sys/kern/kern_sx.c

   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("$FreeBSD$");
  45
  46 #include <sys/param.h>
  47 #include <sys/ktr.h>
  48 #include <sys/lock.h>
  49 #include <sys/mutex.h>
  50 #include <sys/proc.h>
  51 #include <sys/sleepqueue.h>
  52 #include <sys/sx.h>
  53 #include <sys/sysctl.h>
  54 #include <sys/systm.h>
  55
  56 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
  57 #include <machine/cpu.h>
  58 #endif
  59
  60 #ifdef DDB
  61 #include <ddb/ddb.h>
  62 #endif
  63
  64 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
  65 #define ADAPTIVE_SX
  66 #endif
  67
  68 CTASSERT((SX_NOADAPTIVE & LO_CLASSFLAGS) == SX_NOADAPTIVE);
  69
  70 /* Handy macros for sleep queues. */
  71 #define SQ_EXCLUSIVE_QUEUE      0
  72 #define SQ_SHARED_QUEUE         1
  73
  74 /*
  75  * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file.  We
  76  * drop Giant anytime we have to sleep or if we adaptively spin.
  77  */
  78 #define GIANT_DECLARE                                                   \
  79         int _giantcnt = 0;                                              \
  80         WITNESS_SAVE_DECL(Giant)                                        \
  81
  82 #define GIANT_SAVE() do {                                               \
  83         if (mtx_owned(&Giant)) {                                        \
  84                 WITNESS_SAVE(&Giant.lock_object, Giant);                \
  85                 while (mtx_owned(&Giant)) {                             \
  86                         _giantcnt++;                                    \
  87                         mtx_unlock(&Giant);                             \
  88                 }                                                       \
  89         }                                                               \
  90 } while (0)
  91
  92 #define GIANT_RESTORE() do {                                            \
  93         if (_giantcnt > 0) {                                            \
  94                 mtx_assert(&Giant, MA_NOTOWNED);                        \
  95                 while (_giantcnt--)                                     \
  96                         mtx_lock(&Giant);                               \
  97                 WITNESS_RESTORE(&Giant.lock_object, Giant);             \
  98         }                                                               \
  99 } while (0)
 100
 101 /*
 102  * Returns true if an exclusive lock is recursed.  It assumes
 103  * curthread currently has an exclusive lock.
 104  */
 105 #define sx_recurse              lock_object.lo_data
 106 #define sx_recursed(sx)         ((sx)->sx_recurse != 0)
 107
 108 static void     assert_sx(struct lock_object *lock, int what);
 109 #ifdef DDB
 110 static void     db_show_sx(struct lock_object *lock);
 111 #endif
 112 static void     lock_sx(struct lock_object *lock, int how);
 113 #ifdef KDTRACE_HOOKS
 114 static int      owner_sx(struct lock_object *lock, struct thread **owner);
 115 #endif
 116 static int      unlock_sx(struct lock_object *lock);
 117
 118 struct lock_class lock_class_sx = {
 119         .lc_name = "sx",
 120         .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
 121         .lc_assert = assert_sx,
 122 #ifdef DDB
 123         .lc_ddb_show = db_show_sx,
 124 #endif
 125         .lc_lock = lock_sx,
 126         .lc_unlock = unlock_sx,
 127 #ifdef KDTRACE_HOOKS
 128         .lc_owner = owner_sx,
 129 #endif
 130 };
 131
 132 #ifndef INVARIANTS
 133 #define _sx_assert(sx, what, file, line)
 134 #endif
 135
 136 #ifdef ADAPTIVE_SX
 137 static u_int asx_retries = 10;
 138 static u_int asx_loops = 10000;
 139 SYSCTL_NODE(_debug, OID_AUTO, sx, CTLFLAG_RD, NULL, "sxlock debugging");
 140 SYSCTL_UINT(_debug_sx, OID_AUTO, retries, CTLFLAG_RW, &asx_retries, 0, "");
 141 SYSCTL_UINT(_debug_sx, OID_AUTO, loops, CTLFLAG_RW, &asx_loops, 0, "");
 142 #endif
 143
 144 void
 145 assert_sx(struct lock_object *lock, int what)
 146 {
 147
 148         sx_assert((struct sx *)lock, what);
 149 }
 150
 151 void
 152 lock_sx(struct lock_object *lock, int how)
 153 {
 154         struct sx *sx;
 155
 156         sx = (struct sx *)lock;
 157         if (how)
 158                 sx_xlock(sx);
 159         else
 160                 sx_slock(sx);
 161 }
 162
 163 int
 164 unlock_sx(struct lock_object *lock)
 165 {
 166         struct sx *sx;
 167
 168         sx = (struct sx *)lock;
 169         sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
 170         if (sx_xlocked(sx)) {
 171                 sx_xunlock(sx);
 172                 return (1);
 173         } else {
 174                 sx_sunlock(sx);
 175                 return (0);
 176         }
 177 }
 178
 179 #ifdef KDTRACE_HOOKS
 180 int
 181 owner_sx(struct lock_object *lock, struct thread **owner)
 182 {
 183         struct sx *sx = (struct sx *)lock;
 184         uintptr_t x = sx->sx_lock;
 185
 186         *owner = (struct thread *)SX_OWNER(x);
 187         return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
 188             (*owner != NULL));
 189 }
 190 #endif
 191
 192 void
 193 sx_sysinit(void *arg)
 194 {
 195         struct sx_args *sargs = arg;
 196
 197         sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
 198 }
 199
 200 void
 201 sx_init_flags(struct sx *sx, const char *description, int opts)
 202 {
 203         int flags;
 204
 205         MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
 206             SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
 207         ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
 208             ("%s: sx_lock not aligned for %s: %p", __func__, description,
 209             &sx->sx_lock));
 210
 211         flags = LO_SLEEPABLE | LO_UPGRADABLE;
 212         if (opts & SX_DUPOK)
 213                 flags |= LO_DUPOK;
 214         if (opts & SX_NOPROFILE)
 215                 flags |= LO_NOPROFILE;
 216         if (!(opts & SX_NOWITNESS))
 217                 flags |= LO_WITNESS;
 218         if (opts & SX_RECURSE)
 219                 flags |= LO_RECURSABLE;
 220         if (opts & SX_QUIET)
 221                 flags |= LO_QUIET;
 222
 223         flags |= opts & SX_NOADAPTIVE;
 224         sx->sx_lock = SX_LOCK_UNLOCKED;
 225         sx->sx_recurse = 0;
 226         lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
 227 }
 228
 229 void
 230 sx_destroy(struct sx *sx)
 231 {
 232
 233         KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
 234         KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
 235         sx->sx_lock = SX_LOCK_DESTROYED;
 236         lock_destroy(&sx->lock_object);
 237 }
 238
 239 int
 240 _sx_slock(struct sx *sx, int opts, const char *file, int line)
 241 {
 242         int error = 0;
 243
 244         MPASS(curthread != NULL);
 245         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 246             ("sx_slock() of destroyed sx @ %s:%d", file, line));
 247         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
 248         error = __sx_slock(sx, opts, file, line);
 249         if (!error) {
 250                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
 251                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
 252                 curthread->td_locks++;
 253         }
 254
 255         return (error);
 256 }
 257
 258 int
 259 _sx_try_slock(struct sx *sx, const char *file, int line)
 260 {
 261         uintptr_t x;
 262
 263         for (;;) {
 264                 x = sx->sx_lock;
 265                 KASSERT(x != SX_LOCK_DESTROYED,
 266                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
 267                 if (!(x & SX_LOCK_SHARED))
 268                         break;
 269                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
 270                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
 271                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
 272                         curthread->td_locks++;
 273                         return (1);
 274                 }
 275         }
 276
 277         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
 278         return (0);
 279 }
 280
 281 int
 282 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
 283 {
 284         int error = 0;
 285
 286         MPASS(curthread != NULL);
 287         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 288             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
 289         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 290             line, NULL);
 291         error = __sx_xlock(sx, curthread, opts, file, line);
 292         if (!error) {
 293                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
 294                     file, line);
 295                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 296                 curthread->td_locks++;
 297         }
 298
 299         return (error);
 300 }
 301
 302 int
 303 _sx_try_xlock(struct sx *sx, const char *file, int line)
 304 {
 305         int rval;
 306
 307         MPASS(curthread != NULL);
 308         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 309             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
 310
 311         if (sx_xlocked(sx) &&
 312             (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 313                 sx->sx_recurse++;
 314                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 315                 rval = 1;
 316         } else
 317                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
 318                     (uintptr_t)curthread);
 319         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
 320         if (rval) {
 321                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 322                     file, line);
 323                 curthread->td_locks++;
 324         }
 325
 326         return (rval);
 327 }
 328
 329 void
 330 _sx_sunlock(struct sx *sx, const char *file, int line)
 331 {
 332
 333         MPASS(curthread != NULL);
 334         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 335             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
 336         _sx_assert(sx, SA_SLOCKED, file, line);
 337         curthread->td_locks--;
 338         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
 339         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
 340         __sx_sunlock(sx, file, line);
 341         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
 342 }
 343
 344 void
 345 _sx_xunlock(struct sx *sx, const char *file, int line)
 346 {
 347
 348         MPASS(curthread != NULL);
 349         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 350             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
 351         _sx_assert(sx, SA_XLOCKED, file, line);
 352         curthread->td_locks--;
 353         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 354         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
 355             line);
 356         if (!sx_recursed(sx))
 357                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
 358         __sx_xunlock(sx, curthread, file, line);
 359 }
 360
 361 /*
 362  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
 363  * This will only succeed if this thread holds a single shared lock.
 364  * Return 1 if if the upgrade succeed, 0 otherwise.
 365  */
 366 int
 367 _sx_try_upgrade(struct sx *sx, const char *file, int line)
 368 {
 369         uintptr_t x;
 370         int success;
 371
 372         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 373             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
 374         _sx_assert(sx, SA_SLOCKED, file, line);
 375
 376         /*
 377          * Try to switch from one shared lock to an exclusive lock.  We need
 378          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
 379          * we will wake up the exclusive waiters when we drop the lock.
 380          */
 381         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
 382         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
 383             (uintptr_t)curthread | x);
 384         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
 385         if (success) {
 386                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 387                     file, line);
 388                 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
 389         }
 390         return (success);
 391 }
 392
 393 /*
 394  * Downgrade an unrecursed exclusive lock into a single shared lock.
 395  */
 396 void
 397 _sx_downgrade(struct sx *sx, const char *file, int line)
 398 {
 399         uintptr_t x;
 400         int wakeup_swapper;
 401
 402         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 403             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
 404         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
 405 #ifndef INVARIANTS
 406         if (sx_recursed(sx))
 407                 panic("downgrade of a recursed lock");
 408 #endif
 409
 410         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
 411
 412         /*
 413          * Try to switch from an exclusive lock with no shared waiters
 414          * to one sharer with no shared waiters.  If there are
 415          * exclusive waiters, we don't need to lock the sleep queue so
 416          * long as we preserve the flag.  We do one quick try and if
 417          * that fails we grab the sleepq lock to keep the flags from
 418          * changing and do it the slow way.
 419          *
 420          * We have to lock the sleep queue if there are shared waiters
 421          * so we can wake them up.
 422          */
 423         x = sx->sx_lock;
 424         if (!(x & SX_LOCK_SHARED_WAITERS) &&
 425             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
 426             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
 427                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 428                 return;
 429         }
 430
 431         /*
 432          * Lock the sleep queue so we can read the waiters bits
 433          * without any races and wakeup any shared waiters.
 434          */
 435         sleepq_lock(&sx->lock_object);
 436
 437         /*
 438          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
 439          * shared lock.  If there are any shared waiters, wake them up.
 440          */
 441         wakeup_swapper = 0;
 442         x = sx->sx_lock;
 443         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
 444             (x & SX_LOCK_EXCLUSIVE_WAITERS));
 445         if (x & SX_LOCK_SHARED_WAITERS)
 446                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
 447                     0, SQ_SHARED_QUEUE);
 448         sleepq_release(&sx->lock_object);
 449
 450         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 451         LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
 452
 453         if (wakeup_swapper)
 454                 kick_proc0();
 455 }
 456
 457 /*
 458  * This function represents the so-called 'hard case' for sx_xlock
 459  * operation.  All 'easy case' failures are redirected to this.  Note
 460  * that ideally this would be a static function, but it needs to be
 461  * accessible from at least sx.h.
 462  */
 463 int
 464 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
 465     int line)
 466 {
 467         GIANT_DECLARE;
 468 #ifdef ADAPTIVE_SX
 469         volatile struct thread *owner;
 470         u_int i, spintries = 0;
 471 #endif
 472         uintptr_t x;
 473 #ifdef LOCK_PROFILING
 474         uint64_t waittime = 0;
 475         int contested = 0;
 476 #endif
 477         int error = 0;
 478 #ifdef  KDTRACE_HOOKS
 479         uint64_t spin_cnt = 0;
 480         uint64_t sleep_cnt = 0;
 481         int64_t sleep_time = 0;
 482 #endif
 483
 484         /* If we already hold an exclusive lock, then recurse. */
 485         if (sx_xlocked(sx)) {
 486                 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
 487             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
 488                     sx->lock_object.lo_name, file, line));
 489                 sx->sx_recurse++;
 490                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 491                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 492                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
 493                 return (0);
 494         }
 495
 496         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 497                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 498                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
 499
 500         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
 501 #ifdef KDTRACE_HOOKS
 502                 spin_cnt++;
 503 #endif
 504                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 505                     &waittime);
 506 #ifdef ADAPTIVE_SX
 507                 /*
 508                  * If the lock is write locked and the owner is
 509                  * running on another CPU, spin until the owner stops
 510                  * running or the state of the lock changes.
 511                  */
 512                 x = sx->sx_lock;
 513                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 514                         if ((x & SX_LOCK_SHARED) == 0) {
 515                                 x = SX_OWNER(x);
 516                                 owner = (struct thread *)x;
 517                                 if (TD_IS_RUNNING(owner)) {
 518                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 519                                                 CTR3(KTR_LOCK,
 520                                             "%s: spinning on %p held by %p",
 521                                                     __func__, sx, owner);
 522                                         GIANT_SAVE();
 523                                         while (SX_OWNER(sx->sx_lock) == x &&
 524                                             TD_IS_RUNNING(owner)) {
 525                                                 cpu_spinwait();
 526 #ifdef KDTRACE_HOOKS
 527                                                 spin_cnt++;
 528 #endif
 529                                         }
 530                                         continue;
 531                                 }
 532                         } else if (SX_SHARERS(x) && spintries < asx_retries) {
 533                                 GIANT_SAVE();
 534                                 spintries++;
 535                                 for (i = 0; i < asx_loops; i++) {
 536                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 537                                                 CTR4(KTR_LOCK,
 538                                     "%s: shared spinning on %p with %u and %u",
 539                                                     __func__, sx, spintries, i);
 540                                         x = sx->sx_lock;
 541                                         if ((x & SX_LOCK_SHARED) == 0 ||
 542                                             SX_SHARERS(x) == 0)
 543                                                 break;
 544                                         cpu_spinwait();
 545 #ifdef KDTRACE_HOOKS
 546                                         spin_cnt++;
 547 #endif
 548                                 }
 549                                 if (i != asx_loops)
 550                                         continue;
 551                         }
 552                 }
 553 #endif
 554
 555                 sleepq_lock(&sx->lock_object);
 556                 x = sx->sx_lock;
 557
 558                 /*
 559                  * If the lock was released while spinning on the
 560                  * sleep queue chain lock, try again.
 561                  */
 562                 if (x == SX_LOCK_UNLOCKED) {
 563                         sleepq_release(&sx->lock_object);
 564                         continue;
 565                 }
 566
 567 #ifdef ADAPTIVE_SX
 568                 /*
 569                  * The current lock owner might have started executing
 570                  * on another CPU (or the lock could have changed
 571                  * owners) while we were waiting on the sleep queue
 572                  * chain lock.  If so, drop the sleep queue lock and try
 573                  * again.
 574                  */
 575                 if (!(x & SX_LOCK_SHARED) &&
 576                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 577                         owner = (struct thread *)SX_OWNER(x);
 578                         if (TD_IS_RUNNING(owner)) {
 579                                 sleepq_release(&sx->lock_object);
 580                                 continue;
 581                         }
 582                 }
 583 #endif
 584
 585                 /*
 586                  * If an exclusive lock was released with both shared
 587                  * and exclusive waiters and a shared waiter hasn't
 588                  * woken up and acquired the lock yet, sx_lock will be
 589                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
 590                  * If we see that value, try to acquire it once.  Note
 591                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
 592                  * as there are other exclusive waiters still.  If we
 593                  * fail, restart the loop.
 594                  */
 595                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
 596                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
 597                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
 598                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
 599                                 sleepq_release(&sx->lock_object);
 600                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
 601                                     __func__, sx);
 602                                 break;
 603                         }
 604                         sleepq_release(&sx->lock_object);
 605                         continue;
 606                 }
 607
 608                 /*
 609                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
 610                  * than loop back and retry.
 611                  */
 612                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 613                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 614                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
 615                                 sleepq_release(&sx->lock_object);
 616                                 continue;
 617                         }
 618                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 619                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
 620                                     __func__, sx);
 621                 }
 622
 623                 /*
 624                  * Since we have been unable to acquire the exclusive
 625                  * lock and the exclusive waiters flag is set, we have
 626                  * to sleep.
 627                  */
 628                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 629                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 630                             __func__, sx);
 631
 632 #ifdef KDTRACE_HOOKS
 633                 sleep_time -= lockstat_nsecs();
 634 #endif
 635                 GIANT_SAVE();
 636                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 637                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 638                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
 639                 if (!(opts & SX_INTERRUPTIBLE))
 640                         sleepq_wait(&sx->lock_object, 0);
 641                 else
 642                         error = sleepq_wait_sig(&sx->lock_object, 0);
 643 #ifdef KDTRACE_HOOKS
 644                 sleep_time += lockstat_nsecs();
 645                 sleep_cnt++;
 646 #endif
 647                 if (error) {
 648                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 649                                 CTR2(KTR_LOCK,
 650                         "%s: interruptible sleep by %p suspended by signal",
 651                                     __func__, sx);
 652                         break;
 653                 }
 654                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 655                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 656                             __func__, sx);
 657         }
 658
 659         GIANT_RESTORE();
 660         if (!error)
 661                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
 662                     contested, waittime, file, line);
 663 #ifdef KDTRACE_HOOKS
 664         if (sleep_time)
 665                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 666         if (spin_cnt > sleep_cnt)
 667                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 668 #endif
 669         return (error);
 670 }
 671
 672 /*
 673  * This function represents the so-called 'hard case' for sx_xunlock
 674  * operation.  All 'easy case' failures are redirected to this.  Note
 675  * that ideally this would be a static function, but it needs to be
 676  * accessible from at least sx.h.
 677  */
 678 void
 679 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
 680 {
 681         uintptr_t x;
 682         int queue, wakeup_swapper;
 683
 684         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
 685
 686         /* If the lock is recursed, then unrecurse one level. */
 687         if (sx_xlocked(sx) && sx_recursed(sx)) {
 688                 if ((--sx->sx_recurse) == 0)
 689                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 690                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 691                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
 692                 return;
 693         }
 694         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
 695             SX_LOCK_EXCLUSIVE_WAITERS));
 696         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 697                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
 698
 699         sleepq_lock(&sx->lock_object);
 700         x = SX_LOCK_UNLOCKED;
 701
 702         /*
 703          * The wake up algorithm here is quite simple and probably not
 704          * ideal.  It gives precedence to shared waiters if they are
 705          * present.  For this condition, we have to preserve the
 706          * state of the exclusive waiters flag.
 707          * If interruptible sleeps left the shared queue empty avoid a
 708          * starvation for the threads sleeping on the exclusive queue by giving
 709          * them precedence and cleaning up the shared waiters bit anyway.
 710          */
 711         if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
 712             sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
 713                 queue = SQ_SHARED_QUEUE;
 714                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
 715         } else
 716                 queue = SQ_EXCLUSIVE_QUEUE;
 717
 718         /* Wake up all the waiters for the specific queue. */
 719         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 720                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
 721                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
 722                     "exclusive");
 723         atomic_store_rel_ptr(&sx->sx_lock, x);
 724         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
 725             queue);
 726         sleepq_release(&sx->lock_object);
 727         if (wakeup_swapper)
 728                 kick_proc0();
 729 }
 730
 731 /*
 732  * This function represents the so-called 'hard case' for sx_slock
 733  * operation.  All 'easy case' failures are redirected to this.  Note
 734  * that ideally this would be a static function, but it needs to be
 735  * accessible from at least sx.h.
 736  */
 737 int
 738 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
 739 {
 740         GIANT_DECLARE;
 741 #ifdef ADAPTIVE_SX
 742         volatile struct thread *owner;
 743 #endif
 744 #ifdef LOCK_PROFILING
 745         uint64_t waittime = 0;
 746         int contested = 0;
 747 #endif
 748         uintptr_t x;
 749         int error = 0;
 750 #ifdef KDTRACE_HOOKS
 751         uint64_t spin_cnt = 0;
 752         uint64_t sleep_cnt = 0;
 753         int64_t sleep_time = 0;
 754 #endif
 755
 756         /*
 757          * As with rwlocks, we don't make any attempt to try to block
 758          * shared locks once there is an exclusive waiter.
 759          */
 760         for (;;) {
 761 #ifdef KDTRACE_HOOKS
 762                 spin_cnt++;
 763 #endif
 764                 x = sx->sx_lock;
 765
 766                 /*
 767                  * If no other thread has an exclusive lock then try to bump up
 768                  * the count of sharers.  Since we have to preserve the state
 769                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
 770                  * shared lock loop back and retry.
 771                  */
 772                 if (x & SX_LOCK_SHARED) {
 773                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
 774                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
 775                             x + SX_ONE_SHARER)) {
 776                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 777                                         CTR4(KTR_LOCK,
 778                                             "%s: %p succeed %p -> %p", __func__,
 779                                             sx, (void *)x,
 780                                             (void *)(x + SX_ONE_SHARER));
 781                                 break;
 782                         }
 783                         continue;
 784                 }
 785                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 786                     &waittime);
 787
 788 #ifdef ADAPTIVE_SX
 789                 /*
 790                  * If the owner is running on another CPU, spin until
 791                  * the owner stops running or the state of the lock
 792                  * changes.
 793                  */
 794                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 795                         x = SX_OWNER(x);
 796                         owner = (struct thread *)x;
 797                         if (TD_IS_RUNNING(owner)) {
 798                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 799                                         CTR3(KTR_LOCK,
 800                                             "%s: spinning on %p held by %p",
 801                                             __func__, sx, owner);
 802                                 GIANT_SAVE();
 803                                 while (SX_OWNER(sx->sx_lock) == x &&
 804                                     TD_IS_RUNNING(owner)) {
 805 #ifdef KDTRACE_HOOKS
 806                                         spin_cnt++;
 807 #endif
 808                                         cpu_spinwait();
 809                                 }
 810                                 continue;
 811                         }
 812                 }
 813 #endif
 814
 815                 /*
 816                  * Some other thread already has an exclusive lock, so
 817                  * start the process of blocking.
 818                  */
 819                 sleepq_lock(&sx->lock_object);
 820                 x = sx->sx_lock;
 821
 822                 /*
 823                  * The lock could have been released while we spun.
 824                  * In this case loop back and retry.
 825                  */
 826                 if (x & SX_LOCK_SHARED) {
 827                         sleepq_release(&sx->lock_object);
 828                         continue;
 829                 }
 830
 831 #ifdef ADAPTIVE_SX
 832                 /*
 833                  * If the owner is running on another CPU, spin until
 834                  * the owner stops running or the state of the lock
 835                  * changes.
 836                  */
 837                 if (!(x & SX_LOCK_SHARED) &&
 838                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 839                         owner = (struct thread *)SX_OWNER(x);
 840                         if (TD_IS_RUNNING(owner)) {
 841                                 sleepq_release(&sx->lock_object);
 842                                 continue;
 843                         }
 844                 }
 845 #endif
 846
 847                 /*
 848                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
 849                  * fail to set it drop the sleep queue lock and loop
 850                  * back.
 851                  */
 852                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
 853                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 854                             x | SX_LOCK_SHARED_WAITERS)) {
 855                                 sleepq_release(&sx->lock_object);
 856                                 continue;
 857                         }
 858                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 859                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
 860                                     __func__, sx);
 861                 }
 862
 863                 /*
 864                  * Since we have been unable to acquire the shared lock,
 865                  * we have to sleep.
 866                  */
 867                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 868                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 869                             __func__, sx);
 870
 871 #ifdef KDTRACE_HOOKS
 872                 sleep_time -= lockstat_nsecs();
 873 #endif
 874                 GIANT_SAVE();
 875                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 876                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 877                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
 878                 if (!(opts & SX_INTERRUPTIBLE))
 879                         sleepq_wait(&sx->lock_object, 0);
 880                 else
 881                         error = sleepq_wait_sig(&sx->lock_object, 0);
 882 #ifdef KDTRACE_HOOKS
 883                 sleep_time += lockstat_nsecs();
 884                 sleep_cnt++;
 885 #endif
 886                 if (error) {
 887                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 888                                 CTR2(KTR_LOCK,
 889                         "%s: interruptible sleep by %p suspended by signal",
 890                                     __func__, sx);
 891                         break;
 892                 }
 893                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 894                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 895                             __func__, sx);
 896         }
 897         if (error == 0)
 898                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
 899                     contested, waittime, file, line);
 900 #ifdef KDTRACE_HOOKS
 901         if (sleep_time)
 902                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 903         if (spin_cnt > sleep_cnt)
 904                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 905 #endif
 906         GIANT_RESTORE();
 907         return (error);
 908 }
 909
 910 /*
 911  * This function represents the so-called 'hard case' for sx_sunlock
 912  * operation.  All 'easy case' failures are redirected to this.  Note
 913  * that ideally this would be a static function, but it needs to be
 914  * accessible from at least sx.h.
 915  */
 916 void
 917 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
 918 {
 919         uintptr_t x;
 920         int wakeup_swapper;
 921
 922         for (;;) {
 923                 x = sx->sx_lock;
 924
 925                 /*
 926                  * We should never have sharers while at least one thread
 927                  * holds a shared lock.
 928                  */
 929                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
 930                     ("%s: waiting sharers", __func__));
 931
 932                 /*
 933                  * See if there is more than one shared lock held.  If
 934                  * so, just drop one and return.
 935                  */
 936                 if (SX_SHARERS(x) > 1) {
 937                         if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
 938                             x - SX_ONE_SHARER)) {
 939                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 940                                         CTR4(KTR_LOCK,
 941                                             "%s: %p succeeded %p -> %p",
 942                                             __func__, sx, (void *)x,
 943                                             (void *)(x - SX_ONE_SHARER));
 944                                 break;
 945                         }
 946                         continue;
 947                 }
 948
 949                 /*
 950                  * If there aren't any waiters for an exclusive lock,
 951                  * then try to drop it quickly.
 952                  */
 953                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 954                         MPASS(x == SX_SHARERS_LOCK(1));
 955                         if (atomic_cmpset_rel_ptr(&sx->sx_lock,
 956                             SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
 957                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 958                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 959                                             __func__, sx);
 960                                 break;
 961                         }
 962                         continue;
 963                 }
 964
 965                 /*
 966                  * At this point, there should just be one sharer with
 967                  * exclusive waiters.
 968                  */
 969                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
 970
 971                 sleepq_lock(&sx->lock_object);
 972
 973                 /*
 974                  * Wake up semantic here is quite simple:
 975                  * Just wake up all the exclusive waiters.
 976                  * Note that the state of the lock could have changed,
 977                  * so if it fails loop back and retry.
 978                  */
 979                 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
 980                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
 981                     SX_LOCK_UNLOCKED)) {
 982                         sleepq_release(&sx->lock_object);
 983                         continue;
 984                 }
 985                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 986                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
 987                             "exclusive queue", __func__, sx);
 988                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
 989                     0, SQ_EXCLUSIVE_QUEUE);
 990                 sleepq_release(&sx->lock_object);
 991                 if (wakeup_swapper)
 992                         kick_proc0();
 993                 break;
 994         }
 995 }
 996
 997 #ifdef INVARIANT_SUPPORT
 998 #ifndef INVARIANTS
 999 #undef  _sx_assert
1000 #endif
1001
1002 /*
1003  * In the non-WITNESS case, sx_assert() can only detect that at least
1004  * *some* thread owns an slock, but it cannot guarantee that *this*
1005  * thread owns an slock.
1006  */
1007 void
1008 _sx_assert(struct sx *sx, int what, const char *file, int line)
1009 {
1010 #ifndef WITNESS
1011         int slocked = 0;
1012 #endif
1013
1014         if (panicstr != NULL)
1015                 return;
1016         switch (what) {
1017         case SA_SLOCKED:
1018         case SA_SLOCKED | SA_NOTRECURSED:
1019         case SA_SLOCKED | SA_RECURSED:
1020 #ifndef WITNESS
1021                 slocked = 1;
1022                 /* FALLTHROUGH */
1023 #endif
1024         case SA_LOCKED:
1025         case SA_LOCKED | SA_NOTRECURSED:
1026         case SA_LOCKED | SA_RECURSED:
1027 #ifdef WITNESS
1028                 witness_assert(&sx->lock_object, what, file, line);
1029 #else
1030                 /*
1031                  * If some other thread has an exclusive lock or we
1032                  * have one and are asserting a shared lock, fail.
1033                  * Also, if no one has a lock at all, fail.
1034                  */
1035                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1036                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1037                     sx_xholder(sx) != curthread)))
1038                         panic("Lock %s not %slocked @ %s:%d\n",
1039                             sx->lock_object.lo_name, slocked ? "share " : "",
1040                             file, line);
1041
1042                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1043                         if (sx_recursed(sx)) {
1044                                 if (what & SA_NOTRECURSED)
1045                                         panic("Lock %s recursed @ %s:%d\n",
1046                                             sx->lock_object.lo_name, file,
1047                                             line);
1048                         } else if (what & SA_RECURSED)
1049                                 panic("Lock %s not recursed @ %s:%d\n",
1050                                     sx->lock_object.lo_name, file, line);
1051                 }
1052 #endif
1053                 break;
1054         case SA_XLOCKED:
1055         case SA_XLOCKED | SA_NOTRECURSED:
1056         case SA_XLOCKED | SA_RECURSED:
1057                 if (sx_xholder(sx) != curthread)
1058                         panic("Lock %s not exclusively locked @ %s:%d\n",
1059                             sx->lock_object.lo_name, file, line);
1060                 if (sx_recursed(sx)) {
1061                         if (what & SA_NOTRECURSED)
1062                                 panic("Lock %s recursed @ %s:%d\n",
1063                                     sx->lock_object.lo_name, file, line);
1064                 } else if (what & SA_RECURSED)
1065                         panic("Lock %s not recursed @ %s:%d\n",
1066                             sx->lock_object.lo_name, file, line);
1067                 break;
1068         case SA_UNLOCKED:
1069 #ifdef WITNESS
1070                 witness_assert(&sx->lock_object, what, file, line);
1071 #else
1072                 /*
1073                  * If we hold an exclusve lock fail.  We can't
1074                  * reliably check to see if we hold a shared lock or
1075                  * not.
1076                  */
1077                 if (sx_xholder(sx) == curthread)
1078                         panic("Lock %s exclusively locked @ %s:%d\n",
1079                             sx->lock_object.lo_name, file, line);
1080 #endif
1081                 break;
1082         default:
1083                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1084                     line);
1085         }
1086 }
1087 #endif  /* INVARIANT_SUPPORT */
1088
1089 #ifdef DDB
1090 static void
1091 db_show_sx(struct lock_object *lock)
1092 {
1093         struct thread *td;
1094         struct sx *sx;
1095
1096         sx = (struct sx *)lock;
1097
1098         db_printf(" state: ");
1099         if (sx->sx_lock == SX_LOCK_UNLOCKED)
1100                 db_printf("UNLOCKED\n");
1101         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1102                 db_printf("DESTROYED\n");
1103                 return;
1104         } else if (sx->sx_lock & SX_LOCK_SHARED)
1105                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1106         else {
1107                 td = sx_xholder(sx);
1108                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1109                     td->td_tid, td->td_proc->p_pid, td->td_name);
1110                 if (sx_recursed(sx))
1111                         db_printf(" recursed: %d\n", sx->sx_recurse);
1112         }
1113
1114         db_printf(" waiters: ");
1115         switch(sx->sx_lock &
1116             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1117         case SX_LOCK_SHARED_WAITERS:
1118                 db_printf("shared\n");
1119                 break;
1120         case SX_LOCK_EXCLUSIVE_WAITERS:
1121                 db_printf("exclusive\n");
1122                 break;
1123         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1124                 db_printf("exclusive and shared\n");
1125                 break;
1126         default:
1127                 db_printf("none\n");
1128         }
1129 }
1130
1131 /*
1132  * Check to see if a thread that is blocked on a sleep queue is actually
1133  * blocked on an sx lock.  If so, output some details and return true.
1134  * If the lock has an exclusive owner, return that in *ownerp.
1135  */
1136 int
1137 sx_chain(struct thread *td, struct thread **ownerp)
1138 {
1139         struct sx *sx;
1140
1141         /*
1142          * Check to see if this thread is blocked on an sx lock.
1143          * First, we check the lock class.  If that is ok, then we
1144          * compare the lock name against the wait message.
1145          */
1146         sx = td->td_wchan;
1147         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1148             sx->lock_object.lo_name != td->td_wmesg)
1149                 return (0);
1150
1151         /* We think we have an sx lock, so output some details. */
1152         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1153         *ownerp = sx_xholder(sx);
1154         if (sx->sx_lock & SX_LOCK_SHARED)
1155                 db_printf("SLOCK (count %ju)\n",
1156                     (uintmax_t)SX_SHARERS(sx->sx_lock));
1157         else
1158                 db_printf("XLOCK\n");
1159         return (1);
1160 }
1161 #endif