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_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/kdb.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_recursed(sx)         ((sx)->sx_recurse != 0)
 112
 113 static void     assert_sx(const struct lock_object *lock, int what);
 114 #ifdef DDB
 115 static void     db_show_sx(const struct lock_object *lock);
 116 #endif
 117 static void     lock_sx(struct lock_object *lock, uintptr_t how);
 118 #ifdef KDTRACE_HOOKS
 119 static int      owner_sx(const struct lock_object *lock, struct thread **owner);
 120 #endif
 121 static uintptr_t unlock_sx(struct lock_object *lock);
 122
 123 struct lock_class lock_class_sx = {
 124         .lc_name = "sx",
 125         .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
 126         .lc_assert = assert_sx,
 127 #ifdef DDB
 128         .lc_ddb_show = db_show_sx,
 129 #endif
 130         .lc_lock = lock_sx,
 131         .lc_unlock = unlock_sx,
 132 #ifdef KDTRACE_HOOKS
 133         .lc_owner = owner_sx,
 134 #endif
 135 };
 136
 137 #ifndef INVARIANTS
 138 #define _sx_assert(sx, what, file, line)
 139 #endif
 140
 141 #ifdef ADAPTIVE_SX
 142 static u_int asx_retries = 10;
 143 static u_int asx_loops = 10000;
 144 static SYSCTL_NODE(_debug, OID_AUTO, sx, CTLFLAG_RD, NULL, "sxlock debugging");
 145 SYSCTL_UINT(_debug_sx, OID_AUTO, retries, CTLFLAG_RW, &asx_retries, 0, "");
 146 SYSCTL_UINT(_debug_sx, OID_AUTO, loops, CTLFLAG_RW, &asx_loops, 0, "");
 147 #endif
 148
 149 void
 150 assert_sx(const struct lock_object *lock, int what)
 151 {
 152
 153         sx_assert((const struct sx *)lock, what);
 154 }
 155
 156 void
 157 lock_sx(struct lock_object *lock, uintptr_t how)
 158 {
 159         struct sx *sx;
 160
 161         sx = (struct sx *)lock;
 162         if (how)
 163                 sx_slock(sx);
 164         else
 165                 sx_xlock(sx);
 166 }
 167
 168 uintptr_t
 169 unlock_sx(struct lock_object *lock)
 170 {
 171         struct sx *sx;
 172
 173         sx = (struct sx *)lock;
 174         sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
 175         if (sx_xlocked(sx)) {
 176                 sx_xunlock(sx);
 177                 return (0);
 178         } else {
 179                 sx_sunlock(sx);
 180                 return (1);
 181         }
 182 }
 183
 184 #ifdef KDTRACE_HOOKS
 185 int
 186 owner_sx(const struct lock_object *lock, struct thread **owner)
 187 {
 188         const struct sx *sx = (const struct sx *)lock;
 189         uintptr_t x = sx->sx_lock;
 190
 191         *owner = (struct thread *)SX_OWNER(x);
 192         return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
 193             (*owner != NULL));
 194 }
 195 #endif
 196
 197 void
 198 sx_sysinit(void *arg)
 199 {
 200         struct sx_args *sargs = arg;
 201
 202         sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
 203 }
 204
 205 void
 206 sx_init_flags(struct sx *sx, const char *description, int opts)
 207 {
 208         int flags;
 209
 210         MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
 211             SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
 212         ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
 213             ("%s: sx_lock not aligned for %s: %p", __func__, description,
 214             &sx->sx_lock));
 215
 216         flags = LO_SLEEPABLE | LO_UPGRADABLE;
 217         if (opts & SX_DUPOK)
 218                 flags |= LO_DUPOK;
 219         if (opts & SX_NOPROFILE)
 220                 flags |= LO_NOPROFILE;
 221         if (!(opts & SX_NOWITNESS))
 222                 flags |= LO_WITNESS;
 223         if (opts & SX_RECURSE)
 224                 flags |= LO_RECURSABLE;
 225         if (opts & SX_QUIET)
 226                 flags |= LO_QUIET;
 227
 228         flags |= opts & SX_NOADAPTIVE;
 229         lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
 230         sx->sx_lock = SX_LOCK_UNLOCKED;
 231         sx->sx_recurse = 0;
 232 }
 233
 234 void
 235 sx_destroy(struct sx *sx)
 236 {
 237
 238         KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
 239         KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
 240         sx->sx_lock = SX_LOCK_DESTROYED;
 241         lock_destroy(&sx->lock_object);
 242 }
 243
 244 int
 245 _sx_slock(struct sx *sx, int opts, const char *file, int line)
 246 {
 247         int error = 0;
 248
 249         if (SCHEDULER_STOPPED())
 250                 return (0);
 251         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 252             ("sx_slock() by idle thread %p on sx %s @ %s:%d",
 253             curthread, sx->lock_object.lo_name, file, line));
 254         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 255             ("sx_slock() of destroyed sx @ %s:%d", file, line));
 256         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
 257         error = __sx_slock(sx, opts, file, line);
 258         if (!error) {
 259                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
 260                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
 261                 curthread->td_locks++;
 262         }
 263
 264         return (error);
 265 }
 266
 267 int
 268 sx_try_slock_(struct sx *sx, const char *file, int line)
 269 {
 270         uintptr_t x;
 271
 272         if (SCHEDULER_STOPPED())
 273                 return (1);
 274
 275         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 276             ("sx_try_slock() by idle thread %p on sx %s @ %s:%d",
 277             curthread, sx->lock_object.lo_name, file, line));
 278
 279         for (;;) {
 280                 x = sx->sx_lock;
 281                 KASSERT(x != SX_LOCK_DESTROYED,
 282                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
 283                 if (!(x & SX_LOCK_SHARED))
 284                         break;
 285                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
 286                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
 287                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
 288                         curthread->td_locks++;
 289                         return (1);
 290                 }
 291         }
 292
 293         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
 294         return (0);
 295 }
 296
 297 int
 298 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
 299 {
 300         int error = 0;
 301
 302         if (SCHEDULER_STOPPED())
 303                 return (0);
 304         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 305             ("sx_xlock() by idle thread %p on sx %s @ %s:%d",
 306             curthread, sx->lock_object.lo_name, file, line));
 307         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 308             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
 309         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 310             line, NULL);
 311         error = __sx_xlock(sx, curthread, opts, file, line);
 312         if (!error) {
 313                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
 314                     file, line);
 315                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 316                 curthread->td_locks++;
 317         }
 318
 319         return (error);
 320 }
 321
 322 int
 323 sx_try_xlock_(struct sx *sx, const char *file, int line)
 324 {
 325         int rval;
 326
 327         if (SCHEDULER_STOPPED())
 328                 return (1);
 329
 330         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 331             ("sx_try_xlock() by idle thread %p on sx %s @ %s:%d",
 332             curthread, sx->lock_object.lo_name, file, line));
 333         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 334             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
 335
 336         if (sx_xlocked(sx) &&
 337             (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 338                 sx->sx_recurse++;
 339                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 340                 rval = 1;
 341         } else
 342                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
 343                     (uintptr_t)curthread);
 344         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
 345         if (rval) {
 346                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 347                     file, line);
 348                 curthread->td_locks++;
 349         }
 350
 351         return (rval);
 352 }
 353
 354 void
 355 _sx_sunlock(struct sx *sx, const char *file, int line)
 356 {
 357
 358         if (SCHEDULER_STOPPED())
 359                 return;
 360         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 361             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
 362         _sx_assert(sx, SA_SLOCKED, file, line);
 363         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
 364         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
 365         __sx_sunlock(sx, file, line);
 366         curthread->td_locks--;
 367 }
 368
 369 void
 370 _sx_xunlock(struct sx *sx, const char *file, int line)
 371 {
 372
 373         if (SCHEDULER_STOPPED())
 374                 return;
 375         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 376             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
 377         _sx_assert(sx, SA_XLOCKED, file, line);
 378         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 379         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
 380             line);
 381         __sx_xunlock(sx, curthread, file, line);
 382         curthread->td_locks--;
 383 }
 384
 385 /*
 386  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
 387  * This will only succeed if this thread holds a single shared lock.
 388  * Return 1 if if the upgrade succeed, 0 otherwise.
 389  */
 390 int
 391 sx_try_upgrade_(struct sx *sx, const char *file, int line)
 392 {
 393         uintptr_t x;
 394         int success;
 395
 396         if (SCHEDULER_STOPPED())
 397                 return (1);
 398
 399         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 400             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
 401         _sx_assert(sx, SA_SLOCKED, file, line);
 402
 403         /*
 404          * Try to switch from one shared lock to an exclusive lock.  We need
 405          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
 406          * we will wake up the exclusive waiters when we drop the lock.
 407          */
 408         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
 409         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
 410             (uintptr_t)curthread | x);
 411         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
 412         if (success) {
 413                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 414                     file, line);
 415                 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
 416         }
 417         return (success);
 418 }
 419
 420 /*
 421  * Downgrade an unrecursed exclusive lock into a single shared lock.
 422  */
 423 void
 424 sx_downgrade_(struct sx *sx, const char *file, int line)
 425 {
 426         uintptr_t x;
 427         int wakeup_swapper;
 428
 429         if (SCHEDULER_STOPPED())
 430                 return;
 431
 432         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 433             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
 434         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
 435 #ifndef INVARIANTS
 436         if (sx_recursed(sx))
 437                 panic("downgrade of a recursed lock");
 438 #endif
 439
 440         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
 441
 442         /*
 443          * Try to switch from an exclusive lock with no shared waiters
 444          * to one sharer with no shared waiters.  If there are
 445          * exclusive waiters, we don't need to lock the sleep queue so
 446          * long as we preserve the flag.  We do one quick try and if
 447          * that fails we grab the sleepq lock to keep the flags from
 448          * changing and do it the slow way.
 449          *
 450          * We have to lock the sleep queue if there are shared waiters
 451          * so we can wake them up.
 452          */
 453         x = sx->sx_lock;
 454         if (!(x & SX_LOCK_SHARED_WAITERS) &&
 455             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
 456             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
 457                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 458                 return;
 459         }
 460
 461         /*
 462          * Lock the sleep queue so we can read the waiters bits
 463          * without any races and wakeup any shared waiters.
 464          */
 465         sleepq_lock(&sx->lock_object);
 466
 467         /*
 468          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
 469          * shared lock.  If there are any shared waiters, wake them up.
 470          */
 471         wakeup_swapper = 0;
 472         x = sx->sx_lock;
 473         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
 474             (x & SX_LOCK_EXCLUSIVE_WAITERS));
 475         if (x & SX_LOCK_SHARED_WAITERS)
 476                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
 477                     0, SQ_SHARED_QUEUE);
 478         sleepq_release(&sx->lock_object);
 479
 480         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 481         LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
 482
 483         if (wakeup_swapper)
 484                 kick_proc0();
 485 }
 486
 487 /*
 488  * This function represents the so-called 'hard case' for sx_xlock
 489  * operation.  All 'easy case' failures are redirected to this.  Note
 490  * that ideally this would be a static function, but it needs to be
 491  * accessible from at least sx.h.
 492  */
 493 int
 494 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
 495     int line)
 496 {
 497         GIANT_DECLARE;
 498 #ifdef ADAPTIVE_SX
 499         volatile struct thread *owner;
 500         u_int i, spintries = 0;
 501 #endif
 502         uintptr_t x;
 503 #ifdef LOCK_PROFILING
 504         uint64_t waittime = 0;
 505         int contested = 0;
 506 #endif
 507         int error = 0;
 508 #ifdef  KDTRACE_HOOKS
 509         uint64_t spin_cnt = 0;
 510         uint64_t sleep_cnt = 0;
 511         int64_t sleep_time = 0;
 512 #endif
 513
 514         if (SCHEDULER_STOPPED())
 515                 return (0);
 516
 517         /* If we already hold an exclusive lock, then recurse. */
 518         if (sx_xlocked(sx)) {
 519                 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
 520             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
 521                     sx->lock_object.lo_name, file, line));
 522                 sx->sx_recurse++;
 523                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 524                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 525                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
 526                 return (0);
 527         }
 528
 529         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 530                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 531                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
 532
 533         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
 534 #ifdef KDTRACE_HOOKS
 535                 spin_cnt++;
 536 #endif
 537 #ifdef HWPMC_HOOKS
 538                 PMC_SOFT_CALL( , , lock, failed);
 539 #endif
 540                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 541                     &waittime);
 542 #ifdef ADAPTIVE_SX
 543                 /*
 544                  * If the lock is write locked and the owner is
 545                  * running on another CPU, spin until the owner stops
 546                  * running or the state of the lock changes.
 547                  */
 548                 x = sx->sx_lock;
 549                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 550                         if ((x & SX_LOCK_SHARED) == 0) {
 551                                 x = SX_OWNER(x);
 552                                 owner = (struct thread *)x;
 553                                 if (TD_IS_RUNNING(owner)) {
 554                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 555                                                 CTR3(KTR_LOCK,
 556                                             "%s: spinning on %p held by %p",
 557                                                     __func__, sx, owner);
 558                                         GIANT_SAVE();
 559                                         while (SX_OWNER(sx->sx_lock) == x &&
 560                                             TD_IS_RUNNING(owner)) {
 561                                                 cpu_spinwait();
 562 #ifdef KDTRACE_HOOKS
 563                                                 spin_cnt++;
 564 #endif
 565                                         }
 566                                         continue;
 567                                 }
 568                         } else if (SX_SHARERS(x) && spintries < asx_retries) {
 569                                 GIANT_SAVE();
 570                                 spintries++;
 571                                 for (i = 0; i < asx_loops; i++) {
 572                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 573                                                 CTR4(KTR_LOCK,
 574                                     "%s: shared spinning on %p with %u and %u",
 575                                                     __func__, sx, spintries, i);
 576                                         x = sx->sx_lock;
 577                                         if ((x & SX_LOCK_SHARED) == 0 ||
 578                                             SX_SHARERS(x) == 0)
 579                                                 break;
 580                                         cpu_spinwait();
 581 #ifdef KDTRACE_HOOKS
 582                                         spin_cnt++;
 583 #endif
 584                                 }
 585                                 if (i != asx_loops)
 586                                         continue;
 587                         }
 588                 }
 589 #endif
 590
 591                 sleepq_lock(&sx->lock_object);
 592                 x = sx->sx_lock;
 593
 594                 /*
 595                  * If the lock was released while spinning on the
 596                  * sleep queue chain lock, try again.
 597                  */
 598                 if (x == SX_LOCK_UNLOCKED) {
 599                         sleepq_release(&sx->lock_object);
 600                         continue;
 601                 }
 602
 603 #ifdef ADAPTIVE_SX
 604                 /*
 605                  * The current lock owner might have started executing
 606                  * on another CPU (or the lock could have changed
 607                  * owners) while we were waiting on the sleep queue
 608                  * chain lock.  If so, drop the sleep queue lock and try
 609                  * again.
 610                  */
 611                 if (!(x & SX_LOCK_SHARED) &&
 612                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 613                         owner = (struct thread *)SX_OWNER(x);
 614                         if (TD_IS_RUNNING(owner)) {
 615                                 sleepq_release(&sx->lock_object);
 616                                 continue;
 617                         }
 618                 }
 619 #endif
 620
 621                 /*
 622                  * If an exclusive lock was released with both shared
 623                  * and exclusive waiters and a shared waiter hasn't
 624                  * woken up and acquired the lock yet, sx_lock will be
 625                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
 626                  * If we see that value, try to acquire it once.  Note
 627                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
 628                  * as there are other exclusive waiters still.  If we
 629                  * fail, restart the loop.
 630                  */
 631                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
 632                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
 633                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
 634                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
 635                                 sleepq_release(&sx->lock_object);
 636                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
 637                                     __func__, sx);
 638                                 break;
 639                         }
 640                         sleepq_release(&sx->lock_object);
 641                         continue;
 642                 }
 643
 644                 /*
 645                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
 646                  * than loop back and retry.
 647                  */
 648                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 649                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 650                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
 651                                 sleepq_release(&sx->lock_object);
 652                                 continue;
 653                         }
 654                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 655                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
 656                                     __func__, sx);
 657                 }
 658
 659                 /*
 660                  * Since we have been unable to acquire the exclusive
 661                  * lock and the exclusive waiters flag is set, we have
 662                  * to sleep.
 663                  */
 664                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 665                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 666                             __func__, sx);
 667
 668 #ifdef KDTRACE_HOOKS
 669                 sleep_time -= lockstat_nsecs();
 670 #endif
 671                 GIANT_SAVE();
 672                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 673                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 674                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
 675                 if (!(opts & SX_INTERRUPTIBLE))
 676                         sleepq_wait(&sx->lock_object, 0);
 677                 else
 678                         error = sleepq_wait_sig(&sx->lock_object, 0);
 679 #ifdef KDTRACE_HOOKS
 680                 sleep_time += lockstat_nsecs();
 681                 sleep_cnt++;
 682 #endif
 683                 if (error) {
 684                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 685                                 CTR2(KTR_LOCK,
 686                         "%s: interruptible sleep by %p suspended by signal",
 687                                     __func__, sx);
 688                         break;
 689                 }
 690                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 691                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 692                             __func__, sx);
 693         }
 694
 695         GIANT_RESTORE();
 696         if (!error)
 697                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
 698                     contested, waittime, file, line);
 699 #ifdef KDTRACE_HOOKS
 700         if (sleep_time)
 701                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 702         if (spin_cnt > sleep_cnt)
 703                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 704 #endif
 705         return (error);
 706 }
 707
 708 /*
 709  * This function represents the so-called 'hard case' for sx_xunlock
 710  * operation.  All 'easy case' failures are redirected to this.  Note
 711  * that ideally this would be a static function, but it needs to be
 712  * accessible from at least sx.h.
 713  */
 714 void
 715 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
 716 {
 717         uintptr_t x;
 718         int queue, wakeup_swapper;
 719
 720         if (SCHEDULER_STOPPED())
 721                 return;
 722
 723         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
 724
 725         /* If the lock is recursed, then unrecurse one level. */
 726         if (sx_xlocked(sx) && sx_recursed(sx)) {
 727                 if ((--sx->sx_recurse) == 0)
 728                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 729                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 730                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
 731                 return;
 732         }
 733         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
 734             SX_LOCK_EXCLUSIVE_WAITERS));
 735         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 736                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
 737
 738         sleepq_lock(&sx->lock_object);
 739         x = SX_LOCK_UNLOCKED;
 740
 741         /*
 742          * The wake up algorithm here is quite simple and probably not
 743          * ideal.  It gives precedence to shared waiters if they are
 744          * present.  For this condition, we have to preserve the
 745          * state of the exclusive waiters flag.
 746          * If interruptible sleeps left the shared queue empty avoid a
 747          * starvation for the threads sleeping on the exclusive queue by giving
 748          * them precedence and cleaning up the shared waiters bit anyway.
 749          */
 750         if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
 751             sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
 752                 queue = SQ_SHARED_QUEUE;
 753                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
 754         } else
 755                 queue = SQ_EXCLUSIVE_QUEUE;
 756
 757         /* Wake up all the waiters for the specific queue. */
 758         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 759                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
 760                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
 761                     "exclusive");
 762         atomic_store_rel_ptr(&sx->sx_lock, x);
 763         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
 764             queue);
 765         sleepq_release(&sx->lock_object);
 766         if (wakeup_swapper)
 767                 kick_proc0();
 768 }
 769
 770 /*
 771  * This function represents the so-called 'hard case' for sx_slock
 772  * operation.  All 'easy case' failures are redirected to this.  Note
 773  * that ideally this would be a static function, but it needs to be
 774  * accessible from at least sx.h.
 775  */
 776 int
 777 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
 778 {
 779         GIANT_DECLARE;
 780 #ifdef ADAPTIVE_SX
 781         volatile struct thread *owner;
 782 #endif
 783 #ifdef LOCK_PROFILING
 784         uint64_t waittime = 0;
 785         int contested = 0;
 786 #endif
 787         uintptr_t x;
 788         int error = 0;
 789 #ifdef KDTRACE_HOOKS
 790         uint64_t spin_cnt = 0;
 791         uint64_t sleep_cnt = 0;
 792         int64_t sleep_time = 0;
 793 #endif
 794
 795         if (SCHEDULER_STOPPED())
 796                 return (0);
 797
 798         /*
 799          * As with rwlocks, we don't make any attempt to try to block
 800          * shared locks once there is an exclusive waiter.
 801          */
 802         for (;;) {
 803 #ifdef KDTRACE_HOOKS
 804                 spin_cnt++;
 805 #endif
 806                 x = sx->sx_lock;
 807
 808                 /*
 809                  * If no other thread has an exclusive lock then try to bump up
 810                  * the count of sharers.  Since we have to preserve the state
 811                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
 812                  * shared lock loop back and retry.
 813                  */
 814                 if (x & SX_LOCK_SHARED) {
 815                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
 816                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
 817                             x + SX_ONE_SHARER)) {
 818                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 819                                         CTR4(KTR_LOCK,
 820                                             "%s: %p succeed %p -> %p", __func__,
 821                                             sx, (void *)x,
 822                                             (void *)(x + SX_ONE_SHARER));
 823                                 break;
 824                         }
 825                         continue;
 826                 }
 827 #ifdef HWPMC_HOOKS
 828                 PMC_SOFT_CALL( , , lock, failed);
 829 #endif
 830                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 831                     &waittime);
 832
 833 #ifdef ADAPTIVE_SX
 834                 /*
 835                  * If the owner is running on another CPU, spin until
 836                  * the owner stops running or the state of the lock
 837                  * changes.
 838                  */
 839                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 840                         x = SX_OWNER(x);
 841                         owner = (struct thread *)x;
 842                         if (TD_IS_RUNNING(owner)) {
 843                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 844                                         CTR3(KTR_LOCK,
 845                                             "%s: spinning on %p held by %p",
 846                                             __func__, sx, owner);
 847                                 GIANT_SAVE();
 848                                 while (SX_OWNER(sx->sx_lock) == x &&
 849                                     TD_IS_RUNNING(owner)) {
 850 #ifdef KDTRACE_HOOKS
 851                                         spin_cnt++;
 852 #endif
 853                                         cpu_spinwait();
 854                                 }
 855                                 continue;
 856                         }
 857                 }
 858 #endif
 859
 860                 /*
 861                  * Some other thread already has an exclusive lock, so
 862                  * start the process of blocking.
 863                  */
 864                 sleepq_lock(&sx->lock_object);
 865                 x = sx->sx_lock;
 866
 867                 /*
 868                  * The lock could have been released while we spun.
 869                  * In this case loop back and retry.
 870                  */
 871                 if (x & SX_LOCK_SHARED) {
 872                         sleepq_release(&sx->lock_object);
 873                         continue;
 874                 }
 875
 876 #ifdef ADAPTIVE_SX
 877                 /*
 878                  * If the owner is running on another CPU, spin until
 879                  * the owner stops running or the state of the lock
 880                  * changes.
 881                  */
 882                 if (!(x & SX_LOCK_SHARED) &&
 883                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 884                         owner = (struct thread *)SX_OWNER(x);
 885                         if (TD_IS_RUNNING(owner)) {
 886                                 sleepq_release(&sx->lock_object);
 887                                 continue;
 888                         }
 889                 }
 890 #endif
 891
 892                 /*
 893                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
 894                  * fail to set it drop the sleep queue lock and loop
 895                  * back.
 896                  */
 897                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
 898                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 899                             x | SX_LOCK_SHARED_WAITERS)) {
 900                                 sleepq_release(&sx->lock_object);
 901                                 continue;
 902                         }
 903                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 904                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
 905                                     __func__, sx);
 906                 }
 907
 908                 /*
 909                  * Since we have been unable to acquire the shared lock,
 910                  * we have to sleep.
 911                  */
 912                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 913                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 914                             __func__, sx);
 915
 916 #ifdef KDTRACE_HOOKS
 917                 sleep_time -= lockstat_nsecs();
 918 #endif
 919                 GIANT_SAVE();
 920                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 921                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 922                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
 923                 if (!(opts & SX_INTERRUPTIBLE))
 924                         sleepq_wait(&sx->lock_object, 0);
 925                 else
 926                         error = sleepq_wait_sig(&sx->lock_object, 0);
 927 #ifdef KDTRACE_HOOKS
 928                 sleep_time += lockstat_nsecs();
 929                 sleep_cnt++;
 930 #endif
 931                 if (error) {
 932                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 933                                 CTR2(KTR_LOCK,
 934                         "%s: interruptible sleep by %p suspended by signal",
 935                                     __func__, sx);
 936                         break;
 937                 }
 938                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 939                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 940                             __func__, sx);
 941         }
 942         if (error == 0)
 943                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
 944                     contested, waittime, file, line);
 945 #ifdef KDTRACE_HOOKS
 946         if (sleep_time)
 947                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 948         if (spin_cnt > sleep_cnt)
 949                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 950 #endif
 951         GIANT_RESTORE();
 952         return (error);
 953 }
 954
 955 /*
 956  * This function represents the so-called 'hard case' for sx_sunlock
 957  * operation.  All 'easy case' failures are redirected to this.  Note
 958  * that ideally this would be a static function, but it needs to be
 959  * accessible from at least sx.h.
 960  */
 961 void
 962 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
 963 {
 964         uintptr_t x;
 965         int wakeup_swapper;
 966
 967         if (SCHEDULER_STOPPED())
 968                 return;
 969
 970         for (;;) {
 971                 x = sx->sx_lock;
 972
 973                 /*
 974                  * We should never have sharers while at least one thread
 975                  * holds a shared lock.
 976                  */
 977                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
 978                     ("%s: waiting sharers", __func__));
 979
 980                 /*
 981                  * See if there is more than one shared lock held.  If
 982                  * so, just drop one and return.
 983                  */
 984                 if (SX_SHARERS(x) > 1) {
 985                         if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
 986                             x - SX_ONE_SHARER)) {
 987                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 988                                         CTR4(KTR_LOCK,
 989                                             "%s: %p succeeded %p -> %p",
 990                                             __func__, sx, (void *)x,
 991                                             (void *)(x - SX_ONE_SHARER));
 992                                 break;
 993                         }
 994                         continue;
 995                 }
 996
 997                 /*
 998                  * If there aren't any waiters for an exclusive lock,
 999                  * then try to drop it quickly.
1000                  */
1001                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
1002                         MPASS(x == SX_SHARERS_LOCK(1));
1003                         if (atomic_cmpset_rel_ptr(&sx->sx_lock,
1004                             SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
1005                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1006                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
1007                                             __func__, sx);
1008                                 break;
1009                         }
1010                         continue;
1011                 }
1012
1013                 /*
1014                  * At this point, there should just be one sharer with
1015                  * exclusive waiters.
1016                  */
1017                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
1018
1019                 sleepq_lock(&sx->lock_object);
1020
1021                 /*
1022                  * Wake up semantic here is quite simple:
1023                  * Just wake up all the exclusive waiters.
1024                  * Note that the state of the lock could have changed,
1025                  * so if it fails loop back and retry.
1026                  */
1027                 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
1028                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
1029                     SX_LOCK_UNLOCKED)) {
1030                         sleepq_release(&sx->lock_object);
1031                         continue;
1032                 }
1033                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1034                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1035                             "exclusive queue", __func__, sx);
1036                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1037                     0, SQ_EXCLUSIVE_QUEUE);
1038                 sleepq_release(&sx->lock_object);
1039                 if (wakeup_swapper)
1040                         kick_proc0();
1041                 break;
1042         }
1043 }
1044
1045 #ifdef INVARIANT_SUPPORT
1046 #ifndef INVARIANTS
1047 #undef  _sx_assert
1048 #endif
1049
1050 /*
1051  * In the non-WITNESS case, sx_assert() can only detect that at least
1052  * *some* thread owns an slock, but it cannot guarantee that *this*
1053  * thread owns an slock.
1054  */
1055 void
1056 _sx_assert(const struct sx *sx, int what, const char *file, int line)
1057 {
1058 #ifndef WITNESS
1059         int slocked = 0;
1060 #endif
1061
1062         if (panicstr != NULL)
1063                 return;
1064         switch (what) {
1065         case SA_SLOCKED:
1066         case SA_SLOCKED | SA_NOTRECURSED:
1067         case SA_SLOCKED | SA_RECURSED:
1068 #ifndef WITNESS
1069                 slocked = 1;
1070                 /* FALLTHROUGH */
1071 #endif
1072         case SA_LOCKED:
1073         case SA_LOCKED | SA_NOTRECURSED:
1074         case SA_LOCKED | SA_RECURSED:
1075 #ifdef WITNESS
1076                 witness_assert(&sx->lock_object, what, file, line);
1077 #else
1078                 /*
1079                  * If some other thread has an exclusive lock or we
1080                  * have one and are asserting a shared lock, fail.
1081                  * Also, if no one has a lock at all, fail.
1082                  */
1083                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1084                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1085                     sx_xholder(sx) != curthread)))
1086                         panic("Lock %s not %slocked @ %s:%d\n",
1087                             sx->lock_object.lo_name, slocked ? "share " : "",
1088                             file, line);
1089
1090                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1091                         if (sx_recursed(sx)) {
1092                                 if (what & SA_NOTRECURSED)
1093                                         panic("Lock %s recursed @ %s:%d\n",
1094                                             sx->lock_object.lo_name, file,
1095                                             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                 }
1100 #endif
1101                 break;
1102         case SA_XLOCKED:
1103         case SA_XLOCKED | SA_NOTRECURSED:
1104         case SA_XLOCKED | SA_RECURSED:
1105                 if (sx_xholder(sx) != curthread)
1106                         panic("Lock %s not exclusively locked @ %s:%d\n",
1107                             sx->lock_object.lo_name, file, line);
1108                 if (sx_recursed(sx)) {
1109                         if (what & SA_NOTRECURSED)
1110                                 panic("Lock %s recursed @ %s:%d\n",
1111                                     sx->lock_object.lo_name, file, line);
1112                 } else if (what & SA_RECURSED)
1113                         panic("Lock %s not recursed @ %s:%d\n",
1114                             sx->lock_object.lo_name, file, line);
1115                 break;
1116         case SA_UNLOCKED:
1117 #ifdef WITNESS
1118                 witness_assert(&sx->lock_object, what, file, line);
1119 #else
1120                 /*
1121                  * If we hold an exclusve lock fail.  We can't
1122                  * reliably check to see if we hold a shared lock or
1123                  * not.
1124                  */
1125                 if (sx_xholder(sx) == curthread)
1126                         panic("Lock %s exclusively locked @ %s:%d\n",
1127                             sx->lock_object.lo_name, file, line);
1128 #endif
1129                 break;
1130         default:
1131                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1132                     line);
1133         }
1134 }
1135 #endif  /* INVARIANT_SUPPORT */
1136
1137 #ifdef DDB
1138 static void
1139 db_show_sx(const struct lock_object *lock)
1140 {
1141         struct thread *td;
1142         const struct sx *sx;
1143
1144         sx = (const struct sx *)lock;
1145
1146         db_printf(" state: ");
1147         if (sx->sx_lock == SX_LOCK_UNLOCKED)
1148                 db_printf("UNLOCKED\n");
1149         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1150                 db_printf("DESTROYED\n");
1151                 return;
1152         } else if (sx->sx_lock & SX_LOCK_SHARED)
1153                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1154         else {
1155                 td = sx_xholder(sx);
1156                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1157                     td->td_tid, td->td_proc->p_pid, td->td_name);
1158                 if (sx_recursed(sx))
1159                         db_printf(" recursed: %d\n", sx->sx_recurse);
1160         }
1161
1162         db_printf(" waiters: ");
1163         switch(sx->sx_lock &
1164             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1165         case SX_LOCK_SHARED_WAITERS:
1166                 db_printf("shared\n");
1167                 break;
1168         case SX_LOCK_EXCLUSIVE_WAITERS:
1169                 db_printf("exclusive\n");
1170                 break;
1171         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1172                 db_printf("exclusive and shared\n");
1173                 break;
1174         default:
1175                 db_printf("none\n");
1176         }
1177 }
1178
1179 /*
1180  * Check to see if a thread that is blocked on a sleep queue is actually
1181  * blocked on an sx lock.  If so, output some details and return true.
1182  * If the lock has an exclusive owner, return that in *ownerp.
1183  */
1184 int
1185 sx_chain(struct thread *td, struct thread **ownerp)
1186 {
1187         struct sx *sx;
1188
1189         /*
1190          * Check to see if this thread is blocked on an sx lock.
1191          * First, we check the lock class.  If that is ok, then we
1192          * compare the lock name against the wait message.
1193          */
1194         sx = td->td_wchan;
1195         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1196             sx->lock_object.lo_name != td->td_wmesg)
1197                 return (0);
1198
1199         /* We think we have an sx lock, so output some details. */
1200         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1201         *ownerp = sx_xholder(sx);
1202         if (sx->sx_lock & SX_LOCK_SHARED)
1203                 db_printf("SLOCK (count %ju)\n",
1204                     (uintmax_t)SX_SHARERS(sx->sx_lock));
1205         else
1206                 db_printf("XLOCK\n");
1207         return (1);
1208 }
1209 #endif