS_ANCESTOR_MASK) == 0 && 
                            (j != ci))
                                continue;
                        w_rmatrix[i][j] |= WITNESS_ANCESTOR;
                        w_rmatrix[j][i] |= WITNESS_DESCENDANT;
                        w_data[i].w_num_descendants++;
                        w_data[j].w_num_ancestors++;

                        /* 
                         * Make sure we aren't marking a node as both an
                         * ancestor and descendant. We should have caught 
                         * this as a lock order reversal earlier.
                         */
                        if ((w_rmatrix[i][j] & WITNESS_ANCESTOR_MASK) &&
                            (w_rmatrix[i][j] & WITNESS_DESCENDANT_MASK)) {
                                printf("witness rmatrix paradox! [%d][%d]=%d "
                                    "both ancestor and descendant\n",
                                    i, j, w_rmatrix[i][j]); 
                                kdb_backtrace();
                                printf("Witness disabled.\n");
                                witness_watch = -1;
                        }
                        if ((w_rmatrix[j][i] & WITNESS_ANCESTOR_MASK) &&
                            (w_rmatrix[j][i] & WITNESS_DESCENDANT_MASK)) {
                                printf("witness rmatrix paradox! [%d][%d]=%d "
                                    "both ancestor and descendant\n",
                                    j, i, w_rmatrix[j][i]); 
                                kdb_backtrace();
                                printf("Witness disabled.\n");
                                witness_watch = -1;
                        }
                }
        }
}

static void
itismychild(struct witness *parent, struct witness *child)
{
        int unlocked;

        MPASS(child != NULL && parent != NULL);
        if (witness_cold == 0)
                mtx_assert(&w_mtx, MA_OWNED);

        if (!witness_lock_type_equal(parent, child)) {
                if (witness_cold == 0) {
                        unlocked = 1;
                        mtx_unlock_spin(&w_mtx);
                } else {
                        unlocked = 0;
                }
                kassert_panic(
                    "%s: parent \"%s\" (%s) and child \"%s\" (%s) are not "
                    "the same lock type", __func__, parent->w_name,
                    parent->w_class->lc_name, child->w_name,
                    child->w_class->lc_name);
                if (unlocked)
                        mtx_lock_spin(&w_mtx);
        }
        adopt(parent, child);
}

/*
 * Generic code for the isitmy*() functions. The rmask parameter is the
 * expected relationship of w1 to w2.
 */
static int
_isitmyx(struct witness *w1, struct witness *w2, int rmask, const char *fname)
{
        unsigned char r1, r2;
        int i1, i2;

        i1 = w1->w_index;
        i2 = w2->w_index;
        WITNESS_INDEX_ASSERT(i1);
        WITNESS_INDEX_ASSERT(i2);
        r1 = w_rmatrix[i1][i2] & WITNESS_RELATED_MASK;
        r2 = w_rmatrix[i2][i1] & WITNESS_RELATED_MASK;

        /* The flags on one better be the inverse of the flags on the other */
        if (!((WITNESS_ATOD(r1) == r2 && WITNESS_DTOA(r2) == r1) ||
            (WITNESS_DTOA(r1) == r2 && WITNESS_ATOD(r2) == r1))) {
                /* Don't squawk if we're potentially racing with an update. */
                if (!mtx_owned(&w_mtx))
                        return (0);
                printf("%s: rmatrix mismatch between %s (index %d) and %s "
                    "(index %d): w_rmatrix[%d][%d] == %hhx but "
                    "w_rmatrix[%d][%d] == %hhx\n",
                    fname, w1->w_name, i1, w2->w_name, i2, i1, i2, r1,
                    i2, i1, r2);
                kdb_backtrace();
                printf("Witness disabled.\n");
                witness_watch = -1;
        }
        return (r1 & rmask);
}

/*
 * Checks if @child is a direct child of @parent.
 */
static int
isitmychild(struct witness *parent, struct witness *child)
{

        return (_isitmyx(parent, child, WITNESS_PARENT, __func__));
}

/*
 * Checks if @descendant is a direct or inderect descendant of @ancestor.
 */
static int
isitmydescendant(struct witness *ancestor, struct witness *descendant)
{

        return (_isitmyx(ancestor, descendant, WITNESS_ANCESTOR_MASK,
            __func__));
}

static int
blessed(struct witness *w1, struct witness *w2)
{
        int i;
        struct witness_blessed *b;

        for (i = 0; i < nitems(blessed_list); i++) {
                b = &blessed_list[i];
                if (strcmp(w1->w_name, b->b_lock1) == 0) {
                        if (strcmp(w2->w_name, b->b_lock2) == 0)
                                return (1);
                        continue;
                }
                if (strcmp(w1->w_name, b->b_lock2) == 0)
                        if (strcmp(w2->w_name, b->b_lock1) == 0)
                                return (1);
        }
        return (0);
}

static struct witness *
witness_get(void)
{
        struct witness *w;
        int index;

        if (witness_cold == 0)
                mtx_assert(&w_mtx, MA_OWNED);

        if (witness_watch == -1) {
                mtx_unlock_spin(&w_mtx);
                return (NULL);
        }
        if (STAILQ_EMPTY(&w_free)) {
                witness_watch = -1;
                mtx_unlock_spin(&w_mtx);
                printf("WITNESS: unable to allocate a new witness object\n");
                return (NULL);
        }
        w = STAILQ_FIRST(&w_free);
        STAILQ_REMOVE_HEAD(&w_free, w_list);
        w_free_cnt--;
        index = w->w_index;
        MPASS(index > 0 && index == w_max_used_index+1 &&
            index < witness_count);
        bzero(w, sizeof(*w));
        w->w_index = index;
        if (index > w_max_used_index)
                w_max_used_index = index;
        return (w);
}

static void
witness_free(struct witness *w)
{

        STAILQ_INSERT_HEAD(&w_free, w, w_list);
        w_free_cnt++;
}

static struct lock_list_entry *
witness_lock_list_get(void)
{
        struct lock_list_entry *lle;

        if (witness_watch == -1)
                return (NULL);
        mtx_lock_spin(&w_mtx);
        lle = w_lock_list_free;
        if (lle == NULL) {
                witness_watch = -1;
                mtx_unlock_spin(&w_mtx);
                printf("%s: witness exhausted\n", __func__);
                return (NULL);
        }
        w_lock_list_free = lle->ll_next;
        mtx_unlock_spin(&w_mtx);
        bzero(lle, sizeof(*lle));
        return (lle);
}
                
static void
witness_lock_list_free(struct lock_list_entry *lle)
{

        mtx_lock_spin(&w_mtx);
        lle->ll_next = w_lock_list_free;
        w_lock_list_free = lle;
        mtx_unlock_spin(&w_mtx);
}

static struct lock_instance *
find_instance(struct lock_list_entry *list, const struct lock_object *lock)
{
        struct lock_list_entry *lle;
        struct lock_instance *instance;
        int i;

        for (lle = list; lle != NULL; lle = lle->ll_next)
                for (i = lle->ll_count - 1; i >= 0; i--) {
                        instance = &lle->ll_children[i];
                        if (instance->li_lock == lock)
                                return (instance);
                }
        return (NULL);
}

static void
witness_list_lock(struct lock_instance *instance,
    int (*prnt)(const char *fmt, ...))
{
        struct lock_object *lock;

        lock = instance->li_lock;
        prnt("%s %s %s", (instance->li_flags & LI_EXCLUSIVE) != 0 ?
            "exclusive" : "shared", LOCK_CLASS(lock)->lc_name, lock->lo_name);
        if (lock->lo_witness->w_name != lock->lo_name)
                prnt(" (%s)", lock->lo_witness->w_name);
        prnt(" r = %d (%p) locked @ %s:%d\n",
            instance->li_flags & LI_RECURSEMASK, lock,
            fixup_filename(instance->li_file), instance->li_line);
}

static int
witness_output(const char *fmt, ...)
{
        va_list ap;
        int ret;

        va_start(ap, fmt);
        ret = witness_voutput(fmt, ap);
        va_end(ap);
        return (ret);
}

static int
witness_voutput(const char *fmt, va_list ap)
{
        int ret;

        ret = 0;
        switch (witness_channel) {
        case WITNESS_CONSOLE:
                ret = vprintf(fmt, ap);
                break;
        case WITNESS_LOG:
                vlog(LOG_NOTICE, fmt, ap);
                break;
        case WITNESS_NONE:
                break;
        }
        return (ret);
}

#ifdef DDB
static int
witness_thread_has_locks(struct thread *td)
{

        if (td->td_sleeplocks == NULL)
                return (0);
        return (td->td_sleeplocks->ll_count != 0);
}

static int
witness_proc_has_locks(struct proc *p)
{
        struct thread *td;

        FOREACH_THREAD_IN_PROC(p, td) {
                if (witness_thread_has_locks(td))
                        return (1);
        }
        return (0);
}
#endif

int
witness_list_locks(struct lock_list_entry **lock_list,
    int (*prnt)(const char *fmt, ...))
{
        struct lock_list_entry *lle;
        int i, nheld;

        nheld = 0;
        for (lle = *lock_list; lle != NULL; lle = lle->ll_next)
                for (i = lle->ll_count - 1; i >= 0; i--) {
                        witness_list_lock(&lle->ll_children[i], prnt);
                        nheld++;
                }
        return (nheld);
}

/*
 * This is a bit risky at best.  We call this function when we have timed
 * out acquiring a spin lock, and we assume that the other CPU is stuck
 * with this lock held.  So, we go groveling around in the other CPU's
 * per-cpu data to try to find the lock instance for this spin lock to
 * see when it was last acquired.
 */
void
witness_display_spinlock(struct lock_object *lock, struct thread *owner,
    int (*prnt)(const char *fmt, ...))
{
        struct lock_instance *instance;
        struct pcpu *pc;

        if (owner->td_critnest == 0 || owner->td_oncpu == NOCPU)
                return;
        pc = pcpu_find(owner->td_oncpu);
        instance = find_instance(pc->pc_spinlocks, lock);
        if (instance != NULL)
                witness_list_lock(instance, prnt);
}

void
witness_save(struct lock_object *lock, const char **filep, int *linep)
{
        struct lock_list_entry *lock_list;
        struct lock_instance *instance;
        struct lock_class *class;

        /*
         * This function is used independently in locking code to deal with
         * Giant, SCHEDULER_STOPPED() check can be removed here after Giant
         * is gone.
         */
        if (SCHEDULER_STOPPED())
                return;
        KASSERT(witness_cold == 0, ("%s: witness_cold", __func__));
        if (lock->lo_witness == NULL || witness_watch == -1 || KERNEL_PANICKED())
                return;
        class = LOCK_CLASS(lock);
        if (class->lc_flags & LC_SLEEPLOCK)
                lock_list = curthread->td_sleeplocks;
        else {
                if (witness_skipspin)
                        return;
                lock_list = PCPU_GET(spinlocks);
        }
        instance = find_instance(lock_list, lock);
        if (instance == NULL) {
                kassert_panic("%s: lock (%s) %s not locked", __func__,
                    class->lc_name, lock->lo_name);
                return;
        }
        *filep = instance->li_file;
        *linep = instance->li_line;
}

void
witness_restore(struct lock_object *lock, const char *file, int line)
{
        struct lock_list_entry *lock_list;
        struct lock_instance *instance;
        struct lock_class *class;

        /*
         * This function is used independently in locking code to deal with
         * Giant, SCHEDULER_STOPPED() check can be removed here after Giant
         * is gone.
         */
        if (SCHEDULER_STOPPED())
                return;
        KASSERT(witness_cold == 0, ("%s: witness_cold", __func__));
        if (lock->lo_witness == NULL || witness_watch == -1 || KERNEL_PANICKED())
                return;
        class = LOCK_CLASS(lock);
        if (class->lc_flags & LC_SLEEPLOCK)
                lock_list = curthread->td_sleeplocks;
        else {
                if (witness_skipspin)
                        return;
                lock_list = PCPU_GET(spinlocks);
        }
        instance = find_instance(lock_list, lock);
        if (instance == NULL)
                kassert_panic("%s: lock (%s) %s not locked", __func__,
                    class->lc_name, lock->lo_name);
        lock->lo_witness->w_file = file;
        lock->lo_witness->w_line = line;
        if (instance == NULL)
                return;
        instance->li_file = file;
        instance->li_line = line;
}

void
witness_assert(const struct lock_object *lock, int flags, const char *file,
    int line)
{
#ifdef INVARIANT_SUPPORT
        struct lock_instance *instance;
        struct lock_class *class;

        if (lock->lo_witness == NULL || witness_watch < 1 || KERNEL_PANICKED())
                return;
        class = LOCK_CLASS(lock);
        if ((class->lc_flags & LC_SLEEPLOCK) != 0)
                instance = find_instance(curthread->td_sleeplocks, lock);
        else if ((class->lc_flags & LC_SPINLOCK) != 0)
                instance = find_instance(PCPU_GET(spinlocks), lock);
        else {
                kassert_panic("Lock (%s) %s is not sleep or spin!",
                    class->lc_name, lock->lo_name);
                return;
        }
        switch (flags) {
        case LA_UNLOCKED:
                if (instance != NULL)
                        kassert_panic("Lock (%s) %s locked @ %s:%d.",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                break;
        case LA_LOCKED:
        case LA_LOCKED | LA_RECURSED:
        case LA_LOCKED | LA_NOTRECURSED:
        case LA_SLOCKED:
        case LA_SLOCKED | LA_RECURSED:
        case LA_SLOCKED | LA_NOTRECURSED:
        case LA_XLOCKED:
        case LA_XLOCKED | LA_RECURSED:
        case LA_XLOCKED | LA_NOTRECURSED:
                if (instance == NULL) {
                        kassert_panic("Lock (%s) %s not locked @ %s:%d.",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                        break;
                }
                if ((flags & LA_XLOCKED) != 0 &&
                    (instance->li_flags & LI_EXCLUSIVE) == 0)
                        kassert_panic(
                            "Lock (%s) %s not exclusively locked @ %s:%d.",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                if ((flags & LA_SLOCKED) != 0 &&
                    (instance->li_flags & LI_EXCLUSIVE) != 0)
                        kassert_panic(
                            "Lock (%s) %s exclusively locked @ %s:%d.",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                if ((flags & LA_RECURSED) != 0 &&
                    (instance->li_flags & LI_RECURSEMASK) == 0)
                        kassert_panic("Lock (%s) %s not recursed @ %s:%d.",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                if ((flags & LA_NOTRECURSED) != 0 &&
                    (instance->li_flags & LI_RECURSEMASK) != 0)
                        kassert_panic("Lock (%s) %s recursed @ %s:%d.",
                            class->lc_name, lock->lo_name,
                            fixup_filename(file), line);
                break;
        default:
                kassert_panic("Invalid lock assertion at %s:%d.",
                    fixup_filename(file), line);
        }
#endif  /* INVARIANT_SUPPORT */
}

static void
witness_setflag(struct lock_object *lock, int flag, int set)
{
        struct lock_list_entry *lock_list;
        struct lock_instance *instance;
        struct lock_class *class;

        if (lock->lo_witness == NULL || witness_watch == -1 || KERNEL_PANICKED())
                return;
        class = LOCK_CLASS(lock);
        if (class->lc_flags & LC_SLEEPLOCK)
                lock_list = curthread->td_sleeplocks;
        else {
                if (witness_skipspin)
                        return;
                lock_list = PCPU_GET(spinlocks);
        }
        instance = find_instance(lock_list, lock);
        if (instance == NULL) {
                kassert_panic("%s: lock (%s) %s not locked", __func__,
                    class->lc_name, lock->lo_name);
                return;
        }

        if (set)
                instance->li_flags |= flag;
        else
                instance->li_flags &= ~flag;
}

void
witness_norelease(struct lock_object *lock)
{

        witness_setflag(lock, LI_NORELEASE, 1);
}

void
witness_releaseok(struct lock_object *lock)
{

        witness_setflag(lock, LI_NORELEASE, 0);
}

#ifdef DDB
static void
witness_ddb_list(struct thread *td)
{

        KASSERT(witness_cold == 0, ("%s: witness_cold", __func__));
        KASSERT(kdb_active, ("%s: not in the debugger", __func__));

        if (witness_watch < 1)
                return;

        witness_list_locks(&td->td_sleeplocks, db_printf);

        /*
         * We only handle spinlocks if td == curthread.  This is somewhat broken
         * if td is currently executing on some other CPU and holds spin locks
         * as we won't display those locks.  If we had a MI way of getting
         * the per-cpu data for a given cpu then we could use
         * td->td_oncpu to get the list of spinlocks for this thread
         * and "fix" this.
         *
         * That still wouldn't really fix this unless we locked the scheduler
         * lock or stopped the other CPU to make sure it wasn't changing the
         * list out from under us.  It is probably best to just not try to
         * handle threads on other CPU's for now.
         */
        if (td == curthread && PCPU_GET(spinlocks) != NULL)
                witness_list_locks(PCPU_PTR(spinlocks), db_printf);
}

DB_SHOW_COMMAND(locks, db_witness_list)
{
        struct thread *td;

        if (have_addr)
                td = db_lookup_thread(addr, true);
        else
                td = kdb_thread;
        witness_ddb_list(td);
}

DB_SHOW_ALL_COMMAND(locks, db_witness_list_all)
{
        struct thread *td;
        struct proc *p;

        /*
         * It would be nice to list only threads and processes that actually
         * held sleep locks, but that information is currently not exported
         * by WITNESS.
         */
        FOREACH_PROC_IN_SYSTEM(p) {
                if (!witness_proc_has_locks(p))
                        continue;
                FOREACH_THREAD_IN_PROC(p, td) {
                        if (!witness_thread_has_locks(td))
                                continue;
                        db_printf("Process %d (%s) thread %p (%d)\n", p->p_pid,
                            p->p_comm, td, td->td_tid);
                        witness_ddb_list(td);
                        if (db_pager_quit)
                                return;
                }
        }
}
DB_SHOW_ALIAS(alllocks, db_witness_list_all)

DB_SHOW_COMMAND(witness, db_witness_display)
{

        witness_ddb_display(db_printf);
}
#endif

static void
sbuf_print_witness_badstacks(struct sbuf *sb, size_t *oldidx)
{
        struct witness_lock_order_data *data1, *data2, *tmp_data1, *tmp_data2;
        struct witness *tmp_w1, *tmp_w2, *w1, *w2;
        int generation, i, j;

        tmp_data1 = NULL;
        tmp_data2 = NULL;
        tmp_w1 = NULL;
        tmp_w2 = NULL;

        /* Allocate and init temporary storage space. */
        tmp_w1 = malloc(sizeof(struct witness), M_TEMP, M_WAITOK | M_ZERO);
        tmp_w2 = malloc(sizeof(struct witness), M_TEMP, M_WAITOK | M_ZERO);
        tmp_data1 = malloc(sizeof(struct witness_lock_order_data), M_TEMP, 
            M_WAITOK | M_ZERO);
        tmp_data2 = malloc(sizeof(struct witness_lock_order_data), M_TEMP, 
            M_WAITOK | M_ZERO);
        stack_zero(&tmp_data1->wlod_stack);
        stack_zero(&tmp_data2->wlod_stack);

restart:
        mtx_lock_spin(&w_mtx);
        generation = w_generation;
        mtx_unlock_spin(&w_mtx);
        sbuf_printf(sb, "Number of known direct relationships is %d\n",
            w_lohash.wloh_count);
        for (i = 1; i < w_max_used_index; i++) {
                mtx_lock_spin(&w_mtx);
                if (generation != w_generation) {
                        mtx_unlock_spin(&w_mtx);

                        /* The graph has changed, try again. */
                        *oldidx = 0;
                        sbuf_clear(sb);
                        goto restart;
                }

                w1 = &w_data[i];
                if (w1->w_reversed == 0) {
                        mtx_unlock_spin(&w_mtx);
                        continue;
                }

                /* Copy w1 locally so we can release the spin lock. */
                *tmp_w1 = *w1;
                mtx_unlock_spin(&w_mtx);

                if (tmp_w1->w_reversed == 0)
                        continue;
                for (j = 1; j < w_max_used_index; j++) {
                        if ((w_rmatrix[i][j] & WITNESS_REVERSAL) == 0 || i > j)
                                continue;

                        mtx_lock_spin(&w_mtx);
                        if (generation != w_generation) {
                                mtx_unlock_spin(&w_mtx);

                                /* The graph has changed, try again. */
                                *oldidx = 0;
                                sbuf_clear(sb);
                                goto restart;
                        }

                        w2 = &w_data[j];
                        data1 = witness_lock_order_get(w1, w2);
                        data2 = witness_lock_order_get(w2, w1);

                        /*
                         * Copy information locally so we can release the
                         * spin lock.
                         */
                        *tmp_w2 = *w2;

                        if (data1) {
                                stack_zero(&tmp_data1->wlod_stack);
                                stack_copy(&data1->wlod_stack,
                                    &tmp_data1->wlod_stack);
                        }
                        if (data2 && data2 != data1) {
                                stack_zero(&tmp_data2->wlod_stack);
                                stack_copy(&data2->wlod_stack,
                                    &tmp_data2->wlod_stack);
                        }
                        mtx_unlock_spin(&w_mtx);

                        if (blessed(tmp_w1, tmp_w2))
                                continue;

                        sbuf_printf(sb,
            "\nLock order reversal between \"%s\"(%s) and \"%s\"(%s)!\n",
                            tmp_w1->w_name, tmp_w1->w_class->lc_name, 
                            tmp_w2->w_name, tmp_w2->w_class->lc_name);
                        if (data1) {
                                sbuf_printf(sb,
                        "Lock order \"%s\"(%s) -> \"%s\"(%s) first seen at:\n",
                                    tmp_w1->w_name, tmp_w1->w_class->lc_name, 
                                    tmp_w2->w_name, tmp_w2->w_class->lc_name);
                                stack_sbuf_print(sb, &tmp_data1->wlod_stack);
                                sbuf_printf(sb, "\n");
                        }
                        if (data2 && data2 != data1) {
                                sbuf_printf(sb,
                        "Lock order \"%s\"(%s) -> \"%s\"(%s) first seen at:\n",
                                    tmp_w2->w_name, tmp_w2->w_class->lc_name, 
                                    tmp_w1->w_name, tmp_w1->w_class->lc_name);
                                stack_sbuf_print(sb, &tmp_data2->wlod_stack);
                                sbuf_printf(sb, "\n");
                        }
                }
        }
        mtx_lock_spin(&w_mtx);
        if (generation != w_generation) {
                mtx_unlock_spin(&w_mtx);

                /*
                 * The graph changed while we were printing stack data,
                 * try again.
                 */
                *oldidx = 0;
                sbuf_clear(sb);
                goto restart;
        }
        mtx_unlock_spin(&w_mtx);

        /* Free temporary storage space. */
        free(tmp_data1, M_TEMP);
        free(tmp_data2, M_TEMP);
        free(tmp_w1, M_TEMP);
        free(tmp_w2, M_TEMP);
}

static int
sysctl_debug_witness_badstacks(SYSCTL_HANDLER_ARGS)
{
        struct sbuf *sb;
        int error;

        if (witness_watch < 1) {
                error = SYSCTL_OUT(req, w_notrunning, sizeof(w_notrunning));
                return (error);
        }
        if (witness_cold) {
                error = SYSCTL_OUT(req, w_stillcold, sizeof(w_stillcold));
                return (error);
        }
        error = 0;
        sb = sbuf_new(NULL, NULL, badstack_sbuf_size, SBUF_AUTOEXTEND);
        if (sb == NULL)
                return (ENOMEM);

        sbuf_print_witness_badstacks(sb, &req->oldidx);

        sbuf_finish(sb);
        error = SYSCTL_OUT(req, sbuf_data(sb), sbuf_len(sb) + 1);
        sbuf_delete(sb);

        return (error);
}

#ifdef DDB
static int
sbuf_db_printf_drain(void *arg __unused, const char *data, int len)
{

        return (db_printf("%.*s", len, data));
}

DB_SHOW_COMMAND(badstacks, db_witness_badstacks)
{
        struct sbuf sb;
        char buffer[128];
        size_t dummy;

        sbuf_new(&sb, buffer, sizeof(buffer), SBUF_FIXEDLEN);
        sbuf_set_drain(&sb, sbuf_db_printf_drain, NULL);
        sbuf_print_witness_badstacks(&sb, &dummy);
        sbuf_finish(&sb);
}
#endif

static int
sysctl_debug_witness_channel(SYSCTL_HANDLER_ARGS)
{
        static const struct {
                enum witness_channel channel;
                const char *name;
        } channels[] = {
                { WITNESS_CONSOLE, "console" },
                { WITNESS_LOG, "log" },
                { WITNESS_NONE, "none" },
        };
        char buf[16];
        u_int i;
        int error;

        buf[0] = '\0';
        for (i = 0; i < nitems(channels); i++)
                if (witness_channel == channels[i].channel) {
                        snprintf(buf, sizeof(buf), "%s", channels[i].name);
                        break;
                }

        error = sysctl_handle_string(oidp, buf, sizeof(buf), req);
        if (error != 0 || req->newptr == NULL)
                return (error);

        error = EINVAL;
        for (i = 0; i < nitems(channels); i++)
                if (strcmp(channels[i].name, buf) == 0) {
                        witness_channel = channels[i].channel;
                        error = 0;
                        break;
                }
        return (error);
}

static int
sysctl_debug_witness_fullgraph(SYSCTL_HANDLER_ARGS)
{
        struct witness *w;
        struct sbuf *sb;
        int error;

#ifdef __i386__
        error = SYSCTL_OUT(req, w_notallowed, sizeof(w_notallowed));
        return (error);
#endif

        if (witness_watch < 1) {
                error = SYSCTL_OUT(req, w_notrunning, sizeof(w_notrunning));
                return (error);
        }
        if (witness_cold) {
                error = SYSCTL_OUT(req, w_stillcold, sizeof(w_stillcold));
                return (error);
        }
        error = 0;

        error = sysctl_wire_old_buffer(req, 0);
        if (error != 0)
                return (error);
        sb = sbuf_new_for_sysctl(NULL, NULL, FULLGRAPH_SBUF_SIZE, req);
        if (sb == NULL)
                return (ENOMEM);
        sbuf_printf(sb, "\n");

        mtx_lock_spin(&w_mtx);
        STAILQ_FOREACH(w, &w_all, w_list)
                w->w_displayed = 0;
        STAILQ_FOREACH(w, &w_all, w_list)
                witness_add_fullgraph(sb, w);
        mtx_unlock_spin(&w_mtx);

        /*
         * Close the sbuf and return to userland.
         */
        error = sbuf_finish(sb);
        sbuf_delete(sb);

        return (error);
}

static int
sysctl_debug_witness_watch(SYSCTL_HANDLER_ARGS)
{
        int error, value;

        value = witness_watch;
        error = sysctl_handle_int(oidp, &value, 0, req);
        if (error != 0 || req->newptr == NULL)
                return (error);
        if (value > 1 || value < -1 ||
            (witness_watch == -1 && value != witness_watch))
                return (EINVAL);
        witness_watch = value;
        return (0);
}

static void
witness_add_fullgraph(struct sbuf *sb, struct witness *w)
{
        int i;

        if (w->w_displayed != 0 || (w->w_file == NULL && w->w_line == 0))
                return;
        w->w_displayed = 1;

        WITNESS_INDEX_ASSERT(w->w_index);
        for (i = 1; i <= w_max_used_index; i++) {
                if (w_rmatrix[w->w_index][i] & WITNESS_PARENT) {
                        sbuf_printf(sb, "\"%s\",\"%s\"\n", w->w_name,
                            w_data[i].w_name);
                        witness_add_fullgraph(sb, &w_data[i]);
                }
        }
}

/*
 * A simple hash function. Takes a key pointer and a key size. If size == 0,
 * interprets the key as a string and reads until the null
 * terminator. Otherwise, reads the first size bytes. Returns an unsigned 32-bit
 * hash value computed from the key.
 */
static uint32_t
witness_hash_djb2(const uint8_t *key, uint32_t size)
{
        unsigned int hash = 5381;
        int i;

        /* hash = hash * 33 + key[i] */
        if (size)
                for (i = 0; i < size; i++)
                        hash = ((hash << 5) + hash) + (unsigned int)key[i];
        else
                for (i = 0; key[i] != 0; i++)
                        hash = ((hash << 5) + hash) + (unsigned int)key[i];

        return (hash);
}

/*
 * Initializes the two witness hash tables. Called exactly once from
 * witness_initialize().
 */
static void
witness_init_hash_tables(void)
{
        int i;

        MPASS(witness_cold);

        /* Initialize the hash tables. */
        for (i = 0; i < WITNESS_HASH_SIZE; i++)
                w_hash.wh_array[i] = NULL;

        w_hash.wh_size = WITNESS_HASH_SIZE;
        w_hash.wh_count = 0;

        /* Initialize the lock order data hash. */
        w_lofree = NULL;
        for (i = 0; i < WITNESS_LO_DATA_COUNT; i++) {
                memset(&w_lodata[i], 0, sizeof(w_lodata[i]));
                w_lodata[i].wlod_next = w_lofree;
                w_lofree = &w_lodata[i];
        }
        w_lohash.wloh_size = WITNESS_LO_HASH_SIZE;
        w_lohash.wloh_count = 0;
        for (i = 0; i < WITNESS_LO_HASH_SIZE; i++)
                w_lohash.wloh_array[i] = NULL;
}

static struct witness *
witness_hash_get(const char *key)
{
        struct witness *w;
        uint32_t hash;

        MPASS(key != NULL);
        if (witness_cold == 0)
                mtx_assert(&w_mtx, MA_OWNED);
        hash = witness_hash_djb2(key, 0) % w_hash.wh_size;
        w = w_hash.wh_array[hash];
        while (w != NULL) {
                if (strcmp(w->w_name, key) == 0)
                        goto out;
                w = w->w_hash_next;
        }

out:
        return (w);
}

static void
witness_hash_put(struct witness *w)
{
        uint32_t hash;

        MPASS(w != NULL);
        MPASS(w->w_name != NULL);
        if (witness_cold == 0)
                mtx_assert(&w_mtx, MA_OWNED);
        KASSERT(witness_hash_get(w->w_name) == NULL,
            ("%s: trying to add a hash entry that already exists!", __func__));
        KASSERT(w->w_hash_next == NULL,
            ("%s: w->w_hash_next != NULL", __func__));

        hash = witness_hash_djb2(w->w_name, 0) % w_hash.wh_size;
        w->w_hash_next = w_hash.wh_array[hash];
        w_hash.wh_array[hash] = w;
        w_hash.wh_count++;
}

static struct witness_lock_order_data *
witness_lock_order_get(struct witness *parent, struct witness *child)
{
        struct witness_lock_order_data *data = NULL;
        struct witness_lock_order_key key;
        unsigned int hash;

        MPASS(parent != NULL && child != NULL);
        key.from = parent->w_index;
        key.to = child->w_index;
        WITNESS_INDEX_ASSERT(key.from);
        WITNESS_INDEX_ASSERT(key.to);
        if ((w_rmatrix[parent->w_index][child->w_index]
            & WITNESS_LOCK_ORDER_KNOWN) == 0)
                goto out;

        hash = witness_hash_djb2((const char*)&key,
            sizeof(key)) % w_lohash.wloh_size;
        data = w_lohash.wloh_array[hash];
        while (data != NULL) {
                if (witness_lock_order_key_equal(&data->wlod_key, &key))
                        break;
                data = data->wlod_next;
        }

out:
        return (data);
}

/*
 * Verify that parent and child have a known relationship, are not the same,
 * and child is actually a child of parent.  This is done without w_mtx
 * to avoid contention in the common case.
 */
static int
witness_lock_order_check(struct witness *parent, struct witness *child)
{

        if (parent != child &&
            w_rmatrix[parent->w_index][child->w_index]
            & WITNESS_LOCK_ORDER_KNOWN &&
            isitmychild(parent, child))
                return (1);

        return (0);
}

static int
witness_lock_order_add(struct witness *parent, struct witness *child)
{
        struct witness_lock_order_data *data = NULL;
        struct witness_lock_order_key key;
        unsigned int hash;

        MPASS(parent != NULL && child != NULL);
        key.from = parent->w_index;
        key.to = child->w_index;
        WITNESS_INDEX_ASSERT(key.from);
        WITNESS_INDEX_ASSERT(key.to);
        if (w_rmatrix[parent->w_index][child->w_index]
            & WITNESS_LOCK_ORDER_KNOWN)
                return (1);

        hash = witness_hash_djb2((const char*)&key,
            sizeof(key)) % w_lohash.wloh_size;
        w_rmatrix[parent->w_index][child->w_index] |= WITNESS_LOCK_ORDER_KNOWN;
        data = w_lofree;
        if (data == NULL)
                return (0);
        w_lofree = data->wlod_next;
        data->wlod_next = w_lohash.wloh_array[hash];
        data->wlod_key = key;
        w_lohash.wloh_array[hash] = data;
        w_lohash.wloh_count++;
        stack_zero(&data->wlod_stack);
        stack_save(&data->wlod_stack);
        return (1);
}

/* Call this whenever the structure of the witness graph changes. */
static void
witness_increment_graph_generation(void)
{

        if (witness_cold == 0)
                mtx_assert(&w_mtx, MA_OWNED);
        w_generation++;
}

static int
witness_output_drain(void *arg __unused, const char *data, int len)
{

        witness_output("%.*s", len, data);
        return (len);
}

static void
witness_debugger(int cond, const char *msg)
{
        char buf[32];
        struct sbuf sb;
        struct stack st;

        if (!cond)
                return;

        if (witness_trace) {
                sbuf_new(&sb, buf, sizeof(buf), SBUF_FIXEDLEN);
                sbuf_set_drain(&sb, witness_output_drain, NULL);

                stack_zero(&st);
                stack_save(&st);
                witness_output("stack backtrace:\n");
                stack_sbuf_print_ddb(&sb, &st);

                sbuf_finish(&sb);
        }

        witness_enter_debugger(msg);
}

static void
witness_enter_debugger(const char *msg)
{
#ifdef KDB
        if (witness_kdb)
                kdb_enter(KDB_WHY_WITNESS, msg);
#endif
}