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