The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/kern/subr_turnstile.c

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    1 /*-
    2  * Copyright (c) 1998 Berkeley Software Design, Inc. All rights reserved.
    3  *
    4  * Redistribution and use in source and binary forms, with or without
    5  * modification, are permitted provided that the following conditions
    6  * are met:
    7  * 1. Redistributions of source code must retain the above copyright
    8  *    notice, this list of conditions and the following disclaimer.
    9  * 2. Redistributions in binary form must reproduce the above copyright
   10  *    notice, this list of conditions and the following disclaimer in the
   11  *    documentation and/or other materials provided with the distribution.
   12  * 3. Berkeley Software Design Inc's name may not be used to endorse or
   13  *    promote products derived from this software without specific prior
   14  *    written permission.
   15  *
   16  * THIS SOFTWARE IS PROVIDED BY BERKELEY SOFTWARE DESIGN INC ``AS IS'' AND
   17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   19  * ARE DISCLAIMED.  IN NO EVENT SHALL BERKELEY SOFTWARE DESIGN INC BE LIABLE
   20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   26  * SUCH DAMAGE.
   27  *
   28  *      from BSDI $Id: mutex_witness.c,v 1.1.2.20 2000/04/27 03:10:27 cp Exp $
   29  *      and BSDI $Id: synch_machdep.c,v 2.3.2.39 2000/04/27 03:10:25 cp Exp $
   30  */
   31 
   32 /*
   33  * Implementation of turnstiles used to hold queue of threads blocked on
   34  * non-sleepable locks.  Sleepable locks use condition variables to
   35  * implement their queues.  Turnstiles differ from a sleep queue in that
   36  * turnstile queue's are assigned to a lock held by an owning thread.  Thus,
   37  * when one thread is enqueued onto a turnstile, it can lend its priority
   38  * to the owning thread.
   39  *
   40  * We wish to avoid bloating locks with an embedded turnstile and we do not
   41  * want to use back-pointers in the locks for the same reason.  Thus, we
   42  * use a similar approach to that of Solaris 7 as described in Solaris
   43  * Internals by Jim Mauro and Richard McDougall.  Turnstiles are looked up
   44  * in a hash table based on the address of the lock.  Each entry in the
   45  * hash table is a linked-lists of turnstiles and is called a turnstile
   46  * chain.  Each chain contains a spin mutex that protects all of the
   47  * turnstiles in the chain.
   48  *
   49  * Each time a thread is created, a turnstile is malloc'd and attached to
   50  * that thread.  When a thread blocks on a lock, if it is the first thread
   51  * to block, it lends its turnstile to the lock.  If the lock already has
   52  * a turnstile, then it gives its turnstile to the lock's turnstile's free
   53  * list.  When a thread is woken up, it takes a turnstile from the free list
   54  * if there are any other waiters.  If it is the only thread blocked on the
   55  * lock, then it reclaims the turnstile associated with the lock and removes
   56  * it from the hash table.
   57  */
   58 
   59 #include "opt_turnstile_profiling.h"
   60 
   61 #include <sys/cdefs.h>
   62 __FBSDID("$FreeBSD: releng/5.3/sys/kern/subr_turnstile.c 136588 2004-10-16 08:43:07Z cvs2svn $");
   63 
   64 #include <sys/param.h>
   65 #include <sys/systm.h>
   66 #include <sys/kernel.h>
   67 #include <sys/ktr.h>
   68 #include <sys/lock.h>
   69 #include <sys/malloc.h>
   70 #include <sys/mutex.h>
   71 #include <sys/proc.h>
   72 #include <sys/queue.h>
   73 #include <sys/resourcevar.h>
   74 #include <sys/sched.h>
   75 #include <sys/sysctl.h>
   76 #include <sys/turnstile.h>
   77 
   78 /*
   79  * Constants for the hash table of turnstile chains.  TC_SHIFT is a magic
   80  * number chosen because the sleep queue's use the same value for the
   81  * shift.  Basically, we ignore the lower 8 bits of the address.
   82  * TC_TABLESIZE must be a power of two for TC_MASK to work properly.
   83  */
   84 #define TC_TABLESIZE    128                     /* Must be power of 2. */
   85 #define TC_MASK         (TC_TABLESIZE - 1)
   86 #define TC_SHIFT        8
   87 #define TC_HASH(lock)   (((uintptr_t)(lock) >> TC_SHIFT) & TC_MASK)
   88 #define TC_LOOKUP(lock) &turnstile_chains[TC_HASH(lock)]
   89 
   90 /*
   91  * There are three different lists of turnstiles as follows.  The list
   92  * connected by ts_link entries is a per-thread list of all the turnstiles
   93  * attached to locks that we own.  This is used to fixup our priority when
   94  * a lock is released.  The other two lists use the ts_hash entries.  The
   95  * first of these two is the turnstile chain list that a turnstile is on
   96  * when it is attached to a lock.  The second list to use ts_hash is the
   97  * free list hung off of a turnstile that is attached to a lock.
   98  *
   99  * Each turnstile contains two lists of threads.  The ts_blocked list is
  100  * a linked list of threads blocked on the turnstile's lock.  The
  101  * ts_pending list is a linked list of threads previously awakened by
  102  * turnstile_signal() or turnstile_wait() that are waiting to be put on
  103  * the run queue.
  104  *
  105  * Locking key:
  106  *  c - turnstile chain lock
  107  *  q - td_contested lock
  108  */
  109 struct turnstile {
  110         TAILQ_HEAD(, thread) ts_blocked;        /* (c + q) Blocked threads. */
  111         TAILQ_HEAD(, thread) ts_pending;        /* (c) Pending threads. */
  112         LIST_ENTRY(turnstile) ts_hash;          /* (c) Chain and free list. */
  113         LIST_ENTRY(turnstile) ts_link;          /* (q) Contested locks. */
  114         LIST_HEAD(, turnstile) ts_free;         /* (c) Free turnstiles. */
  115         struct lock_object *ts_lockobj;         /* (c) Lock we reference. */
  116         struct thread *ts_owner;                /* (c + q) Who owns the lock. */
  117 };
  118 
  119 struct turnstile_chain {
  120         LIST_HEAD(, turnstile) tc_turnstiles;   /* List of turnstiles. */
  121         struct mtx tc_lock;                     /* Spin lock for this chain. */
  122 #ifdef TURNSTILE_PROFILING
  123         u_int   tc_depth;                       /* Length of tc_queues. */
  124         u_int   tc_max_depth;                   /* Max length of tc_queues. */
  125 #endif
  126 };
  127 
  128 #ifdef TURNSTILE_PROFILING
  129 u_int turnstile_max_depth;
  130 SYSCTL_NODE(_debug, OID_AUTO, turnstile, CTLFLAG_RD, 0, "turnstile profiling");
  131 SYSCTL_NODE(_debug_turnstile, OID_AUTO, chains, CTLFLAG_RD, 0,
  132     "turnstile chain stats");
  133 SYSCTL_UINT(_debug_turnstile, OID_AUTO, max_depth, CTLFLAG_RD,
  134     &turnstile_max_depth, 0, "maxmimum depth achieved of a single chain");
  135 #endif
  136 static struct mtx td_contested_lock;
  137 static struct turnstile_chain turnstile_chains[TC_TABLESIZE];
  138 
  139 MALLOC_DEFINE(M_TURNSTILE, "turnstiles", "turnstiles");
  140 
  141 /*
  142  * Prototypes for non-exported routines.
  143  */
  144 static void     init_turnstile0(void *dummy);
  145 #ifdef TURNSTILE_PROFILING
  146 static void     init_turnstile_profiling(void *arg);
  147 #endif
  148 static void     propagate_priority(struct thread *);
  149 static void     turnstile_setowner(struct turnstile *ts, struct thread *owner);
  150 
  151 /*
  152  * Walks the chain of turnstiles and their owners to propagate the priority
  153  * of the thread being blocked to all the threads holding locks that have to
  154  * release their locks before this thread can run again.
  155  */
  156 static void
  157 propagate_priority(struct thread *td)
  158 {
  159         struct turnstile_chain *tc;
  160         struct turnstile *ts;
  161         struct thread *td1;
  162         int pri;
  163 
  164         mtx_assert(&sched_lock, MA_OWNED);
  165         pri = td->td_priority;
  166         ts = td->td_blocked;
  167         for (;;) {
  168                 td = ts->ts_owner;
  169 
  170                 if (td == NULL) {
  171                         /*
  172                          * This really isn't quite right. Really
  173                          * ought to bump priority of thread that
  174                          * next acquires the lock.
  175                          */
  176                         return;
  177                 }
  178 
  179                 MPASS(td->td_proc != NULL);
  180                 MPASS(td->td_proc->p_magic == P_MAGIC);
  181 
  182                 /*
  183                  * XXX: The owner of a turnstile can be stale if it is the
  184                  * first thread to grab a slock of a sx lock.  In that case
  185                  * it is possible for us to be at SSLEEP or some other
  186                  * weird state.  We should probably just return if the state
  187                  * isn't SRUN or SLOCK.
  188                  */
  189                 KASSERT(!TD_IS_SLEEPING(td),
  190                     ("sleeping thread (pid %d) owns a non-sleepable lock",
  191                     td->td_proc->p_pid));
  192 
  193                 /*
  194                  * If this thread already has higher priority than the
  195                  * thread that is being blocked, we are finished.
  196                  */
  197                 if (td->td_priority <= pri)
  198                         return;
  199 
  200                 /*
  201                  * If lock holder is actually running, just bump priority.
  202                  */
  203                 if (TD_IS_RUNNING(td)) {
  204                         td->td_priority = pri;
  205                         return;
  206                 }
  207 
  208 #ifndef SMP
  209                 /*
  210                  * For UP, we check to see if td is curthread (this shouldn't
  211                  * ever happen however as it would mean we are in a deadlock.)
  212                  */
  213                 KASSERT(td != curthread, ("Deadlock detected"));
  214 #endif
  215 
  216                 /*
  217                  * If on run queue move to new run queue, and quit.
  218                  * XXXKSE this gets a lot more complicated under threads
  219                  * but try anyhow.
  220                  */
  221                 if (TD_ON_RUNQ(td)) {
  222                         MPASS(td->td_blocked == NULL);
  223                         sched_prio(td, pri);
  224                         return;
  225                 }
  226 
  227                 /*
  228                  * Bump this thread's priority.
  229                  */
  230                 td->td_priority = pri;
  231 
  232                 /*
  233                  * If we aren't blocked on a lock, we should be.
  234                  */
  235                 KASSERT(TD_ON_LOCK(td), (
  236                     "process %d(%s):%d holds %s but isn't blocked on a lock\n",
  237                     td->td_proc->p_pid, td->td_proc->p_comm, td->td_state,
  238                     ts->ts_lockobj->lo_name));
  239 
  240                 /*
  241                  * Pick up the lock that td is blocked on.
  242                  */
  243                 ts = td->td_blocked;
  244                 MPASS(ts != NULL);
  245                 tc = TC_LOOKUP(ts->ts_lockobj);
  246                 mtx_lock_spin(&tc->tc_lock);
  247 
  248                 /*
  249                  * This thread may not be blocked on this turnstile anymore
  250                  * but instead might already be woken up on another CPU
  251                  * that is waiting on sched_lock in turnstile_unpend() to
  252                  * finish waking this thread up.  We can detect this case
  253                  * by checking to see if this thread has been given a
  254                  * turnstile by either turnstile_signal() or
  255                  * turnstile_broadcast().  In this case, treat the thread as
  256                  * if it was already running.
  257                  */
  258                 if (td->td_turnstile != NULL) {
  259                         mtx_unlock_spin(&tc->tc_lock);
  260                         return;
  261                 }
  262 
  263                 /*
  264                  * Check if the thread needs to be moved up on
  265                  * the blocked chain.  It doesn't need to be moved
  266                  * if it is already at the head of the list or if
  267                  * the item in front of it still has a higher priority.
  268                  */
  269                 if (td == TAILQ_FIRST(&ts->ts_blocked)) {
  270                         mtx_unlock_spin(&tc->tc_lock);
  271                         continue;
  272                 }
  273 
  274                 td1 = TAILQ_PREV(td, threadqueue, td_lockq);
  275                 if (td1->td_priority <= pri) {
  276                         mtx_unlock_spin(&tc->tc_lock);
  277                         continue;
  278                 }
  279 
  280                 /*
  281                  * Remove thread from blocked chain and determine where
  282                  * it should be moved up to.  Since we know that td1 has
  283                  * a lower priority than td, we know that at least one
  284                  * thread in the chain has a lower priority and that
  285                  * td1 will thus not be NULL after the loop.
  286                  */
  287                 mtx_lock_spin(&td_contested_lock);
  288                 TAILQ_REMOVE(&ts->ts_blocked, td, td_lockq);
  289                 TAILQ_FOREACH(td1, &ts->ts_blocked, td_lockq) {
  290                         MPASS(td1->td_proc->p_magic == P_MAGIC);
  291                         if (td1->td_priority > pri)
  292                                 break;
  293                 }
  294 
  295                 MPASS(td1 != NULL);
  296                 TAILQ_INSERT_BEFORE(td1, td, td_lockq);
  297                 mtx_unlock_spin(&td_contested_lock);
  298                 CTR4(KTR_LOCK,
  299                     "propagate_priority: td %p moved before %p on [%p] %s",
  300                     td, td1, ts->ts_lockobj, ts->ts_lockobj->lo_name);
  301                 mtx_unlock_spin(&tc->tc_lock);
  302         }
  303 }
  304 
  305 /*
  306  * Early initialization of turnstiles.  This is not done via a SYSINIT()
  307  * since this needs to be initialized very early when mutexes are first
  308  * initialized.
  309  */
  310 void
  311 init_turnstiles(void)
  312 {
  313         int i;
  314 
  315         for (i = 0; i < TC_TABLESIZE; i++) {
  316                 LIST_INIT(&turnstile_chains[i].tc_turnstiles);
  317                 mtx_init(&turnstile_chains[i].tc_lock, "turnstile chain",
  318                     NULL, MTX_SPIN);
  319         }
  320         mtx_init(&td_contested_lock, "td_contested", NULL, MTX_SPIN);
  321         thread0.td_turnstile = NULL;
  322 }
  323 
  324 #ifdef TURNSTILE_PROFILING
  325 static void
  326 init_turnstile_profiling(void *arg)
  327 {
  328         struct sysctl_oid *chain_oid;
  329         char chain_name[10];
  330         int i;
  331 
  332         for (i = 0; i < TC_TABLESIZE; i++) {
  333                 snprintf(chain_name, sizeof(chain_name), "%d", i);
  334                 chain_oid = SYSCTL_ADD_NODE(NULL, 
  335                     SYSCTL_STATIC_CHILDREN(_debug_turnstile_chains), OID_AUTO,
  336                     chain_name, CTLFLAG_RD, NULL, "turnstile chain stats");
  337                 SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
  338                     "depth", CTLFLAG_RD, &turnstile_chains[i].tc_depth, 0,
  339                     NULL);
  340                 SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO,
  341                     "max_depth", CTLFLAG_RD, &turnstile_chains[i].tc_max_depth,
  342                     0, NULL);
  343         }
  344 }
  345 SYSINIT(turnstile_profiling, SI_SUB_LOCK, SI_ORDER_ANY,
  346     init_turnstile_profiling, NULL);
  347 #endif
  348 
  349 static void
  350 init_turnstile0(void *dummy)
  351 {
  352 
  353         thread0.td_turnstile = turnstile_alloc();
  354 }
  355 SYSINIT(turnstile0, SI_SUB_LOCK, SI_ORDER_ANY, init_turnstile0, NULL);
  356 
  357 /*
  358  * Set the owner of the lock this turnstile is attached to.
  359  */
  360 static void
  361 turnstile_setowner(struct turnstile *ts, struct thread *owner)
  362 {
  363 
  364         mtx_assert(&td_contested_lock, MA_OWNED);
  365         MPASS(owner->td_proc->p_magic == P_MAGIC);
  366         MPASS(ts->ts_owner == NULL);
  367         ts->ts_owner = owner;
  368         LIST_INSERT_HEAD(&owner->td_contested, ts, ts_link);
  369 }
  370 
  371 /*
  372  * Malloc a turnstile for a new thread, initialize it and return it.
  373  */
  374 struct turnstile *
  375 turnstile_alloc(void)
  376 {
  377         struct turnstile *ts;
  378 
  379         ts = malloc(sizeof(struct turnstile), M_TURNSTILE, M_WAITOK | M_ZERO);
  380         TAILQ_INIT(&ts->ts_blocked);
  381         TAILQ_INIT(&ts->ts_pending);
  382         LIST_INIT(&ts->ts_free);
  383         return (ts);
  384 }
  385 
  386 /*
  387  * Free a turnstile when a thread is destroyed.
  388  */
  389 void
  390 turnstile_free(struct turnstile *ts)
  391 {
  392 
  393         MPASS(ts != NULL);
  394         MPASS(TAILQ_EMPTY(&ts->ts_blocked));
  395         MPASS(TAILQ_EMPTY(&ts->ts_pending));
  396         free(ts, M_TURNSTILE);
  397 }
  398 
  399 /*
  400  * Look up the turnstile for a lock in the hash table locking the associated
  401  * turnstile chain along the way.  Return with the turnstile chain locked.
  402  * If no turnstile is found in the hash table, NULL is returned.
  403  */
  404 struct turnstile *
  405 turnstile_lookup(struct lock_object *lock)
  406 {
  407         struct turnstile_chain *tc;
  408         struct turnstile *ts;
  409 
  410         tc = TC_LOOKUP(lock);
  411         mtx_lock_spin(&tc->tc_lock);
  412         LIST_FOREACH(ts, &tc->tc_turnstiles, ts_hash)
  413                 if (ts->ts_lockobj == lock)
  414                         return (ts);
  415         return (NULL);
  416 }
  417 
  418 /*
  419  * Unlock the turnstile chain associated with a given lock.
  420  */
  421 void
  422 turnstile_release(struct lock_object *lock)
  423 {
  424         struct turnstile_chain *tc;
  425 
  426         tc = TC_LOOKUP(lock);
  427         mtx_unlock_spin(&tc->tc_lock);
  428 }
  429 
  430 /*
  431  * Take ownership of a turnstile and adjust the priority of the new
  432  * owner appropriately.
  433  */
  434 void
  435 turnstile_claim(struct turnstile *ts)
  436 {
  437         struct turnstile_chain *tc;
  438         struct thread *td, *owner;
  439 
  440         tc = TC_LOOKUP(ts->ts_lockobj);
  441         mtx_assert(&tc->tc_lock, MA_OWNED);
  442 
  443         owner = curthread;
  444         mtx_lock_spin(&td_contested_lock);
  445         turnstile_setowner(ts, owner);
  446         mtx_unlock_spin(&td_contested_lock);
  447 
  448         td = TAILQ_FIRST(&ts->ts_blocked);
  449         MPASS(td != NULL);
  450         MPASS(td->td_proc->p_magic == P_MAGIC);
  451         mtx_unlock_spin(&tc->tc_lock);
  452 
  453         /*
  454          * Update the priority of the new owner if needed.
  455          */
  456         mtx_lock_spin(&sched_lock);
  457         if (td->td_priority < owner->td_priority)
  458                 owner->td_priority = td->td_priority; 
  459         mtx_unlock_spin(&sched_lock);
  460 }
  461 
  462 /*
  463  * Block the current thread on the turnstile ts.  This function will context
  464  * switch and not return until this thread has been woken back up.  This
  465  * function must be called with the appropriate turnstile chain locked and
  466  * will return with it unlocked.
  467  */
  468 void
  469 turnstile_wait(struct turnstile *ts, struct lock_object *lock,
  470     struct thread *owner)
  471 {
  472         struct turnstile_chain *tc;
  473         struct thread *td, *td1;
  474 
  475         td = curthread;
  476         tc = TC_LOOKUP(lock);
  477         mtx_assert(&tc->tc_lock, MA_OWNED);
  478         MPASS(td->td_turnstile != NULL);
  479         MPASS(owner != NULL);
  480         MPASS(owner->td_proc->p_magic == P_MAGIC);
  481 
  482         /* If the passed in turnstile is NULL, use this thread's turnstile. */
  483         if (ts == NULL) {
  484 #ifdef TURNSTILE_PROFILING
  485                 tc->tc_depth++;
  486                 if (tc->tc_depth > tc->tc_max_depth) {
  487                         tc->tc_max_depth = tc->tc_depth;
  488                         if (tc->tc_max_depth > turnstile_max_depth)
  489                                 turnstile_max_depth = tc->tc_max_depth;
  490                 }
  491 #endif
  492                 ts = td->td_turnstile;
  493                 LIST_INSERT_HEAD(&tc->tc_turnstiles, ts, ts_hash);
  494                 KASSERT(TAILQ_EMPTY(&ts->ts_pending),
  495                     ("thread's turnstile has pending threads"));
  496                 KASSERT(TAILQ_EMPTY(&ts->ts_blocked),
  497                     ("thread's turnstile has a non-empty queue"));
  498                 KASSERT(LIST_EMPTY(&ts->ts_free),
  499                     ("thread's turnstile has a non-empty free list"));
  500                 KASSERT(ts->ts_lockobj == NULL, ("stale ts_lockobj pointer"));
  501                 ts->ts_lockobj = lock;
  502                 mtx_lock_spin(&td_contested_lock);
  503                 TAILQ_INSERT_TAIL(&ts->ts_blocked, td, td_lockq);
  504                 turnstile_setowner(ts, owner);
  505                 mtx_unlock_spin(&td_contested_lock);
  506         } else {
  507                 TAILQ_FOREACH(td1, &ts->ts_blocked, td_lockq)
  508                         if (td1->td_priority > td->td_priority)
  509                                 break;
  510                 mtx_lock_spin(&td_contested_lock);
  511                 if (td1 != NULL)
  512                         TAILQ_INSERT_BEFORE(td1, td, td_lockq);
  513                 else
  514                         TAILQ_INSERT_TAIL(&ts->ts_blocked, td, td_lockq);
  515                 mtx_unlock_spin(&td_contested_lock);
  516                 MPASS(td->td_turnstile != NULL);
  517                 LIST_INSERT_HEAD(&ts->ts_free, td->td_turnstile, ts_hash);
  518                 MPASS(owner == ts->ts_owner);
  519         }
  520         td->td_turnstile = NULL;
  521         mtx_unlock_spin(&tc->tc_lock);
  522 
  523         mtx_lock_spin(&sched_lock);
  524         /*
  525          * Handle race condition where a thread on another CPU that owns
  526          * lock 'lock' could have woken us in between us dropping the
  527          * turnstile chain lock and acquiring the sched_lock.
  528          */
  529         if (td->td_flags & TDF_TSNOBLOCK) {
  530                 td->td_flags &= ~TDF_TSNOBLOCK;
  531                 mtx_unlock_spin(&sched_lock);
  532                 return;
  533         }
  534                 
  535 #ifdef notyet
  536         /*
  537          * If we're borrowing an interrupted thread's VM context, we
  538          * must clean up before going to sleep.
  539          */
  540         if (td->td_ithd != NULL) {
  541                 struct ithd *it = td->td_ithd;
  542 
  543                 if (it->it_interrupted) {
  544                         if (LOCK_LOG_TEST(lock, 0))
  545                                 CTR3(KTR_LOCK, "%s: %p interrupted %p",
  546                                     __func__, it, it->it_interrupted);
  547                         intr_thd_fixup(it);
  548                 }
  549         }
  550 #endif
  551 
  552         /* Save who we are blocked on and switch. */
  553         td->td_blocked = ts;
  554         td->td_lockname = lock->lo_name;
  555         TD_SET_LOCK(td);
  556         propagate_priority(td);
  557 
  558         if (LOCK_LOG_TEST(lock, 0))
  559                 CTR4(KTR_LOCK, "%s: td %p blocked on [%p] %s", __func__, td,
  560                     lock, lock->lo_name);
  561 
  562         mi_switch(SW_VOL, NULL);
  563 
  564         if (LOCK_LOG_TEST(lock, 0))
  565                 CTR4(KTR_LOCK, "%s: td %p free from blocked on [%p] %s",
  566                     __func__, td, lock, lock->lo_name);
  567 
  568         mtx_unlock_spin(&sched_lock);
  569 }
  570 
  571 /*
  572  * Pick the highest priority thread on this turnstile and put it on the
  573  * pending list.  This must be called with the turnstile chain locked.
  574  */
  575 int
  576 turnstile_signal(struct turnstile *ts)
  577 {
  578         struct turnstile_chain *tc;
  579         struct thread *td;
  580         int empty;
  581 
  582         MPASS(ts != NULL);
  583         MPASS(curthread->td_proc->p_magic == P_MAGIC);
  584         MPASS(ts->ts_owner == curthread);
  585         tc = TC_LOOKUP(ts->ts_lockobj);
  586         mtx_assert(&tc->tc_lock, MA_OWNED);
  587 
  588         /*
  589          * Pick the highest priority thread blocked on this lock and
  590          * move it to the pending list.
  591          */
  592         td = TAILQ_FIRST(&ts->ts_blocked);
  593         MPASS(td->td_proc->p_magic == P_MAGIC);
  594         mtx_lock_spin(&td_contested_lock);
  595         TAILQ_REMOVE(&ts->ts_blocked, td, td_lockq);
  596         mtx_unlock_spin(&td_contested_lock);
  597         TAILQ_INSERT_TAIL(&ts->ts_pending, td, td_lockq);
  598 
  599         /*
  600          * If the turnstile is now empty, remove it from its chain and
  601          * give it to the about-to-be-woken thread.  Otherwise take a
  602          * turnstile from the free list and give it to the thread.
  603          */
  604         empty = TAILQ_EMPTY(&ts->ts_blocked);
  605         if (empty) {
  606                 MPASS(LIST_EMPTY(&ts->ts_free));
  607 #ifdef TURNSTILE_PROFILING
  608                 tc->tc_depth--;
  609 #endif
  610         } else
  611                 ts = LIST_FIRST(&ts->ts_free);
  612         MPASS(ts != NULL);
  613         LIST_REMOVE(ts, ts_hash);
  614         td->td_turnstile = ts;
  615 
  616         return (empty);
  617 }
  618         
  619 /*
  620  * Put all blocked threads on the pending list.  This must be called with
  621  * the turnstile chain locked.
  622  */
  623 void
  624 turnstile_broadcast(struct turnstile *ts)
  625 {
  626         struct turnstile_chain *tc;
  627         struct turnstile *ts1;
  628         struct thread *td;
  629 
  630         MPASS(ts != NULL);
  631         MPASS(curthread->td_proc->p_magic == P_MAGIC);
  632         MPASS(ts->ts_owner == curthread);
  633         tc = TC_LOOKUP(ts->ts_lockobj);
  634         mtx_assert(&tc->tc_lock, MA_OWNED);
  635 
  636         /*
  637          * Transfer the blocked list to the pending list.
  638          */
  639         mtx_lock_spin(&td_contested_lock);
  640         TAILQ_CONCAT(&ts->ts_pending, &ts->ts_blocked, td_lockq);
  641         mtx_unlock_spin(&td_contested_lock);
  642 
  643         /*
  644          * Give a turnstile to each thread.  The last thread gets
  645          * this turnstile.
  646          */
  647         TAILQ_FOREACH(td, &ts->ts_pending, td_lockq) {
  648                 if (LIST_EMPTY(&ts->ts_free)) {
  649                         MPASS(TAILQ_NEXT(td, td_lockq) == NULL);
  650                         ts1 = ts;
  651 #ifdef TURNSTILE_PROFILING
  652                         tc->tc_depth--;
  653 #endif
  654                 } else
  655                         ts1 = LIST_FIRST(&ts->ts_free);
  656                 MPASS(ts1 != NULL);
  657                 LIST_REMOVE(ts1, ts_hash);
  658                 td->td_turnstile = ts1;
  659         }
  660 }
  661 
  662 /*
  663  * Wakeup all threads on the pending list and adjust the priority of the
  664  * current thread appropriately.  This must be called with the turnstile
  665  * chain locked.
  666  */
  667 void
  668 turnstile_unpend(struct turnstile *ts)
  669 {
  670         TAILQ_HEAD( ,thread) pending_threads;
  671         struct turnstile_chain *tc;
  672         struct thread *td;
  673         int cp, pri;
  674 
  675         MPASS(ts != NULL);
  676         MPASS(ts->ts_owner == curthread);
  677         tc = TC_LOOKUP(ts->ts_lockobj);
  678         mtx_assert(&tc->tc_lock, MA_OWNED);
  679         MPASS(!TAILQ_EMPTY(&ts->ts_pending));
  680 
  681         /*
  682          * Move the list of pending threads out of the turnstile and
  683          * into a local variable.
  684          */
  685         TAILQ_INIT(&pending_threads);
  686         TAILQ_CONCAT(&pending_threads, &ts->ts_pending, td_lockq);
  687 #ifdef INVARIANTS
  688         if (TAILQ_EMPTY(&ts->ts_blocked))
  689                 ts->ts_lockobj = NULL;
  690 #endif
  691 
  692         /*
  693          * Remove the turnstile from this thread's list of contested locks
  694          * since this thread doesn't own it anymore.  New threads will
  695          * not be blocking on the turnstile until it is claimed by a new
  696          * owner.
  697          */
  698         mtx_lock_spin(&td_contested_lock);
  699         ts->ts_owner = NULL;
  700         LIST_REMOVE(ts, ts_link);
  701         mtx_unlock_spin(&td_contested_lock);
  702         critical_enter();
  703         mtx_unlock_spin(&tc->tc_lock);
  704 
  705         /*
  706          * Adjust the priority of curthread based on other contested
  707          * locks it owns.  Don't lower the priority below the base
  708          * priority however.
  709          */
  710         td = curthread;
  711         pri = PRI_MAX;
  712         mtx_lock_spin(&sched_lock);
  713         mtx_lock_spin(&td_contested_lock);
  714         LIST_FOREACH(ts, &td->td_contested, ts_link) {
  715                 cp = TAILQ_FIRST(&ts->ts_blocked)->td_priority;
  716                 if (cp < pri)
  717                         pri = cp;
  718         }
  719         mtx_unlock_spin(&td_contested_lock);
  720         if (pri > td->td_base_pri)
  721                 pri = td->td_base_pri;
  722         td->td_priority = pri;
  723 
  724         /*
  725          * Wake up all the pending threads.  If a thread is not blocked
  726          * on a lock, then it is currently executing on another CPU in
  727          * turnstile_wait() or sitting on a run queue waiting to resume
  728          * in turnstile_wait().  Set a flag to force it to try to acquire
  729          * the lock again instead of blocking.
  730          */
  731         while (!TAILQ_EMPTY(&pending_threads)) {
  732                 td = TAILQ_FIRST(&pending_threads);
  733                 TAILQ_REMOVE(&pending_threads, td, td_lockq);
  734                 MPASS(td->td_proc->p_magic == P_MAGIC);
  735                 if (TD_ON_LOCK(td)) {
  736                         td->td_blocked = NULL;
  737                         td->td_lockname = NULL;
  738                         TD_CLR_LOCK(td);
  739                         MPASS(TD_CAN_RUN(td));
  740                         setrunqueue(td, SRQ_BORING);
  741                 } else {
  742                         td->td_flags |= TDF_TSNOBLOCK;
  743                         MPASS(TD_IS_RUNNING(td) || TD_ON_RUNQ(td));
  744                 }
  745         }
  746         critical_exit();
  747         mtx_unlock_spin(&sched_lock);
  748 }
  749 
  750 /*
  751  * Return the first thread in a turnstile.
  752  */
  753 struct thread *
  754 turnstile_head(struct turnstile *ts)
  755 {
  756 #ifdef INVARIANTS
  757         struct turnstile_chain *tc;
  758 
  759         MPASS(ts != NULL);
  760         tc = TC_LOOKUP(ts->ts_lockobj);
  761         mtx_assert(&tc->tc_lock, MA_OWNED);
  762 #endif
  763         return (TAILQ_FIRST(&ts->ts_blocked));
  764 }
  765 
  766 /*
  767  * Returns true if a turnstile is empty.
  768  */
  769 int
  770 turnstile_empty(struct turnstile *ts)
  771 {
  772 #ifdef INVARIANTS
  773         struct turnstile_chain *tc;
  774 
  775         MPASS(ts != NULL);
  776         tc = TC_LOOKUP(ts->ts_lockobj);
  777         mtx_assert(&tc->tc_lock, MA_OWNED);
  778 #endif
  779         return (TAILQ_EMPTY(&ts->ts_blocked));
  780 }

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