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/systm.h>
  48 #include <sys/ktr.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
  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(sargs->sa_sx, sargs->sa_desc);
 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         if (SCHEDULER_STOPPED())
 242                 return (0);
 243         MPASS(curthread != NULL);
 244         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 245             ("sx_slock() of destroyed sx @ %s:%d", file, line));
 246         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
 247         error = __sx_slock(sx, opts, file, line);
 248         if (!error) {
 249                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
 250                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
 251                 curthread->td_locks++;
 252         }
 253
 254         return (error);
 255 }
 256
 257 int
 258 _sx_try_slock(struct sx *sx, const char *file, int line)
 259 {
 260         uintptr_t x;
 261
 262         if (SCHEDULER_STOPPED())
 263                 return (1);
 264
 265         for (;;) {
 266                 x = sx->sx_lock;
 267                 KASSERT(x != SX_LOCK_DESTROYED,
 268                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
 269                 if (!(x & SX_LOCK_SHARED))
 270                         break;
 271                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
 272                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
 273                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
 274                         curthread->td_locks++;
 275                         return (1);
 276                 }
 277         }
 278
 279         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
 280         return (0);
 281 }
 282
 283 int
 284 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
 285 {
 286         int error = 0;
 287
 288         if (SCHEDULER_STOPPED())
 289                 return (0);
 290         MPASS(curthread != NULL);
 291         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 292             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
 293         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 294             line, NULL);
 295         error = __sx_xlock(sx, curthread, opts, file, line);
 296         if (!error) {
 297                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
 298                     file, line);
 299                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 300                 curthread->td_locks++;
 301         }
 302
 303         return (error);
 304 }
 305
 306 int
 307 _sx_try_xlock(struct sx *sx, const char *file, int line)
 308 {
 309         int rval;
 310
 311         if (SCHEDULER_STOPPED())
 312                 return (1);
 313
 314         MPASS(curthread != NULL);
 315         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 316             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
 317
 318         if (sx_xlocked(sx) &&
 319             (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 320                 sx->sx_recurse++;
 321                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 322                 rval = 1;
 323         } else
 324                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
 325                     (uintptr_t)curthread);
 326         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
 327         if (rval) {
 328                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 329                     file, line);
 330                 curthread->td_locks++;
 331         }
 332
 333         return (rval);
 334 }
 335
 336 void
 337 _sx_sunlock(struct sx *sx, const char *file, int line)
 338 {
 339
 340         if (SCHEDULER_STOPPED())
 341                 return;
 342         MPASS(curthread != NULL);
 343         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 344             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
 345         _sx_assert(sx, SA_SLOCKED, file, line);
 346         curthread->td_locks--;
 347         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
 348         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
 349         __sx_sunlock(sx, file, line);
 350         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
 351 }
 352
 353 void
 354 _sx_xunlock(struct sx *sx, const char *file, int line)
 355 {
 356
 357         if (SCHEDULER_STOPPED())
 358                 return;
 359         MPASS(curthread != NULL);
 360         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 361             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
 362         _sx_assert(sx, SA_XLOCKED, file, line);
 363         curthread->td_locks--;
 364         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 365         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
 366             line);
 367         if (!sx_recursed(sx))
 368                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
 369         __sx_xunlock(sx, curthread, file, line);
 370 }
 371
 372 /*
 373  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
 374  * This will only succeed if this thread holds a single shared lock.
 375  * Return 1 if if the upgrade succeed, 0 otherwise.
 376  */
 377 int
 378 _sx_try_upgrade(struct sx *sx, const char *file, int line)
 379 {
 380         uintptr_t x;
 381         int success;
 382
 383         if (SCHEDULER_STOPPED())
 384                 return (1);
 385
 386         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 387             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
 388         _sx_assert(sx, SA_SLOCKED, file, line);
 389
 390         /*
 391          * Try to switch from one shared lock to an exclusive lock.  We need
 392          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
 393          * we will wake up the exclusive waiters when we drop the lock.
 394          */
 395         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
 396         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
 397             (uintptr_t)curthread | x);
 398         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
 399         if (success) {
 400                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 401                     file, line);
 402                 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
 403         }
 404         return (success);
 405 }
 406
 407 /*
 408  * Downgrade an unrecursed exclusive lock into a single shared lock.
 409  */
 410 void
 411 _sx_downgrade(struct sx *sx, const char *file, int line)
 412 {
 413         uintptr_t x;
 414         int wakeup_swapper;
 415
 416         if (SCHEDULER_STOPPED())
 417                 return;
 418
 419         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 420             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
 421         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
 422 #ifndef INVARIANTS
 423         if (sx_recursed(sx))
 424                 panic("downgrade of a recursed lock");
 425 #endif
 426
 427         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
 428
 429         /*
 430          * Try to switch from an exclusive lock with no shared waiters
 431          * to one sharer with no shared waiters.  If there are
 432          * exclusive waiters, we don't need to lock the sleep queue so
 433          * long as we preserve the flag.  We do one quick try and if
 434          * that fails we grab the sleepq lock to keep the flags from
 435          * changing and do it the slow way.
 436          *
 437          * We have to lock the sleep queue if there are shared waiters
 438          * so we can wake them up.
 439          */
 440         x = sx->sx_lock;
 441         if (!(x & SX_LOCK_SHARED_WAITERS) &&
 442             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
 443             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
 444                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 445                 return;
 446         }
 447
 448         /*
 449          * Lock the sleep queue so we can read the waiters bits
 450          * without any races and wakeup any shared waiters.
 451          */
 452         sleepq_lock(&sx->lock_object);
 453
 454         /*
 455          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
 456          * shared lock.  If there are any shared waiters, wake them up.
 457          */
 458         wakeup_swapper = 0;
 459         x = sx->sx_lock;
 460         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
 461             (x & SX_LOCK_EXCLUSIVE_WAITERS));
 462         if (x & SX_LOCK_SHARED_WAITERS)
 463                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
 464                     0, SQ_SHARED_QUEUE);
 465         sleepq_release(&sx->lock_object);
 466
 467         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 468         LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
 469
 470         if (wakeup_swapper)
 471                 kick_proc0();
 472 }
 473
 474 /*
 475  * This function represents the so-called 'hard case' for sx_xlock
 476  * operation.  All 'easy case' failures are redirected to this.  Note
 477  * that ideally this would be a static function, but it needs to be
 478  * accessible from at least sx.h.
 479  */
 480 int
 481 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
 482     int line)
 483 {
 484         GIANT_DECLARE;
 485 #ifdef ADAPTIVE_SX
 486         volatile struct thread *owner;
 487         u_int i, spintries = 0;
 488 #endif
 489         uintptr_t x;
 490 #ifdef LOCK_PROFILING
 491         uint64_t waittime = 0;
 492         int contested = 0;
 493 #endif
 494         int error = 0;
 495 #ifdef  KDTRACE_HOOKS
 496         uint64_t spin_cnt = 0;
 497         uint64_t sleep_cnt = 0;
 498         int64_t sleep_time = 0;
 499 #endif
 500
 501         if (SCHEDULER_STOPPED())
 502                 return (0);
 503
 504         /* If we already hold an exclusive lock, then recurse. */
 505         if (sx_xlocked(sx)) {
 506                 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
 507             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
 508                     sx->lock_object.lo_name, file, line));
 509                 sx->sx_recurse++;
 510                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 511                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 512                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
 513                 return (0);
 514         }
 515
 516         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 517                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 518                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
 519
 520         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
 521 #ifdef KDTRACE_HOOKS
 522                 spin_cnt++;
 523 #endif
 524                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 525                     &waittime);
 526 #ifdef ADAPTIVE_SX
 527                 /*
 528                  * If the lock is write locked and the owner is
 529                  * running on another CPU, spin until the owner stops
 530                  * running or the state of the lock changes.
 531                  */
 532                 x = sx->sx_lock;
 533                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 534                         if ((x & SX_LOCK_SHARED) == 0) {
 535                                 x = SX_OWNER(x);
 536                                 owner = (struct thread *)x;
 537                                 if (TD_IS_RUNNING(owner)) {
 538                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 539                                                 CTR3(KTR_LOCK,
 540                                             "%s: spinning on %p held by %p",
 541                                                     __func__, sx, owner);
 542                                         GIANT_SAVE();
 543                                         while (SX_OWNER(sx->sx_lock) == x &&
 544                                             TD_IS_RUNNING(owner)) {
 545                                                 cpu_spinwait();
 546 #ifdef KDTRACE_HOOKS
 547                                                 spin_cnt++;
 548 #endif
 549                                         }
 550                                         continue;
 551                                 }
 552                         } else if (SX_SHARERS(x) && spintries < ASX_RETRIES) {
 553                                 GIANT_SAVE();
 554                                 spintries++;
 555                                 for (i = 0; i < ASX_LOOPS; i++) {
 556                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 557                                                 CTR4(KTR_LOCK,
 558                                     "%s: shared spinning on %p with %u and %u",
 559                                                     __func__, sx, spintries, i);
 560                                         x = sx->sx_lock;
 561                                         if ((x & SX_LOCK_SHARED) == 0 ||
 562                                             SX_SHARERS(x) == 0)
 563                                                 break;
 564                                         cpu_spinwait();
 565 #ifdef KDTRACE_HOOKS
 566                                         spin_cnt++;
 567 #endif
 568                                 }
 569                                 if (i != ASX_LOOPS)
 570                                         continue;
 571                         }
 572                 }
 573 #endif
 574
 575                 sleepq_lock(&sx->lock_object);
 576                 x = sx->sx_lock;
 577
 578                 /*
 579                  * If the lock was released while spinning on the
 580                  * sleep queue chain lock, try again.
 581                  */
 582                 if (x == SX_LOCK_UNLOCKED) {
 583                         sleepq_release(&sx->lock_object);
 584                         continue;
 585                 }
 586
 587 #ifdef ADAPTIVE_SX
 588                 /*
 589                  * The current lock owner might have started executing
 590                  * on another CPU (or the lock could have changed
 591                  * owners) while we were waiting on the sleep queue
 592                  * chain lock.  If so, drop the sleep queue lock and try
 593                  * again.
 594                  */
 595                 if (!(x & SX_LOCK_SHARED) &&
 596                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 597                         owner = (struct thread *)SX_OWNER(x);
 598                         if (TD_IS_RUNNING(owner)) {
 599                                 sleepq_release(&sx->lock_object);
 600                                 continue;
 601                         }
 602                 }
 603 #endif
 604
 605                 /*
 606                  * If an exclusive lock was released with both shared
 607                  * and exclusive waiters and a shared waiter hasn't
 608                  * woken up and acquired the lock yet, sx_lock will be
 609                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
 610                  * If we see that value, try to acquire it once.  Note
 611                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
 612                  * as there are other exclusive waiters still.  If we
 613                  * fail, restart the loop.
 614                  */
 615                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
 616                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
 617                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
 618                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
 619                                 sleepq_release(&sx->lock_object);
 620                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
 621                                     __func__, sx);
 622                                 break;
 623                         }
 624                         sleepq_release(&sx->lock_object);
 625                         continue;
 626                 }
 627
 628                 /*
 629                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
 630                  * than loop back and retry.
 631                  */
 632                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 633                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 634                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
 635                                 sleepq_release(&sx->lock_object);
 636                                 continue;
 637                         }
 638                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 639                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
 640                                     __func__, sx);
 641                 }
 642
 643                 /*
 644                  * Since we have been unable to acquire the exclusive
 645                  * lock and the exclusive waiters flag is set, we have
 646                  * to sleep.
 647                  */
 648                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 649                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 650                             __func__, sx);
 651
 652 #ifdef KDTRACE_HOOKS
 653                 sleep_time -= lockstat_nsecs();
 654 #endif
 655                 GIANT_SAVE();
 656                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 657                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 658                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
 659                 if (!(opts & SX_INTERRUPTIBLE))
 660                         sleepq_wait(&sx->lock_object, 0);
 661                 else
 662                         error = sleepq_wait_sig(&sx->lock_object, 0);
 663 #ifdef KDTRACE_HOOKS
 664                 sleep_time += lockstat_nsecs();
 665                 sleep_cnt++;
 666 #endif
 667                 if (error) {
 668                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 669                                 CTR2(KTR_LOCK,
 670                         "%s: interruptible sleep by %p suspended by signal",
 671                                     __func__, sx);
 672                         break;
 673                 }
 674                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 675                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 676                             __func__, sx);
 677         }
 678
 679         GIANT_RESTORE();
 680         if (!error)
 681                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
 682                     contested, waittime, file, line);
 683 #ifdef KDTRACE_HOOKS
 684         if (sleep_time)
 685                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 686         if (spin_cnt > sleep_cnt)
 687                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 688 #endif
 689         return (error);
 690 }
 691
 692 /*
 693  * This function represents the so-called 'hard case' for sx_xunlock
 694  * operation.  All 'easy case' failures are redirected to this.  Note
 695  * that ideally this would be a static function, but it needs to be
 696  * accessible from at least sx.h.
 697  */
 698 void
 699 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
 700 {
 701         uintptr_t x;
 702         int queue, wakeup_swapper;
 703
 704         if (SCHEDULER_STOPPED())
 705                 return;
 706
 707         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
 708
 709         /* If the lock is recursed, then unrecurse one level. */
 710         if (sx_xlocked(sx) && sx_recursed(sx)) {
 711                 if ((--sx->sx_recurse) == 0)
 712                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 713                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 714                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
 715                 return;
 716         }
 717         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
 718             SX_LOCK_EXCLUSIVE_WAITERS));
 719         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 720                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
 721
 722         sleepq_lock(&sx->lock_object);
 723         x = SX_LOCK_UNLOCKED;
 724
 725         /*
 726          * The wake up algorithm here is quite simple and probably not
 727          * ideal.  It gives precedence to shared waiters if they are
 728          * present.  For this condition, we have to preserve the
 729          * state of the exclusive waiters flag.
 730          * If interruptible sleeps left the shared queue empty avoid a
 731          * starvation for the threads sleeping on the exclusive queue by giving
 732          * them precedence and cleaning up the shared waiters bit anyway.
 733          */
 734         if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
 735             sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
 736                 queue = SQ_SHARED_QUEUE;
 737                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
 738         } else
 739                 queue = SQ_EXCLUSIVE_QUEUE;
 740
 741         /* Wake up all the waiters for the specific queue. */
 742         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 743                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
 744                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
 745                     "exclusive");
 746         atomic_store_rel_ptr(&sx->sx_lock, x);
 747         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
 748             queue);
 749         sleepq_release(&sx->lock_object);
 750         if (wakeup_swapper)
 751                 kick_proc0();
 752 }
 753
 754 /*
 755  * This function represents the so-called 'hard case' for sx_slock
 756  * operation.  All 'easy case' failures are redirected to this.  Note
 757  * that ideally this would be a static function, but it needs to be
 758  * accessible from at least sx.h.
 759  */
 760 int
 761 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
 762 {
 763         GIANT_DECLARE;
 764 #ifdef ADAPTIVE_SX
 765         volatile struct thread *owner;
 766 #endif
 767 #ifdef LOCK_PROFILING
 768         uint64_t waittime = 0;
 769         int contested = 0;
 770 #endif
 771         uintptr_t x;
 772         int error = 0;
 773 #ifdef KDTRACE_HOOKS
 774         uint64_t spin_cnt = 0;
 775         uint64_t sleep_cnt = 0;
 776         int64_t sleep_time = 0;
 777 #endif
 778
 779         if (SCHEDULER_STOPPED())
 780                 return (0);
 781
 782         /*
 783          * As with rwlocks, we don't make any attempt to try to block
 784          * shared locks once there is an exclusive waiter.
 785          */
 786         for (;;) {
 787 #ifdef KDTRACE_HOOKS
 788                 spin_cnt++;
 789 #endif
 790                 x = sx->sx_lock;
 791
 792                 /*
 793                  * If no other thread has an exclusive lock then try to bump up
 794                  * the count of sharers.  Since we have to preserve the state
 795                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
 796                  * shared lock loop back and retry.
 797                  */
 798                 if (x & SX_LOCK_SHARED) {
 799                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
 800                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
 801                             x + SX_ONE_SHARER)) {
 802                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 803                                         CTR4(KTR_LOCK,
 804                                             "%s: %p succeed %p -> %p", __func__,
 805                                             sx, (void *)x,
 806                                             (void *)(x + SX_ONE_SHARER));
 807                                 break;
 808                         }
 809                         continue;
 810                 }
 811                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 812                     &waittime);
 813
 814 #ifdef ADAPTIVE_SX
 815                 /*
 816                  * If the owner is running on another CPU, spin until
 817                  * the owner stops running or the state of the lock
 818                  * changes.
 819                  */
 820                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 821                         x = SX_OWNER(x);
 822                         owner = (struct thread *)x;
 823                         if (TD_IS_RUNNING(owner)) {
 824                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 825                                         CTR3(KTR_LOCK,
 826                                             "%s: spinning on %p held by %p",
 827                                             __func__, sx, owner);
 828                                 GIANT_SAVE();
 829                                 while (SX_OWNER(sx->sx_lock) == x &&
 830                                     TD_IS_RUNNING(owner)) {
 831 #ifdef KDTRACE_HOOKS
 832                                         spin_cnt++;
 833 #endif
 834                                         cpu_spinwait();
 835                                 }
 836                                 continue;
 837                         }
 838                 }
 839 #endif
 840
 841                 /*
 842                  * Some other thread already has an exclusive lock, so
 843                  * start the process of blocking.
 844                  */
 845                 sleepq_lock(&sx->lock_object);
 846                 x = sx->sx_lock;
 847
 848                 /*
 849                  * The lock could have been released while we spun.
 850                  * In this case loop back and retry.
 851                  */
 852                 if (x & SX_LOCK_SHARED) {
 853                         sleepq_release(&sx->lock_object);
 854                         continue;
 855                 }
 856
 857 #ifdef ADAPTIVE_SX
 858                 /*
 859                  * If the owner is running on another CPU, spin until
 860                  * the owner stops running or the state of the lock
 861                  * changes.
 862                  */
 863                 if (!(x & SX_LOCK_SHARED) &&
 864                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 865                         owner = (struct thread *)SX_OWNER(x);
 866                         if (TD_IS_RUNNING(owner)) {
 867                                 sleepq_release(&sx->lock_object);
 868                                 continue;
 869                         }
 870                 }
 871 #endif
 872
 873                 /*
 874                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
 875                  * fail to set it drop the sleep queue lock and loop
 876                  * back.
 877                  */
 878                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
 879                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 880                             x | SX_LOCK_SHARED_WAITERS)) {
 881                                 sleepq_release(&sx->lock_object);
 882                                 continue;
 883                         }
 884                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 885                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
 886                                     __func__, sx);
 887                 }
 888
 889                 /*
 890                  * Since we have been unable to acquire the shared lock,
 891                  * we have to sleep.
 892                  */
 893                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 894                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 895                             __func__, sx);
 896
 897 #ifdef KDTRACE_HOOKS
 898                 sleep_time -= lockstat_nsecs();
 899 #endif
 900                 GIANT_SAVE();
 901                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 902                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 903                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
 904                 if (!(opts & SX_INTERRUPTIBLE))
 905                         sleepq_wait(&sx->lock_object, 0);
 906                 else
 907                         error = sleepq_wait_sig(&sx->lock_object, 0);
 908 #ifdef KDTRACE_HOOKS
 909                 sleep_time += lockstat_nsecs();
 910                 sleep_cnt++;
 911 #endif
 912                 if (error) {
 913                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 914                                 CTR2(KTR_LOCK,
 915                         "%s: interruptible sleep by %p suspended by signal",
 916                                     __func__, sx);
 917                         break;
 918                 }
 919                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 920                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 921                             __func__, sx);
 922         }
 923         if (error == 0)
 924                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
 925                     contested, waittime, file, line);
 926 #ifdef KDTRACE_HOOKS
 927         if (sleep_time)
 928                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 929         if (spin_cnt > sleep_cnt)
 930                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 931 #endif
 932         GIANT_RESTORE();
 933         return (error);
 934 }
 935
 936 /*
 937  * This function represents the so-called 'hard case' for sx_sunlock
 938  * operation.  All 'easy case' failures are redirected to this.  Note
 939  * that ideally this would be a static function, but it needs to be
 940  * accessible from at least sx.h.
 941  */
 942 void
 943 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
 944 {
 945         uintptr_t x;
 946         int wakeup_swapper;
 947
 948         if (SCHEDULER_STOPPED())
 949                 return;
 950
 951         for (;;) {
 952                 x = sx->sx_lock;
 953
 954                 /*
 955                  * We should never have sharers while at least one thread
 956                  * holds a shared lock.
 957                  */
 958                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
 959                     ("%s: waiting sharers", __func__));
 960
 961                 /*
 962                  * See if there is more than one shared lock held.  If
 963                  * so, just drop one and return.
 964                  */
 965                 if (SX_SHARERS(x) > 1) {
 966                         if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
 967                             x - SX_ONE_SHARER)) {
 968                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 969                                         CTR4(KTR_LOCK,
 970                                             "%s: %p succeeded %p -> %p",
 971                                             __func__, sx, (void *)x,
 972                                             (void *)(x - SX_ONE_SHARER));
 973                                 break;
 974                         }
 975                         continue;
 976                 }
 977
 978                 /*
 979                  * If there aren't any waiters for an exclusive lock,
 980                  * then try to drop it quickly.
 981                  */
 982                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 983                         MPASS(x == SX_SHARERS_LOCK(1));
 984                         if (atomic_cmpset_rel_ptr(&sx->sx_lock,
 985                             SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
 986                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 987                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 988                                             __func__, sx);
 989                                 break;
 990                         }
 991                         continue;
 992                 }
 993
 994                 /*
 995                  * At this point, there should just be one sharer with
 996                  * exclusive waiters.
 997                  */
 998                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
 999
1000                 sleepq_lock(&sx->lock_object);
1001
1002                 /*
1003                  * Wake up semantic here is quite simple:
1004                  * Just wake up all the exclusive waiters.
1005                  * Note that the state of the lock could have changed,
1006                  * so if it fails loop back and retry.
1007                  */
1008                 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
1009                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
1010                     SX_LOCK_UNLOCKED)) {
1011                         sleepq_release(&sx->lock_object);
1012                         continue;
1013                 }
1014                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1015                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1016                             "exclusive queue", __func__, sx);
1017                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1018                     0, SQ_EXCLUSIVE_QUEUE);
1019                 sleepq_release(&sx->lock_object);
1020                 if (wakeup_swapper)
1021                         kick_proc0();
1022                 break;
1023         }
1024 }
1025
1026 #ifdef INVARIANT_SUPPORT
1027 #ifndef INVARIANTS
1028 #undef  _sx_assert
1029 #endif
1030
1031 /*
1032  * In the non-WITNESS case, sx_assert() can only detect that at least
1033  * *some* thread owns an slock, but it cannot guarantee that *this*
1034  * thread owns an slock.
1035  */
1036 void
1037 _sx_assert(struct sx *sx, int what, const char *file, int line)
1038 {
1039 #ifndef WITNESS
1040         int slocked = 0;
1041 #endif
1042
1043         if (panicstr != NULL)
1044                 return;
1045         switch (what) {
1046         case SA_SLOCKED:
1047         case SA_SLOCKED | SA_NOTRECURSED:
1048         case SA_SLOCKED | SA_RECURSED:
1049 #ifndef WITNESS
1050                 slocked = 1;
1051                 /* FALLTHROUGH */
1052 #endif
1053         case SA_LOCKED:
1054         case SA_LOCKED | SA_NOTRECURSED:
1055         case SA_LOCKED | SA_RECURSED:
1056 #ifdef WITNESS
1057                 witness_assert(&sx->lock_object, what, file, line);
1058 #else
1059                 /*
1060                  * If some other thread has an exclusive lock or we
1061                  * have one and are asserting a shared lock, fail.
1062                  * Also, if no one has a lock at all, fail.
1063                  */
1064                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1065                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1066                     sx_xholder(sx) != curthread)))
1067                         panic("Lock %s not %slocked @ %s:%d\n",
1068                             sx->lock_object.lo_name, slocked ? "share " : "",
1069                             file, line);
1070
1071                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1072                         if (sx_recursed(sx)) {
1073                                 if (what & SA_NOTRECURSED)
1074                                         panic("Lock %s recursed @ %s:%d\n",
1075                                             sx->lock_object.lo_name, file,
1076                                             line);
1077                         } else if (what & SA_RECURSED)
1078                                 panic("Lock %s not recursed @ %s:%d\n",
1079                                     sx->lock_object.lo_name, file, line);
1080                 }
1081 #endif
1082                 break;
1083         case SA_XLOCKED:
1084         case SA_XLOCKED | SA_NOTRECURSED:
1085         case SA_XLOCKED | SA_RECURSED:
1086                 if (sx_xholder(sx) != curthread)
1087                         panic("Lock %s not exclusively locked @ %s:%d\n",
1088                             sx->lock_object.lo_name, file, line);
1089                 if (sx_recursed(sx)) {
1090                         if (what & SA_NOTRECURSED)
1091                                 panic("Lock %s recursed @ %s:%d\n",
1092                                     sx->lock_object.lo_name, file, line);
1093                 } else if (what & SA_RECURSED)
1094                         panic("Lock %s not recursed @ %s:%d\n",
1095                             sx->lock_object.lo_name, file, line);
1096                 break;
1097         case SA_UNLOCKED:
1098 #ifdef WITNESS
1099                 witness_assert(&sx->lock_object, what, file, line);
1100 #else
1101                 /*
1102                  * If we hold an exclusve lock fail.  We can't
1103                  * reliably check to see if we hold a shared lock or
1104                  * not.
1105                  */
1106                 if (sx_xholder(sx) == curthread)
1107                         panic("Lock %s exclusively locked @ %s:%d\n",
1108                             sx->lock_object.lo_name, file, line);
1109 #endif
1110                 break;
1111         default:
1112                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1113                     line);
1114         }
1115 }
1116 #endif  /* INVARIANT_SUPPORT */
1117
1118 #ifdef DDB
1119 static void
1120 db_show_sx(struct lock_object *lock)
1121 {
1122         struct thread *td;
1123         struct sx *sx;
1124
1125         sx = (struct sx *)lock;
1126
1127         db_printf(" state: ");
1128         if (sx->sx_lock == SX_LOCK_UNLOCKED)
1129                 db_printf("UNLOCKED\n");
1130         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1131                 db_printf("DESTROYED\n");
1132                 return;
1133         } else if (sx->sx_lock & SX_LOCK_SHARED)
1134                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1135         else {
1136                 td = sx_xholder(sx);
1137                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1138                     td->td_tid, td->td_proc->p_pid, td->td_name);
1139                 if (sx_recursed(sx))
1140                         db_printf(" recursed: %d\n", sx->sx_recurse);
1141         }
1142
1143         db_printf(" waiters: ");
1144         switch(sx->sx_lock &
1145             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1146         case SX_LOCK_SHARED_WAITERS:
1147                 db_printf("shared\n");
1148                 break;
1149         case SX_LOCK_EXCLUSIVE_WAITERS:
1150                 db_printf("exclusive\n");
1151                 break;
1152         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1153                 db_printf("exclusive and shared\n");
1154                 break;
1155         default:
1156                 db_printf("none\n");
1157         }
1158 }
1159
1160 /*
1161  * Check to see if a thread that is blocked on a sleep queue is actually
1162  * blocked on an sx lock.  If so, output some details and return true.
1163  * If the lock has an exclusive owner, return that in *ownerp.
1164  */
1165 int
1166 sx_chain(struct thread *td, struct thread **ownerp)
1167 {
1168         struct sx *sx;
1169
1170         /*
1171          * Check to see if this thread is blocked on an sx lock.
1172          * First, we check the lock class.  If that is ok, then we
1173          * compare the lock name against the wait message.
1174          */
1175         sx = td->td_wchan;
1176         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1177             sx->lock_object.lo_name != td->td_wmesg)
1178                 return (0);
1179
1180         /* We think we have an sx lock, so output some details. */
1181         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1182         *ownerp = sx_xholder(sx);
1183         if (sx->sx_lock & SX_LOCK_SHARED)
1184                 db_printf("SLOCK (count %ju)\n",
1185                     (uintmax_t)SX_SHARERS(sx->sx_lock));
1186         else
1187                 db_printf("XLOCK\n");
1188         return (1);
1189 }
1190 #endif