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_hwpmc_hooks.h"
  41 #include "opt_kdtrace.h"
  42 #include "opt_no_adaptive_sx.h"
  43
  44 #include <sys/cdefs.h>
  45 __FBSDID("$FreeBSD$");
  46
  47 #include <sys/param.h>
  48 #include <sys/systm.h>
  49 #include <sys/ktr.h>
  50 #include <sys/lock.h>
  51 #include <sys/mutex.h>
  52 #include <sys/proc.h>
  53 #include <sys/sleepqueue.h>
  54 #include <sys/sx.h>
  55 #include <sys/sysctl.h>
  56
  57 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
  58 #include <machine/cpu.h>
  59 #endif
  60
  61 #ifdef DDB
  62 #include <ddb/ddb.h>
  63 #endif
  64
  65 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
  66 #define ADAPTIVE_SX
  67 #endif
  68
  69 CTASSERT((SX_NOADAPTIVE & LO_CLASSFLAGS) == SX_NOADAPTIVE);
  70
  71 #ifdef HWPMC_HOOKS
  72 #include <sys/pmckern.h>
  73 PMC_SOFT_DECLARE( , , lock, failed);
  74 #endif
  75
  76 /* Handy macros for sleep queues. */
  77 #define SQ_EXCLUSIVE_QUEUE      0
  78 #define SQ_SHARED_QUEUE         1
  79
  80 #ifdef ADAPTIVE_SX
  81 #define ASX_RETRIES             10
  82 #define ASX_LOOPS               10000
  83 #endif
  84
  85 /*
  86  * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file.  We
  87  * drop Giant anytime we have to sleep or if we adaptively spin.
  88  */
  89 #define GIANT_DECLARE                                                   \
  90         int _giantcnt = 0;                                              \
  91         WITNESS_SAVE_DECL(Giant)                                        \
  92
  93 #define GIANT_SAVE() do {                                               \
  94         if (mtx_owned(&Giant)) {                                        \
  95                 WITNESS_SAVE(&Giant.lock_object, Giant);                \
  96                 while (mtx_owned(&Giant)) {                             \
  97                         _giantcnt++;                                    \
  98                         mtx_unlock(&Giant);                             \
  99                 }                                                       \
 100         }                                                               \
 101 } while (0)
 102
 103 #define GIANT_RESTORE() do {                                            \
 104         if (_giantcnt > 0) {                                            \
 105                 mtx_assert(&Giant, MA_NOTOWNED);                        \
 106                 while (_giantcnt--)                                     \
 107                         mtx_lock(&Giant);                               \
 108                 WITNESS_RESTORE(&Giant.lock_object, Giant);             \
 109         }                                                               \
 110 } while (0)
 111
 112 /*
 113  * Returns true if an exclusive lock is recursed.  It assumes
 114  * curthread currently has an exclusive lock.
 115  */
 116 #define sx_recurse              lock_object.lo_data
 117 #define sx_recursed(sx)         ((sx)->sx_recurse != 0)
 118
 119 static void     assert_sx(struct lock_object *lock, int what);
 120 #ifdef DDB
 121 static void     db_show_sx(struct lock_object *lock);
 122 #endif
 123 static void     lock_sx(struct lock_object *lock, int how);
 124 #ifdef KDTRACE_HOOKS
 125 static int      owner_sx(struct lock_object *lock, struct thread **owner);
 126 #endif
 127 static int      unlock_sx(struct lock_object *lock);
 128
 129 struct lock_class lock_class_sx = {
 130         .lc_name = "sx",
 131         .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
 132         .lc_assert = assert_sx,
 133 #ifdef DDB
 134         .lc_ddb_show = db_show_sx,
 135 #endif
 136         .lc_lock = lock_sx,
 137         .lc_unlock = unlock_sx,
 138 #ifdef KDTRACE_HOOKS
 139         .lc_owner = owner_sx,
 140 #endif
 141 };
 142
 143 #ifndef INVARIANTS
 144 #define _sx_assert(sx, what, file, line)
 145 #endif
 146
 147 void
 148 assert_sx(struct lock_object *lock, int what)
 149 {
 150
 151         sx_assert((struct sx *)lock, what);
 152 }
 153
 154 void
 155 lock_sx(struct lock_object *lock, int how)
 156 {
 157         struct sx *sx;
 158
 159         sx = (struct sx *)lock;
 160         if (how)
 161                 sx_xlock(sx);
 162         else
 163                 sx_slock(sx);
 164 }
 165
 166 int
 167 unlock_sx(struct lock_object *lock)
 168 {
 169         struct sx *sx;
 170
 171         sx = (struct sx *)lock;
 172         sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
 173         if (sx_xlocked(sx)) {
 174                 sx_xunlock(sx);
 175                 return (1);
 176         } else {
 177                 sx_sunlock(sx);
 178                 return (0);
 179         }
 180 }
 181
 182 #ifdef KDTRACE_HOOKS
 183 int
 184 owner_sx(struct lock_object *lock, struct thread **owner)
 185 {
 186         struct sx *sx = (struct sx *)lock;
 187         uintptr_t x = sx->sx_lock;
 188
 189         *owner = (struct thread *)SX_OWNER(x);
 190         return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
 191             (*owner != NULL));
 192 }
 193 #endif
 194
 195 void
 196 sx_sysinit(void *arg)
 197 {
 198         struct sx_args *sargs = arg;
 199
 200         sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
 201 }
 202
 203 void
 204 sx_init_flags(struct sx *sx, const char *description, int opts)
 205 {
 206         int flags;
 207
 208         MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
 209             SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
 210         ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
 211             ("%s: sx_lock not aligned for %s: %p", __func__, description,
 212             &sx->sx_lock));
 213
 214         flags = LO_SLEEPABLE | LO_UPGRADABLE;
 215         if (opts & SX_DUPOK)
 216                 flags |= LO_DUPOK;
 217         if (opts & SX_NOPROFILE)
 218                 flags |= LO_NOPROFILE;
 219         if (!(opts & SX_NOWITNESS))
 220                 flags |= LO_WITNESS;
 221         if (opts & SX_RECURSE)
 222                 flags |= LO_RECURSABLE;
 223         if (opts & SX_QUIET)
 224                 flags |= LO_QUIET;
 225
 226         flags |= opts & SX_NOADAPTIVE;
 227         sx->sx_lock = SX_LOCK_UNLOCKED;
 228         sx->sx_recurse = 0;
 229         lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
 230 }
 231
 232 void
 233 sx_destroy(struct sx *sx)
 234 {
 235
 236         KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
 237         KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
 238         sx->sx_lock = SX_LOCK_DESTROYED;
 239         lock_destroy(&sx->lock_object);
 240 }
 241
 242 int
 243 _sx_slock(struct sx *sx, int opts, const char *file, int line)
 244 {
 245         int error = 0;
 246
 247         if (SCHEDULER_STOPPED())
 248                 return (0);
 249         MPASS(curthread != NULL);
 250         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 251             ("sx_slock() of destroyed sx @ %s:%d", file, line));
 252         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
 253         error = __sx_slock(sx, opts, file, line);
 254         if (!error) {
 255                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
 256                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
 257                 curthread->td_locks++;
 258         }
 259
 260         return (error);
 261 }
 262
 263 int
 264 _sx_try_slock(struct sx *sx, const char *file, int line)
 265 {
 266         uintptr_t x;
 267
 268         if (SCHEDULER_STOPPED())
 269                 return (1);
 270
 271         for (;;) {
 272                 x = sx->sx_lock;
 273                 KASSERT(x != SX_LOCK_DESTROYED,
 274                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
 275                 if (!(x & SX_LOCK_SHARED))
 276                         break;
 277                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
 278                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
 279                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
 280                         curthread->td_locks++;
 281                         return (1);
 282                 }
 283         }
 284
 285         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
 286         return (0);
 287 }
 288
 289 int
 290 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
 291 {
 292         int error = 0;
 293
 294         if (SCHEDULER_STOPPED())
 295                 return (0);
 296         MPASS(curthread != NULL);
 297         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 298             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
 299         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 300             line, NULL);
 301         error = __sx_xlock(sx, curthread, opts, file, line);
 302         if (!error) {
 303                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
 304                     file, line);
 305                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 306                 curthread->td_locks++;
 307         }
 308
 309         return (error);
 310 }
 311
 312 int
 313 _sx_try_xlock(struct sx *sx, const char *file, int line)
 314 {
 315         int rval;
 316
 317         if (SCHEDULER_STOPPED())
 318                 return (1);
 319
 320         MPASS(curthread != NULL);
 321         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 322             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
 323
 324         if (sx_xlocked(sx) &&
 325             (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 326                 sx->sx_recurse++;
 327                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 328                 rval = 1;
 329         } else
 330                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
 331                     (uintptr_t)curthread);
 332         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
 333         if (rval) {
 334                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 335                     file, line);
 336                 curthread->td_locks++;
 337         }
 338
 339         return (rval);
 340 }
 341
 342 void
 343 _sx_sunlock(struct sx *sx, const char *file, int line)
 344 {
 345
 346         if (SCHEDULER_STOPPED())
 347                 return;
 348         MPASS(curthread != NULL);
 349         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 350             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
 351         _sx_assert(sx, SA_SLOCKED, file, line);
 352         curthread->td_locks--;
 353         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
 354         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
 355         __sx_sunlock(sx, file, line);
 356         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
 357 }
 358
 359 void
 360 _sx_xunlock(struct sx *sx, const char *file, int line)
 361 {
 362
 363         if (SCHEDULER_STOPPED())
 364                 return;
 365         MPASS(curthread != NULL);
 366         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 367             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
 368         _sx_assert(sx, SA_XLOCKED, file, line);
 369         curthread->td_locks--;
 370         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 371         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
 372             line);
 373         if (!sx_recursed(sx))
 374                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
 375         __sx_xunlock(sx, curthread, file, line);
 376 }
 377
 378 /*
 379  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
 380  * This will only succeed if this thread holds a single shared lock.
 381  * Return 1 if if the upgrade succeed, 0 otherwise.
 382  */
 383 int
 384 _sx_try_upgrade(struct sx *sx, const char *file, int line)
 385 {
 386         uintptr_t x;
 387         int success;
 388
 389         if (SCHEDULER_STOPPED())
 390                 return (1);
 391
 392         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 393             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
 394         _sx_assert(sx, SA_SLOCKED, file, line);
 395
 396         /*
 397          * Try to switch from one shared lock to an exclusive lock.  We need
 398          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
 399          * we will wake up the exclusive waiters when we drop the lock.
 400          */
 401         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
 402         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
 403             (uintptr_t)curthread | x);
 404         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
 405         if (success) {
 406                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 407                     file, line);
 408                 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
 409         }
 410         return (success);
 411 }
 412
 413 /*
 414  * Downgrade an unrecursed exclusive lock into a single shared lock.
 415  */
 416 void
 417 _sx_downgrade(struct sx *sx, const char *file, int line)
 418 {
 419         uintptr_t x;
 420         int wakeup_swapper;
 421
 422         if (SCHEDULER_STOPPED())
 423                 return;
 424
 425         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 426             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
 427         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
 428 #ifndef INVARIANTS
 429         if (sx_recursed(sx))
 430                 panic("downgrade of a recursed lock");
 431 #endif
 432
 433         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
 434
 435         /*
 436          * Try to switch from an exclusive lock with no shared waiters
 437          * to one sharer with no shared waiters.  If there are
 438          * exclusive waiters, we don't need to lock the sleep queue so
 439          * long as we preserve the flag.  We do one quick try and if
 440          * that fails we grab the sleepq lock to keep the flags from
 441          * changing and do it the slow way.
 442          *
 443          * We have to lock the sleep queue if there are shared waiters
 444          * so we can wake them up.
 445          */
 446         x = sx->sx_lock;
 447         if (!(x & SX_LOCK_SHARED_WAITERS) &&
 448             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
 449             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
 450                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 451                 return;
 452         }
 453
 454         /*
 455          * Lock the sleep queue so we can read the waiters bits
 456          * without any races and wakeup any shared waiters.
 457          */
 458         sleepq_lock(&sx->lock_object);
 459
 460         /*
 461          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
 462          * shared lock.  If there are any shared waiters, wake them up.
 463          */
 464         wakeup_swapper = 0;
 465         x = sx->sx_lock;
 466         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
 467             (x & SX_LOCK_EXCLUSIVE_WAITERS));
 468         if (x & SX_LOCK_SHARED_WAITERS)
 469                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
 470                     0, SQ_SHARED_QUEUE);
 471         sleepq_release(&sx->lock_object);
 472
 473         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 474         LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
 475
 476         if (wakeup_swapper)
 477                 kick_proc0();
 478 }
 479
 480 /*
 481  * This function represents the so-called 'hard case' for sx_xlock
 482  * operation.  All 'easy case' failures are redirected to this.  Note
 483  * that ideally this would be a static function, but it needs to be
 484  * accessible from at least sx.h.
 485  */
 486 int
 487 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
 488     int line)
 489 {
 490         GIANT_DECLARE;
 491 #ifdef ADAPTIVE_SX
 492         volatile struct thread *owner;
 493         u_int i, spintries = 0;
 494 #endif
 495         uintptr_t x;
 496 #ifdef LOCK_PROFILING
 497         uint64_t waittime = 0;
 498         int contested = 0;
 499 #endif
 500         int error = 0;
 501 #ifdef  KDTRACE_HOOKS
 502         uint64_t spin_cnt = 0;
 503         uint64_t sleep_cnt = 0;
 504         int64_t sleep_time = 0;
 505 #endif
 506
 507         if (SCHEDULER_STOPPED())
 508                 return (0);
 509
 510         /* If we already hold an exclusive lock, then recurse. */
 511         if (sx_xlocked(sx)) {
 512                 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
 513             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
 514                     sx->lock_object.lo_name, file, line));
 515                 sx->sx_recurse++;
 516                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 517                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 518                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
 519                 return (0);
 520         }
 521
 522         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 523                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 524                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
 525
 526         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
 527 #ifdef KDTRACE_HOOKS
 528                 spin_cnt++;
 529 #endif
 530 #ifdef HWPMC_HOOKS
 531                 PMC_SOFT_CALL( , , lock, failed);
 532 #endif
 533                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 534                     &waittime);
 535 #ifdef ADAPTIVE_SX
 536                 /*
 537                  * If the lock is write locked and the owner is
 538                  * running on another CPU, spin until the owner stops
 539                  * running or the state of the lock changes.
 540                  */
 541                 x = sx->sx_lock;
 542                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 543                         if ((x & SX_LOCK_SHARED) == 0) {
 544                                 x = SX_OWNER(x);
 545                                 owner = (struct thread *)x;
 546                                 if (TD_IS_RUNNING(owner)) {
 547                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 548                                                 CTR3(KTR_LOCK,
 549                                             "%s: spinning on %p held by %p",
 550                                                     __func__, sx, owner);
 551                                         GIANT_SAVE();
 552                                         while (SX_OWNER(sx->sx_lock) == x &&
 553                                             TD_IS_RUNNING(owner)) {
 554                                                 cpu_spinwait();
 555 #ifdef KDTRACE_HOOKS
 556                                                 spin_cnt++;
 557 #endif
 558                                         }
 559                                         continue;
 560                                 }
 561                         } else if (SX_SHARERS(x) && spintries < ASX_RETRIES) {
 562                                 GIANT_SAVE();
 563                                 spintries++;
 564                                 for (i = 0; i < ASX_LOOPS; i++) {
 565                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 566                                                 CTR4(KTR_LOCK,
 567                                     "%s: shared spinning on %p with %u and %u",
 568                                                     __func__, sx, spintries, i);
 569                                         x = sx->sx_lock;
 570                                         if ((x & SX_LOCK_SHARED) == 0 ||
 571                                             SX_SHARERS(x) == 0)
 572                                                 break;
 573                                         cpu_spinwait();
 574 #ifdef KDTRACE_HOOKS
 575                                         spin_cnt++;
 576 #endif
 577                                 }
 578                                 if (i != ASX_LOOPS)
 579                                         continue;
 580                         }
 581                 }
 582 #endif
 583
 584                 sleepq_lock(&sx->lock_object);
 585                 x = sx->sx_lock;
 586
 587                 /*
 588                  * If the lock was released while spinning on the
 589                  * sleep queue chain lock, try again.
 590                  */
 591                 if (x == SX_LOCK_UNLOCKED) {
 592                         sleepq_release(&sx->lock_object);
 593                         continue;
 594                 }
 595
 596 #ifdef ADAPTIVE_SX
 597                 /*
 598                  * The current lock owner might have started executing
 599                  * on another CPU (or the lock could have changed
 600                  * owners) while we were waiting on the sleep queue
 601                  * chain lock.  If so, drop the sleep queue lock and try
 602                  * again.
 603                  */
 604                 if (!(x & SX_LOCK_SHARED) &&
 605                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 606                         owner = (struct thread *)SX_OWNER(x);
 607                         if (TD_IS_RUNNING(owner)) {
 608                                 sleepq_release(&sx->lock_object);
 609                                 continue;
 610                         }
 611                 }
 612 #endif
 613
 614                 /*
 615                  * If an exclusive lock was released with both shared
 616                  * and exclusive waiters and a shared waiter hasn't
 617                  * woken up and acquired the lock yet, sx_lock will be
 618                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
 619                  * If we see that value, try to acquire it once.  Note
 620                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
 621                  * as there are other exclusive waiters still.  If we
 622                  * fail, restart the loop.
 623                  */
 624                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
 625                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
 626                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
 627                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
 628                                 sleepq_release(&sx->lock_object);
 629                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
 630                                     __func__, sx);
 631                                 break;
 632                         }
 633                         sleepq_release(&sx->lock_object);
 634                         continue;
 635                 }
 636
 637                 /*
 638                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
 639                  * than loop back and retry.
 640                  */
 641                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 642                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 643                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
 644                                 sleepq_release(&sx->lock_object);
 645                                 continue;
 646                         }
 647                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 648                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
 649                                     __func__, sx);
 650                 }
 651
 652                 /*
 653                  * Since we have been unable to acquire the exclusive
 654                  * lock and the exclusive waiters flag is set, we have
 655                  * to sleep.
 656                  */
 657                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 658                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 659                             __func__, sx);
 660
 661 #ifdef KDTRACE_HOOKS
 662                 sleep_time -= lockstat_nsecs();
 663 #endif
 664                 GIANT_SAVE();
 665                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 666                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 667                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
 668                 if (!(opts & SX_INTERRUPTIBLE))
 669                         sleepq_wait(&sx->lock_object, 0);
 670                 else
 671                         error = sleepq_wait_sig(&sx->lock_object, 0);
 672 #ifdef KDTRACE_HOOKS
 673                 sleep_time += lockstat_nsecs();
 674                 sleep_cnt++;
 675 #endif
 676                 if (error) {
 677                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 678                                 CTR2(KTR_LOCK,
 679                         "%s: interruptible sleep by %p suspended by signal",
 680                                     __func__, sx);
 681                         break;
 682                 }
 683                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 684                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 685                             __func__, sx);
 686         }
 687
 688         GIANT_RESTORE();
 689         if (!error)
 690                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
 691                     contested, waittime, file, line);
 692 #ifdef KDTRACE_HOOKS
 693         if (sleep_time)
 694                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 695         if (spin_cnt > sleep_cnt)
 696                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 697 #endif
 698         return (error);
 699 }
 700
 701 /*
 702  * This function represents the so-called 'hard case' for sx_xunlock
 703  * operation.  All 'easy case' failures are redirected to this.  Note
 704  * that ideally this would be a static function, but it needs to be
 705  * accessible from at least sx.h.
 706  */
 707 void
 708 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
 709 {
 710         uintptr_t x;
 711         int queue, wakeup_swapper;
 712
 713         if (SCHEDULER_STOPPED())
 714                 return;
 715
 716         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
 717
 718         /* If the lock is recursed, then unrecurse one level. */
 719         if (sx_xlocked(sx) && sx_recursed(sx)) {
 720                 if ((--sx->sx_recurse) == 0)
 721                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 722                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 723                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
 724                 return;
 725         }
 726         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
 727             SX_LOCK_EXCLUSIVE_WAITERS));
 728         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 729                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
 730
 731         sleepq_lock(&sx->lock_object);
 732         x = SX_LOCK_UNLOCKED;
 733
 734         /*
 735          * The wake up algorithm here is quite simple and probably not
 736          * ideal.  It gives precedence to shared waiters if they are
 737          * present.  For this condition, we have to preserve the
 738          * state of the exclusive waiters flag.
 739          * If interruptible sleeps left the shared queue empty avoid a
 740          * starvation for the threads sleeping on the exclusive queue by giving
 741          * them precedence and cleaning up the shared waiters bit anyway.
 742          */
 743         if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
 744             sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
 745                 queue = SQ_SHARED_QUEUE;
 746                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
 747         } else
 748                 queue = SQ_EXCLUSIVE_QUEUE;
 749
 750         /* Wake up all the waiters for the specific queue. */
 751         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 752                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
 753                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
 754                     "exclusive");
 755         atomic_store_rel_ptr(&sx->sx_lock, x);
 756         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
 757             queue);
 758         sleepq_release(&sx->lock_object);
 759         if (wakeup_swapper)
 760                 kick_proc0();
 761 }
 762
 763 /*
 764  * This function represents the so-called 'hard case' for sx_slock
 765  * operation.  All 'easy case' failures are redirected to this.  Note
 766  * that ideally this would be a static function, but it needs to be
 767  * accessible from at least sx.h.
 768  */
 769 int
 770 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
 771 {
 772         GIANT_DECLARE;
 773 #ifdef ADAPTIVE_SX
 774         volatile struct thread *owner;
 775 #endif
 776 #ifdef LOCK_PROFILING
 777         uint64_t waittime = 0;
 778         int contested = 0;
 779 #endif
 780         uintptr_t x;
 781         int error = 0;
 782 #ifdef KDTRACE_HOOKS
 783         uint64_t spin_cnt = 0;
 784         uint64_t sleep_cnt = 0;
 785         int64_t sleep_time = 0;
 786 #endif
 787
 788         if (SCHEDULER_STOPPED())
 789                 return (0);
 790
 791         /*
 792          * As with rwlocks, we don't make any attempt to try to block
 793          * shared locks once there is an exclusive waiter.
 794          */
 795         for (;;) {
 796 #ifdef KDTRACE_HOOKS
 797                 spin_cnt++;
 798 #endif
 799                 x = sx->sx_lock;
 800
 801                 /*
 802                  * If no other thread has an exclusive lock then try to bump up
 803                  * the count of sharers.  Since we have to preserve the state
 804                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
 805                  * shared lock loop back and retry.
 806                  */
 807                 if (x & SX_LOCK_SHARED) {
 808                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
 809                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
 810                             x + SX_ONE_SHARER)) {
 811                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 812                                         CTR4(KTR_LOCK,
 813                                             "%s: %p succeed %p -> %p", __func__,
 814                                             sx, (void *)x,
 815                                             (void *)(x + SX_ONE_SHARER));
 816                                 break;
 817                         }
 818                         continue;
 819                 }
 820 #ifdef HWPMC_HOOKS
 821                 PMC_SOFT_CALL( , , lock, failed);
 822 #endif
 823                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 824                     &waittime);
 825
 826 #ifdef ADAPTIVE_SX
 827                 /*
 828                  * If the owner is running on another CPU, spin until
 829                  * the owner stops running or the state of the lock
 830                  * changes.
 831                  */
 832                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 833                         x = SX_OWNER(x);
 834                         owner = (struct thread *)x;
 835                         if (TD_IS_RUNNING(owner)) {
 836                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 837                                         CTR3(KTR_LOCK,
 838                                             "%s: spinning on %p held by %p",
 839                                             __func__, sx, owner);
 840                                 GIANT_SAVE();
 841                                 while (SX_OWNER(sx->sx_lock) == x &&
 842                                     TD_IS_RUNNING(owner)) {
 843 #ifdef KDTRACE_HOOKS
 844                                         spin_cnt++;
 845 #endif
 846                                         cpu_spinwait();
 847                                 }
 848                                 continue;
 849                         }
 850                 }
 851 #endif
 852
 853                 /*
 854                  * Some other thread already has an exclusive lock, so
 855                  * start the process of blocking.
 856                  */
 857                 sleepq_lock(&sx->lock_object);
 858                 x = sx->sx_lock;
 859
 860                 /*
 861                  * The lock could have been released while we spun.
 862                  * In this case loop back and retry.
 863                  */
 864                 if (x & SX_LOCK_SHARED) {
 865                         sleepq_release(&sx->lock_object);
 866                         continue;
 867                 }
 868
 869 #ifdef ADAPTIVE_SX
 870                 /*
 871                  * If the owner is running on another CPU, spin until
 872                  * the owner stops running or the state of the lock
 873                  * changes.
 874                  */
 875                 if (!(x & SX_LOCK_SHARED) &&
 876                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 877                         owner = (struct thread *)SX_OWNER(x);
 878                         if (TD_IS_RUNNING(owner)) {
 879                                 sleepq_release(&sx->lock_object);
 880                                 continue;
 881                         }
 882                 }
 883 #endif
 884
 885                 /*
 886                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
 887                  * fail to set it drop the sleep queue lock and loop
 888                  * back.
 889                  */
 890                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
 891                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 892                             x | SX_LOCK_SHARED_WAITERS)) {
 893                                 sleepq_release(&sx->lock_object);
 894                                 continue;
 895                         }
 896                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 897                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
 898                                     __func__, sx);
 899                 }
 900
 901                 /*
 902                  * Since we have been unable to acquire the shared lock,
 903                  * we have to sleep.
 904                  */
 905                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 906                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 907                             __func__, sx);
 908
 909 #ifdef KDTRACE_HOOKS
 910                 sleep_time -= lockstat_nsecs();
 911 #endif
 912                 GIANT_SAVE();
 913                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 914                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 915                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
 916                 if (!(opts & SX_INTERRUPTIBLE))
 917                         sleepq_wait(&sx->lock_object, 0);
 918                 else
 919                         error = sleepq_wait_sig(&sx->lock_object, 0);
 920 #ifdef KDTRACE_HOOKS
 921                 sleep_time += lockstat_nsecs();
 922                 sleep_cnt++;
 923 #endif
 924                 if (error) {
 925                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 926                                 CTR2(KTR_LOCK,
 927                         "%s: interruptible sleep by %p suspended by signal",
 928                                     __func__, sx);
 929                         break;
 930                 }
 931                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 932                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 933                             __func__, sx);
 934         }
 935         if (error == 0)
 936                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
 937                     contested, waittime, file, line);
 938 #ifdef KDTRACE_HOOKS
 939         if (sleep_time)
 940                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 941         if (spin_cnt > sleep_cnt)
 942                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 943 #endif
 944         GIANT_RESTORE();
 945         return (error);
 946 }
 947
 948 /*
 949  * This function represents the so-called 'hard case' for sx_sunlock
 950  * operation.  All 'easy case' failures are redirected to this.  Note
 951  * that ideally this would be a static function, but it needs to be
 952  * accessible from at least sx.h.
 953  */
 954 void
 955 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
 956 {
 957         uintptr_t x;
 958         int wakeup_swapper;
 959
 960         if (SCHEDULER_STOPPED())
 961                 return;
 962
 963         for (;;) {
 964                 x = sx->sx_lock;
 965
 966                 /*
 967                  * We should never have sharers while at least one thread
 968                  * holds a shared lock.
 969                  */
 970                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
 971                     ("%s: waiting sharers", __func__));
 972
 973                 /*
 974                  * See if there is more than one shared lock held.  If
 975                  * so, just drop one and return.
 976                  */
 977                 if (SX_SHARERS(x) > 1) {
 978                         if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
 979                             x - SX_ONE_SHARER)) {
 980                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 981                                         CTR4(KTR_LOCK,
 982                                             "%s: %p succeeded %p -> %p",
 983                                             __func__, sx, (void *)x,
 984                                             (void *)(x - SX_ONE_SHARER));
 985                                 break;
 986                         }
 987                         continue;
 988                 }
 989
 990                 /*
 991                  * If there aren't any waiters for an exclusive lock,
 992                  * then try to drop it quickly.
 993                  */
 994                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 995                         MPASS(x == SX_SHARERS_LOCK(1));
 996                         if (atomic_cmpset_rel_ptr(&sx->sx_lock,
 997                             SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
 998                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 999                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
1000                                             __func__, sx);
1001                                 break;
1002                         }
1003                         continue;
1004                 }
1005
1006                 /*
1007                  * At this point, there should just be one sharer with
1008                  * exclusive waiters.
1009                  */
1010                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
1011
1012                 sleepq_lock(&sx->lock_object);
1013
1014                 /*
1015                  * Wake up semantic here is quite simple:
1016                  * Just wake up all the exclusive waiters.
1017                  * Note that the state of the lock could have changed,
1018                  * so if it fails loop back and retry.
1019                  */
1020                 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
1021                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
1022                     SX_LOCK_UNLOCKED)) {
1023                         sleepq_release(&sx->lock_object);
1024                         continue;
1025                 }
1026                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1027                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1028                             "exclusive queue", __func__, sx);
1029                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1030                     0, SQ_EXCLUSIVE_QUEUE);
1031                 sleepq_release(&sx->lock_object);
1032                 if (wakeup_swapper)
1033                         kick_proc0();
1034                 break;
1035         }
1036 }
1037
1038 #ifdef INVARIANT_SUPPORT
1039 #ifndef INVARIANTS
1040 #undef  _sx_assert
1041 #endif
1042
1043 /*
1044  * In the non-WITNESS case, sx_assert() can only detect that at least
1045  * *some* thread owns an slock, but it cannot guarantee that *this*
1046  * thread owns an slock.
1047  */
1048 void
1049 _sx_assert(struct sx *sx, int what, const char *file, int line)
1050 {
1051 #ifndef WITNESS
1052         int slocked = 0;
1053 #endif
1054
1055         if (panicstr != NULL)
1056                 return;
1057         switch (what) {
1058         case SA_SLOCKED:
1059         case SA_SLOCKED | SA_NOTRECURSED:
1060         case SA_SLOCKED | SA_RECURSED:
1061 #ifndef WITNESS
1062                 slocked = 1;
1063                 /* FALLTHROUGH */
1064 #endif
1065         case SA_LOCKED:
1066         case SA_LOCKED | SA_NOTRECURSED:
1067         case SA_LOCKED | SA_RECURSED:
1068 #ifdef WITNESS
1069                 witness_assert(&sx->lock_object, what, file, line);
1070 #else
1071                 /*
1072                  * If some other thread has an exclusive lock or we
1073                  * have one and are asserting a shared lock, fail.
1074                  * Also, if no one has a lock at all, fail.
1075                  */
1076                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1077                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1078                     sx_xholder(sx) != curthread)))
1079                         panic("Lock %s not %slocked @ %s:%d\n",
1080                             sx->lock_object.lo_name, slocked ? "share " : "",
1081                             file, line);
1082
1083                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1084                         if (sx_recursed(sx)) {
1085                                 if (what & SA_NOTRECURSED)
1086                                         panic("Lock %s recursed @ %s:%d\n",
1087                                             sx->lock_object.lo_name, file,
1088                                             line);
1089                         } else if (what & SA_RECURSED)
1090                                 panic("Lock %s not recursed @ %s:%d\n",
1091                                     sx->lock_object.lo_name, file, line);
1092                 }
1093 #endif
1094                 break;
1095         case SA_XLOCKED:
1096         case SA_XLOCKED | SA_NOTRECURSED:
1097         case SA_XLOCKED | SA_RECURSED:
1098                 if (sx_xholder(sx) != curthread)
1099                         panic("Lock %s not exclusively locked @ %s:%d\n",
1100                             sx->lock_object.lo_name, file, line);
1101                 if (sx_recursed(sx)) {
1102                         if (what & SA_NOTRECURSED)
1103                                 panic("Lock %s recursed @ %s:%d\n",
1104                                     sx->lock_object.lo_name, file, line);
1105                 } else if (what & SA_RECURSED)
1106                         panic("Lock %s not recursed @ %s:%d\n",
1107                             sx->lock_object.lo_name, file, line);
1108                 break;
1109         case SA_UNLOCKED:
1110 #ifdef WITNESS
1111                 witness_assert(&sx->lock_object, what, file, line);
1112 #else
1113                 /*
1114                  * If we hold an exclusve lock fail.  We can't
1115                  * reliably check to see if we hold a shared lock or
1116                  * not.
1117                  */
1118                 if (sx_xholder(sx) == curthread)
1119                         panic("Lock %s exclusively locked @ %s:%d\n",
1120                             sx->lock_object.lo_name, file, line);
1121 #endif
1122                 break;
1123         default:
1124                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1125                     line);
1126         }
1127 }
1128 #endif  /* INVARIANT_SUPPORT */
1129
1130 #ifdef DDB
1131 static void
1132 db_show_sx(struct lock_object *lock)
1133 {
1134         struct thread *td;
1135         struct sx *sx;
1136
1137         sx = (struct sx *)lock;
1138
1139         db_printf(" state: ");
1140         if (sx->sx_lock == SX_LOCK_UNLOCKED)
1141                 db_printf("UNLOCKED\n");
1142         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1143                 db_printf("DESTROYED\n");
1144                 return;
1145         } else if (sx->sx_lock & SX_LOCK_SHARED)
1146                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1147         else {
1148                 td = sx_xholder(sx);
1149                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1150                     td->td_tid, td->td_proc->p_pid, td->td_name);
1151                 if (sx_recursed(sx))
1152                         db_printf(" recursed: %d\n", sx->sx_recurse);
1153         }
1154
1155         db_printf(" waiters: ");
1156         switch(sx->sx_lock &
1157             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1158         case SX_LOCK_SHARED_WAITERS:
1159                 db_printf("shared\n");
1160                 break;
1161         case SX_LOCK_EXCLUSIVE_WAITERS:
1162                 db_printf("exclusive\n");
1163                 break;
1164         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1165                 db_printf("exclusive and shared\n");
1166                 break;
1167         default:
1168                 db_printf("none\n");
1169         }
1170 }
1171
1172 /*
1173  * Check to see if a thread that is blocked on a sleep queue is actually
1174  * blocked on an sx lock.  If so, output some details and return true.
1175  * If the lock has an exclusive owner, return that in *ownerp.
1176  */
1177 int
1178 sx_chain(struct thread *td, struct thread **ownerp)
1179 {
1180         struct sx *sx;
1181
1182         /*
1183          * Check to see if this thread is blocked on an sx lock.
1184          * First, we check the lock class.  If that is ok, then we
1185          * compare the lock name against the wait message.
1186          */
1187         sx = td->td_wchan;
1188         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1189             sx->lock_object.lo_name != td->td_wmesg)
1190                 return (0);
1191
1192         /* We think we have an sx lock, so output some details. */
1193         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1194         *ownerp = sx_xholder(sx);
1195         if (sx->sx_lock & SX_LOCK_SHARED)
1196                 db_printf("SLOCK (count %ju)\n",
1197                     (uintmax_t)SX_SHARERS(sx->sx_lock));
1198         else
1199                 db_printf("XLOCK\n");
1200         return (1);
1201 }
1202 #endif