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