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

     /*-
      * SPDX-License-Identifier: BSD-3-Clause
      *
      * 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 turnstiles used to hold queue of threads blocked on
     * non-sleepable locks.  Sleepable locks use condition variables to
     * implement their queues.  Turnstiles differ from a sleep queue in that
     * turnstile queue's are assigned to a lock held by an owning thread.  Thus,
     * when one thread is enqueued onto a turnstile, it can lend its priority
     * to the owning thread.
     *
     * We wish to avoid bloating locks with an embedded turnstile and we do not
     * want to use back-pointers in the locks for the same reason.  Thus, we
     * use a similar approach to that of Solaris 7 as described in Solaris
     * Internals by Jim Mauro and Richard McDougall.  Turnstiles are looked up
     * in a hash table based on the address of the lock.  Each entry in the
     * hash table is a linked-lists of turnstiles and is called a turnstile
     * chain.  Each chain contains a spin mutex that protects all of the
     * turnstiles in the chain.
     *
     * Each time a thread is created, a turnstile is allocated from a UMA zone
     * and attached to that thread.  When a thread blocks on a lock, if it is the
     * first thread to block, it lends its turnstile to the lock.  If the lock
     * already has a turnstile, then it gives its turnstile to the lock's
     * turnstile's free list.  When a thread is woken up, it takes a turnstile from
     * the free list if there are any other waiters.  If it is the only thread
     * blocked on the lock, then it reclaims the turnstile associated with the lock
     * and removes it from the hash table.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_ddb.h"
    #include "opt_turnstile_profiling.h"
    #include "opt_sched.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kdb.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/lock.h>
    #include <sys/mutex.h>
    #include <sys/proc.h>
    #include <sys/queue.h>
    #include <sys/sched.h>
    #include <sys/sdt.h>
    #include <sys/sysctl.h>
    #include <sys/turnstile.h>
    
    #include <vm/uma.h>
    
    #ifdef DDB
    #include <ddb/ddb.h>
    #include <sys/lockmgr.h>
    #include <sys/sx.h>
    #endif
    
    /*
     * Constants for the hash table of turnstile chains.  TC_SHIFT is a magic
     * number chosen because the sleep queue's use the same value for the
     * shift.  Basically, we ignore the lower 8 bits of the address.
     * TC_TABLESIZE must be a power of two for TC_MASK to work properly.
     */
    #define TC_TABLESIZE    128                     /* Must be power of 2. */
    #define TC_MASK         (TC_TABLESIZE - 1)
    #define TC_SHIFT        8
    #define TC_HASH(lock)   (((uintptr_t)(lock) >> TC_SHIFT) & TC_MASK)
   #define TC_LOOKUP(lock) &turnstile_chains[TC_HASH(lock)]
   
   /*
    * There are three different lists of turnstiles as follows.  The list
    * connected by ts_link entries is a per-thread list of all the turnstiles
    * attached to locks that we own.  This is used to fixup our priority when
    * a lock is released.  The other two lists use the ts_hash entries.  The
    * first of these two is the turnstile chain list that a turnstile is on
    * when it is attached to a lock.  The second list to use ts_hash is the
    * free list hung off of a turnstile that is attached to a lock.
    *
    * Each turnstile contains three lists of threads.  The two ts_blocked lists
    * are linked list of threads blocked on the turnstile's lock.  One list is
    * for exclusive waiters, and the other is for shared waiters.  The
    * ts_pending list is a linked list of threads previously awakened by
    * turnstile_signal() or turnstile_wait() that are waiting to be put on
    * the run queue.
    *
    * Locking key:
    *  c - turnstile chain lock
    *  q - td_contested lock
    */
   struct turnstile {
           struct mtx ts_lock;                     /* Spin lock for self. */
           struct threadqueue ts_blocked[2];       /* (c + q) Blocked threads. */
           struct threadqueue ts_pending;          /* (c) Pending threads. */
           LIST_ENTRY(turnstile) ts_hash;          /* (c) Chain and free list. */
           LIST_ENTRY(turnstile) ts_link;          /* (q) Contested locks. */
           LIST_HEAD(, turnstile) ts_free;         /* (c) Free turnstiles. */
           struct lock_object *ts_lockobj;         /* (c) Lock we reference. */
           struct thread *ts_owner;                /* (c + q) Who owns the lock. */
   };
   
   struct turnstile_chain {
           LIST_HEAD(, turnstile) tc_turnstiles;   /* List of turnstiles. */
           struct mtx tc_lock;                     /* Spin lock for this chain. */
   #ifdef TURNSTILE_PROFILING
           u_int   tc_depth;                       /* Length of tc_queues. */
           u_int   tc_max_depth;                   /* Max length of tc_queues. */
   #endif
   };
   
   #ifdef TURNSTILE_PROFILING
   u_int turnstile_max_depth;
   static SYSCTL_NODE(_debug, OID_AUTO, turnstile, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
       "turnstile profiling");
   static SYSCTL_NODE(_debug_turnstile, OID_AUTO, chains,
       CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
       "turnstile chain stats");
   SYSCTL_UINT(_debug_turnstile, OID_AUTO, max_depth, CTLFLAG_RD,
       &turnstile_max_depth, 0, "maximum depth achieved of a single chain");
   #endif
   static struct mtx td_contested_lock;
   static struct turnstile_chain turnstile_chains[TC_TABLESIZE];
   static uma_zone_t turnstile_zone;
   
   /*
    * Prototypes for non-exported routines.
    */
   static void     init_turnstile0(void *dummy);
   #ifdef TURNSTILE_PROFILING
   static void     init_turnstile_profiling(void *arg);
   #endif
   static void     propagate_priority(struct thread *td);
   static int      turnstile_adjust_thread(struct turnstile *ts,
                       struct thread *td);
   static struct thread *turnstile_first_waiter(struct turnstile *ts);
   static void     turnstile_setowner(struct turnstile *ts, struct thread *owner);
   #ifdef INVARIANTS
   static void     turnstile_dtor(void *mem, int size, void *arg);
   #endif
   static int      turnstile_init(void *mem, int size, int flags);
   static void     turnstile_fini(void *mem, int size);
   
   SDT_PROVIDER_DECLARE(sched);
   SDT_PROBE_DEFINE(sched, , , sleep);
   SDT_PROBE_DEFINE2(sched, , , wakeup, "struct thread *", 
       "struct proc *");
   
   static inline void
   propagate_unlock_ts(struct turnstile *top, struct turnstile *ts)
   {
   
           if (ts != top)
                   mtx_unlock_spin(&ts->ts_lock);
   }
   
   static inline void
   propagate_unlock_td(struct turnstile *top, struct thread *td)
   {
   
           if (td->td_lock != &top->ts_lock)
                   thread_unlock(td);
   }
   
   /*
    * Walks the chain of turnstiles and their owners to propagate the priority
    * of the thread being blocked to all the threads holding locks that have to
    * release their locks before this thread can run again.
    */
   static void
   propagate_priority(struct thread *td)
   {
           struct turnstile *ts, *top;
           int pri;
   
           THREAD_LOCK_ASSERT(td, MA_OWNED);
           pri = td->td_priority;
           top = ts = td->td_blocked;
           THREAD_LOCKPTR_ASSERT(td, &ts->ts_lock);
   
           /*
            * The original turnstile lock is held across the entire
            * operation.  We only ever lock down the chain so the lock
            * order is constant.
            */
           for (;;) {
                   td = ts->ts_owner;
   
                   if (td == NULL) {
                           /*
                            * This might be a read lock with no owner.  There's
                            * not much we can do, so just bail.
                            */
                           propagate_unlock_ts(top, ts);
                           return;
                   }
   
                   /*
                    * Wait for the thread lock to be stable and then only
                    * acquire if it is not the turnstile lock.
                    */
                   thread_lock_block_wait(td);
                   if (td->td_lock != &ts->ts_lock) {
                           thread_lock_flags(td, MTX_DUPOK);
                           propagate_unlock_ts(top, ts);
                   }
                   MPASS(td->td_proc != NULL);
                   MPASS(td->td_proc->p_magic == P_MAGIC);
   
                   /*
                    * If the thread is asleep, then we are probably about
                    * to deadlock.  To make debugging this easier, show
                    * backtrace of misbehaving thread and panic to not
                    * leave the kernel deadlocked.
                    */
                   if (TD_IS_SLEEPING(td)) {
                           printf(
                   "Sleeping thread (tid %d, pid %d) owns a non-sleepable lock\n",
                               td->td_tid, td->td_proc->p_pid);
                           kdb_backtrace_thread(td);
                           panic("sleeping thread");
                   }
   
                   /*
                    * If this thread already has higher priority than the
                    * thread that is being blocked, we are finished.
                    */
                   if (td->td_priority <= pri) {
                           propagate_unlock_td(top, td);
                           return;
                   }
   
                   /*
                    * Bump this thread's priority.
                    */
                   sched_lend_prio(td, pri);
   
                   /*
                    * If lock holder is actually running or on the run queue
                    * then we are done.
                    */
                   if (TD_IS_RUNNING(td) || TD_ON_RUNQ(td)) {
                           MPASS(td->td_blocked == NULL);
                           propagate_unlock_td(top, td);
                           return;
                   }
   
   #ifndef SMP
                   /*
                    * For UP, we check to see if td is curthread (this shouldn't
                    * ever happen however as it would mean we are in a deadlock.)
                    */
                   KASSERT(td != curthread, ("Deadlock detected"));
   #endif
   
                   /*
                    * If we aren't blocked on a lock, we should be.
                    */
                   KASSERT(TD_ON_LOCK(td), (
                       "thread %d(%s):%d holds %s but isn't blocked on a lock\n",
                       td->td_tid, td->td_name, td->td_state,
                       ts->ts_lockobj->lo_name));
   
                   /*
                    * Pick up the lock that td is blocked on.
                    */
                   ts = td->td_blocked;
                   MPASS(ts != NULL);
                   THREAD_LOCKPTR_ASSERT(td, &ts->ts_lock);
                   /* Resort td on the list if needed. */
                   if (!turnstile_adjust_thread(ts, td)) {
                           propagate_unlock_ts(top, ts);
                           return;
                   }
                   /* The thread lock is released as ts lock above. */
           }
   }
   
   /*
    * Adjust the thread's position on a turnstile after its priority has been
    * changed.
    */
   static int
   turnstile_adjust_thread(struct turnstile *ts, struct thread *td)
   {
           struct thread *td1, *td2;
           int queue;
   
           THREAD_LOCK_ASSERT(td, MA_OWNED);
           MPASS(TD_ON_LOCK(td));
   
           /*
            * This thread may not be blocked on this turnstile anymore
            * but instead might already be woken up on another CPU
            * that is waiting on the thread lock in turnstile_unpend() to
            * finish waking this thread up.  We can detect this case
            * by checking to see if this thread has been given a
            * turnstile by either turnstile_signal() or
            * turnstile_broadcast().  In this case, treat the thread as
            * if it was already running.
            */
           if (td->td_turnstile != NULL)
                   return (0);
   
           /*
            * Check if the thread needs to be moved on the blocked chain.
            * It needs to be moved if either its priority is lower than
            * the previous thread or higher than the next thread.
            */
           THREAD_LOCKPTR_BLOCKED_ASSERT(td, &ts->ts_lock);
           td1 = TAILQ_PREV(td, threadqueue, td_lockq);
           td2 = TAILQ_NEXT(td, td_lockq);
           if ((td1 != NULL && td->td_priority < td1->td_priority) ||
               (td2 != NULL && td->td_priority > td2->td_priority)) {
                   /*
                    * Remove thread from blocked chain and determine where
                    * it should be moved to.
                    */
                   queue = td->td_tsqueue;
                   MPASS(queue == TS_EXCLUSIVE_QUEUE || queue == TS_SHARED_QUEUE);
                   mtx_lock_spin(&td_contested_lock);
                   TAILQ_REMOVE(&ts->ts_blocked[queue], td, td_lockq);
                   TAILQ_FOREACH(td1, &ts->ts_blocked[queue], td_lockq) {
                           MPASS(td1->td_proc->p_magic == P_MAGIC);
                           if (td1->td_priority > td->td_priority)
                                   break;
                   }
   
                   if (td1 == NULL)
                           TAILQ_INSERT_TAIL(&ts->ts_blocked[queue], td, td_lockq);
                   else
                           TAILQ_INSERT_BEFORE(td1, td, td_lockq);
                   mtx_unlock_spin(&td_contested_lock);
                   if (td1 == NULL)
                           CTR3(KTR_LOCK,
                       "turnstile_adjust_thread: td %d put at tail on [%p] %s",
                               td->td_tid, ts->ts_lockobj, ts->ts_lockobj->lo_name);
                   else
                           CTR4(KTR_LOCK,
                       "turnstile_adjust_thread: td %d moved before %d on [%p] %s",
                               td->td_tid, td1->td_tid, ts->ts_lockobj,
                               ts->ts_lockobj->lo_name);
           }
           return (1);
   }
   
   /*
    * Early initialization of turnstiles.  This is not done via a SYSINIT()
    * since this needs to be initialized very early when mutexes are first
    * initialized.
    */
   void
   init_turnstiles(void)
   {
           int i;
   
           for (i = 0; i < TC_TABLESIZE; i++) {
                   LIST_INIT(&turnstile_chains[i].tc_turnstiles);
                   mtx_init(&turnstile_chains[i].tc_lock, "turnstile chain",
                       NULL, MTX_SPIN);
           }
           mtx_init(&td_contested_lock, "td_contested", NULL, MTX_SPIN);
           LIST_INIT(&thread0.td_contested);
           thread0.td_turnstile = NULL;
   }
   
   #ifdef TURNSTILE_PROFILING
   static void
   init_turnstile_profiling(void *arg)
   {
           struct sysctl_oid *chain_oid;
           char chain_name[10];
           int i;
   
           for (i = 0; i < TC_TABLESIZE; i++) {
                   snprintf(chain_name, sizeof(chain_name), "%d", i);
                   chain_oid = SYSCTL_ADD_NODE(NULL, 
                       SYSCTL_STATIC_CHILDREN(_debug_turnstile_chains), OID_AUTO,
                       chain_name, CTLFLAG_RD | CTLFLAG_MPSAFE, NULL,
                       "turnstile chain stats");
                   SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
                       "depth", CTLFLAG_RD, &turnstile_chains[i].tc_depth, 0,
                       NULL);
                   SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
                       "max_depth", CTLFLAG_RD, &turnstile_chains[i].tc_max_depth,
                       0, NULL);
           }
   }
   SYSINIT(turnstile_profiling, SI_SUB_LOCK, SI_ORDER_ANY,
       init_turnstile_profiling, NULL);
   #endif
   
   static void
   init_turnstile0(void *dummy)
   {
   
           turnstile_zone = uma_zcreate("TURNSTILE", sizeof(struct turnstile),
               NULL,
   #ifdef INVARIANTS
               turnstile_dtor,
   #else
               NULL,
   #endif
               turnstile_init, turnstile_fini, UMA_ALIGN_CACHE, UMA_ZONE_NOFREE);
           thread0.td_turnstile = turnstile_alloc();
   }
   SYSINIT(turnstile0, SI_SUB_LOCK, SI_ORDER_ANY, init_turnstile0, NULL);
   
   /*
    * Update a thread on the turnstile list after it's priority has been changed.
    * The old priority is passed in as an argument.
    */
   void
   turnstile_adjust(struct thread *td, u_char oldpri)
   {
           struct turnstile *ts;
   
           MPASS(TD_ON_LOCK(td));
   
           /*
            * Pick up the lock that td is blocked on.
            */
           ts = td->td_blocked;
           MPASS(ts != NULL);
           THREAD_LOCKPTR_BLOCKED_ASSERT(td, &ts->ts_lock);
           mtx_assert(&ts->ts_lock, MA_OWNED);
   
           /* Resort the turnstile on the list. */
           if (!turnstile_adjust_thread(ts, td))
                   return;
           /*
            * If our priority was lowered and we are at the head of the
            * turnstile, then propagate our new priority up the chain.
            * Note that we currently don't try to revoke lent priorities
            * when our priority goes up.
            */
           MPASS(td->td_tsqueue == TS_EXCLUSIVE_QUEUE ||
               td->td_tsqueue == TS_SHARED_QUEUE);
           if (td == TAILQ_FIRST(&ts->ts_blocked[td->td_tsqueue]) &&
               td->td_priority < oldpri) {
                   propagate_priority(td);
           }
   }
   
   /*
    * Set the owner of the lock this turnstile is attached to.
    */
   static void
   turnstile_setowner(struct turnstile *ts, struct thread *owner)
   {
   
           mtx_assert(&td_contested_lock, MA_OWNED);
           MPASS(ts->ts_owner == NULL);
   
           /* A shared lock might not have an owner. */
           if (owner == NULL)
                   return;
   
           MPASS(owner->td_proc->p_magic == P_MAGIC);
           ts->ts_owner = owner;
           LIST_INSERT_HEAD(&owner->td_contested, ts, ts_link);
   }
   
   #ifdef INVARIANTS
   /*
    * UMA zone item deallocator.
    */
   static void
   turnstile_dtor(void *mem, int size, void *arg)
   {
           struct turnstile *ts;
   
           ts = mem;
           MPASS(TAILQ_EMPTY(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]));
           MPASS(TAILQ_EMPTY(&ts->ts_blocked[TS_SHARED_QUEUE]));
           MPASS(TAILQ_EMPTY(&ts->ts_pending));
   }
   #endif
   
   /*
    * UMA zone item initializer.
    */
   static int
   turnstile_init(void *mem, int size, int flags)
   {
           struct turnstile *ts;
   
           bzero(mem, size);
           ts = mem;
           TAILQ_INIT(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]);
           TAILQ_INIT(&ts->ts_blocked[TS_SHARED_QUEUE]);
           TAILQ_INIT(&ts->ts_pending);
           LIST_INIT(&ts->ts_free);
           mtx_init(&ts->ts_lock, "turnstile lock", NULL, MTX_SPIN);
           return (0);
   }
   
   static void
   turnstile_fini(void *mem, int size)
   {
           struct turnstile *ts;
   
           ts = mem;
           mtx_destroy(&ts->ts_lock);
   }
   
   /*
    * Get a turnstile for a new thread.
    */
   struct turnstile *
   turnstile_alloc(void)
   {
   
           return (uma_zalloc(turnstile_zone, M_WAITOK));
   }
   
   /*
    * Free a turnstile when a thread is destroyed.
    */
   void
   turnstile_free(struct turnstile *ts)
   {
   
           uma_zfree(turnstile_zone, ts);
   }
   
   /*
    * Lock the turnstile chain associated with the specified lock.
    */
   void
   turnstile_chain_lock(struct lock_object *lock)
   {
           struct turnstile_chain *tc;
   
           tc = TC_LOOKUP(lock);
           mtx_lock_spin(&tc->tc_lock);
   }
   
   struct turnstile *
   turnstile_trywait(struct lock_object *lock)
   {
           struct turnstile_chain *tc;
           struct turnstile *ts;
   
           tc = TC_LOOKUP(lock);
           mtx_lock_spin(&tc->tc_lock);
           LIST_FOREACH(ts, &tc->tc_turnstiles, ts_hash)
                   if (ts->ts_lockobj == lock) {
                           mtx_lock_spin(&ts->ts_lock);
                           return (ts);
                   }
   
           ts = curthread->td_turnstile;
           MPASS(ts != NULL);
           mtx_lock_spin(&ts->ts_lock);
           KASSERT(ts->ts_lockobj == NULL, ("stale ts_lockobj pointer"));
           ts->ts_lockobj = lock;
   
           return (ts);
   }
   
   bool
   turnstile_lock(struct turnstile *ts, struct lock_object **lockp,
       struct thread **tdp)
   {
           struct turnstile_chain *tc;
           struct lock_object *lock;
   
           if ((lock = ts->ts_lockobj) == NULL)
                   return (false);
           tc = TC_LOOKUP(lock);
           mtx_lock_spin(&tc->tc_lock);
           mtx_lock_spin(&ts->ts_lock);
           if (__predict_false(lock != ts->ts_lockobj)) {
                   mtx_unlock_spin(&tc->tc_lock);
                   mtx_unlock_spin(&ts->ts_lock);
                   return (false);
           }
           *lockp = lock;
           *tdp = ts->ts_owner;
           return (true);
   }
   
   void
   turnstile_unlock(struct turnstile *ts, struct lock_object *lock)
   {
           struct turnstile_chain *tc;
   
           mtx_assert(&ts->ts_lock, MA_OWNED);
           mtx_unlock_spin(&ts->ts_lock);
           if (ts == curthread->td_turnstile)
                   ts->ts_lockobj = NULL;
           tc = TC_LOOKUP(lock);
           mtx_unlock_spin(&tc->tc_lock);
   }
   
   void
   turnstile_assert(struct turnstile *ts)
   {
           MPASS(ts->ts_lockobj == NULL);
   }
   
   void
   turnstile_cancel(struct turnstile *ts)
   {
           struct turnstile_chain *tc;
           struct lock_object *lock;
   
           mtx_assert(&ts->ts_lock, MA_OWNED);
   
           mtx_unlock_spin(&ts->ts_lock);
           lock = ts->ts_lockobj;
           if (ts == curthread->td_turnstile)
                   ts->ts_lockobj = NULL;
           tc = TC_LOOKUP(lock);
           mtx_unlock_spin(&tc->tc_lock);
   }
   
   /*
    * Look up the turnstile for a lock in the hash table locking the associated
    * turnstile chain along the way.  If no turnstile is found in the hash
    * table, NULL is returned.
    */
   struct turnstile *
   turnstile_lookup(struct lock_object *lock)
   {
           struct turnstile_chain *tc;
           struct turnstile *ts;
   
           tc = TC_LOOKUP(lock);
           mtx_assert(&tc->tc_lock, MA_OWNED);
           LIST_FOREACH(ts, &tc->tc_turnstiles, ts_hash)
                   if (ts->ts_lockobj == lock) {
                           mtx_lock_spin(&ts->ts_lock);
                           return (ts);
                   }
           return (NULL);
   }
   
   /*
    * Unlock the turnstile chain associated with a given lock.
    */
   void
   turnstile_chain_unlock(struct lock_object *lock)
   {
           struct turnstile_chain *tc;
   
           tc = TC_LOOKUP(lock);
           mtx_unlock_spin(&tc->tc_lock);
   }
   
   /*
    * Return a pointer to the thread waiting on this turnstile with the
    * most important priority or NULL if the turnstile has no waiters.
    */
   static struct thread *
   turnstile_first_waiter(struct turnstile *ts)
   {
           struct thread *std, *xtd;
   
           std = TAILQ_FIRST(&ts->ts_blocked[TS_SHARED_QUEUE]);
           xtd = TAILQ_FIRST(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]);
           if (xtd == NULL || (std != NULL && std->td_priority < xtd->td_priority))
                   return (std);
           return (xtd);
   }
   
   /*
    * Take ownership of a turnstile and adjust the priority of the new
    * owner appropriately.
    */
   void
   turnstile_claim(struct turnstile *ts)
   {
           struct thread *td, *owner;
           struct turnstile_chain *tc;
   
           mtx_assert(&ts->ts_lock, MA_OWNED);
           MPASS(ts != curthread->td_turnstile);
   
           owner = curthread;
           mtx_lock_spin(&td_contested_lock);
           turnstile_setowner(ts, owner);
           mtx_unlock_spin(&td_contested_lock);
   
           td = turnstile_first_waiter(ts);
           MPASS(td != NULL);
           MPASS(td->td_proc->p_magic == P_MAGIC);
           THREAD_LOCKPTR_BLOCKED_ASSERT(td, &ts->ts_lock);
   
           /*
            * Update the priority of the new owner if needed.
            */
           thread_lock(owner);
           if (td->td_priority < owner->td_priority)
                   sched_lend_prio(owner, td->td_priority);
           thread_unlock(owner);
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_unlock_spin(&ts->ts_lock);
           mtx_unlock_spin(&tc->tc_lock);
   }
   
   /*
    * Block the current thread on the turnstile assicated with 'lock'.  This
    * function will context switch and not return until this thread has been
    * woken back up.  This function must be called with the appropriate
    * turnstile chain locked and will return with it unlocked.
    */
   void
   turnstile_wait(struct turnstile *ts, struct thread *owner, int queue)
   {
           struct turnstile_chain *tc;
           struct thread *td, *td1;
           struct lock_object *lock;
   
           td = curthread;
           mtx_assert(&ts->ts_lock, MA_OWNED);
           if (owner)
                   MPASS(owner->td_proc->p_magic == P_MAGIC);
           MPASS(queue == TS_SHARED_QUEUE || queue == TS_EXCLUSIVE_QUEUE);
   
           /*
            * If the lock does not already have a turnstile, use this thread's
            * turnstile.  Otherwise insert the current thread into the
            * turnstile already in use by this lock.
            */
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
           if (ts == td->td_turnstile) {
   #ifdef TURNSTILE_PROFILING
                   tc->tc_depth++;
                   if (tc->tc_depth > tc->tc_max_depth) {
                           tc->tc_max_depth = tc->tc_depth;
                           if (tc->tc_max_depth > turnstile_max_depth)
                                   turnstile_max_depth = tc->tc_max_depth;
                   }
   #endif
                   LIST_INSERT_HEAD(&tc->tc_turnstiles, ts, ts_hash);
                   KASSERT(TAILQ_EMPTY(&ts->ts_pending),
                       ("thread's turnstile has pending threads"));
                   KASSERT(TAILQ_EMPTY(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]),
                       ("thread's turnstile has exclusive waiters"));
                   KASSERT(TAILQ_EMPTY(&ts->ts_blocked[TS_SHARED_QUEUE]),
                       ("thread's turnstile has shared waiters"));
                   KASSERT(LIST_EMPTY(&ts->ts_free),
                       ("thread's turnstile has a non-empty free list"));
                   MPASS(ts->ts_lockobj != NULL);
                   mtx_lock_spin(&td_contested_lock);
                   TAILQ_INSERT_TAIL(&ts->ts_blocked[queue], td, td_lockq);
                   turnstile_setowner(ts, owner);
                   mtx_unlock_spin(&td_contested_lock);
           } else {
                   TAILQ_FOREACH(td1, &ts->ts_blocked[queue], td_lockq)
                           if (td1->td_priority > td->td_priority)
                                   break;
                   mtx_lock_spin(&td_contested_lock);
                   if (td1 != NULL)
                           TAILQ_INSERT_BEFORE(td1, td, td_lockq);
                   else
                           TAILQ_INSERT_TAIL(&ts->ts_blocked[queue], td, td_lockq);
                   MPASS(owner == ts->ts_owner);
                   mtx_unlock_spin(&td_contested_lock);
                   MPASS(td->td_turnstile != NULL);
                   LIST_INSERT_HEAD(&ts->ts_free, td->td_turnstile, ts_hash);
           }
           thread_lock(td);
           thread_lock_set(td, &ts->ts_lock);
           td->td_turnstile = NULL;
   
           /* Save who we are blocked on and switch. */
           lock = ts->ts_lockobj;
           td->td_tsqueue = queue;
           td->td_blocked = ts;
           td->td_lockname = lock->lo_name;
           td->td_blktick = ticks;
           TD_SET_LOCK(td);
           mtx_unlock_spin(&tc->tc_lock);
           propagate_priority(td);
   
           if (LOCK_LOG_TEST(lock, 0))
                   CTR4(KTR_LOCK, "%s: td %d blocked on [%p] %s", __func__,
                       td->td_tid, lock, lock->lo_name);
   
           SDT_PROBE0(sched, , , sleep);
   
           THREAD_LOCKPTR_ASSERT(td, &ts->ts_lock);
           mi_switch(SW_VOL | SWT_TURNSTILE);
   
           if (LOCK_LOG_TEST(lock, 0))
                   CTR4(KTR_LOCK, "%s: td %d free from blocked on [%p] %s",
                       __func__, td->td_tid, lock, lock->lo_name);
   }
   
   /*
    * Pick the highest priority thread on this turnstile and put it on the
    * pending list.  This must be called with the turnstile chain locked.
    */
   int
   turnstile_signal(struct turnstile *ts, int queue)
   {
           struct turnstile_chain *tc __unused;
           struct thread *td;
           int empty;
   
           MPASS(ts != NULL);
           mtx_assert(&ts->ts_lock, MA_OWNED);
           MPASS(curthread->td_proc->p_magic == P_MAGIC);
           MPASS(ts->ts_owner == curthread || ts->ts_owner == NULL);
           MPASS(queue == TS_SHARED_QUEUE || queue == TS_EXCLUSIVE_QUEUE);
   
           /*
            * Pick the highest priority thread blocked on this lock and
            * move it to the pending list.
            */
           td = TAILQ_FIRST(&ts->ts_blocked[queue]);
           MPASS(td->td_proc->p_magic == P_MAGIC);
           mtx_lock_spin(&td_contested_lock);
           TAILQ_REMOVE(&ts->ts_blocked[queue], td, td_lockq);
           mtx_unlock_spin(&td_contested_lock);
           TAILQ_INSERT_TAIL(&ts->ts_pending, td, td_lockq);
   
           /*
            * If the turnstile is now empty, remove it from its chain and
            * give it to the about-to-be-woken thread.  Otherwise take a
            * turnstile from the free list and give it to the thread.
            */
           empty = TAILQ_EMPTY(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]) &&
               TAILQ_EMPTY(&ts->ts_blocked[TS_SHARED_QUEUE]);
           if (empty) {
                   tc = TC_LOOKUP(ts->ts_lockobj);
                   mtx_assert(&tc->tc_lock, MA_OWNED);
                   MPASS(LIST_EMPTY(&ts->ts_free));
   #ifdef TURNSTILE_PROFILING
                   tc->tc_depth--;
   #endif
           } else
                   ts = LIST_FIRST(&ts->ts_free);
           MPASS(ts != NULL);
           LIST_REMOVE(ts, ts_hash);
           td->td_turnstile = ts;
   
           return (empty);
   }
   
   /*
    * Put all blocked threads on the pending list.  This must be called with
    * the turnstile chain locked.
    */
   void
   turnstile_broadcast(struct turnstile *ts, int queue)
   {
           struct turnstile_chain *tc __unused;
           struct turnstile *ts1;
           struct thread *td;
   
           MPASS(ts != NULL);
           mtx_assert(&ts->ts_lock, MA_OWNED);
           MPASS(curthread->td_proc->p_magic == P_MAGIC);
           MPASS(ts->ts_owner == curthread || ts->ts_owner == NULL);
           /*
            * We must have the chain locked so that we can remove the empty
            * turnstile from the hash queue.
            */
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
           MPASS(queue == TS_SHARED_QUEUE || queue == TS_EXCLUSIVE_QUEUE);
   
           /*
            * Transfer the blocked list to the pending list.
            */
           mtx_lock_spin(&td_contested_lock);
           TAILQ_CONCAT(&ts->ts_pending, &ts->ts_blocked[queue], td_lockq);
           mtx_unlock_spin(&td_contested_lock);
   
           /*
            * Give a turnstile to each thread.  The last thread gets
            * this turnstile if the turnstile is empty.
            */
           TAILQ_FOREACH(td, &ts->ts_pending, td_lockq) {
                   if (LIST_EMPTY(&ts->ts_free)) {
                           MPASS(TAILQ_NEXT(td, td_lockq) == NULL);
                           ts1 = ts;
   #ifdef TURNSTILE_PROFILING
                           tc->tc_depth--;
   #endif
                   } else
                           ts1 = LIST_FIRST(&ts->ts_free);
                   MPASS(ts1 != NULL);
                   LIST_REMOVE(ts1, ts_hash);
                   td->td_turnstile = ts1;
           }
   }
   
   static u_char
   turnstile_calc_unlend_prio_locked(struct thread *td)
   {
           struct turnstile *nts;
           u_char cp, pri;
   
           THREAD_LOCK_ASSERT(td, MA_OWNED);
           mtx_assert(&td_contested_lock, MA_OWNED);
   
           pri = PRI_MAX;
           LIST_FOREACH(nts, &td->td_contested, ts_link) {
                   cp = turnstile_first_waiter(nts)->td_priority;
                   if (cp < pri)
                           pri = cp;
           }
           return (pri);
   }
   
   /*
    * Wakeup all threads on the pending list and adjust the priority of the
    * current thread appropriately.  This must be called with the turnstile
    * chain locked.
    */
   void
   turnstile_unpend(struct turnstile *ts)
   {
           TAILQ_HEAD( ,thread) pending_threads;
           struct thread *td;
           u_char pri;
   
           MPASS(ts != NULL);
           mtx_assert(&ts->ts_lock, MA_OWNED);
           MPASS(ts->ts_owner == curthread || ts->ts_owner == NULL);
           MPASS(!TAILQ_EMPTY(&ts->ts_pending));
   
           /*
            * Move the list of pending threads out of the turnstile and
            * into a local variable.
            */
           TAILQ_INIT(&pending_threads);
           TAILQ_CONCAT(&pending_threads, &ts->ts_pending, td_lockq);
   #ifdef INVARIANTS
           if (TAILQ_EMPTY(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]) &&
               TAILQ_EMPTY(&ts->ts_blocked[TS_SHARED_QUEUE]))
                   ts->ts_lockobj = NULL;
   #endif
           /*
            * Adjust the priority of curthread based on other contested
            * locks it owns.  Don't lower the priority below the base
            * priority however.
            */
           td = curthread;
           thread_lock(td);
           mtx_lock_spin(&td_contested_lock);
           /*
            * Remove the turnstile from this thread's list of contested locks
            * since this thread doesn't own it anymore.  New threads will
            * not be blocking on the turnstile until it is claimed by a new
            * owner.  There might not be a current owner if this is a shared
            * lock.
            */
           if (ts->ts_owner != NULL) {
                   ts->ts_owner = NULL;
                   LIST_REMOVE(ts, ts_link);
           }
           pri = turnstile_calc_unlend_prio_locked(td);
           mtx_unlock_spin(&td_contested_lock);
           sched_unlend_prio(td, pri);
           thread_unlock(td);
           /*
            * Wake up all the pending threads.  If a thread is not blocked
            * on a lock, then it is currently executing on another CPU in
            * turnstile_wait() or sitting on a run queue waiting to resume
            * in turnstile_wait().  Set a flag to force it to try to acquire
            * the lock again instead of blocking.
            */
          while (!TAILQ_EMPTY(&pending_threads)) {
                  td = TAILQ_FIRST(&pending_threads);
                  TAILQ_REMOVE(&pending_threads, td, td_lockq);
                  SDT_PROBE2(sched, , , wakeup, td, td->td_proc);
                  thread_lock_block_wait(td);
                  THREAD_LOCKPTR_ASSERT(td, &ts->ts_lock);
                  MPASS(td->td_proc->p_magic == P_MAGIC);
                  MPASS(TD_ON_LOCK(td));
                  TD_CLR_LOCK(td);
                  MPASS(TD_CAN_RUN(td));
                  td->td_blocked = NULL;
                  td->td_lockname = NULL;
                  td->td_blktick = 0;
  #ifdef INVARIANTS
                  td->td_tsqueue = 0xff;
  #endif
                  sched_add(td, SRQ_HOLD | SRQ_BORING);
          }
          mtx_unlock_spin(&ts->ts_lock);
  }
  
  /*
   * Give up ownership of a turnstile.  This must be called with the
   * turnstile chain locked.
   */
  void
  turnstile_disown(struct turnstile *ts)
  {
          struct thread *td;
          u_char pri;
  
          MPASS(ts != NULL);
          mtx_assert(&ts->ts_lock, MA_OWNED);
          MPASS(ts->ts_owner == curthread);
          MPASS(TAILQ_EMPTY(&ts->ts_pending));
          MPASS(!TAILQ_EMPTY(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]) ||
              !TAILQ_EMPTY(&ts->ts_blocked[TS_SHARED_QUEUE]));
  
          /*
           * Remove the turnstile from this thread's list of contested locks
           * since this thread doesn't own it anymore.  New threads will
           * not be blocking on the turnstile until it is claimed by a new
           * owner.
           */
          mtx_lock_spin(&td_contested_lock);
          ts->ts_owner = NULL;
          LIST_REMOVE(ts, ts_link);
          mtx_unlock_spin(&td_contested_lock);
  
          /*
           * Adjust the priority of curthread based on other contested
           * locks it owns.  Don't lower the priority below the base
           * priority however.
           */
          td = curthread;
          thread_lock(td);
          mtx_unlock_spin(&ts->ts_lock);
          mtx_lock_spin(&td_contested_lock);
          pri = turnstile_calc_unlend_prio_locked(td);
          mtx_unlock_spin(&td_contested_lock);
          sched_unlend_prio(td, pri);
          thread_unlock(td);
  }
  
  /*
   * Return the first thread in a turnstile.
   */
  struct thread *
  turnstile_head(struct turnstile *ts, int queue)
  {
  #ifdef INVARIANTS
  
          MPASS(ts != NULL);
          MPASS(queue == TS_SHARED_QUEUE || queue == TS_EXCLUSIVE_QUEUE);
          mtx_assert(&ts->ts_lock, MA_OWNED);
  #endif
          return (TAILQ_FIRST(&ts->ts_blocked[queue]));
  }
  
  /*
   * Returns true if a sub-queue of a turnstile is empty.
   */
  int
  turnstile_empty(struct turnstile *ts, int queue)
  {
  #ifdef INVARIANTS
  
          MPASS(ts != NULL);
          MPASS(queue == TS_SHARED_QUEUE || queue == TS_EXCLUSIVE_QUEUE);
          mtx_assert(&ts->ts_lock, MA_OWNED);
  #endif
          return (TAILQ_EMPTY(&ts->ts_blocked[queue]));
  }
  
  #ifdef DDB
  static void
  print_thread(struct thread *td, const char *prefix)
  {
  
          db_printf("%s%p (tid %d, pid %d, \"%s\")\n", prefix, td, td->td_tid,
              td->td_proc->p_pid, td->td_name);
  }
  
  static void
  print_queue(struct threadqueue *queue, const char *header, const char *prefix)
  {
          struct thread *td;
  
          db_printf("%s:\n", header);
          if (TAILQ_EMPTY(queue)) {
                  db_printf("%sempty\n", prefix);
                  return;
          }
          TAILQ_FOREACH(td, queue, td_lockq) {
                  print_thread(td, prefix);
          }
  }
  
  DB_SHOW_COMMAND(turnstile, db_show_turnstile)
  {
          struct turnstile_chain *tc;
          struct turnstile *ts;
          struct lock_object *lock;
          int i;
  
          if (!have_addr)
                  return;
  
          /*
           * First, see if there is an active turnstile for the lock indicated
           * by the address.
           */
          lock = (struct lock_object *)addr;
          tc = TC_LOOKUP(lock);
          LIST_FOREACH(ts, &tc->tc_turnstiles, ts_hash)
                  if (ts->ts_lockobj == lock)
                          goto found;
  
          /*
           * Second, see if there is an active turnstile at the address
           * indicated.
           */
          for (i = 0; i < TC_TABLESIZE; i++)
                  LIST_FOREACH(ts, &turnstile_chains[i].tc_turnstiles, ts_hash) {
                          if (ts == (struct turnstile *)addr)
                                  goto found;
                  }
  
          db_printf("Unable to locate a turnstile via %p\n", (void *)addr);
          return;
  found:
          lock = ts->ts_lockobj;
          db_printf("Lock: %p - (%s) %s\n", lock, LOCK_CLASS(lock)->lc_name,
              lock->lo_name);
          if (ts->ts_owner)
                  print_thread(ts->ts_owner, "Lock Owner: ");
          else
                  db_printf("Lock Owner: none\n");
          print_queue(&ts->ts_blocked[TS_SHARED_QUEUE], "Shared Waiters", "\t");
          print_queue(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE], "Exclusive Waiters",
              "\t");
          print_queue(&ts->ts_pending, "Pending Threads", "\t");
  
  }
  
  /*
   * Show all the threads a particular thread is waiting on based on
   * non-spin locks.
   */
  static void
  print_lockchain(struct thread *td, const char *prefix)
  {
          struct lock_object *lock;
          struct lock_class *class;
          struct turnstile *ts;
          struct thread *owner;
  
          /*
           * Follow the chain.  We keep walking as long as the thread is
           * blocked on a lock that has an owner.
           */
          while (!db_pager_quit) {
                  if (td == (void *)LK_KERNPROC) {
                          db_printf("%sdisowned (LK_KERNPROC)\n", prefix);
                          return;
                  }
                  db_printf("%sthread %d (pid %d, %s) is ", prefix, td->td_tid,
                      td->td_proc->p_pid, td->td_name);
                  switch (td->td_state) {
                  case TDS_INACTIVE:
                          db_printf("inactive\n");
                          return;
                  case TDS_CAN_RUN:
                          db_printf("runnable\n");
                          return;
                  case TDS_RUNQ:
                          db_printf("on a run queue\n");
                          return;
                  case TDS_RUNNING:
                          db_printf("running on CPU %d\n", td->td_oncpu);
                          return;
                  case TDS_INHIBITED:
                          if (TD_ON_LOCK(td)) {
                                  ts = td->td_blocked;
                                  lock = ts->ts_lockobj;
                                  class = LOCK_CLASS(lock);
                                  db_printf("blocked on lock %p (%s) \"%s\"\n",
                                      lock, class->lc_name, lock->lo_name);
                                  if (ts->ts_owner == NULL)
                                          return;
                                  td = ts->ts_owner;
                                  break;
                          } else if (TD_ON_SLEEPQ(td)) {
                                  if (!lockmgr_chain(td, &owner) &&
                                      !sx_chain(td, &owner)) {
                                          db_printf("sleeping on %p \"%s\"\n",
                                              td->td_wchan, td->td_wmesg);
                                          return;
                                  }
                                  if (owner == NULL)
                                          return;
                                  td = owner;
                                  break;
                          }
                          db_printf("inhibited: %s\n", KTDSTATE(td));
                          return;
                  default:
                          db_printf("??? (%#x)\n", td->td_state);
                          return;
                  }
          }
  }
  
  DB_SHOW_COMMAND(lockchain, db_show_lockchain)
  {
          struct thread *td;
  
          /* Figure out which thread to start with. */
          if (have_addr)
                  td = db_lookup_thread(addr, true);
          else
                  td = kdb_thread;
  
          print_lockchain(td, "");
  }
  DB_SHOW_ALIAS(sleepchain, db_show_lockchain);
  
  DB_SHOW_ALL_COMMAND(chains, db_show_allchains)
  {
          struct thread *td;
          struct proc *p;
          int i;
  
          i = 1;
          FOREACH_PROC_IN_SYSTEM(p) {
                  FOREACH_THREAD_IN_PROC(p, td) {
                          if ((TD_ON_LOCK(td) && LIST_EMPTY(&td->td_contested))
                              || (TD_IS_INHIBITED(td) && TD_ON_SLEEPQ(td))) {
                                  db_printf("chain %d:\n", i++);
                                  print_lockchain(td, " ");
                          }
                          if (db_pager_quit)
                                  return;
                  }
          }
  }
  DB_SHOW_ALIAS(allchains, db_show_allchains)
  
  static void     print_waiters(struct turnstile *ts, int indent);
  
  static void
  print_waiter(struct thread *td, int indent)
  {
          struct turnstile *ts;
          int i;
  
          if (db_pager_quit)
                  return;
          for (i = 0; i < indent; i++)
                  db_printf(" ");
          print_thread(td, "thread ");
          LIST_FOREACH(ts, &td->td_contested, ts_link)
                  print_waiters(ts, indent + 1);
  }
  
  static void
  print_waiters(struct turnstile *ts, int indent)
  {
          struct lock_object *lock;
          struct lock_class *class;
          struct thread *td;
          int i;
  
          if (db_pager_quit)
                  return;
          lock = ts->ts_lockobj;
          class = LOCK_CLASS(lock);
          for (i = 0; i < indent; i++)
                  db_printf(" ");
          db_printf("lock %p (%s) \"%s\"\n", lock, class->lc_name, lock->lo_name);
          TAILQ_FOREACH(td, &ts->ts_blocked[TS_EXCLUSIVE_QUEUE], td_lockq)
                  print_waiter(td, indent + 1);
          TAILQ_FOREACH(td, &ts->ts_blocked[TS_SHARED_QUEUE], td_lockq)
                  print_waiter(td, indent + 1);
          TAILQ_FOREACH(td, &ts->ts_pending, td_lockq)
                  print_waiter(td, indent + 1);
  }
  
  DB_SHOW_COMMAND(locktree, db_show_locktree)
  {
          struct lock_object *lock;
          struct lock_class *class;
          struct turnstile_chain *tc;
          struct turnstile *ts;
  
          if (!have_addr)
                  return;
          lock = (struct lock_object *)addr;
          tc = TC_LOOKUP(lock);
          LIST_FOREACH(ts, &tc->tc_turnstiles, ts_hash)
                  if (ts->ts_lockobj == lock)
                          break;
          if (ts == NULL) {
                  class = LOCK_CLASS(lock);
                  db_printf("lock %p (%s) \"%s\"\n", lock, class->lc_name,
                      lock->lo_name);
          } else
                  print_waiters(ts, 0);
  }
  #endif

Cache object: 1eed0a8fd7062bee9e74c1b02558de10