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

     /*-
      * 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 malloc'd 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 "opt_turnstile_profiling.h"
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/6.1/sys/kern/subr_turnstile.c 158179 2006-04-30 16:44:43Z cvs2svn $");
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/lock.h>
    #include <sys/malloc.h>
    #include <sys/mutex.h>
    #include <sys/proc.h>
    #include <sys/queue.h>
    #include <sys/sched.h>
    #include <sys/sysctl.h>
    #include <sys/turnstile.h>
    
    /*
     * 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 two lists of threads.  The ts_blocked list is
     * a linked list of threads blocked on the turnstile's lock.  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 {
           TAILQ_HEAD(, thread) ts_blocked;        /* (c + q) Blocked threads. */
           TAILQ_HEAD(, thread) 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;
   SYSCTL_NODE(_debug, OID_AUTO, turnstile, CTLFLAG_RD, 0, "turnstile profiling");
   SYSCTL_NODE(_debug_turnstile, OID_AUTO, chains, CTLFLAG_RD, 0,
       "turnstile chain stats");
   SYSCTL_UINT(_debug_turnstile, OID_AUTO, max_depth, CTLFLAG_RD,
       &turnstile_max_depth, 0, "maxmimum depth achieved of a single chain");
   #endif
   static struct mtx td_contested_lock;
   static struct turnstile_chain turnstile_chains[TC_TABLESIZE];
   
   static MALLOC_DEFINE(M_TURNSTILE, "turnstiles", "turnstiles");
   
   /*
    * 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 void     turnstile_setowner(struct turnstile *ts, struct thread *owner);
   
   /*
    * 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_chain *tc;
           struct turnstile *ts;
           int pri;
   
           mtx_assert(&sched_lock, MA_OWNED);
           pri = td->td_priority;
           ts = td->td_blocked;
           for (;;) {
                   td = ts->ts_owner;
   
                   if (td == NULL) {
                           /*
                            * This really isn't quite right. Really
                            * ought to bump priority of thread that
                            * next acquires the lock.
                            */
                           return;
                   }
   
                   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, just
                    * panic and tell the user which thread misbehaved so
                    * they can hopefully get a stack trace from the truly
                    * misbehaving thread.
                    */
                   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);
   #ifdef DDB
                           db_trace_thread(td, -1);
   #endif
                           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)
                           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);
                           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_proc->p_comm, td->td_state,
                       ts->ts_lockobj->lo_name));
   
                   /*
                    * Pick up the lock that td is blocked on.
                    */
                   ts = td->td_blocked;
                   MPASS(ts != NULL);
                   tc = TC_LOOKUP(ts->ts_lockobj);
                   mtx_lock_spin(&tc->tc_lock);
   
                   /* Resort td on the list if needed. */
                   if (!turnstile_adjust_thread(ts, td)) {
                           mtx_unlock_spin(&tc->tc_lock);
                           return;
                   }
                   mtx_unlock_spin(&tc->tc_lock);
           }
   }
   
   /*
    * 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 turnstile_chain *tc;
           struct thread *td1, *td2;
   
           mtx_assert(&sched_lock, 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 sched_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.
            */
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
           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.
                    */
                   mtx_lock_spin(&td_contested_lock);
                   TAILQ_REMOVE(&ts->ts_blocked, td, td_lockq);
                   TAILQ_FOREACH(td1, &ts->ts_blocked, 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, 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);
           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, 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)
   {
   
           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_chain *tc;
           struct turnstile *ts;
   
           mtx_assert(&sched_lock, MA_OWNED);
           MPASS(TD_ON_LOCK(td));
   
           /*
            * Pick up the lock that td is blocked on.
            */
           ts = td->td_blocked;
           MPASS(ts != NULL);
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_lock_spin(&tc->tc_lock);
   
           /* Resort the turnstile on the list. */
           if (!turnstile_adjust_thread(ts, td)) {
                   mtx_unlock_spin(&tc->tc_lock);
                   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.
            */
           if (td == TAILQ_FIRST(&ts->ts_blocked) && td->td_priority < oldpri) {
                   mtx_unlock_spin(&tc->tc_lock);
                   propagate_priority(td);
           } else
                   mtx_unlock_spin(&tc->tc_lock);
   }
   
   /*
    * 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(owner->td_proc->p_magic == P_MAGIC);
           MPASS(ts->ts_owner == NULL);
           ts->ts_owner = owner;
           LIST_INSERT_HEAD(&owner->td_contested, ts, ts_link);
   }
   
   /*
    * Malloc a turnstile for a new thread, initialize it and return it.
    */
   struct turnstile *
   turnstile_alloc(void)
   {
           struct turnstile *ts;
   
           ts = malloc(sizeof(struct turnstile), M_TURNSTILE, M_WAITOK | M_ZERO);
           TAILQ_INIT(&ts->ts_blocked);
           TAILQ_INIT(&ts->ts_pending);
           LIST_INIT(&ts->ts_free);
           return (ts);
   }
   
   /*
    * Free a turnstile when a thread is destroyed.
    */
   void
   turnstile_free(struct turnstile *ts)
   {
   
           MPASS(ts != NULL);
           MPASS(TAILQ_EMPTY(&ts->ts_blocked));
           MPASS(TAILQ_EMPTY(&ts->ts_pending));
           free(ts, M_TURNSTILE);
   }
   
   /*
    * Lock the turnstile chain associated with the specified lock.
    */
   void
   turnstile_lock(struct lock_object *lock)
   {
           struct turnstile_chain *tc;
   
           tc = TC_LOOKUP(lock);
           mtx_lock_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)
                           return (ts);
           return (NULL);
   }
   
   /*
    * Unlock the turnstile chain associated with a given lock.
    */
   void
   turnstile_release(struct lock_object *lock)
   {
           struct turnstile_chain *tc;
   
           tc = TC_LOOKUP(lock);
           mtx_unlock_spin(&tc->tc_lock);
   }
   
   /*
    * Take ownership of a turnstile and adjust the priority of the new
    * owner appropriately.
    */
   void
   turnstile_claim(struct lock_object *lock)
   {
           struct turnstile_chain *tc;
           struct turnstile *ts;
           struct thread *td, *owner;
   
           tc = TC_LOOKUP(lock);
           mtx_assert(&tc->tc_lock, MA_OWNED);
           ts = turnstile_lookup(lock);
           MPASS(ts != NULL);
   
           owner = curthread;
           mtx_lock_spin(&td_contested_lock);
           turnstile_setowner(ts, owner);
           mtx_unlock_spin(&td_contested_lock);
   
           td = TAILQ_FIRST(&ts->ts_blocked);
           MPASS(td != NULL);
           MPASS(td->td_proc->p_magic == P_MAGIC);
           mtx_unlock_spin(&tc->tc_lock);
   
           /*
            * Update the priority of the new owner if needed.
            */
           mtx_lock_spin(&sched_lock);
           if (td->td_priority < owner->td_priority)
                   sched_lend_prio(owner, td->td_priority);
           mtx_unlock_spin(&sched_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 lock_object *lock, struct thread *owner)
   {
           struct turnstile_chain *tc;
           struct turnstile *ts;
           struct thread *td, *td1;
   
           td = curthread;
           tc = TC_LOOKUP(lock);
           mtx_assert(&tc->tc_lock, MA_OWNED);
           MPASS(td->td_turnstile != NULL);
           MPASS(owner != NULL);
           MPASS(owner->td_proc->p_magic == P_MAGIC);
   
           /* Look up the turnstile associated with the lock 'lock'. */
           ts = turnstile_lookup(lock);
   
           /*
            * 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.
            */
           if (ts == NULL) {
   #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
                   ts = td->td_turnstile;
                   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),
                       ("thread's turnstile has a non-empty queue"));
                   KASSERT(LIST_EMPTY(&ts->ts_free),
                       ("thread's turnstile has a non-empty free list"));
                   KASSERT(ts->ts_lockobj == NULL, ("stale ts_lockobj pointer"));
                   ts->ts_lockobj = lock;
                   mtx_lock_spin(&td_contested_lock);
                   TAILQ_INSERT_TAIL(&ts->ts_blocked, td, td_lockq);
                   turnstile_setowner(ts, owner);
                   mtx_unlock_spin(&td_contested_lock);
           } else {
                   TAILQ_FOREACH(td1, &ts->ts_blocked, 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, td, td_lockq);
                   mtx_unlock_spin(&td_contested_lock);
                   MPASS(td->td_turnstile != NULL);
                   LIST_INSERT_HEAD(&ts->ts_free, td->td_turnstile, ts_hash);
                   MPASS(owner == ts->ts_owner);
           }
           td->td_turnstile = NULL;
           mtx_unlock_spin(&tc->tc_lock);
   
           mtx_lock_spin(&sched_lock);
           /*
            * Handle race condition where a thread on another CPU that owns
            * lock 'lock' could have woken us in between us dropping the
            * turnstile chain lock and acquiring the sched_lock.
            */
           if (td->td_flags & TDF_TSNOBLOCK) {
                   td->td_flags &= ~TDF_TSNOBLOCK;
                   mtx_unlock_spin(&sched_lock);
                   return;
           }
                   
   #ifdef notyet
           /*
            * If we're borrowing an interrupted thread's VM context, we
            * must clean up before going to sleep.
            */
           if (td->td_ithd != NULL) {
                   struct ithd *it = td->td_ithd;
   
                   if (it->it_interrupted) {
                           if (LOCK_LOG_TEST(lock, 0))
                                   CTR3(KTR_LOCK, "%s: %p interrupted %p",
                                       __func__, it, it->it_interrupted);
                           intr_thd_fixup(it);
                   }
           }
   #endif
   
           /* Save who we are blocked on and switch. */
           td->td_blocked = ts;
           td->td_lockname = lock->lo_name;
           TD_SET_LOCK(td);
           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);
   
           mi_switch(SW_VOL, NULL);
   
           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);
   
           mtx_unlock_spin(&sched_lock);
   }
   
   /*
    * 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)
   {
           struct turnstile_chain *tc;
           struct thread *td;
           int empty;
   
           MPASS(ts != NULL);
           MPASS(curthread->td_proc->p_magic == P_MAGIC);
           MPASS(ts->ts_owner == curthread);
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
   
           /*
            * Pick the highest priority thread blocked on this lock and
            * move it to the pending list.
            */
           td = TAILQ_FIRST(&ts->ts_blocked);
           MPASS(td->td_proc->p_magic == P_MAGIC);
           mtx_lock_spin(&td_contested_lock);
           TAILQ_REMOVE(&ts->ts_blocked, 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);
           if (empty) {
                   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)
   {
           struct turnstile_chain *tc;
           struct turnstile *ts1;
           struct thread *td;
   
           MPASS(ts != NULL);
           MPASS(curthread->td_proc->p_magic == P_MAGIC);
           MPASS(ts->ts_owner == curthread);
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
   
           /*
            * Transfer the blocked list to the pending list.
            */
           mtx_lock_spin(&td_contested_lock);
           TAILQ_CONCAT(&ts->ts_pending, &ts->ts_blocked, td_lockq);
           mtx_unlock_spin(&td_contested_lock);
   
           /*
            * Give a turnstile to each thread.  The last thread gets
            * this turnstile.
            */
           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;
           }
   }
   
   /*
    * 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 turnstile_chain *tc;
           struct thread *td;
           u_char cp, pri;
   
           MPASS(ts != NULL);
           MPASS(ts->ts_owner == curthread);
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
           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->ts_lockobj = NULL;
   #endif
   
           /*
            * 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);
           critical_enter();
           mtx_unlock_spin(&tc->tc_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;
           pri = PRI_MAX;
           mtx_lock_spin(&sched_lock);
           mtx_lock_spin(&td_contested_lock);
           LIST_FOREACH(ts, &td->td_contested, ts_link) {
                   cp = TAILQ_FIRST(&ts->ts_blocked)->td_priority;
                   if (cp < pri)
                           pri = cp;
           }
           mtx_unlock_spin(&td_contested_lock);
           sched_unlend_prio(td, pri);
   
           /*
            * 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);
                   MPASS(td->td_proc->p_magic == P_MAGIC);
                   if (TD_ON_LOCK(td)) {
                           td->td_blocked = NULL;
                           td->td_lockname = NULL;
                           TD_CLR_LOCK(td);
                           MPASS(TD_CAN_RUN(td));
                           setrunqueue(td, SRQ_BORING);
                   } else {
                           td->td_flags |= TDF_TSNOBLOCK;
                           MPASS(TD_IS_RUNNING(td) || TD_ON_RUNQ(td));
                   }
           }
           critical_exit();
           mtx_unlock_spin(&sched_lock);
   }
   
   /*
    * Return the first thread in a turnstile.
    */
   struct thread *
   turnstile_head(struct turnstile *ts)
   {
   #ifdef INVARIANTS
           struct turnstile_chain *tc;
   
           MPASS(ts != NULL);
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
   #endif
           return (TAILQ_FIRST(&ts->ts_blocked));
   }
   
   /*
    * Returns true if a turnstile is empty.
    */
   int
   turnstile_empty(struct turnstile *ts)
   {
   #ifdef INVARIANTS
           struct turnstile_chain *tc;
   
           MPASS(ts != NULL);
           tc = TC_LOOKUP(ts->ts_lockobj);
           mtx_assert(&tc->tc_lock, MA_OWNED);
   #endif
           return (TAILQ_EMPTY(&ts->ts_blocked));
   }

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