sys/kern/kern_sx.c

   1 /*-
   2  * Copyright (c) 2007 Attilio Rao <attilio@freebsd.org>
   3  * Copyright (c) 2001 Jason Evans <jasone@freebsd.org>
   4  * All rights reserved.
   5  *
   6  * Redistribution and use in source and binary forms, with or without
   7  * modification, are permitted provided that the following conditions
   8  * are met:
   9  * 1. Redistributions of source code must retain the above copyright
  10  *    notice(s), this list of conditions and the following disclaimer as
  11  *    the first lines of this file unmodified other than the possible
  12  *    addition of one or more copyright notices.
  13  * 2. Redistributions in binary form must reproduce the above copyright
  14  *    notice(s), this list of conditions and the following disclaimer in the
  15  *    documentation and/or other materials provided with the distribution.
  16  *
  17  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
  18  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  19  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  20  * DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
  21  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  22  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  23  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  24  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
  27  * DAMAGE.
  28  */
  29
  30 /*
  31  * Shared/exclusive locks.  This implementation attempts to ensure
  32  * deterministic lock granting behavior, so that slocks and xlocks are
  33  * interleaved.
  34  *
  35  * Priority propagation will not generally raise the priority of lock holders,
  36  * so should not be relied upon in combination with sx locks.
  37  */
  38
  39 #include "opt_ddb.h"
  40 #include "opt_hwpmc_hooks.h"
  41 #include "opt_kdtrace.h"
  42 #include "opt_no_adaptive_sx.h"
  43
  44 #include <sys/cdefs.h>
  45 __FBSDID("$FreeBSD$");
  46
  47 #include <sys/param.h>
  48 #include <sys/systm.h>
  49 #include <sys/ktr.h>
  50 #include <sys/lock.h>
  51 #include <sys/mutex.h>
  52 #include <sys/proc.h>
  53 #include <sys/sleepqueue.h>
  54 #include <sys/sx.h>
  55 #include <sys/sysctl.h>
  56
  57 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
  58 #include <machine/cpu.h>
  59 #endif
  60
  61 #ifdef DDB
  62 #include <ddb/ddb.h>
  63 #endif
  64
  65 #if defined(SMP) && !defined(NO_ADAPTIVE_SX)
  66 #define ADAPTIVE_SX
  67 #endif
  68
  69 CTASSERT((SX_NOADAPTIVE & LO_CLASSFLAGS) == SX_NOADAPTIVE);
  70
  71 #ifdef HWPMC_HOOKS
  72 #include <sys/pmckern.h>
  73 PMC_SOFT_DECLARE( , , lock, failed);
  74 #endif
  75
  76 /* Handy macros for sleep queues. */
  77 #define SQ_EXCLUSIVE_QUEUE      0
  78 #define SQ_SHARED_QUEUE         1
  79
  80 #ifdef ADAPTIVE_SX
  81 #define ASX_RETRIES             10
  82 #define ASX_LOOPS               10000
  83 #endif
  84
  85 /*
  86  * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file.  We
  87  * drop Giant anytime we have to sleep or if we adaptively spin.
  88  */
  89 #define GIANT_DECLARE                                                   \
  90         int _giantcnt = 0;                                              \
  91         WITNESS_SAVE_DECL(Giant)                                        \
  92
  93 #define GIANT_SAVE() do {                                               \
  94         if (mtx_owned(&Giant)) {                                        \
  95                 WITNESS_SAVE(&Giant.lock_object, Giant);                \
  96                 while (mtx_owned(&Giant)) {                             \
  97                         _giantcnt++;                                    \
  98                         mtx_unlock(&Giant);                             \
  99                 }                                                       \
 100         }                                                               \
 101 } while (0)
 102
 103 #define GIANT_RESTORE() do {                                            \
 104         if (_giantcnt > 0) {                                            \
 105                 mtx_assert(&Giant, MA_NOTOWNED);                        \
 106                 while (_giantcnt--)                                     \
 107                         mtx_lock(&Giant);                               \
 108                 WITNESS_RESTORE(&Giant.lock_object, Giant);             \
 109         }                                                               \
 110 } while (0)
 111
 112 /*
 113  * Returns true if an exclusive lock is recursed.  It assumes
 114  * curthread currently has an exclusive lock.
 115  */
 116 #define sx_recurse              lock_object.lo_data
 117 #define sx_recursed(sx)         ((sx)->sx_recurse != 0)
 118
 119 static void     assert_sx(struct lock_object *lock, int what);
 120 #ifdef DDB
 121 static void     db_show_sx(struct lock_object *lock);
 122 #endif
 123 static void     lock_sx(struct lock_object *lock, int how);
 124 #ifdef KDTRACE_HOOKS
 125 static int      owner_sx(struct lock_object *lock, struct thread **owner);
 126 #endif
 127 static int      unlock_sx(struct lock_object *lock);
 128
 129 struct lock_class lock_class_sx = {
 130         .lc_name = "sx",
 131         .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
 132         .lc_assert = assert_sx,
 133 #ifdef DDB
 134         .lc_ddb_show = db_show_sx,
 135 #endif
 136         .lc_lock = lock_sx,
 137         .lc_unlock = unlock_sx,
 138 #ifdef KDTRACE_HOOKS
 139         .lc_owner = owner_sx,
 140 #endif
 141 };
 142
 143 #ifndef INVARIANTS
 144 #define _sx_assert(sx, what, file, line)
 145 #endif
 146
 147 void
 148 assert_sx(struct lock_object *lock, int what)
 149 {
 150
 151         sx_assert((struct sx *)lock, what);
 152 }
 153
 154 void
 155 lock_sx(struct lock_object *lock, int how)
 156 {
 157         struct sx *sx;
 158
 159         sx = (struct sx *)lock;
 160         if (how)
 161                 sx_xlock(sx);
 162         else
 163                 sx_slock(sx);
 164 }
 165
 166 int
 167 unlock_sx(struct lock_object *lock)
 168 {
 169         struct sx *sx;
 170
 171         sx = (struct sx *)lock;
 172         sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
 173         if (sx_xlocked(sx)) {
 174                 sx_xunlock(sx);
 175                 return (1);
 176         } else {
 177                 sx_sunlock(sx);
 178                 return (0);
 179         }
 180 }
 181
 182 #ifdef KDTRACE_HOOKS
 183 int
 184 owner_sx(struct lock_object *lock, struct thread **owner)
 185 {
 186         struct sx *sx = (struct sx *)lock;
 187         uintptr_t x = sx->sx_lock;
 188
 189         *owner = (struct thread *)SX_OWNER(x);
 190         return ((x & SX_LOCK_SHARED) != 0 ? (SX_SHARERS(x) != 0) :
 191             (*owner != NULL));
 192 }
 193 #endif
 194
 195 void
 196 sx_sysinit(void *arg)
 197 {
 198         struct sx_args *sargs = arg;
 199
 200         sx_init_flags(sargs->sa_sx, sargs->sa_desc, sargs->sa_flags);
 201 }
 202
 203 void
 204 sx_init_flags(struct sx *sx, const char *description, int opts)
 205 {
 206         int flags;
 207
 208         MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
 209             SX_NOPROFILE | SX_NOADAPTIVE)) == 0);
 210         ASSERT_ATOMIC_LOAD_PTR(sx->sx_lock,
 211             ("%s: sx_lock not aligned for %s: %p", __func__, description,
 212             &sx->sx_lock));
 213
 214         flags = LO_SLEEPABLE | LO_UPGRADABLE;
 215         if (opts & SX_DUPOK)
 216                 flags |= LO_DUPOK;
 217         if (opts & SX_NOPROFILE)
 218                 flags |= LO_NOPROFILE;
 219         if (!(opts & SX_NOWITNESS))
 220                 flags |= LO_WITNESS;
 221         if (opts & SX_RECURSE)
 222                 flags |= LO_RECURSABLE;
 223         if (opts & SX_QUIET)
 224                 flags |= LO_QUIET;
 225
 226         flags |= opts & SX_NOADAPTIVE;
 227         sx->sx_lock = SX_LOCK_UNLOCKED;
 228         sx->sx_recurse = 0;
 229         lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
 230 }
 231
 232 void
 233 sx_destroy(struct sx *sx)
 234 {
 235
 236         KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
 237         KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
 238         sx->sx_lock = SX_LOCK_DESTROYED;
 239         lock_destroy(&sx->lock_object);
 240 }
 241
 242 int
 243 _sx_slock(struct sx *sx, int opts, const char *file, int line)
 244 {
 245         int error = 0;
 246
 247         if (SCHEDULER_STOPPED())
 248                 return (0);
 249         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 250             ("sx_slock() of destroyed sx @ %s:%d", file, line));
 251         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line, NULL);
 252         error = __sx_slock(sx, opts, file, line);
 253         if (!error) {
 254                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
 255                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
 256                 curthread->td_locks++;
 257         }
 258
 259         return (error);
 260 }
 261
 262 int
 263 _sx_try_slock(struct sx *sx, const char *file, int line)
 264 {
 265         uintptr_t x;
 266
 267         if (SCHEDULER_STOPPED())
 268                 return (1);
 269
 270         for (;;) {
 271                 x = sx->sx_lock;
 272                 KASSERT(x != SX_LOCK_DESTROYED,
 273                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
 274                 if (!(x & SX_LOCK_SHARED))
 275                         break;
 276                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
 277                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
 278                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
 279                         curthread->td_locks++;
 280                         return (1);
 281                 }
 282         }
 283
 284         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
 285         return (0);
 286 }
 287
 288 int
 289 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
 290 {
 291         int error = 0;
 292
 293         if (SCHEDULER_STOPPED())
 294                 return (0);
 295         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 296             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
 297         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 298             line, NULL);
 299         error = __sx_xlock(sx, curthread, opts, file, line);
 300         if (!error) {
 301                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
 302                     file, line);
 303                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 304                 curthread->td_locks++;
 305         }
 306
 307         return (error);
 308 }
 309
 310 int
 311 _sx_try_xlock(struct sx *sx, const char *file, int line)
 312 {
 313         int rval;
 314
 315         if (SCHEDULER_STOPPED())
 316                 return (1);
 317
 318         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 319             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
 320
 321         if (sx_xlocked(sx) &&
 322             (sx->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 323                 sx->sx_recurse++;
 324                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 325                 rval = 1;
 326         } else
 327                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
 328                     (uintptr_t)curthread);
 329         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
 330         if (rval) {
 331                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 332                     file, line);
 333                 curthread->td_locks++;
 334         }
 335
 336         return (rval);
 337 }
 338
 339 void
 340 _sx_sunlock(struct sx *sx, const char *file, int line)
 341 {
 342
 343         if (SCHEDULER_STOPPED())
 344                 return;
 345         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 346             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
 347         _sx_assert(sx, SA_SLOCKED, file, line);
 348         curthread->td_locks--;
 349         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
 350         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
 351         __sx_sunlock(sx, file, line);
 352         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_SUNLOCK_RELEASE, sx);
 353 }
 354
 355 void
 356 _sx_xunlock(struct sx *sx, const char *file, int line)
 357 {
 358
 359         if (SCHEDULER_STOPPED())
 360                 return;
 361         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 362             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
 363         _sx_assert(sx, SA_XLOCKED, file, line);
 364         curthread->td_locks--;
 365         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
 366         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
 367             line);
 368         if (!sx_recursed(sx))
 369                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_SX_XUNLOCK_RELEASE, sx);
 370         __sx_xunlock(sx, curthread, file, line);
 371 }
 372
 373 /*
 374  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
 375  * This will only succeed if this thread holds a single shared lock.
 376  * Return 1 if if the upgrade succeed, 0 otherwise.
 377  */
 378 int
 379 _sx_try_upgrade(struct sx *sx, const char *file, int line)
 380 {
 381         uintptr_t x;
 382         int success;
 383
 384         if (SCHEDULER_STOPPED())
 385                 return (1);
 386
 387         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 388             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
 389         _sx_assert(sx, SA_SLOCKED, file, line);
 390
 391         /*
 392          * Try to switch from one shared lock to an exclusive lock.  We need
 393          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
 394          * we will wake up the exclusive waiters when we drop the lock.
 395          */
 396         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
 397         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
 398             (uintptr_t)curthread | x);
 399         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
 400         if (success) {
 401                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 402                     file, line);
 403                 LOCKSTAT_RECORD0(LS_SX_TRYUPGRADE_UPGRADE, sx);
 404         }
 405         return (success);
 406 }
 407
 408 /*
 409  * Downgrade an unrecursed exclusive lock into a single shared lock.
 410  */
 411 void
 412 _sx_downgrade(struct sx *sx, const char *file, int line)
 413 {
 414         uintptr_t x;
 415         int wakeup_swapper;
 416
 417         if (SCHEDULER_STOPPED())
 418                 return;
 419
 420         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
 421             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
 422         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
 423 #ifndef INVARIANTS
 424         if (sx_recursed(sx))
 425                 panic("downgrade of a recursed lock");
 426 #endif
 427
 428         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
 429
 430         /*
 431          * Try to switch from an exclusive lock with no shared waiters
 432          * to one sharer with no shared waiters.  If there are
 433          * exclusive waiters, we don't need to lock the sleep queue so
 434          * long as we preserve the flag.  We do one quick try and if
 435          * that fails we grab the sleepq lock to keep the flags from
 436          * changing and do it the slow way.
 437          *
 438          * We have to lock the sleep queue if there are shared waiters
 439          * so we can wake them up.
 440          */
 441         x = sx->sx_lock;
 442         if (!(x & SX_LOCK_SHARED_WAITERS) &&
 443             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
 444             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
 445                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 446                 return;
 447         }
 448
 449         /*
 450          * Lock the sleep queue so we can read the waiters bits
 451          * without any races and wakeup any shared waiters.
 452          */
 453         sleepq_lock(&sx->lock_object);
 454
 455         /*
 456          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
 457          * shared lock.  If there are any shared waiters, wake them up.
 458          */
 459         wakeup_swapper = 0;
 460         x = sx->sx_lock;
 461         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
 462             (x & SX_LOCK_EXCLUSIVE_WAITERS));
 463         if (x & SX_LOCK_SHARED_WAITERS)
 464                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
 465                     0, SQ_SHARED_QUEUE);
 466         sleepq_release(&sx->lock_object);
 467
 468         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
 469         LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
 470
 471         if (wakeup_swapper)
 472                 kick_proc0();
 473 }
 474
 475 /*
 476  * This function represents the so-called 'hard case' for sx_xlock
 477  * operation.  All 'easy case' failures are redirected to this.  Note
 478  * that ideally this would be a static function, but it needs to be
 479  * accessible from at least sx.h.
 480  */
 481 int
 482 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
 483     int line)
 484 {
 485         GIANT_DECLARE;
 486 #ifdef ADAPTIVE_SX
 487         volatile struct thread *owner;
 488         u_int i, spintries = 0;
 489 #endif
 490         uintptr_t x;
 491 #ifdef LOCK_PROFILING
 492         uint64_t waittime = 0;
 493         int contested = 0;
 494 #endif
 495         int error = 0;
 496 #ifdef  KDTRACE_HOOKS
 497         uint64_t spin_cnt = 0;
 498         uint64_t sleep_cnt = 0;
 499         int64_t sleep_time = 0;
 500 #endif
 501
 502         if (SCHEDULER_STOPPED())
 503                 return (0);
 504
 505         /* If we already hold an exclusive lock, then recurse. */
 506         if (sx_xlocked(sx)) {
 507                 KASSERT((sx->lock_object.lo_flags & LO_RECURSABLE) != 0,
 508             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
 509                     sx->lock_object.lo_name, file, line));
 510                 sx->sx_recurse++;
 511                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 512                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 513                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
 514                 return (0);
 515         }
 516
 517         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 518                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 519                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
 520
 521         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
 522 #ifdef KDTRACE_HOOKS
 523                 spin_cnt++;
 524 #endif
 525 #ifdef HWPMC_HOOKS
 526                 PMC_SOFT_CALL( , , lock, failed);
 527 #endif
 528                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 529                     &waittime);
 530 #ifdef ADAPTIVE_SX
 531                 /*
 532                  * If the lock is write locked and the owner is
 533                  * running on another CPU, spin until the owner stops
 534                  * running or the state of the lock changes.
 535                  */
 536                 x = sx->sx_lock;
 537                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 538                         if ((x & SX_LOCK_SHARED) == 0) {
 539                                 x = SX_OWNER(x);
 540                                 owner = (struct thread *)x;
 541                                 if (TD_IS_RUNNING(owner)) {
 542                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 543                                                 CTR3(KTR_LOCK,
 544                                             "%s: spinning on %p held by %p",
 545                                                     __func__, sx, owner);
 546                                         GIANT_SAVE();
 547                                         while (SX_OWNER(sx->sx_lock) == x &&
 548                                             TD_IS_RUNNING(owner)) {
 549                                                 cpu_spinwait();
 550 #ifdef KDTRACE_HOOKS
 551                                                 spin_cnt++;
 552 #endif
 553                                         }
 554                                         continue;
 555                                 }
 556                         } else if (SX_SHARERS(x) && spintries < ASX_RETRIES) {
 557                                 GIANT_SAVE();
 558                                 spintries++;
 559                                 for (i = 0; i < ASX_LOOPS; i++) {
 560                                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 561                                                 CTR4(KTR_LOCK,
 562                                     "%s: shared spinning on %p with %u and %u",
 563                                                     __func__, sx, spintries, i);
 564                                         x = sx->sx_lock;
 565                                         if ((x & SX_LOCK_SHARED) == 0 ||
 566                                             SX_SHARERS(x) == 0)
 567                                                 break;
 568                                         cpu_spinwait();
 569 #ifdef KDTRACE_HOOKS
 570                                         spin_cnt++;
 571 #endif
 572                                 }
 573                                 if (i != ASX_LOOPS)
 574                                         continue;
 575                         }
 576                 }
 577 #endif
 578
 579                 sleepq_lock(&sx->lock_object);
 580                 x = sx->sx_lock;
 581
 582                 /*
 583                  * If the lock was released while spinning on the
 584                  * sleep queue chain lock, try again.
 585                  */
 586                 if (x == SX_LOCK_UNLOCKED) {
 587                         sleepq_release(&sx->lock_object);
 588                         continue;
 589                 }
 590
 591 #ifdef ADAPTIVE_SX
 592                 /*
 593                  * The current lock owner might have started executing
 594                  * on another CPU (or the lock could have changed
 595                  * owners) while we were waiting on the sleep queue
 596                  * chain lock.  If so, drop the sleep queue lock and try
 597                  * again.
 598                  */
 599                 if (!(x & SX_LOCK_SHARED) &&
 600                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 601                         owner = (struct thread *)SX_OWNER(x);
 602                         if (TD_IS_RUNNING(owner)) {
 603                                 sleepq_release(&sx->lock_object);
 604                                 continue;
 605                         }
 606                 }
 607 #endif
 608
 609                 /*
 610                  * If an exclusive lock was released with both shared
 611                  * and exclusive waiters and a shared waiter hasn't
 612                  * woken up and acquired the lock yet, sx_lock will be
 613                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
 614                  * If we see that value, try to acquire it once.  Note
 615                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
 616                  * as there are other exclusive waiters still.  If we
 617                  * fail, restart the loop.
 618                  */
 619                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
 620                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
 621                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
 622                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
 623                                 sleepq_release(&sx->lock_object);
 624                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
 625                                     __func__, sx);
 626                                 break;
 627                         }
 628                         sleepq_release(&sx->lock_object);
 629                         continue;
 630                 }
 631
 632                 /*
 633                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
 634                  * than loop back and retry.
 635                  */
 636                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 637                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 638                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
 639                                 sleepq_release(&sx->lock_object);
 640                                 continue;
 641                         }
 642                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 643                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
 644                                     __func__, sx);
 645                 }
 646
 647                 /*
 648                  * Since we have been unable to acquire the exclusive
 649                  * lock and the exclusive waiters flag is set, we have
 650                  * to sleep.
 651                  */
 652                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 653                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 654                             __func__, sx);
 655
 656 #ifdef KDTRACE_HOOKS
 657                 sleep_time -= lockstat_nsecs();
 658 #endif
 659                 GIANT_SAVE();
 660                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 661                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 662                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
 663                 if (!(opts & SX_INTERRUPTIBLE))
 664                         sleepq_wait(&sx->lock_object, 0);
 665                 else
 666                         error = sleepq_wait_sig(&sx->lock_object, 0);
 667 #ifdef KDTRACE_HOOKS
 668                 sleep_time += lockstat_nsecs();
 669                 sleep_cnt++;
 670 #endif
 671                 if (error) {
 672                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 673                                 CTR2(KTR_LOCK,
 674                         "%s: interruptible sleep by %p suspended by signal",
 675                                     __func__, sx);
 676                         break;
 677                 }
 678                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 679                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 680                             __func__, sx);
 681         }
 682
 683         GIANT_RESTORE();
 684         if (!error)
 685                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_XLOCK_ACQUIRE, sx,
 686                     contested, waittime, file, line);
 687 #ifdef KDTRACE_HOOKS
 688         if (sleep_time)
 689                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 690         if (spin_cnt > sleep_cnt)
 691                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 692 #endif
 693         return (error);
 694 }
 695
 696 /*
 697  * This function represents the so-called 'hard case' for sx_xunlock
 698  * operation.  All 'easy case' failures are redirected to this.  Note
 699  * that ideally this would be a static function, but it needs to be
 700  * accessible from at least sx.h.
 701  */
 702 void
 703 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
 704 {
 705         uintptr_t x;
 706         int queue, wakeup_swapper;
 707
 708         if (SCHEDULER_STOPPED())
 709                 return;
 710
 711         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
 712
 713         /* If the lock is recursed, then unrecurse one level. */
 714         if (sx_xlocked(sx) && sx_recursed(sx)) {
 715                 if ((--sx->sx_recurse) == 0)
 716                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
 717                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 718                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
 719                 return;
 720         }
 721         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
 722             SX_LOCK_EXCLUSIVE_WAITERS));
 723         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 724                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
 725
 726         sleepq_lock(&sx->lock_object);
 727         x = SX_LOCK_UNLOCKED;
 728
 729         /*
 730          * The wake up algorithm here is quite simple and probably not
 731          * ideal.  It gives precedence to shared waiters if they are
 732          * present.  For this condition, we have to preserve the
 733          * state of the exclusive waiters flag.
 734          * If interruptible sleeps left the shared queue empty avoid a
 735          * starvation for the threads sleeping on the exclusive queue by giving
 736          * them precedence and cleaning up the shared waiters bit anyway.
 737          */
 738         if ((sx->sx_lock & SX_LOCK_SHARED_WAITERS) != 0 &&
 739             sleepq_sleepcnt(&sx->lock_object, SQ_SHARED_QUEUE) != 0) {
 740                 queue = SQ_SHARED_QUEUE;
 741                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
 742         } else
 743                 queue = SQ_EXCLUSIVE_QUEUE;
 744
 745         /* Wake up all the waiters for the specific queue. */
 746         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 747                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
 748                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
 749                     "exclusive");
 750         atomic_store_rel_ptr(&sx->sx_lock, x);
 751         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, 0,
 752             queue);
 753         sleepq_release(&sx->lock_object);
 754         if (wakeup_swapper)
 755                 kick_proc0();
 756 }
 757
 758 /*
 759  * This function represents the so-called 'hard case' for sx_slock
 760  * operation.  All 'easy case' failures are redirected to this.  Note
 761  * that ideally this would be a static function, but it needs to be
 762  * accessible from at least sx.h.
 763  */
 764 int
 765 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
 766 {
 767         GIANT_DECLARE;
 768 #ifdef ADAPTIVE_SX
 769         volatile struct thread *owner;
 770 #endif
 771 #ifdef LOCK_PROFILING
 772         uint64_t waittime = 0;
 773         int contested = 0;
 774 #endif
 775         uintptr_t x;
 776         int error = 0;
 777 #ifdef KDTRACE_HOOKS
 778         uint64_t spin_cnt = 0;
 779         uint64_t sleep_cnt = 0;
 780         int64_t sleep_time = 0;
 781 #endif
 782
 783         if (SCHEDULER_STOPPED())
 784                 return (0);
 785
 786         /*
 787          * As with rwlocks, we don't make any attempt to try to block
 788          * shared locks once there is an exclusive waiter.
 789          */
 790         for (;;) {
 791 #ifdef KDTRACE_HOOKS
 792                 spin_cnt++;
 793 #endif
 794                 x = sx->sx_lock;
 795
 796                 /*
 797                  * If no other thread has an exclusive lock then try to bump up
 798                  * the count of sharers.  Since we have to preserve the state
 799                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
 800                  * shared lock loop back and retry.
 801                  */
 802                 if (x & SX_LOCK_SHARED) {
 803                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
 804                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
 805                             x + SX_ONE_SHARER)) {
 806                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 807                                         CTR4(KTR_LOCK,
 808                                             "%s: %p succeed %p -> %p", __func__,
 809                                             sx, (void *)x,
 810                                             (void *)(x + SX_ONE_SHARER));
 811                                 break;
 812                         }
 813                         continue;
 814                 }
 815 #ifdef HWPMC_HOOKS
 816                 PMC_SOFT_CALL( , , lock, failed);
 817 #endif
 818                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
 819                     &waittime);
 820
 821 #ifdef ADAPTIVE_SX
 822                 /*
 823                  * If the owner is running on another CPU, spin until
 824                  * the owner stops running or the state of the lock
 825                  * changes.
 826                  */
 827                 if ((sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 828                         x = SX_OWNER(x);
 829                         owner = (struct thread *)x;
 830                         if (TD_IS_RUNNING(owner)) {
 831                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 832                                         CTR3(KTR_LOCK,
 833                                             "%s: spinning on %p held by %p",
 834                                             __func__, sx, owner);
 835                                 GIANT_SAVE();
 836                                 while (SX_OWNER(sx->sx_lock) == x &&
 837                                     TD_IS_RUNNING(owner)) {
 838 #ifdef KDTRACE_HOOKS
 839                                         spin_cnt++;
 840 #endif
 841                                         cpu_spinwait();
 842                                 }
 843                                 continue;
 844                         }
 845                 }
 846 #endif
 847
 848                 /*
 849                  * Some other thread already has an exclusive lock, so
 850                  * start the process of blocking.
 851                  */
 852                 sleepq_lock(&sx->lock_object);
 853                 x = sx->sx_lock;
 854
 855                 /*
 856                  * The lock could have been released while we spun.
 857                  * In this case loop back and retry.
 858                  */
 859                 if (x & SX_LOCK_SHARED) {
 860                         sleepq_release(&sx->lock_object);
 861                         continue;
 862                 }
 863
 864 #ifdef ADAPTIVE_SX
 865                 /*
 866                  * If the owner is running on another CPU, spin until
 867                  * the owner stops running or the state of the lock
 868                  * changes.
 869                  */
 870                 if (!(x & SX_LOCK_SHARED) &&
 871                     (sx->lock_object.lo_flags & SX_NOADAPTIVE) == 0) {
 872                         owner = (struct thread *)SX_OWNER(x);
 873                         if (TD_IS_RUNNING(owner)) {
 874                                 sleepq_release(&sx->lock_object);
 875                                 continue;
 876                         }
 877                 }
 878 #endif
 879
 880                 /*
 881                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
 882                  * fail to set it drop the sleep queue lock and loop
 883                  * back.
 884                  */
 885                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
 886                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
 887                             x | SX_LOCK_SHARED_WAITERS)) {
 888                                 sleepq_release(&sx->lock_object);
 889                                 continue;
 890                         }
 891                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 892                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
 893                                     __func__, sx);
 894                 }
 895
 896                 /*
 897                  * Since we have been unable to acquire the shared lock,
 898                  * we have to sleep.
 899                  */
 900                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 901                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
 902                             __func__, sx);
 903
 904 #ifdef KDTRACE_HOOKS
 905                 sleep_time -= lockstat_nsecs();
 906 #endif
 907                 GIANT_SAVE();
 908                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
 909                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
 910                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
 911                 if (!(opts & SX_INTERRUPTIBLE))
 912                         sleepq_wait(&sx->lock_object, 0);
 913                 else
 914                         error = sleepq_wait_sig(&sx->lock_object, 0);
 915 #ifdef KDTRACE_HOOKS
 916                 sleep_time += lockstat_nsecs();
 917                 sleep_cnt++;
 918 #endif
 919                 if (error) {
 920                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
 921                                 CTR2(KTR_LOCK,
 922                         "%s: interruptible sleep by %p suspended by signal",
 923                                     __func__, sx);
 924                         break;
 925                 }
 926                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 927                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
 928                             __func__, sx);
 929         }
 930         if (error == 0)
 931                 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_SX_SLOCK_ACQUIRE, sx,
 932                     contested, waittime, file, line);
 933 #ifdef KDTRACE_HOOKS
 934         if (sleep_time)
 935                 LOCKSTAT_RECORD1(LS_SX_XLOCK_BLOCK, sx, sleep_time);
 936         if (spin_cnt > sleep_cnt)
 937                 LOCKSTAT_RECORD1(LS_SX_XLOCK_SPIN, sx, (spin_cnt - sleep_cnt));
 938 #endif
 939         GIANT_RESTORE();
 940         return (error);
 941 }
 942
 943 /*
 944  * This function represents the so-called 'hard case' for sx_sunlock
 945  * operation.  All 'easy case' failures are redirected to this.  Note
 946  * that ideally this would be a static function, but it needs to be
 947  * accessible from at least sx.h.
 948  */
 949 void
 950 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
 951 {
 952         uintptr_t x;
 953         int wakeup_swapper;
 954
 955         if (SCHEDULER_STOPPED())
 956                 return;
 957
 958         for (;;) {
 959                 x = sx->sx_lock;
 960
 961                 /*
 962                  * We should never have sharers while at least one thread
 963                  * holds a shared lock.
 964                  */
 965                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
 966                     ("%s: waiting sharers", __func__));
 967
 968                 /*
 969                  * See if there is more than one shared lock held.  If
 970                  * so, just drop one and return.
 971                  */
 972                 if (SX_SHARERS(x) > 1) {
 973                         if (atomic_cmpset_rel_ptr(&sx->sx_lock, x,
 974                             x - SX_ONE_SHARER)) {
 975                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 976                                         CTR4(KTR_LOCK,
 977                                             "%s: %p succeeded %p -> %p",
 978                                             __func__, sx, (void *)x,
 979                                             (void *)(x - SX_ONE_SHARER));
 980                                 break;
 981                         }
 982                         continue;
 983                 }
 984
 985                 /*
 986                  * If there aren't any waiters for an exclusive lock,
 987                  * then try to drop it quickly.
 988                  */
 989                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
 990                         MPASS(x == SX_SHARERS_LOCK(1));
 991                         if (atomic_cmpset_rel_ptr(&sx->sx_lock,
 992                             SX_SHARERS_LOCK(1), SX_LOCK_UNLOCKED)) {
 993                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
 994                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 995                                             __func__, sx);
 996                                 break;
 997                         }
 998                         continue;
 999                 }
1000
1001                 /*
1002                  * At this point, there should just be one sharer with
1003                  * exclusive waiters.
1004                  */
1005                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
1006
1007                 sleepq_lock(&sx->lock_object);
1008
1009                 /*
1010                  * Wake up semantic here is quite simple:
1011                  * Just wake up all the exclusive waiters.
1012                  * Note that the state of the lock could have changed,
1013                  * so if it fails loop back and retry.
1014                  */
1015                 if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
1016                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
1017                     SX_LOCK_UNLOCKED)) {
1018                         sleepq_release(&sx->lock_object);
1019                         continue;
1020                 }
1021                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
1022                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
1023                             "exclusive queue", __func__, sx);
1024                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
1025                     0, SQ_EXCLUSIVE_QUEUE);
1026                 sleepq_release(&sx->lock_object);
1027                 if (wakeup_swapper)
1028                         kick_proc0();
1029                 break;
1030         }
1031 }
1032
1033 #ifdef INVARIANT_SUPPORT
1034 #ifndef INVARIANTS
1035 #undef  _sx_assert
1036 #endif
1037
1038 /*
1039  * In the non-WITNESS case, sx_assert() can only detect that at least
1040  * *some* thread owns an slock, but it cannot guarantee that *this*
1041  * thread owns an slock.
1042  */
1043 void
1044 _sx_assert(struct sx *sx, int what, const char *file, int line)
1045 {
1046 #ifndef WITNESS
1047         int slocked = 0;
1048 #endif
1049
1050         if (panicstr != NULL)
1051                 return;
1052         switch (what) {
1053         case SA_SLOCKED:
1054         case SA_SLOCKED | SA_NOTRECURSED:
1055         case SA_SLOCKED | SA_RECURSED:
1056 #ifndef WITNESS
1057                 slocked = 1;
1058                 /* FALLTHROUGH */
1059 #endif
1060         case SA_LOCKED:
1061         case SA_LOCKED | SA_NOTRECURSED:
1062         case SA_LOCKED | SA_RECURSED:
1063 #ifdef WITNESS
1064                 witness_assert(&sx->lock_object, what, file, line);
1065 #else
1066                 /*
1067                  * If some other thread has an exclusive lock or we
1068                  * have one and are asserting a shared lock, fail.
1069                  * Also, if no one has a lock at all, fail.
1070                  */
1071                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
1072                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
1073                     sx_xholder(sx) != curthread)))
1074                         panic("Lock %s not %slocked @ %s:%d\n",
1075                             sx->lock_object.lo_name, slocked ? "share " : "",
1076                             file, line);
1077
1078                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
1079                         if (sx_recursed(sx)) {
1080                                 if (what & SA_NOTRECURSED)
1081                                         panic("Lock %s recursed @ %s:%d\n",
1082                                             sx->lock_object.lo_name, file,
1083                                             line);
1084                         } else if (what & SA_RECURSED)
1085                                 panic("Lock %s not recursed @ %s:%d\n",
1086                                     sx->lock_object.lo_name, file, line);
1087                 }
1088 #endif
1089                 break;
1090         case SA_XLOCKED:
1091         case SA_XLOCKED | SA_NOTRECURSED:
1092         case SA_XLOCKED | SA_RECURSED:
1093                 if (sx_xholder(sx) != curthread)
1094                         panic("Lock %s not exclusively locked @ %s:%d\n",
1095                             sx->lock_object.lo_name, file, line);
1096                 if (sx_recursed(sx)) {
1097                         if (what & SA_NOTRECURSED)
1098                                 panic("Lock %s recursed @ %s:%d\n",
1099                                     sx->lock_object.lo_name, file, line);
1100                 } else if (what & SA_RECURSED)
1101                         panic("Lock %s not recursed @ %s:%d\n",
1102                             sx->lock_object.lo_name, file, line);
1103                 break;
1104         case SA_UNLOCKED:
1105 #ifdef WITNESS
1106                 witness_assert(&sx->lock_object, what, file, line);
1107 #else
1108                 /*
1109                  * If we hold an exclusve lock fail.  We can't
1110                  * reliably check to see if we hold a shared lock or
1111                  * not.
1112                  */
1113                 if (sx_xholder(sx) == curthread)
1114                         panic("Lock %s exclusively locked @ %s:%d\n",
1115                             sx->lock_object.lo_name, file, line);
1116 #endif
1117                 break;
1118         default:
1119                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
1120                     line);
1121         }
1122 }
1123 #endif  /* INVARIANT_SUPPORT */
1124
1125 #ifdef DDB
1126 static void
1127 db_show_sx(struct lock_object *lock)
1128 {
1129         struct thread *td;
1130         struct sx *sx;
1131
1132         sx = (struct sx *)lock;
1133
1134         db_printf(" state: ");
1135         if (sx->sx_lock == SX_LOCK_UNLOCKED)
1136                 db_printf("UNLOCKED\n");
1137         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
1138                 db_printf("DESTROYED\n");
1139                 return;
1140         } else if (sx->sx_lock & SX_LOCK_SHARED)
1141                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
1142         else {
1143                 td = sx_xholder(sx);
1144                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1145                     td->td_tid, td->td_proc->p_pid, td->td_name);
1146                 if (sx_recursed(sx))
1147                         db_printf(" recursed: %d\n", sx->sx_recurse);
1148         }
1149
1150         db_printf(" waiters: ");
1151         switch(sx->sx_lock &
1152             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
1153         case SX_LOCK_SHARED_WAITERS:
1154                 db_printf("shared\n");
1155                 break;
1156         case SX_LOCK_EXCLUSIVE_WAITERS:
1157                 db_printf("exclusive\n");
1158                 break;
1159         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
1160                 db_printf("exclusive and shared\n");
1161                 break;
1162         default:
1163                 db_printf("none\n");
1164         }
1165 }
1166
1167 /*
1168  * Check to see if a thread that is blocked on a sleep queue is actually
1169  * blocked on an sx lock.  If so, output some details and return true.
1170  * If the lock has an exclusive owner, return that in *ownerp.
1171  */
1172 int
1173 sx_chain(struct thread *td, struct thread **ownerp)
1174 {
1175         struct sx *sx;
1176
1177         /*
1178          * Check to see if this thread is blocked on an sx lock.
1179          * First, we check the lock class.  If that is ok, then we
1180          * compare the lock name against the wait message.
1181          */
1182         sx = td->td_wchan;
1183         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
1184             sx->lock_object.lo_name != td->td_wmesg)
1185                 return (0);
1186
1187         /* We think we have an sx lock, so output some details. */
1188         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
1189         *ownerp = sx_xholder(sx);
1190         if (sx->sx_lock & SX_LOCK_SHARED)
1191                 db_printf("SLOCK (count %ju)\n",
1192                     (uintmax_t)SX_SHARERS(sx->sx_lock));
1193         else
1194                 db_printf("XLOCK\n");
1195         return (1);
1196 }
1197 #endif