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/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