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

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    1 /*-
    2  * Copyright (c) 2007 Attilio Rao <attilio@freebsd.org>
    3  * Copyright (c) 2001 Jason Evans <jasone@freebsd.org>
    4  * All rights reserved.
    5  *
    6  * Redistribution and use in source and binary forms, with or without
    7  * modification, are permitted provided that the following conditions
    8  * are met:
    9  * 1. Redistributions of source code must retain the above copyright
   10  *    notice(s), this list of conditions and the following disclaimer as
   11  *    the first lines of this file unmodified other than the possible
   12  *    addition of one or more copyright notices.
   13  * 2. Redistributions in binary form must reproduce the above copyright
   14  *    notice(s), this list of conditions and the following disclaimer in the
   15  *    documentation and/or other materials provided with the distribution.
   16  *
   17  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
   18  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
   19  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
   20  * DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
   21  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
   22  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
   23  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   24  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
   27  * DAMAGE.
   28  */
   29 
   30 /*
   31  * Shared/exclusive locks.  This implementation attempts to ensure
   32  * deterministic lock granting behavior, so that slocks and xlocks are
   33  * interleaved.
   34  *
   35  * Priority propagation will not generally raise the priority of lock holders,
   36  * so should not be relied upon in combination with sx locks.
   37  */
   38 
   39 #include "opt_ddb.h"
   40 #include "opt_kdtrace.h"
   41 #include "opt_no_adaptive_sx.h"
   42 
   43 #include <sys/cdefs.h>
   44 __FBSDID("$FreeBSD: releng/8.1/sys/kern/kern_sx.c 209002 2010-06-10 20:04:44Z jhb $");
   45 
   46 #include <sys/param.h>
   47 #include <sys/ktr.h>
   48 #include <sys/linker_set.h>
   49 #include <sys/lock.h>
   50 #include <sys/mutex.h>
   51 #include <sys/proc.h>
   52 #include <sys/sleepqueue.h>
   53 #include <sys/sx.h>
   54 #include <sys/sysctl.h>
   55 #include <sys/systm.h>
   56 
   57 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
   58 #include <machine/cpu.h>
   59 #endif
   60 
   61 #ifdef DDB
   62 #include <ddb/ddb.h>
   63 #endif
   64 
   65 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
   66 #define ADAPTIVE_SX
   67 #endif
   68 
   69 CTASSERT((SX_NOADAPTIVE & LO_CLASSFLAGS) == SX_NOADAPTIVE);
   70 
   71 /* Handy macros for sleep queues. */
   72 #define SQ_EXCLUSIVE_QUEUE      0
   73 #define SQ_SHARED_QUEUE         1
   74 
   75 #ifdef ADAPTIVE_SX
   76 #define ASX_RETRIES             10
   77 #define ASX_LOOPS               10000
   78 #endif
   79 
   80 /*
   81  * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file.  We
   82  * drop Giant anytime we have to sleep or if we adaptively spin.
   83  */
   84 #define GIANT_DECLARE                                                   \
   85         int _giantcnt = 0;                                              \
   86         WITNESS_SAVE_DECL(Giant)                                        \
   87 
   88 #define GIANT_SAVE() do {                                               \
   89         if (mtx_owned(&Giant)) {                                        \
   90                 WITNESS_SAVE(&Giant.lock_object, Giant);                \
   91                 while (mtx_owned(&Giant)) {                             \
   92                         _giantcnt++;                                    \
   93                         mtx_unlock(&Giant);                             \
   94                 }                                                       \
   95         }                                                               \
   96 } while (0)
   97 
   98 #define GIANT_RESTORE() do {                                            \
   99         if (_giantcnt > 0) {                                            \
  100                 mtx_assert(&Giant, MA_NOTOWNED);                        \
  101                 while (_giantcnt--)                                     \
  102                         mtx_lock(&Giant);                               \
  103                 WITNESS_RESTORE(&Giant.lock_object, Giant);             \
  104         }                                                               \
  105 } while (0)
  106 
  107 /*
  108  * Returns true if an exclusive lock is recursed.  It assumes
  109  * curthread currently has an exclusive lock.
  110  */
  111 #define sx_recurse              lock_object.lo_data
  112 #define sx_recursed(sx)         ((sx)->sx_recurse != 0)
  113 
  114 static void     assert_sx(struct lock_object *lock, int what);
  115 #ifdef DDB
  116 static void     db_show_sx(struct lock_object *lock);
  117 #endif
  118 static void     lock_sx(struct lock_object *lock, int how);
  119 #ifdef KDTRACE_HOOKS
  120 static int      owner_sx(struct lock_object *lock, struct thread **owner);
  121 #endif
  122 static int      unlock_sx(struct lock_object *lock);
  123 
  124 struct lock_class lock_class_sx = {
  125         .lc_name = "sx",
  126         .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
  127         .lc_assert = assert_sx,
  128 #ifdef DDB
  129         .lc_ddb_show = db_show_sx,
  130 #endif
  131         .lc_lock = lock_sx,
  132         .lc_unlock = unlock_sx,
  133 #ifdef KDTRACE_HOOKS
  134         .lc_owner = owner_sx,
  135 #endif
  136 };
  137 
  138 #ifndef INVARIANTS
  139 #define _sx_assert(sx, what, file, line)
  140 #endif
  141 
  142 void
  143 assert_sx(struct lock_object *lock, int what)
  144 {
  145 
  146         sx_assert((struct sx *)lock, what);
  147 }
  148 
  149 void
  150 lock_sx(struct lock_object *lock, int how)
  151 {
  152         struct sx *sx;
  153 
  154         sx = (struct sx *)lock;
  155         if (how)
  156                 sx_xlock(sx);
  157         else
  158                 sx_slock(sx);
  159 }
  160 
  161 int
  162 unlock_sx(struct lock_object *lock)
  163 {
  164         struct sx *sx;
  165 
  166         sx = (struct sx *)lock;
  167         sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
  168         if (sx_xlocked(sx)) {
  169                 sx_xunlock(sx);
  170                 return (1);
  171         } else {
  172                 sx_sunlock(sx);
  173                 return (0);
  174         }
  175 }
  176 
  177 #ifdef KDTRACE_HOOKS
  178 int
  179 owner_sx(struct lock_object *lock, struct thread **owner)
  180 {
  181         struct sx *sx = (struct sx *)lock;
  182         uintptr_t x = sx->sx_lock;
  183 
  184         *owner = (struct thread *)SX_OWNER(x);
  185         return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
  186             (*owner != NULL));
  187 }
  188 #endif
  189 
  190 void
  191 sx_sysinit(void *arg)
  192 {
  193         struct sx_args *sargs = arg;
  194 
  195         sx_init(sargs->sa_sx, sargs->sa_desc);
  196 }
  197 
  198 void
  199 sx_init_flags(struct sx *sx, const char *description, int opts)
  200 {
  201         int flags;
  202 
  203         MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
  204             SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
  205         ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
  206             ("%s: sx_lock not aligned for %s: %p", __func__, description,
  207             &sx->sx_lock));
  208 
  209         flags = LO_SLEEPABLE | LO_UPGRADABLE;
  210         if (opts & SX_DUPOK)
  211                 flags |= LO_DUPOK;
  212         if (opts & SX_NOPROFILE)
  213                 flags |= LO_NOPROFILE;
  214         if (!(opts & SX_NOWITNESS))
  215                 flags |= LO_WITNESS;
  216         if (opts & SX_RECURSE)
  217                 flags |= LO_RECURSABLE;
  218         if (opts & SX_QUIET)
  219                 flags |= LO_QUIET;
  220 
  221         flags |= opts & SX_NOADAPTIVE;
  222         sx->sx_lock = SX_LOCK_UNLOCKED;
  223         sx->sx_recurse = 0;
  224         lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
  225 }
  226 
  227 void
  228 sx_destroy(struct sx *sx)
  229 {
  230 
  231         KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
  232         KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
  233         sx->sx_lock = SX_LOCK_DESTROYED;
  234         lock_destroy(&sx->lock_object);
  235 }
  236 
  237 int
  238 _sx_slock(struct sx *sx, int opts, const char *file, int line)
  239 {
  240         int error = 0;
  241 
  242         MPASS(curthread != NULL);
  243         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  244             ("sx_slock() of destroyed sx @ %s:%d", file, line));
  245         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
  246         error = __sx_slock(sx, opts, file, line);
  247         if (!error) {
  248                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
  249                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
  250                 curthread->td_locks++;
  251         }
  252 
  253         return (error);
  254 }
  255 
  256 int
  257 _sx_try_slock(struct sx *sx, const char *file, int line)
  258 {
  259         uintptr_t x;
  260 
  261         for (;;) {
  262                 x = sx->sx_lock;
  263                 KASSERT(x != SX_LOCK_DESTROYED,
  264                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
  265                 if (!(x & SX_LOCK_SHARED))
  266                         break;
  267                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
  268                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
  269                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
  270                         curthread->td_locks++;
  271                         return (1);
  272                 }
  273         }
  274 
  275         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
  276         return (0);
  277 }
  278 
  279 int
  280 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
  281 {
  282         int error = 0;
  283 
  284         MPASS(curthread != NULL);
  285         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  286             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
  287         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
  288             line, NULL);
  289         error = __sx_xlock(sx, curthread, opts, file, line);
  290         if (!error) {
  291                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
  292                     file, line);
  293                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
  294                 curthread->td_locks++;
  295         }
  296 
  297         return (error);
  298 }
  299 
  300 int
  301 _sx_try_xlock(struct sx *sx, const char *file, int line)
  302 {
  303         int rval;
  304 
  305         MPASS(curthread != NULL);
  306         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  307             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
  308 
  309         if (sx_xlocked(sx) &&
  310             (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
  311                 sx->sx_recurse++;
  312                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
  313                 rval = 1;
  314         } else
  315                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
  316                     (uintptr_t)curthread);
  317         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
  318         if (rval) {
  319                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  320                     file, line);
  321                 curthread->td_locks++;
  322         }
  323 
  324         return (rval);
  325 }
  326 
  327 void
  328 _sx_sunlock(struct sx *sx, const char *file, int line)
  329 {
  330 
  331         MPASS(curthread != NULL);
  332         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  333             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
  334         _sx_assert(sx, SA_SLOCKED, file, line);
  335         curthread->td_locks--;
  336         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
  337         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
  338         __sx_sunlock(sx, file, line);
  339         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
  340 }
  341 
  342 void
  343 _sx_xunlock(struct sx *sx, const char *file, int line)
  344 {
  345 
  346         MPASS(curthread != NULL);
  347         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  348             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
  349         _sx_assert(sx, SA_XLOCKED, file, line);
  350         curthread->td_locks--;
  351         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
  352         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
  353             line);
  354         if (!sx_recursed(sx))
  355                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
  356         __sx_xunlock(sx, curthread, file, line);
  357 }
  358 
  359 /*
  360  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
  361  * This will only succeed if this thread holds a single shared lock.
  362  * Return 1 if if the upgrade succeed, 0 otherwise.
  363  */
  364 int
  365 _sx_try_upgrade(struct sx *sx, const char *file, int line)
  366 {
  367         uintptr_t x;
  368         int success;
  369 
  370         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  371             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
  372         _sx_assert(sx, SA_SLOCKED, file, line);
  373 
  374         /*
  375          * Try to switch from one shared lock to an exclusive lock.  We need
  376          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
  377          * we will wake up the exclusive waiters when we drop the lock.
  378          */
  379         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
  380         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
  381             (uintptr_t)curthread | x);
  382         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
  383         if (success) {
  384                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  385                     file, line);
  386                 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
  387         }
  388         return (success);
  389 }
  390 
  391 /*
  392  * Downgrade an unrecursed exclusive lock into a single shared lock.
  393  */
  394 void
  395 _sx_downgrade(struct sx *sx, const char *file, int line)
  396 {
  397         uintptr_t x;
  398         int wakeup_swapper;
  399 
  400         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  401             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
  402         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
  403 #ifndef INVARIANTS
  404         if (sx_recursed(sx))
  405                 panic("downgrade of a recursed lock");
  406 #endif
  407 
  408         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
  409 
  410         /*
  411          * Try to switch from an exclusive lock with no shared waiters
  412          * to one sharer with no shared waiters.  If there are
  413          * exclusive waiters, we don't need to lock the sleep queue so
  414          * long as we preserve the flag.  We do one quick try and if
  415          * that fails we grab the sleepq lock to keep the flags from
  416          * changing and do it the slow way.
  417          *
  418          * We have to lock the sleep queue if there are shared waiters
  419          * so we can wake them up.
  420          */
  421         x = sx->sx_lock;
  422         if (!(x & SX_LOCK_SHARED_WAITERS) &&
  423             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
  424             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
  425                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
  426                 return;
  427         }
  428 
  429         /*
  430          * Lock the sleep queue so we can read the waiters bits
  431          * without any races and wakeup any shared waiters.
  432          */
  433         sleepq_lock(&sx->lock_object);
  434 
  435         /*
  436          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
  437          * shared lock.  If there are any shared waiters, wake them up.
  438          */
  439         wakeup_swapper = 0;
  440         x = sx->sx_lock;
  441         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
  442             (x & SX_LOCK_EXCLUSIVE_WAITERS));
  443         if (x & SX_LOCK_SHARED_WAITERS)
  444                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
  445                     0, SQ_SHARED_QUEUE);
  446         sleepq_release(&sx->lock_object);
  447 
  448         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
  449         LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
  450 
  451         if (wakeup_swapper)
  452                 kick_proc0();
  453 }
  454 
  455 /*
  456  * This function represents the so-called 'hard case' for sx_xlock
  457  * operation.  All 'easy case' failures are redirected to this.  Note
  458  * that ideally this would be a static function, but it needs to be
  459  * accessible from at least sx.h.
  460  */
  461 int
  462 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
  463     int line)
  464 {
  465         GIANT_DECLARE;
  466 #ifdef ADAPTIVE_SX
  467         volatile struct thread *owner;
  468         u_int i, spintries = 0;
  469 #endif
  470         uintptr_t x;
  471 #ifdef LOCK_PROFILING
  472         uint64_t waittime = 0;
  473         int contested = 0;
  474 #endif
  475         int error = 0;
  476 #ifdef  KDTRACE_HOOKS
  477         uint64_t spin_cnt = 0;
  478         uint64_t sleep_cnt = 0;
  479         int64_t sleep_time = 0;
  480 #endif
  481 
  482         /* If we already hold an exclusive lock, then recurse. */
  483         if (sx_xlocked(sx)) {
  484                 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
  485             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
  486                     sx->lock_object.lo_name, file, line));
  487                 sx->sx_recurse++;
  488                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
  489                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  490                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
  491                 return (0);
  492         }
  493 
  494         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  495                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
  496                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
  497 
  498         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
  499 #ifdef KDTRACE_HOOKS
  500                 spin_cnt++;
  501 #endif
  502                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
  503                     &waittime);
  504 #ifdef ADAPTIVE_SX
  505                 /*
  506                  * If the lock is write locked and the owner is
  507                  * running on another CPU, spin until the owner stops
  508                  * running or the state of the lock changes.
  509                  */
  510                 x = sx->sx_lock;
  511                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
  512                         if ((x & SX_LOCK_SHARED) == 0) {
  513                                 x = SX_OWNER(x);
  514                                 owner = (struct thread *)x;
  515                                 if (TD_IS_RUNNING(owner)) {
  516                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  517                                                 CTR3(KTR_LOCK,
  518                                             "%s: spinning on %p held by %p",
  519                                                     __func__, sx, owner);
  520                                         GIANT_SAVE();
  521                                         while (SX_OWNER(sx->sx_lock) == x &&
  522                                             TD_IS_RUNNING(owner)) {
  523                                                 cpu_spinwait();
  524 #ifdef KDTRACE_HOOKS
  525                                                 spin_cnt++;
  526 #endif
  527                                         }
  528                                         continue;
  529                                 }
  530                         } else if (SX_SHARERS(x) && spintries < ASX_RETRIES) {
  531                                 GIANT_SAVE();
  532                                 spintries++;
  533                                 for (i = 0; i < ASX_LOOPS; i++) {
  534                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  535                                                 CTR4(KTR_LOCK,
  536                                     "%s: shared spinning on %p with %u and %u",
  537                                                     __func__, sx, spintries, i);
  538                                         x = sx->sx_lock;
  539                                         if ((x & SX_LOCK_SHARED) == 0 ||
  540                                             SX_SHARERS(x) == 0)
  541                                                 break;
  542                                         cpu_spinwait();
  543 #ifdef KDTRACE_HOOKS
  544                                         spin_cnt++;
  545 #endif
  546                                 }
  547                                 if (i != ASX_LOOPS)
  548                                         continue;
  549                         }
  550                 }
  551 #endif
  552 
  553                 sleepq_lock(&sx->lock_object);
  554                 x = sx->sx_lock;
  555 
  556                 /*
  557                  * If the lock was released while spinning on the
  558                  * sleep queue chain lock, try again.
  559                  */
  560                 if (x == SX_LOCK_UNLOCKED) {
  561                         sleepq_release(&sx->lock_object);
  562                         continue;
  563                 }
  564 
  565 #ifdef ADAPTIVE_SX
  566                 /*
  567                  * The current lock owner might have started executing
  568                  * on another CPU (or the lock could have changed
  569                  * owners) while we were waiting on the sleep queue
  570                  * chain lock.  If so, drop the sleep queue lock and try
  571                  * again.
  572                  */
  573                 if (!(x & SX_LOCK_SHARED) &&
  574                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
  575                         owner = (struct thread *)SX_OWNER(x);
  576                         if (TD_IS_RUNNING(owner)) {
  577                                 sleepq_release(&sx->lock_object);
  578                                 continue;
  579                         }
  580                 }
  581 #endif
  582 
  583                 /*
  584                  * If an exclusive lock was released with both shared
  585                  * and exclusive waiters and a shared waiter hasn't
  586                  * woken up and acquired the lock yet, sx_lock will be
  587                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
  588                  * If we see that value, try to acquire it once.  Note
  589                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
  590                  * as there are other exclusive waiters still.  If we
  591                  * fail, restart the loop.
  592                  */
  593                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
  594                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
  595                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
  596                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
  597                                 sleepq_release(&sx->lock_object);
  598                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
  599                                     __func__, sx);
  600                                 break;
  601                         }
  602                         sleepq_release(&sx->lock_object);
  603                         continue;
  604                 }
  605 
  606                 /*
  607                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
  608                  * than loop back and retry.
  609                  */
  610                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
  611                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
  612                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
  613                                 sleepq_release(&sx->lock_object);
  614                                 continue;
  615                         }
  616                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  617                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
  618                                     __func__, sx);
  619                 }
  620 
  621                 /*
  622                  * Since we have been unable to acquire the exclusive
  623                  * lock and the exclusive waiters flag is set, we have
  624                  * to sleep.
  625                  */
  626                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  627                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
  628                             __func__, sx);
  629 
  630 #ifdef KDTRACE_HOOKS
  631                 sleep_time -= lockstat_nsecs();
  632 #endif
  633                 GIANT_SAVE();
  634                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
  635                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
  636                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
  637                 if (!(opts & SX_INTERRUPTIBLE))
  638                         sleepq_wait(&sx->lock_object, 0);
  639                 else
  640                         error = sleepq_wait_sig(&sx->lock_object, 0);
  641 #ifdef KDTRACE_HOOKS
  642                 sleep_time += lockstat_nsecs();
  643                 sleep_cnt++;
  644 #endif
  645                 if (error) {
  646                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  647                                 CTR2(KTR_LOCK,
  648                         "%s: interruptible sleep by %p suspended by signal",
  649                                     __func__, sx);
  650                         break;
  651                 }
  652                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  653                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
  654                             __func__, sx);
  655         }
  656 
  657         GIANT_RESTORE();
  658         if (!error)
  659                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
  660                     contested, waittime, file, line);
  661 #ifdef KDTRACE_HOOKS
  662         if (sleep_time)
  663                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
  664         if (spin_cnt > sleep_cnt)
  665                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
  666 #endif
  667         return (error);
  668 }
  669 
  670 /*
  671  * This function represents the so-called 'hard case' for sx_xunlock
  672  * operation.  All 'easy case' failures are redirected to this.  Note
  673  * that ideally this would be a static function, but it needs to be
  674  * accessible from at least sx.h.
  675  */
  676 void
  677 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
  678 {
  679         uintptr_t x;
  680         int queue, wakeup_swapper;
  681 
  682         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
  683 
  684         /* If the lock is recursed, then unrecurse one level. */
  685         if (sx_xlocked(sx) && sx_recursed(sx)) {
  686                 if ((--sx->sx_recurse) == 0)
  687                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
  688                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  689                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
  690                 return;
  691         }
  692         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
  693             SX_LOCK_EXCLUSIVE_WAITERS));
  694         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  695                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
  696 
  697         sleepq_lock(&sx->lock_object);
  698         x = SX_LOCK_UNLOCKED;
  699 
  700         /*
  701          * The wake up algorithm here is quite simple and probably not
  702          * ideal.  It gives precedence to shared waiters if they are
  703          * present.  For this condition, we have to preserve the
  704          * state of the exclusive waiters flag.
  705          * If interruptible sleeps left the shared queue empty avoid a
  706          * starvation for the threads sleeping on the exclusive queue by giving
  707          * them precedence and cleaning up the shared waiters bit anyway.
  708          */
  709         if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
  710             sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
  711                 queue = SQ_SHARED_QUEUE;
  712                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
  713         } else
  714                 queue = SQ_EXCLUSIVE_QUEUE;
  715 
  716         /* Wake up all the waiters for the specific queue. */
  717         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  718                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
  719                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
  720                     "exclusive");
  721         atomic_store_rel_ptr(&sx->sx_lock, x);
  722         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
  723             queue);
  724         sleepq_release(&sx->lock_object);
  725         if (wakeup_swapper)
  726                 kick_proc0();
  727 }
  728 
  729 /*
  730  * This function represents the so-called 'hard case' for sx_slock
  731  * operation.  All 'easy case' failures are redirected to this.  Note
  732  * that ideally this would be a static function, but it needs to be
  733  * accessible from at least sx.h.
  734  */
  735 int
  736 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
  737 {
  738         GIANT_DECLARE;
  739 #ifdef ADAPTIVE_SX
  740         volatile struct thread *owner;
  741 #endif
  742 #ifdef LOCK_PROFILING
  743         uint64_t waittime = 0;
  744         int contested = 0;
  745 #endif
  746         uintptr_t x;
  747         int error = 0;
  748 #ifdef KDTRACE_HOOKS
  749         uint64_t spin_cnt = 0;
  750         uint64_t sleep_cnt = 0;
  751         int64_t sleep_time = 0;
  752 #endif
  753 
  754         /*
  755          * As with rwlocks, we don't make any attempt to try to block
  756          * shared locks once there is an exclusive waiter.
  757          */
  758         for (;;) {
  759 #ifdef KDTRACE_HOOKS
  760                 spin_cnt++;
  761 #endif
  762                 x = sx->sx_lock;
  763 
  764                 /*
  765                  * If no other thread has an exclusive lock then try to bump up
  766                  * the count of sharers.  Since we have to preserve the state
  767                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
  768                  * shared lock loop back and retry.
  769                  */
  770                 if (x & SX_LOCK_SHARED) {
  771                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
  772                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
  773                             x + SX_ONE_SHARER)) {
  774                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  775                                         CTR4(KTR_LOCK,
  776                                             "%s: %p succeed %p -> %p", __func__,
  777                                             sx, (void *)x,
  778                                             (void *)(x + SX_ONE_SHARER));
  779                                 break;
  780                         }
  781                         continue;
  782                 }
  783                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
  784                     &waittime);
  785 
  786 #ifdef ADAPTIVE_SX
  787                 /*
  788                  * If the owner is running on another CPU, spin until
  789                  * the owner stops running or the state of the lock
  790                  * changes.
  791                  */
  792                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
  793                         x = SX_OWNER(x);
  794                         owner = (struct thread *)x;
  795                         if (TD_IS_RUNNING(owner)) {
  796                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  797                                         CTR3(KTR_LOCK,
  798                                             "%s: spinning on %p held by %p",
  799                                             __func__, sx, owner);
  800                                 GIANT_SAVE();
  801                                 while (SX_OWNER(sx->sx_lock) == x &&
  802                                     TD_IS_RUNNING(owner)) {
  803 #ifdef KDTRACE_HOOKS
  804                                         spin_cnt++;
  805 #endif
  806                                         cpu_spinwait();
  807                                 }
  808                                 continue;
  809                         }
  810                 }
  811 #endif
  812 
  813                 /*
  814                  * Some other thread already has an exclusive lock, so
  815                  * start the process of blocking.
  816                  */
  817                 sleepq_lock(&sx->lock_object);
  818                 x = sx->sx_lock;
  819 
  820                 /*
  821                  * The lock could have been released while we spun.
  822                  * In this case loop back and retry.
  823                  */
  824                 if (x & SX_LOCK_SHARED) {
  825                         sleepq_release(&sx->lock_object);
  826                         continue;
  827                 }
  828 
  829 #ifdef ADAPTIVE_SX
  830                 /*
  831                  * If the owner is running on another CPU, spin until
  832                  * the owner stops running or the state of the lock
  833                  * changes.
  834                  */
  835                 if (!(x & SX_LOCK_SHARED) &&
  836                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
  837                         owner = (struct thread *)SX_OWNER(x);
  838                         if (TD_IS_RUNNING(owner)) {
  839                                 sleepq_release(&sx->lock_object);
  840                                 continue;
  841                         }
  842                 }
  843 #endif
  844 
  845                 /*
  846                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
  847                  * fail to set it drop the sleep queue lock and loop
  848                  * back.
  849                  */
  850                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
  851                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
  852                             x | SX_LOCK_SHARED_WAITERS)) {
  853                                 sleepq_release(&sx->lock_object);
  854                                 continue;
  855                         }
  856                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  857                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
  858                                     __func__, sx);
  859                 }
  860 
  861                 /*
  862                  * Since we have been unable to acquire the shared lock,
  863                  * we have to sleep.
  864                  */
  865                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  866                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
  867                             __func__, sx);
  868 
  869 #ifdef KDTRACE_HOOKS
  870                 sleep_time -= lockstat_nsecs();
  871 #endif
  872                 GIANT_SAVE();
  873                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
  874                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
  875                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
  876                 if (!(opts & SX_INTERRUPTIBLE))
  877                         sleepq_wait(&sx->lock_object, 0);
  878                 else
  879                         error = sleepq_wait_sig(&sx->lock_object, 0);
  880 #ifdef KDTRACE_HOOKS
  881                 sleep_time += lockstat_nsecs();
  882                 sleep_cnt++;
  883 #endif
  884                 if (error) {
  885                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  886                                 CTR2(KTR_LOCK,
  887                         "%s: interruptible sleep by %p suspended by signal",
  888                                     __func__, sx);
  889                         break;
  890                 }
  891                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  892                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
  893                             __func__, sx);
  894         }
  895         if (error == 0)
  896                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
  897                     contested, waittime, file, line);
  898 #ifdef KDTRACE_HOOKS
  899         if (sleep_time)
  900                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
  901         if (spin_cnt > sleep_cnt)
  902                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
  903 #endif
  904         GIANT_RESTORE();
  905         return (error);
  906 }
  907 
  908 /*
  909  * This function represents the so-called 'hard case' for sx_sunlock
  910  * operation.  All 'easy case' failures are redirected to this.  Note
  911  * that ideally this would be a static function, but it needs to be
  912  * accessible from at least sx.h.
  913  */
  914 void
  915 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
  916 {
  917         uintptr_t x;
  918         int wakeup_swapper;
  919 
  920         for (;;) {
  921                 x = sx->sx_lock;
  922 
  923                 /*
  924                  * We should never have sharers while at least one thread
  925                  * holds a shared lock.
  926                  */
  927                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
  928                     ("%s: waiting sharers", __func__));
  929 
  930                 /*
  931                  * See if there is more than one shared lock held.  If
  932                  * so, just drop one and return.
  933                  */
  934                 if (SX_SHARERS(x) > 1) {
  935                         if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
  936                             x - SX_ONE_SHARER)) {
  937                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  938                                         CTR4(KTR_LOCK,
  939                                             "%s: %p succeeded %p -> %p",
  940                                             __func__, sx, (void *)x,
  941                                             (void *)(x - SX_ONE_SHARER));
  942                                 break;
  943                         }
  944                         continue;
  945                 }
  946 
  947                 /*
  948                  * If there aren't any waiters for an exclusive lock,
  949                  * then try to drop it quickly.
  950                  */
  951                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
  952                         MPASS(x == SX_SHARERS_LOCK(1));
  953                         if (atomic_cmpset_rel_ptr(&sx->sx_lock,
  954                             SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
  955                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  956                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
  957                                             __func__, sx);
  958                                 break;
  959                         }
  960                         continue;
  961                 }
  962 
  963                 /*
  964                  * At this point, there should just be one sharer with
  965                  * exclusive waiters.
  966                  */
  967                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
  968 
  969                 sleepq_lock(&sx->lock_object);
  970 
  971                 /*
  972                  * Wake up semantic here is quite simple:
  973                  * Just wake up all the exclusive waiters.
  974                  * Note that the state of the lock could have changed,
  975                  * so if it fails loop back and retry.
  976                  */
  977                 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
  978                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
  979                     SX_LOCK_UNLOCKED)) {
  980                         sleepq_release(&sx->lock_object);
  981                         continue;
  982                 }
  983                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  984                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
  985                             "exclusive queue", __func__, sx);
  986                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
  987                     0, SQ_EXCLUSIVE_QUEUE);
  988                 sleepq_release(&sx->lock_object);
  989                 if (wakeup_swapper)
  990                         kick_proc0();
  991                 break;
  992         }
  993 }
  994 
  995 #ifdef INVARIANT_SUPPORT
  996 #ifndef INVARIANTS
  997 #undef  _sx_assert
  998 #endif
  999 
 1000 /*
 1001  * In the non-WITNESS case, sx_assert() can only detect that at least
 1002  * *some* thread owns an slock, but it cannot guarantee that *this*
 1003  * thread owns an slock.
 1004  */
 1005 void
 1006 _sx_assert(struct sx *sx, int what, const char *file, int line)
 1007 {
 1008 #ifndef WITNESS
 1009         int slocked = 0;
 1010 #endif
 1011 
 1012         if (panicstr != NULL)
 1013                 return;
 1014         switch (what) {
 1015         case SA_SLOCKED:
 1016         case SA_SLOCKED | SA_NOTRECURSED:
 1017         case SA_SLOCKED | SA_RECURSED:
 1018 #ifndef WITNESS
 1019                 slocked = 1;
 1020                 /* FALLTHROUGH */
 1021 #endif
 1022         case SA_LOCKED:
 1023         case SA_LOCKED | SA_NOTRECURSED:
 1024         case SA_LOCKED | SA_RECURSED:
 1025 #ifdef WITNESS
 1026                 witness_assert(&sx->lock_object, what, file, line);
 1027 #else
 1028                 /*
 1029                  * If some other thread has an exclusive lock or we
 1030                  * have one and are asserting a shared lock, fail.
 1031                  * Also, if no one has a lock at all, fail.
 1032                  */
 1033                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
 1034                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
 1035                     sx_xholder(sx) != curthread)))
 1036                         panic("Lock %s not %slocked @ %s:%d\n",
 1037                             sx->lock_object.lo_name, slocked ? "share " : "",
 1038                             file, line);
 1039 
 1040                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
 1041                         if (sx_recursed(sx)) {
 1042                                 if (what & SA_NOTRECURSED)
 1043                                         panic("Lock %s recursed @ %s:%d\n",
 1044                                             sx->lock_object.lo_name, file,
 1045                                             line);
 1046                         } else if (what & SA_RECURSED)
 1047                                 panic("Lock %s not recursed @ %s:%d\n",
 1048                                     sx->lock_object.lo_name, file, line);
 1049                 }
 1050 #endif
 1051                 break;
 1052         case SA_XLOCKED:
 1053         case SA_XLOCKED | SA_NOTRECURSED:
 1054         case SA_XLOCKED | SA_RECURSED:
 1055                 if (sx_xholder(sx) != curthread)
 1056                         panic("Lock %s not exclusively locked @ %s:%d\n",
 1057                             sx->lock_object.lo_name, file, line);
 1058                 if (sx_recursed(sx)) {
 1059                         if (what & SA_NOTRECURSED)
 1060                                 panic("Lock %s recursed @ %s:%d\n",
 1061                                     sx->lock_object.lo_name, file, line);
 1062                 } else if (what & SA_RECURSED)
 1063                         panic("Lock %s not recursed @ %s:%d\n",
 1064                             sx->lock_object.lo_name, file, line);
 1065                 break;
 1066         case SA_UNLOCKED:
 1067 #ifdef WITNESS
 1068                 witness_assert(&sx->lock_object, what, file, line);
 1069 #else
 1070                 /*
 1071                  * If we hold an exclusve lock fail.  We can't
 1072                  * reliably check to see if we hold a shared lock or
 1073                  * not.
 1074                  */
 1075                 if (sx_xholder(sx) == curthread)
 1076                         panic("Lock %s exclusively locked @ %s:%d\n",
 1077                             sx->lock_object.lo_name, file, line);
 1078 #endif
 1079                 break;
 1080         default:
 1081                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
 1082                     line);
 1083         }
 1084 }
 1085 #endif  /* INVARIANT_SUPPORT */
 1086 
 1087 #ifdef DDB
 1088 static void
 1089 db_show_sx(struct lock_object *lock)
 1090 {
 1091         struct thread *td;
 1092         struct sx *sx;
 1093 
 1094         sx = (struct sx *)lock;
 1095 
 1096         db_printf(" state: ");
 1097         if (sx->sx_lock == SX_LOCK_UNLOCKED)
 1098                 db_printf("UNLOCKED\n");
 1099         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
 1100                 db_printf("DESTROYED\n");
 1101                 return;
 1102         } else if (sx->sx_lock & SX_LOCK_SHARED)
 1103                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
 1104         else {
 1105                 td = sx_xholder(sx);
 1106                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
 1107                     td->td_tid, td->td_proc->p_pid, td->td_name);
 1108                 if (sx_recursed(sx))
 1109                         db_printf(" recursed: %d\n", sx->sx_recurse);
 1110         }
 1111 
 1112         db_printf(" waiters: ");
 1113         switch(sx->sx_lock &
 1114             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
 1115         case SX_LOCK_SHARED_WAITERS:
 1116                 db_printf("shared\n");
 1117                 break;
 1118         case SX_LOCK_EXCLUSIVE_WAITERS:
 1119                 db_printf("exclusive\n");
 1120                 break;
 1121         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
 1122                 db_printf("exclusive and shared\n");
 1123                 break;
 1124         default:
 1125                 db_printf("none\n");
 1126         }
 1127 }
 1128 
 1129 /*
 1130  * Check to see if a thread that is blocked on a sleep queue is actually
 1131  * blocked on an sx lock.  If so, output some details and return true.
 1132  * If the lock has an exclusive owner, return that in *ownerp.
 1133  */
 1134 int
 1135 sx_chain(struct thread *td, struct thread **ownerp)
 1136 {
 1137         struct sx *sx;
 1138 
 1139         /*
 1140          * Check to see if this thread is blocked on an sx lock.
 1141          * First, we check the lock class.  If that is ok, then we
 1142          * compare the lock name against the wait message.
 1143          */
 1144         sx = td->td_wchan;
 1145         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
 1146             sx->lock_object.lo_name != td->td_wmesg)
 1147                 return (0);
 1148 
 1149         /* We think we have an sx lock, so output some details. */
 1150         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
 1151         *ownerp = sx_xholder(sx);
 1152         if (sx->sx_lock & SX_LOCK_SHARED)
 1153                 db_printf("SLOCK (count %ju)\n",
 1154                     (uintmax_t)SX_SHARERS(sx->sx_lock));
 1155         else
 1156                 db_printf("XLOCK\n");
 1157         return (1);
 1158 }
 1159 #endif

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