FreeBSD/Linux Kernel Cross Reference
sys/kern/subr_witness.c

     /*-
      * 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: releng/9.2/sys/kern/subr_witness.c 249132 2013-04-05 08:22:11Z mav $");
    
    #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/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 this to check for blessed mutexes */
   #undef BLESSING
   
   #define WITNESS_COUNT           1024
   #define WITNESS_CHILDCOUNT      (WITNESS_COUNT * 4)
   #define WITNESS_HASH_SIZE       251     /* Prime, gives load factor < 2 */
   #define WITNESS_PENDLIST        768
   
   /* 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 BADSTACK_SBUF_SIZE      (256 * WITNESS_COUNT)
   #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;
   };
   
   #ifdef BLESSING
   struct witness_blessed {
           const char      *b_lock1;
           const char      *b_lock2;
   };
   #endif
   
   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_empty(const struct witness_lock_order_key *key)
   {
   
           return (key->from == 0 && key->to == 0);
   }
   
   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);
   #ifdef KDB
   static void     _witness_debugger(int cond, const char *msg);
   #endif
   static void     adopt(struct witness *parent, struct witness *child);
   #ifdef BLESSING
   static int      blessed(struct witness *, struct witness *);
   #endif
   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,
                                       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 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_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 void     witness_setflag(struct lock_object *lock, int flag, int set);
   
   #ifdef KDB
   #define witness_debugger(c)     _witness_debugger(c, __func__)
   #else
   #define witness_debugger(c)
   #endif
   
   static SYSCTL_NODE(_debug, OID_AUTO, witness, CTLFLAG_RW, 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;
   TUNABLE_INT("debug.witness.watch", &witness_watch);
   SYSCTL_PROC(_debug_witness, OID_AUTO, watch, CTLFLAG_RW | CTLTYPE_INT, 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
   TUNABLE_INT("debug.witness.kdb", &witness_kdb);
   SYSCTL_INT(_debug_witness, OID_AUTO, kdb, CTLFLAG_RW, &witness_kdb, 0, "");
   
   /*
    * When 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;
   TUNABLE_INT("debug.witness.trace", &witness_trace);
   SYSCTL_INT(_debug_witness, OID_AUTO, trace, CTLFLAG_RW, &witness_trace, 0, "");
   #endif /* KDB */
   
   #ifdef WITNESS_SKIPSPIN
   int     witness_skipspin = 1;
   #else
   int     witness_skipspin = 0;
   #endif
   TUNABLE_INT("debug.witness.skipspin", &witness_skipspin);
   SYSCTL_INT(_debug_witness, OID_AUTO, skipspin, CTLFLAG_RDTUN, &witness_skipspin,
       0, "");
   
   /*
    * Call this to print out the relations between locks.
    */
   SYSCTL_PROC(_debug_witness, OID_AUTO, fullgraph, CTLTYPE_STRING | CTLFLAG_RD,
       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,
       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[WITNESS_COUNT+1][WITNESS_COUNT+1];
   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";
   
   
   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 },
           { "process lock", &lock_class_mtx_sleep },
           { "session", &lock_class_mtx_sleep },
           { "uidinfo hash", &lock_class_rw },
   #ifdef  HWPMC_HOOKS
           { "pmc-sleep", &lock_class_mtx_sleep },
   #endif
           { "time 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_rw },
           { "rtentry", &lock_class_mtx_sleep },
           { "ifaddr", &lock_class_mtx_sleep },
           { NULL, NULL },
           /*
            * IPv4 multicast:
            * protocol locks before interface locks, after UDP locks.
            */
           { "udpinp", &lock_class_rw },
           { "in_multi_mtx", &lock_class_mtx_sleep },
           { "igmp_mtx", &lock_class_mtx_sleep },
           { "if_addr_mtx", &lock_class_mtx_sleep },
           { NULL, NULL },
           /*
            * IPv6 multicast:
            * protocol locks before interface locks, after UDP locks.
            */
           { "udpinp", &lock_class_rw },
           { "in6_multi_mtx", &lock_class_mtx_sleep },
           { "mld_mtx", &lock_class_mtx_sleep },
           { "if_addr_mtx", &lock_class_mtx_sleep },
           { NULL, NULL },
           /*
            * UNIX Domain Sockets
            */
           { "unp_global_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_rw },
           { "udpinp", &lock_class_rw },
           { "so_snd", &lock_class_mtx_sleep },
           { NULL, NULL },
           /*
            * TCP/IP
            */
           { "tcp", &lock_class_rw },
           { "tcpinp", &lock_class_rw },
           { "so_snd", &lock_class_mtx_sleep },
           { NULL, NULL },
           /*
            * netatalk
            */
           { "ddp_list_mtx", &lock_class_mtx_sleep },
           { "ddp_mtx", &lock_class_mtx_sleep },
           { NULL, NULL },
           /*
            * BPF
            */
           { "bpf global lock", &lock_class_mtx_sleep },
           { "bpf interface lock", &lock_class_rw },
           { "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 },
           { "vm page queue", &lock_class_mtx_sleep },
           { "vnode interlock", &lock_class_mtx_sleep },
           { "cdev", &lock_class_mtx_sleep },
           { NULL, NULL },
           /*
            * VM
            */
           { "vm map (user)", &lock_class_sx },
           { "vm object", &lock_class_mtx_sleep },
           { "vm page", &lock_class_mtx_sleep },
           { "vm page queue", &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 },
           { 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 },
           /*
            * ZFS locking
            */
           { "dn->dn_mtx", &lock_class_sx },
           { "dr->dt.di.dr_mtx", &lock_class_sx },
           { "db->db_mtx", &lock_class_sx },
           { 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 },
           { "scrlock", &lock_class_mtx_spin },
   #ifdef __i386__
           { "cy", &lock_class_mtx_spin },
   #endif
   #ifdef __sparc64__
           { "pcib_mtx", &lock_class_mtx_spin },
           { "rtc_mtx", &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 },
   #ifdef  HWPMC_HOOKS
           { "pmc-per-proc", &lock_class_mtx_spin },
   #endif
           { "process slock", &lock_class_mtx_spin },
           { "sleepq chain", &lock_class_mtx_spin },
           { "umtx lock", &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 },
           { "syscons video lock", &lock_class_mtx_spin },
   #ifdef SMP
           { "smp rendezvous", &lock_class_mtx_spin },
   #endif
   #ifdef __powerpc__
           { "tlb0", &lock_class_mtx_spin },
   #endif
           /*
            * 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 },
   #ifdef __ia64__
           { "MCA spin lock", &lock_class_mtx_spin },
   #endif
   #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 }
   };
   
   #ifdef BLESSING
   /*
    * Pairs of locks which have been blessed
    * Don't complain about order problems with blessed locks
    */
   static struct witness_blessed blessed_list[] = {
   };
   static int blessed_count =
           sizeof(blessed_list) / sizeof(struct witness_blessed);
   #endif
   
   /*
    * 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);
   }
   
   /*
    * 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.
    */
   static void
   witness_initialize(void *dummy __unused)
   {
           struct lock_object *lock;
           struct witness_order_list_entry *order;
           struct witness *w, *w1;
           int i;
   
           w_data = malloc(sizeof (struct witness) * WITNESS_COUNT, M_WITNESS,
               M_NOWAIT | M_ZERO);
   
           /*
            * 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--;
   
           memset(w_rmatrix, 0,
               (sizeof(**w_rmatrix) * (WITNESS_COUNT+1) * (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);
   }
   SYSINIT(witness_init, SI_SUB_WITNESS, SI_ORDER_FIRST, witness_initialize,
       NULL);
   
   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)
                   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)
                   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)
                   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 || panicstr != NULL ||
               (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, bump it\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 || panicstr != NULL)
                  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;
          struct witness *w, *w1;
          struct thread *td;
          int i, j;
  
          if (witness_cold || witness_watch < 1 || lock->lo_witness == NULL ||
              panicstr != NULL)
                  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)
                          panic("blockable sleep lock (%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) {
                          printf("shared lock of (%s) %s @ %s:%d\n",
                              class->lc_name, lock->lo_name,
                              fixup_filename(file), line);
                          printf("while exclusively locked from %s:%d\n",
                              fixup_filename(lock1->li_file), lock1->li_line);
                          panic("share->excl");
                  }
                  if ((lock1->li_flags & LI_EXCLUSIVE) == 0 &&
                      (flags & LOP_EXCLUSIVE) != 0) {
                          printf("exclusive lock of (%s) %s @ %s:%d\n",
                              class->lc_name, lock->lo_name,
                              fixup_filename(file), line);
                          printf("while share locked from %s:%d\n",
                              fixup_filename(lock1->li_file), lock1->li_line);
                          panic("excl->share");
                  }
                  return;
          }
  
          /*
           * 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.
           */
          w1 = plock->li_lock->lo_witness;
          if (witness_lock_order_check(w1, w))
                  return;
  
          /*
           * 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.
           */
          mtx_lock_spin(&w_mtx);
          witness_lock_order_add(w1, w);
          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);
                          printf(
                              "acquiring duplicate lock of same type: \"%s\"\n", 
                              w->w_name);
                          printf(" 1st %s @ %s:%d\n", plock->li_lock->lo_name,
                              fixup_filename(plock->li_file), plock->li_line);
                          printf(" 2nd %s @ %s:%d\n", lock->lo_name,
                              fixup_filename(file), line);
                          witness_debugger(1);
                  } 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++) {
  
                          MPASS(j < WITNESS_COUNT);
                          lock1 = &lle->ll_children[i];
  
                          /*
                           * Ignore the interlock the first time we see it.
                           */
                          if (interlock != NULL && interlock == lock1->li_lock) {
                                  interlock = NULL;
                                  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_lock->lo_flags & LO_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 &&
                              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 &&
                              (lock1->li_lock->lo_flags & LO_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_lock->lo_flags & LO_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.
                           */
  #ifdef BLESSING
  
                          /*
                           * If the lock order is blessed, just bail.  We don't
                           * look for other lock order violations though, which
                           * may be a bug.
                           */
                          if (blessed(w, w1))
                                  goto out;
  #endif
  
                          /* 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();
                          mtx_unlock_spin(&w_mtx);
                          
                          /*
                           * Ok, yell about it.
                           */
                          if (((lock->lo_flags & LO_SLEEPABLE) != 0 &&
                              (lock1->li_lock->lo_flags & LO_SLEEPABLE) == 0))
                                  printf(
                  "lock order reversal: (sleepable after non-sleepable)\n");
                          else if ((lock1->li_lock->lo_flags & LO_SLEEPABLE) == 0
                              && lock == &Giant.lock_object)
                                  printf(
                  "lock order reversal: (Giant after non-sleepable)\n");
                          else
                                  printf("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) {
                                  printf(" 1st %p %s (%s) @ %s:%d\n",
                                      lock1->li_lock, lock1->li_lock->lo_name,
                                      w1->w_name, fixup_filename(lock1->li_file),
                                      lock1->li_line);
                                  printf(" 2nd %p %s (%s) @ %s:%d\n", lock,
                                      lock->lo_name, w->w_name,
                                      fixup_filename(file), line);
                          } else {
                                  printf(" 1st %p %s (%s) @ %s:%d\n",
                                      lock2->li_lock, lock2->li_lock->lo_name,
                                      lock2->li_lock->lo_witness->w_name,
                                      fixup_filename(lock2->li_file),
                                      lock2->li_line);
                                  printf(" 2nd %p %s (%s) @ %s:%d\n",
                                      lock1->li_lock, lock1->li_lock->lo_name,
                                      w1->w_name, fixup_filename(lock1->li_file),
                                      lock1->li_line);
                                  printf(" 3rd %p %s (%s) @ %s:%d\n", lock,
                                      lock->lo_name, w->w_name,
                                      fixup_filename(file), line);
                          }
                          witness_debugger(1);
                          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)) {
                  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 ||
              panicstr != NULL)
                  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;
          if ((flags & LOP_EXCLUSIVE) != 0)
                  instance->li_flags = LI_EXCLUSIVE;
          else
                  instance->li_flags = 0;
          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 || panicstr != NULL)
                  return;
          class = LOCK_CLASS(lock);
          if (witness_watch) {
                  if ((lock->lo_flags & LO_UPGRADABLE) == 0)
                          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)
                          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)
                  panic("upgrade of unlocked lock (%s) %s @ %s:%d",
                      class->lc_name, lock->lo_name,
                      fixup_filename(file), line);
          if (witness_watch) {
                  if ((instance->li_flags & LI_EXCLUSIVE) != 0)
                          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)
                          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 || panicstr != NULL)
                  return;
          class = LOCK_CLASS(lock);
          if (witness_watch) {
                  if ((lock->lo_flags & LO_UPGRADABLE) == 0)
                  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)
                          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)
                  panic("downgrade of unlocked lock (%s) %s @ %s:%d",
                      class->lc_name, lock->lo_name,
                      fixup_filename(file), line);
          if (witness_watch) {
                  if ((instance->li_flags & LI_EXCLUSIVE) == 0)
                          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)
                          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 || panicstr != NULL)
                  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)
                  panic("lock (%s) %s not locked @ %s:%d", class->lc_name,
                      lock->lo_name, fixup_filename(file), line);
          else
                  return;
  found:
  
          /* First, check for shared/exclusive mismatches. */
          if ((instance->li_flags & LI_EXCLUSIVE) != 0 && witness_watch > 0 &&
              (flags & LOP_EXCLUSIVE) == 0) {
                  printf("shared unlock of (%s) %s @ %s:%d\n", class->lc_name,
                      lock->lo_name, fixup_filename(file), line);
                  printf("while exclusively locked from %s:%d\n",
                      fixup_filename(instance->li_file), instance->li_line);
                  panic("excl->ushare");
          }
          if ((instance->li_flags & LI_EXCLUSIVE) == 0 && witness_watch > 0 &&
              (flags & LOP_EXCLUSIVE) != 0) {
                  printf("exclusive unlock of (%s) %s @ %s:%d\n", class->lc_name,
                      lock->lo_name, fixup_filename(file), line);
                  printf("while share locked from %s:%d\n",
                      fixup_filename(instance->li_file),
                      instance->li_line);
                  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) {
                  printf("forbidden unlock of (%s) %s @ %s:%d\n", class->lc_name,
                      lock->lo_name, fixup_filename(file), line);
                  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 || panicstr != NULL)
                  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)
                  printf("Thread %p exiting with the following locks held:\n",
                                              td);
                                  n++;
                                  witness_list_lock(&lle->ll_children[i], printf);
                                  
                          }
                  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
   * console 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 || panicstr != NULL)
                  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_lock->lo_flags & LO_SLEEPABLE) != 0)
                                  continue;
                          if (n == 0) {
                                  va_start(ap, fmt);
                                  vprintf(fmt, ap);
                                  va_end(ap);
                                  printf(" with the following");
                                  if (flags & WARN_SLEEPOK)
                                          printf(" non-sleepable");
                                  printf(" locks held:\n");
                          }
                          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");
                  if (flags & WARN_SLEEPOK)
                          printf(" non-sleepable");
                  printf(" locks held:\n");
                  n += witness_list_locks(&lock_list, printf);
          } else
                  sched_unpin();
          if (flags & WARN_PANIC && n)
                  panic("%s", __func__);
          else
                  witness_debugger(n);
          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;
          struct witness_list *typelist;
  
          MPASS(description != NULL);
  
          if (witness_watch == -1 || panicstr != NULL)
                  return (NULL);
          if ((lock_class->lc_flags & LC_SPINLOCK)) {
                  if (witness_skipspin)
                          return (NULL);
                  else
                          typelist = &w_spin;
          } else if ((lock_class->lc_flags & LC_SLEEPLOCK))
                  typelist = &w_sleep;
          else
                  panic("lock class %s is not sleep or spin",
                      lock_class->lc_name);
  
          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++;
          mtx_unlock_spin(&w_mtx);
          if (lock_class != w->w_class)
                  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)
  {
          struct witness_list *list;
  
          MPASS(w->w_refcount == 0);
          if (w->w_class->lc_flags & LC_SLEEPLOCK) {
                  list = &w_sleep;
                  w_sleep_cnt--;
          } else {
                  list = &w_spin;
                  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)
  {
  
          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)
                          mtx_unlock_spin(&w_mtx);
                  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);
          }
          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))) {
                  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__));
  }
  
  #ifdef BLESSING
  static int
  blessed(struct witness *w1, struct witness *w2)
  {
          int i;
          struct witness_blessed *b;
  
          for (i = 0; i < blessed_count; 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);
  }
  #endif
  
  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, 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);
  }
  
  #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 || panicstr != NULL)
                  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)
                  panic("%s: lock (%s) %s not locked", __func__,
                      class->lc_name, lock->lo_name);
          *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 || panicstr != NULL)
                  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)
                  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;
          instance->li_file = file;
          instance->li_line = line;
  }
  
  void
  witness_assert(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 || panicstr != NULL)
                  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 {
                  panic("Lock (%s) %s is not sleep or spin!",
                      class->lc_name, lock->lo_name);
          }
          switch (flags) {
          case LA_UNLOCKED:
                  if (instance != NULL)
                          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) {
                          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)
                          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)
                          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)
                          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)
                          panic("Lock (%s) %s recursed @ %s:%d.",
                              class->lc_name, lock->lo_name,
                              fixup_filename(file), line);
                  break;
          default:
                  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 || panicstr != NULL)
                  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)
                  panic("%s: lock (%s) %s not locked", __func__,
                      class->lc_name, lock->lo_name);
  
          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 int
  sysctl_debug_witness_badstacks(SYSCTL_HANDLER_ARGS)
  {
          struct witness_lock_order_data *data1, *data2, *tmp_data1, *tmp_data2;
          struct witness *tmp_w1, *tmp_w2, *w1, *w2;
          struct sbuf *sb;
          u_int w_rmatrix1, w_rmatrix2;
          int error, generation, i, j;
  
          tmp_data1 = NULL;
          tmp_data2 = NULL;
          tmp_w1 = NULL;
          tmp_w2 = NULL;
          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);
  
          /* 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. */
                          req->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. */
                                  req->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;
                          w_rmatrix1 = (unsigned int)w_rmatrix[i][j];
                          w_rmatrix2 = (unsigned int)w_rmatrix[j][i];
  
                          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);
  
                          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 0
                          sbuf_printf(sb,
                          "w_rmatrix[%s][%s] == %x, w_rmatrix[%s][%s] == %x\n",
                              tmp_w1->name, tmp_w2->w_name, w_rmatrix1,
                              tmp_w2->name, tmp_w1->w_name, w_rmatrix2);
  #endif
                          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.
                   */
                  req->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);
  
          sbuf_finish(sb);
          error = SYSCTL_OUT(req, sbuf_data(sb), sbuf_len(sb) + 1);
          sbuf_delete(sb);
  
          return (error);
  }
  
  static int
  sysctl_debug_witness_fullgraph(SYSCTL_HANDLER_ARGS)
  {
          struct witness *w;
          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;
  
          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 whenver 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++;
  }
  
  #ifdef KDB
  static void
  _witness_debugger(int cond, const char *msg)
  {
  
          if (witness_trace && cond)
                  kdb_backtrace();
          if (witness_kdb && cond)
                  kdb_enter(KDB_WHY_WITNESS, msg);
  }
  #endif

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