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 /*
  81  * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file.  We
  82  * drop Giant anytime we have to sleep or if we adaptively spin.
  83  */
  84 #define GIANT_DECLARE                                                   \
  85         int _giantcnt = 0;                                              \
  86         WITNESS_SAVE_DECL(Giant)                                        \
  87
  88 #define GIANT_SAVE() do {                                               \
  89         if (mtx_owned(&Giant)) {                                        \
  90                 WITNESS_SAVE(&Giant.lock_object, Giant);                \
  91                 while (mtx_owned(&Giant)) {                             \
  92                         _giantcnt++;                                    \
  93                         mtx_unlock(&Giant);                             \
  94                 }                                                       \
  95         }                                                               \
  96 } while (0)
  97
  98 #define GIANT_RESTORE() do {                                            \
  99         if (_giantcnt > 0) {                                            \
 100                 mtx_assert(&Giant, MA_NOTOWNED);                        \
 101                 while (_giantcnt--)                                     \
 102                         mtx_lock(&Giant);                               \
 103                 WITNESS_RESTORE(&Giant.lock_object, Giant);             \
 104         }                                                               \
 105 } while (0)
 106
 107 /*
 108  * Returns true if an exclusive lock is recursed.  It assumes
 109  * curthread currently has an exclusive lock.
 110  */
 111 #define sx_recurse              lock_object.lo_data
 112 #define sx_recursed(sx)         ((sx)->sx_recurse != 0)
 113
 114 static void     assert_sx(const struct lock_object *lock, int what);
 115 #ifdef DDB
 116 static void     db_show_sx(const struct lock_object *lock);
 117 #endif
 118 static void     lock_sx(struct lock_object *lock, int how);
 119 #ifdef KDTRACE_HOOKS
 120 static int      owner_sx(const struct lock_object *lock, struct thread **owner);
 121 #endif
 122 static int      unlock_sx(struct lock_object *lock);
 123
 124 struct lock_class lock_class_sx = {
 125         .lc_name = "sx",
 126         .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
 127         .lc_assert = assert_sx,
 128 #ifdef DDB
 129         .lc_ddb_show = db_show_sx,
 130 #endif
 131         .lc_lock = lock_sx,
 132         .lc_unlock = unlock_sx,
 133 #ifdef KDTRACE_HOOKS
 134         .lc_owner = owner_sx,
 135 #endif
 136 };
 137
 138 #ifndef INVARIANTS
 139 #define _sx_assert(sx, what, file, line)
 140 #endif
 141
 142 #ifdef ADAPTIVE_SX
 143 static u_int asx_retries = 10;
 144 static u_int asx_loops = 10000;
 145 static SYSCTL_NODE(_debug, OID_AUTO, sx, CTLFLAG_RD, NULL, "sxlock debugging");
 146 SYSCTL_UINT(_debug_sx, OID_AUTO, retries, CTLFLAG_RW, &asx_retries, 0, "");
 147 SYSCTL_UINT(_debug_sx, OID_AUTO, loops, CTLFLAG_RW, &asx_loops, 0, "");
 148 #endif
 149
 150 void
 151 assert_sx(const struct lock_object *lock, int what)
 152 {
 153
 154         sx_assert((const struct sx *)lock, what);
 155 }
 156
 157 void
 158 lock_sx(struct lock_object *lock, int how)
 159 {
 160         struct sx *sx;
 161
 162         sx = (struct sx *)lock;
 163         if (how)
 164                 sx_xlock(sx);
 165         else
 166                 sx_slock(sx);
 167 }
 168
 169 int
 170 unlock_sx(struct lock_object *lock)
 171 {
 172         struct sx *sx;
 173
 174         sx = (struct sx *)lock;
 175         sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
 176         if (sx_xlocked(sx)) {
 177                 sx_xunlock(sx);
 178                 return (1);
 179         } else {
 180                 sx_sunlock(sx);
 181                 return (0);
 182         }
 183 }
 184
 185 #ifdef KDTRACE_HOOKS
 186 int
 187 owner_sx(const struct lock_object *lock, struct thread **owner)
 188 {
 189         const struct sx *sx = (const struct sx *)lock;
 190         uintptr_t x = sx->sx_lock;
 191
 192         *owner = (struct thread *)SX_OWNER(x);
 193         return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
 194             (*owner != NULL));
 195 }
 196 #endif
 197
 198 void
 199 sx_sysinit(void *arg)
 200 {
 201         struct sx_args *sargs = arg;
 202
 203         sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
 204 }
 205
 206 void
 207 sx_init_flags(struct sx *sx, const char *description, int opts)
 208 {
 209         int flags;
 210
 211         MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
 212             SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
 213         ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
 214             ("%s: sx_lock not aligned for %s: %p", __func__, description,
 215             &sx->sx_lock));
 216
 217         flags = LO_SLEEPABLE | LO_UPGRADABLE;
 218         if (opts & SX_DUPOK)
 219                 flags |= LO_DUPOK;
 220         if (opts & SX_NOPROFILE)
 221                 flags |= LO_NOPROFILE;
 222         if (!(opts & SX_NOWITNESS))
 223                 flags |= LO_WITNESS;
 224         if (opts & SX_RECURSE)
 225                 flags |= LO_RECURSABLE;
 226         if (opts & SX_QUIET)
 227                 flags |= LO_QUIET;
 228
 229         flags |= opts & SX_NOADAPTIVE;
 230         sx->sx_lock = SX_LOCK_UNLOCKED;
 231         sx->sx_recurse = 0;
 232         lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
 233 }
 234
 235 void
 236 sx_destroy(struct sx *sx)
 237 {
 238
 239         KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
 240         KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
 241         sx->sx_lock = SX_LOCK_DESTROYED;
 242         lock_destroy(&sx->lock_object);
 243 }
 244
 245 int
 246 _sx_slock(struct sx *sx, int opts, const char *file, int line)
 247 {
 248         int error = 0;
 249
 250         if (SCHEDULER_STOPPED())
 251                 return (0);
 252         MPASS(curthread != NULL);
 253         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 254             ("sx_slock() of destroyed sx @ %s:%d", file, line));
 255         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
 256         error = __sx_slock(sx, opts, file, line);
 257         if (!error) {
 258                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
 259                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
 260                 curthread->td_locks++;
 261         }
 262
 263         return (error);
 264 }
 265
 266 int
 267 sx_try_slock_(struct sx *sx, const char *file, int line)
 268 {
 269         uintptr_t x;
 270
 271         if (SCHEDULER_STOPPED())
 272                 return (1);
 273
 274         for (;;) {
 275                 x = sx->sx_lock;
 276                 KASSERT(x != SX_LOCK_DESTROYED,
 277                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
 278                 if (!(x & SX_LOCK_SHARED))
 279                         break;
 280                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
 281                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
 282                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
 283                         curthread->td_locks++;
 284                         return (1);
 285                 }
 286         }
 287
 288         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
 289         return (0);
 290 }
 291
 292 int
 293 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
 294 {
 295         int error = 0;
 296
 297         if (SCHEDULER_STOPPED())
 298                 return (0);
 299         MPASS(curthread != NULL);
 300         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 301             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
 302         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 303             line, NULL);
 304         error = __sx_xlock(sx, curthread, opts, file, line);
 305         if (!error) {
 306                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
 307                     file, line);
 308                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 309                 curthread->td_locks++;
 310         }
 311
 312         return (error);
 313 }
 314
 315 int
 316 sx_try_xlock_(struct sx *sx, const char *file, int line)
 317 {
 318         int rval;
 319
 320         if (SCHEDULER_STOPPED())
 321                 return (1);
 322
 323         MPASS(curthread != NULL);
 324         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 325             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
 326
 327         if (sx_xlocked(sx) &&
 328             (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 329                 sx->sx_recurse++;
 330                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 331                 rval = 1;
 332         } else
 333                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
 334                     (uintptr_t)curthread);
 335         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
 336         if (rval) {
 337                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 338                     file, line);
 339                 curthread->td_locks++;
 340         }
 341
 342         return (rval);
 343 }
 344
 345 void
 346 _sx_sunlock(struct sx *sx, const char *file, int line)
 347 {
 348
 349         if (SCHEDULER_STOPPED())
 350                 return;
 351         MPASS(curthread != NULL);
 352         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 353             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
 354         _sx_assert(sx, SA_SLOCKED, file, line);
 355         curthread->td_locks--;
 356         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
 357         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
 358         __sx_sunlock(sx, file, line);
 359         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
 360 }
 361
 362 void
 363 _sx_xunlock(struct sx *sx, const char *file, int line)
 364 {
 365
 366         if (SCHEDULER_STOPPED())
 367                 return;
 368         MPASS(curthread != NULL);
 369         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 370             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
 371         _sx_assert(sx, SA_XLOCKED, file, line);
 372         curthread->td_locks--;
 373         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 374         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
 375             line);
 376         if (!sx_recursed(sx))
 377                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
 378         __sx_xunlock(sx, curthread, file, line);
 379 }
 380
 381 /*
 382  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
 383  * This will only succeed if this thread holds a single shared lock.
 384  * Return 1 if if the upgrade succeed, 0 otherwise.
 385  */
 386 int
 387 sx_try_upgrade_(struct sx *sx, const char *file, int line)
 388 {
 389         uintptr_t x;
 390         int success;
 391
 392         if (SCHEDULER_STOPPED())
 393                 return (1);
 394
 395         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 396             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
 397         _sx_assert(sx, SA_SLOCKED, file, line);
 398
 399         /*
 400          * Try to switch from one shared lock to an exclusive lock.  We need
 401          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
 402          * we will wake up the exclusive waiters when we drop the lock.
 403          */
 404         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
 405         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
 406             (uintptr_t)curthread | x);
 407         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
 408         if (success) {
 409                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 410                     file, line);
 411                 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
 412         }
 413         return (success);
 414 }
 415
 416 /*
 417  * Downgrade an unrecursed exclusive lock into a single shared lock.
 418  */
 419 void
 420 sx_downgrade_(struct sx *sx, const char *file, int line)
 421 {
 422         uintptr_t x;
 423         int wakeup_swapper;
 424
 425         if (SCHEDULER_STOPPED())
 426                 return;
 427
 428         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 429             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
 430         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
 431 #ifndef INVARIANTS
 432         if (sx_recursed(sx))
 433                 panic("downgrade of a recursed lock");
 434 #endif
 435
 436         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
 437
 438         /*
 439          * Try to switch from an exclusive lock with no shared waiters
 440          * to one sharer with no shared waiters.  If there are
 441          * exclusive waiters, we don't need to lock the sleep queue so
 442          * long as we preserve the flag.  We do one quick try and if
 443          * that fails we grab the sleepq lock to keep the flags from
 444          * changing and do it the slow way.
 445          *
 446          * We have to lock the sleep queue if there are shared waiters
 447          * so we can wake them up.
 448          */
 449         x = sx->sx_lock;
 450         if (!(x & SX_LOCK_SHARED_WAITERS) &&
 451             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
 452             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
 453                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 454                 return;
 455         }
 456
 457         /*
 458          * Lock the sleep queue so we can read the waiters bits
 459          * without any races and wakeup any shared waiters.
 460          */
 461         sleepq_lock(&sx->lock_object);
 462
 463         /*
 464          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
 465          * shared lock.  If there are any shared waiters, wake them up.
 466          */
 467         wakeup_swapper = 0;
 468         x = sx->sx_lock;
 469         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
 470             (x & SX_LOCK_EXCLUSIVE_WAITERS));
 471         if (x & SX_LOCK_SHARED_WAITERS)
 472                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
 473                     0, SQ_SHARED_QUEUE);
 474         sleepq_release(&sx->lock_object);
 475
 476         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 477         LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
 478
 479         if (wakeup_swapper)
 480                 kick_proc0();
 481 }
 482
 483 /*
 484  * This function represents the so-called 'hard case' for sx_xlock
 485  * operation.  All 'easy case' failures are redirected to this.  Note
 486  * that ideally this would be a static function, but it needs to be
 487  * accessible from at least sx.h.
 488  */
 489 int
 490 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
 491     int line)
 492 {
 493         GIANT_DECLARE;
 494 #ifdef ADAPTIVE_SX
 495         volatile struct thread *owner;
 496         u_int i, spintries = 0;
 497 #endif
 498         uintptr_t x;
 499 #ifdef LOCK_PROFILING
 500         uint64_t waittime = 0;
 501         int contested = 0;
 502 #endif
 503         int error = 0;
 504 #ifdef  KDTRACE_HOOKS
 505         uint64_t spin_cnt = 0;
 506         uint64_t sleep_cnt = 0;
 507         int64_t sleep_time = 0;
 508 #endif
 509
 510         if (SCHEDULER_STOPPED())
 511                 return (0);
 512
 513         /* If we already hold an exclusive lock, then recurse. */
 514         if (sx_xlocked(sx)) {
 515                 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
 516             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
 517                     sx->lock_object.lo_name, file, line));
 518                 sx->sx_recurse++;
 519                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 520                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 521                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
 522                 return (0);
 523         }
 524
 525         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 526                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 527                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
 528
 529         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
 530 #ifdef KDTRACE_HOOKS
 531                 spin_cnt++;
 532 #endif
 533 #ifdef HWPMC_HOOKS
 534                 PMC_SOFT_CALL( , , lock, failed);
 535 #endif
 536                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 537                     &waittime);
 538 #ifdef ADAPTIVE_SX
 539                 /*
 540                  * If the lock is write locked and the owner is
 541                  * running on another CPU, spin until the owner stops
 542                  * running or the state of the lock changes.
 543                  */
 544                 x = sx->sx_lock;
 545                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 546                         if ((x & SX_LOCK_SHARED) == 0) {
 547                                 x = SX_OWNER(x);
 548                                 owner = (struct thread *)x;
 549                                 if (TD_IS_RUNNING(owner)) {
 550                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 551                                                 CTR3(KTR_LOCK,
 552                                             "%s: spinning on %p held by %p",
 553                                                     __func__, sx, owner);
 554                                         GIANT_SAVE();
 555                                         while (SX_OWNER(sx->sx_lock) == x &&
 556                                             TD_IS_RUNNING(owner)) {
 557                                                 cpu_spinwait();
 558 #ifdef KDTRACE_HOOKS
 559                                                 spin_cnt++;
 560 #endif
 561                                         }
 562                                         continue;
 563                                 }
 564                         } else if (SX_SHARERS(x) && spintries < asx_retries) {
 565                                 GIANT_SAVE();
 566                                 spintries++;
 567                                 for (i = 0; i < asx_loops; i++) {
 568                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 569                                                 CTR4(KTR_LOCK,
 570                                     "%s: shared spinning on %p with %u and %u",
 571                                                     __func__, sx, spintries, i);
 572                                         x = sx->sx_lock;
 573                                         if ((x & SX_LOCK_SHARED) == 0 ||
 574                                             SX_SHARERS(x) == 0)
 575                                                 break;
 576                                         cpu_spinwait();
 577 #ifdef KDTRACE_HOOKS
 578                                         spin_cnt++;
 579 #endif
 580                                 }
 581                                 if (i != asx_loops)
 582                                         continue;
 583                         }
 584                 }
 585 #endif
 586
 587                 sleepq_lock(&sx->lock_object);
 588                 x = sx->sx_lock;
 589
 590                 /*
 591                  * If the lock was released while spinning on the
 592                  * sleep queue chain lock, try again.
 593                  */
 594                 if (x == SX_LOCK_UNLOCKED) {
 595                         sleepq_release(&sx->lock_object);
 596                         continue;
 597                 }
 598
 599 #ifdef ADAPTIVE_SX
 600                 /*
 601                  * The current lock owner might have started executing
 602                  * on another CPU (or the lock could have changed
 603                  * owners) while we were waiting on the sleep queue
 604                  * chain lock.  If so, drop the sleep queue lock and try
 605                  * again.
 606                  */
 607                 if (!(x & SX_LOCK_SHARED) &&
 608                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 609                         owner = (struct thread *)SX_OWNER(x);
 610                         if (TD_IS_RUNNING(owner)) {
 611                                 sleepq_release(&sx->lock_object);
 612                                 continue;
 613                         }
 614                 }
 615 #endif
 616
 617                 /*
 618                  * If an exclusive lock was released with both shared
 619                  * and exclusive waiters and a shared waiter hasn't
 620                  * woken up and acquired the lock yet, sx_lock will be
 621                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
 622                  * If we see that value, try to acquire it once.  Note
 623                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
 624                  * as there are other exclusive waiters still.  If we
 625                  * fail, restart the loop.
 626                  */
 627                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
 628                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
 629                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
 630                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
 631                                 sleepq_release(&sx->lock_object);
 632                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
 633                                     __func__, sx);
 634                                 break;
 635                         }
 636                         sleepq_release(&sx->lock_object);
 637                         continue;
 638                 }
 639
 640                 /*
 641                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
 642                  * than loop back and retry.
 643                  */
 644                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 645                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 646                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
 647                                 sleepq_release(&sx->lock_object);
 648                                 continue;
 649                         }
 650                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 651                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
 652                                     __func__, sx);
 653                 }
 654
 655                 /*
 656                  * Since we have been unable to acquire the exclusive
 657                  * lock and the exclusive waiters flag is set, we have
 658                  * to sleep.
 659                  */
 660                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 661                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 662                             __func__, sx);
 663
 664 #ifdef KDTRACE_HOOKS
 665                 sleep_time -= lockstat_nsecs();
 666 #endif
 667                 GIANT_SAVE();
 668                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 669                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 670                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
 671                 if (!(opts & SX_INTERRUPTIBLE))
 672                         sleepq_wait(&sx->lock_object, 0);
 673                 else
 674                         error = sleepq_wait_sig(&sx->lock_object, 0);
 675 #ifdef KDTRACE_HOOKS
 676                 sleep_time += lockstat_nsecs();
 677                 sleep_cnt++;
 678 #endif
 679                 if (error) {
 680                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 681                                 CTR2(KTR_LOCK,
 682                         "%s: interruptible sleep by %p suspended by signal",
 683                                     __func__, sx);
 684                         break;
 685                 }
 686                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 687                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 688                             __func__, sx);
 689         }
 690
 691         GIANT_RESTORE();
 692         if (!error)
 693                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
 694                     contested, waittime, file, line);
 695 #ifdef KDTRACE_HOOKS
 696         if (sleep_time)
 697                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 698         if (spin_cnt > sleep_cnt)
 699                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 700 #endif
 701         return (error);
 702 }
 703
 704 /*
 705  * This function represents the so-called 'hard case' for sx_xunlock
 706  * operation.  All 'easy case' failures are redirected to this.  Note
 707  * that ideally this would be a static function, but it needs to be
 708  * accessible from at least sx.h.
 709  */
 710 void
 711 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
 712 {
 713         uintptr_t x;
 714         int queue, wakeup_swapper;
 715
 716         if (SCHEDULER_STOPPED())
 717                 return;
 718
 719         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
 720
 721         /* If the lock is recursed, then unrecurse one level. */
 722         if (sx_xlocked(sx) && sx_recursed(sx)) {
 723                 if ((--sx->sx_recurse) == 0)
 724                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 725                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 726                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
 727                 return;
 728         }
 729         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
 730             SX_LOCK_EXCLUSIVE_WAITERS));
 731         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 732                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
 733
 734         sleepq_lock(&sx->lock_object);
 735         x = SX_LOCK_UNLOCKED;
 736
 737         /*
 738          * The wake up algorithm here is quite simple and probably not
 739          * ideal.  It gives precedence to shared waiters if they are
 740          * present.  For this condition, we have to preserve the
 741          * state of the exclusive waiters flag.
 742          * If interruptible sleeps left the shared queue empty avoid a
 743          * starvation for the threads sleeping on the exclusive queue by giving
 744          * them precedence and cleaning up the shared waiters bit anyway.
 745          */
 746         if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
 747             sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
 748                 queue = SQ_SHARED_QUEUE;
 749                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
 750         } else
 751                 queue = SQ_EXCLUSIVE_QUEUE;
 752
 753         /* Wake up all the waiters for the specific queue. */
 754         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 755                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
 756                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
 757                     "exclusive");
 758         atomic_store_rel_ptr(&sx->sx_lock, x);
 759         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
 760             queue);
 761         sleepq_release(&sx->lock_object);
 762         if (wakeup_swapper)
 763                 kick_proc0();
 764 }
 765
 766 /*
 767  * This function represents the so-called 'hard case' for sx_slock
 768  * operation.  All 'easy case' failures are redirected to this.  Note
 769  * that ideally this would be a static function, but it needs to be
 770  * accessible from at least sx.h.
 771  */
 772 int
 773 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
 774 {
 775         GIANT_DECLARE;
 776 #ifdef ADAPTIVE_SX
 777         volatile struct thread *owner;
 778 #endif
 779 #ifdef LOCK_PROFILING
 780         uint64_t waittime = 0;
 781         int contested = 0;
 782 #endif
 783         uintptr_t x;
 784         int error = 0;
 785 #ifdef KDTRACE_HOOKS
 786         uint64_t spin_cnt = 0;
 787         uint64_t sleep_cnt = 0;
 788         int64_t sleep_time = 0;
 789 #endif
 790
 791         if (SCHEDULER_STOPPED())
 792                 return (0);
 793
 794         /*
 795          * As with rwlocks, we don't make any attempt to try to block
 796          * shared locks once there is an exclusive waiter.
 797          */
 798         for (;;) {
 799 #ifdef KDTRACE_HOOKS
 800                 spin_cnt++;
 801 #endif
 802                 x = sx->sx_lock;
 803
 804                 /*
 805                  * If no other thread has an exclusive lock then try to bump up
 806                  * the count of sharers.  Since we have to preserve the state
 807                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
 808                  * shared lock loop back and retry.
 809                  */
 810                 if (x & SX_LOCK_SHARED) {
 811                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
 812                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
 813                             x + SX_ONE_SHARER)) {
 814                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 815                                         CTR4(KTR_LOCK,
 816                                             "%s: %p succeed %p -> %p", __func__,
 817                                             sx, (void *)x,
 818                                             (void *)(x + SX_ONE_SHARER));
 819                                 break;
 820                         }
 821                         continue;
 822                 }
 823 #ifdef HWPMC_HOOKS
 824                 PMC_SOFT_CALL( , , lock, failed);
 825 #endif
 826                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 827                     &waittime);
 828
 829 #ifdef ADAPTIVE_SX
 830                 /*
 831                  * If the owner is running on another CPU, spin until
 832                  * the owner stops running or the state of the lock
 833                  * changes.
 834                  */
 835                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 836                         x = SX_OWNER(x);
 837                         owner = (struct thread *)x;
 838                         if (TD_IS_RUNNING(owner)) {
 839                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 840                                         CTR3(KTR_LOCK,
 841                                             "%s: spinning on %p held by %p",
 842                                             __func__, sx, owner);
 843                                 GIANT_SAVE();
 844                                 while (SX_OWNER(sx->sx_lock) == x &&
 845                                     TD_IS_RUNNING(owner)) {
 846 #ifdef KDTRACE_HOOKS
 847                                         spin_cnt++;
 848 #endif
 849                                         cpu_spinwait();
 850                                 }
 851                                 continue;
 852                         }
 853                 }
 854 #endif
 855
 856                 /*
 857                  * Some other thread already has an exclusive lock, so
 858                  * start the process of blocking.
 859                  */
 860                 sleepq_lock(&sx->lock_object);
 861                 x = sx->sx_lock;
 862
 863                 /*
 864                  * The lock could have been released while we spun.
 865                  * In this case loop back and retry.
 866                  */
 867                 if (x & SX_LOCK_SHARED) {
 868                         sleepq_release(&sx->lock_object);
 869                         continue;
 870                 }
 871
 872 #ifdef ADAPTIVE_SX
 873                 /*
 874                  * If the owner is running on another CPU, spin until
 875                  * the owner stops running or the state of the lock
 876                  * changes.
 877                  */
 878                 if (!(x & SX_LOCK_SHARED) &&
 879                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 880                         owner = (struct thread *)SX_OWNER(x);
 881                         if (TD_IS_RUNNING(owner)) {
 882                                 sleepq_release(&sx->lock_object);
 883                                 continue;
 884                         }
 885                 }
 886 #endif
 887
 888                 /*
 889                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
 890                  * fail to set it drop the sleep queue lock and loop
 891                  * back.
 892                  */
 893                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
 894                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 895                             x | SX_LOCK_SHARED_WAITERS)) {
 896                                 sleepq_release(&sx->lock_object);
 897                                 continue;
 898                         }
 899                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 900                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
 901                                     __func__, sx);
 902                 }
 903
 904                 /*
 905                  * Since we have been unable to acquire the shared lock,
 906                  * we have to sleep.
 907                  */
 908                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 909                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 910                             __func__, sx);
 911
 912 #ifdef KDTRACE_HOOKS
 913                 sleep_time -= lockstat_nsecs();
 914 #endif
 915                 GIANT_SAVE();
 916                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 917                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 918                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
 919                 if (!(opts & SX_INTERRUPTIBLE))
 920                         sleepq_wait(&sx->lock_object, 0);
 921                 else
 922                         error = sleepq_wait_sig(&sx->lock_object, 0);
 923 #ifdef KDTRACE_HOOKS
 924                 sleep_time += lockstat_nsecs();
 925                 sleep_cnt++;
 926 #endif
 927                 if (error) {
 928                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 929                                 CTR2(KTR_LOCK,
 930                         "%s: interruptible sleep by %p suspended by signal",
 931                                     __func__, sx);
 932                         break;
 933                 }
 934                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 935                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 936                             __func__, sx);
 937         }
 938         if (error == 0)
 939                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
 940                     contested, waittime, file, line);
 941 #ifdef KDTRACE_HOOKS
 942         if (sleep_time)
 943                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 944         if (spin_cnt > sleep_cnt)
 945                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 946 #endif
 947         GIANT_RESTORE();
 948         return (error);
 949 }
 950
 951 /*
 952  * This function represents the so-called 'hard case' for sx_sunlock
 953  * operation.  All 'easy case' failures are redirected to this.  Note
 954  * that ideally this would be a static function, but it needs to be
 955  * accessible from at least sx.h.
 956  */
 957 void
 958 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
 959 {
 960         uintptr_t x;
 961         int wakeup_swapper;
 962
 963         if (SCHEDULER_STOPPED())
 964                 return;
 965
 966         for (;;) {
 967                 x = sx->sx_lock;
 968
 969                 /*
 970                  * We should never have sharers while at least one thread
 971                  * holds a shared lock.
 972                  */
 973                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
 974                     ("%s: waiting sharers", __func__));
 975
 976                 /*
 977                  * See if there is more than one shared lock held.  If
 978                  * so, just drop one and return.
 979                  */
 980                 if (SX_SHARERS(x) > 1) {
 981                         if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
 982                             x - SX_ONE_SHARER)) {
 983                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 984                                         CTR4(KTR_LOCK,
 985                                             "%s: %p succeeded %p -> %p",
 986                                             __func__, sx, (void *)x,
 987                                             (void *)(x - SX_ONE_SHARER));
 988                                 break;
 989                         }
 990                         continue;
 991                 }
 992
 993                 /*
 994                  * If there aren't any waiters for an exclusive lock,
 995                  * then try to drop it quickly.
 996                  */
 997                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 998                         MPASS(x == SX_SHARERS_LOCK(1));
 999                         if (atomic_cmpset_rel_ptr(&sx->sx_lock,
1000                             SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
1001                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1002                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
1003                                             __func__, sx);
1004                                 break;
1005                         }
1006                         continue;
1007                 }
1008
1009                 /*
1010                  * At this point, there should just be one sharer with
1011                  * exclusive waiters.
1012                  */
1013                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
1014
1015                 sleepq_lock(&sx->lock_object);
1016
1017                 /*
1018                  * Wake up semantic here is quite simple:
1019                  * Just wake up all the exclusive waiters.
1020                  * Note that the state of the lock could have changed,
1021                  * so if it fails loop back and retry.
1022                  */
1023                 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
1024                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
1025                     SX_LOCK_UNLOCKED)) {
1026                         sleepq_release(&sx->lock_object);
1027                         continue;
1028                 }
1029                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1030                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1031                             "exclusive queue", __func__, sx);
1032                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1033                     0, SQ_EXCLUSIVE_QUEUE);
1034                 sleepq_release(&sx->lock_object);
1035                 if (wakeup_swapper)
1036                         kick_proc0();
1037                 break;
1038         }
1039 }
1040
1041 #ifdef INVARIANT_SUPPORT
1042 #ifndef INVARIANTS
1043 #undef  _sx_assert
1044 #endif
1045
1046 /*
1047  * In the non-WITNESS case, sx_assert() can only detect that at least
1048  * *some* thread owns an slock, but it cannot guarantee that *this*
1049  * thread owns an slock.
1050  */
1051 void
1052 _sx_assert(const struct sx *sx, int what, const char *file, int line)
1053 {
1054 #ifndef WITNESS
1055         int slocked = 0;
1056 #endif
1057
1058         if (panicstr != NULL)
1059                 return;
1060         switch (what) {
1061         case SA_SLOCKED:
1062         case SA_SLOCKED | SA_NOTRECURSED:
1063         case SA_SLOCKED | SA_RECURSED:
1064 #ifndef WITNESS
1065                 slocked = 1;
1066                 /* FALLTHROUGH */
1067 #endif
1068         case SA_LOCKED:
1069         case SA_LOCKED | SA_NOTRECURSED:
1070         case SA_LOCKED | SA_RECURSED:
1071 #ifdef WITNESS
1072                 witness_assert(&sx->lock_object, what, file, line);
1073 #else
1074                 /*
1075                  * If some other thread has an exclusive lock or we
1076                  * have one and are asserting a shared lock, fail.
1077                  * Also, if no one has a lock at all, fail.
1078                  */
1079                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1080                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1081                     sx_xholder(sx) != curthread)))
1082                         panic("Lock %s not %slocked @ %s:%d\n",
1083                             sx->lock_object.lo_name, slocked ? "share " : "",
1084                             file, line);
1085
1086                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1087                         if (sx_recursed(sx)) {
1088                                 if (what & SA_NOTRECURSED)
1089                                         panic("Lock %s recursed @ %s:%d\n",
1090                                             sx->lock_object.lo_name, file,
1091                                             line);
1092                         } else if (what & SA_RECURSED)
1093                                 panic("Lock %s not recursed @ %s:%d\n",
1094                                     sx->lock_object.lo_name, file, line);
1095                 }
1096 #endif
1097                 break;
1098         case SA_XLOCKED:
1099         case SA_XLOCKED | SA_NOTRECURSED:
1100         case SA_XLOCKED | SA_RECURSED:
1101                 if (sx_xholder(sx) != curthread)
1102                         panic("Lock %s not exclusively locked @ %s:%d\n",
1103                             sx->lock_object.lo_name, file, line);
1104                 if (sx_recursed(sx)) {
1105                         if (what & SA_NOTRECURSED)
1106                                 panic("Lock %s recursed @ %s:%d\n",
1107                                     sx->lock_object.lo_name, file, line);
1108                 } else if (what & SA_RECURSED)
1109                         panic("Lock %s not recursed @ %s:%d\n",
1110                             sx->lock_object.lo_name, file, line);
1111                 break;
1112         case SA_UNLOCKED:
1113 #ifdef WITNESS
1114                 witness_assert(&sx->lock_object, what, file, line);
1115 #else
1116                 /*
1117                  * If we hold an exclusve lock fail.  We can't
1118                  * reliably check to see if we hold a shared lock or
1119                  * not.
1120                  */
1121                 if (sx_xholder(sx) == curthread)
1122                         panic("Lock %s exclusively locked @ %s:%d\n",
1123                             sx->lock_object.lo_name, file, line);
1124 #endif
1125                 break;
1126         default:
1127                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1128                     line);
1129         }
1130 }
1131 #endif  /* INVARIANT_SUPPORT */
1132
1133 #ifdef DDB
1134 static void
1135 db_show_sx(const struct lock_object *lock)
1136 {
1137         struct thread *td;
1138         const struct sx *sx;
1139
1140         sx = (const struct sx *)lock;
1141
1142         db_printf(" state: ");
1143         if (sx->sx_lock == SX_LOCK_UNLOCKED)
1144                 db_printf("UNLOCKED\n");
1145         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1146                 db_printf("DESTROYED\n");
1147                 return;
1148         } else if (sx->sx_lock & SX_LOCK_SHARED)
1149                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1150         else {
1151                 td = sx_xholder(sx);
1152                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1153                     td->td_tid, td->td_proc->p_pid, td->td_name);
1154                 if (sx_recursed(sx))
1155                         db_printf(" recursed: %d\n", sx->sx_recurse);
1156         }
1157
1158         db_printf(" waiters: ");
1159         switch(sx->sx_lock &
1160             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1161         case SX_LOCK_SHARED_WAITERS:
1162                 db_printf("shared\n");
1163                 break;
1164         case SX_LOCK_EXCLUSIVE_WAITERS:
1165                 db_printf("exclusive\n");
1166                 break;
1167         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1168                 db_printf("exclusive and shared\n");
1169                 break;
1170         default:
1171                 db_printf("none\n");
1172         }
1173 }
1174
1175 /*
1176  * Check to see if a thread that is blocked on a sleep queue is actually
1177  * blocked on an sx lock.  If so, output some details and return true.
1178  * If the lock has an exclusive owner, return that in *ownerp.
1179  */
1180 int
1181 sx_chain(struct thread *td, struct thread **ownerp)
1182 {
1183         struct sx *sx;
1184
1185         /*
1186          * Check to see if this thread is blocked on an sx lock.
1187          * First, we check the lock class.  If that is ok, then we
1188          * compare the lock name against the wait message.
1189          */
1190         sx = td->td_wchan;
1191         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1192             sx->lock_object.lo_name != td->td_wmesg)
1193                 return (0);
1194
1195         /* We think we have an sx lock, so output some details. */
1196         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1197         *ownerp = sx_xholder(sx);
1198         if (sx->sx_lock & SX_LOCK_SHARED)
1199                 db_printf("SLOCK (count %ju)\n",
1200                     (uintmax_t)SX_SHARERS(sx->sx_lock));
1201         else
1202                 db_printf("XLOCK\n");
1203         return (1);
1204 }
1205 #endif