sys/kern/kern_rwlock.c

   1 /*-
   2  * Copyright (c) 2006 John Baldwin <jhb@FreeBSD.org>
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  *
  14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  15  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  16  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  17  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  18  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  19  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  20  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  21  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  22  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  23  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  24  * SUCH DAMAGE.
  25  */
  26
  27 /*
  28  * Machine independent bits of reader/writer lock implementation.
  29  */
  30
  31 #include <sys/cdefs.h>
  32 __FBSDID("$FreeBSD$");
  33
  34 #include "opt_ddb.h"
  35 #include "opt_hwpmc_hooks.h"
  36 #include "opt_kdtrace.h"
  37 #include "opt_no_adaptive_rwlocks.h"
  38
  39 #include <sys/param.h>
  40 #include <sys/kdb.h>
  41 #include <sys/ktr.h>
  42 #include <sys/kernel.h>
  43 #include <sys/lock.h>
  44 #include <sys/mutex.h>
  45 #include <sys/proc.h>
  46 #include <sys/rwlock.h>
  47 #include <sys/sysctl.h>
  48 #include <sys/systm.h>
  49 #include <sys/turnstile.h>
  50
  51 #include <machine/cpu.h>
  52
  53 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS)
  54 #define ADAPTIVE_RWLOCKS
  55 #endif
  56
  57 #ifdef HWPMC_HOOKS
  58 #include <sys/pmckern.h>
  59 PMC_SOFT_DECLARE( , , lock, failed);
  60 #endif
  61
  62 #ifdef ADAPTIVE_RWLOCKS
  63 #define ROWNER_RETRIES  10
  64 #define ROWNER_LOOPS    10000
  65 #endif
  66
  67 #ifdef DDB
  68 #include <ddb/ddb.h>
  69
  70 static void     db_show_rwlock(struct lock_object *lock);
  71 #endif
  72 static void     assert_rw(struct lock_object *lock, int what);
  73 static void     lock_rw(struct lock_object *lock, int how);
  74 #ifdef KDTRACE_HOOKS
  75 static int      owner_rw(struct lock_object *lock, struct thread **owner);
  76 #endif
  77 static int      unlock_rw(struct lock_object *lock);
  78
  79 struct lock_class lock_class_rw = {
  80         .lc_name = "rw",
  81         .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
  82         .lc_assert = assert_rw,
  83 #ifdef DDB
  84         .lc_ddb_show = db_show_rwlock,
  85 #endif
  86         .lc_lock = lock_rw,
  87         .lc_unlock = unlock_rw,
  88 #ifdef KDTRACE_HOOKS
  89         .lc_owner = owner_rw,
  90 #endif
  91 };
  92
  93 /*
  94  * Return a pointer to the owning thread if the lock is write-locked or
  95  * NULL if the lock is unlocked or read-locked.
  96  */
  97 #define rw_wowner(rw)                                                   \
  98         ((rw)->rw_lock & RW_LOCK_READ ? NULL :                          \
  99             (struct thread *)RW_OWNER((rw)->rw_lock))
 100
 101 /*
 102  * Returns if a write owner is recursed.  Write ownership is not assured
 103  * here and should be previously checked.
 104  */
 105 #define rw_recursed(rw)         ((rw)->rw_recurse != 0)
 106
 107 /*
 108  * Return true if curthread helds the lock.
 109  */
 110 #define rw_wlocked(rw)          (rw_wowner((rw)) == curthread)
 111
 112 /*
 113  * Return a pointer to the owning thread for this lock who should receive
 114  * any priority lent by threads that block on this lock.  Currently this
 115  * is identical to rw_wowner().
 116  */
 117 #define rw_owner(rw)            rw_wowner(rw)
 118
 119 #ifndef INVARIANTS
 120 #define _rw_assert(rw, what, file, line)
 121 #endif
 122
 123 void
 124 assert_rw(struct lock_object *lock, int what)
 125 {
 126
 127         rw_assert((struct rwlock *)lock, what);
 128 }
 129
 130 void
 131 lock_rw(struct lock_object *lock, int how)
 132 {
 133         struct rwlock *rw;
 134
 135         rw = (struct rwlock *)lock;
 136         if (how)
 137                 rw_wlock(rw);
 138         else
 139                 rw_rlock(rw);
 140 }
 141
 142 int
 143 unlock_rw(struct lock_object *lock)
 144 {
 145         struct rwlock *rw;
 146
 147         rw = (struct rwlock *)lock;
 148         rw_assert(rw, RA_LOCKED | LA_NOTRECURSED);
 149         if (rw->rw_lock & RW_LOCK_READ) {
 150                 rw_runlock(rw);
 151                 return (0);
 152         } else {
 153                 rw_wunlock(rw);
 154                 return (1);
 155         }
 156 }
 157
 158 #ifdef KDTRACE_HOOKS
 159 int
 160 owner_rw(struct lock_object *lock, struct thread **owner)
 161 {
 162         struct rwlock *rw = (struct rwlock *)lock;
 163         uintptr_t x = rw->rw_lock;
 164
 165         *owner = rw_wowner(rw);
 166         return ((x & RW_LOCK_READ) != 0 ?  (RW_READERS(x) != 0) :
 167             (*owner != NULL));
 168 }
 169 #endif
 170
 171 void
 172 rw_init_flags(struct rwlock *rw, const char *name, int opts)
 173 {
 174         int flags;
 175
 176         MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
 177             RW_RECURSE)) == 0);
 178         ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock,
 179             ("%s: rw_lock not aligned for %s: %p", __func__, name,
 180             &rw->rw_lock));
 181
 182         flags = LO_UPGRADABLE;
 183         if (opts & RW_DUPOK)
 184                 flags |= LO_DUPOK;
 185         if (opts & RW_NOPROFILE)
 186                 flags |= LO_NOPROFILE;
 187         if (!(opts & RW_NOWITNESS))
 188                 flags |= LO_WITNESS;
 189         if (opts & RW_RECURSE)
 190                 flags |= LO_RECURSABLE;
 191         if (opts & RW_QUIET)
 192                 flags |= LO_QUIET;
 193
 194         rw->rw_lock = RW_UNLOCKED;
 195         rw->rw_recurse = 0;
 196         lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
 197 }
 198
 199 void
 200 rw_destroy(struct rwlock *rw)
 201 {
 202
 203         KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw));
 204         KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw));
 205         rw->rw_lock = RW_DESTROYED;
 206         lock_destroy(&rw->lock_object);
 207 }
 208
 209 void
 210 rw_sysinit(void *arg)
 211 {
 212         struct rw_args *args = arg;
 213
 214         rw_init(args->ra_rw, args->ra_desc);
 215 }
 216
 217 void
 218 rw_sysinit_flags(void *arg)
 219 {
 220         struct rw_args_flags *args = arg;
 221
 222         rw_init_flags(args->ra_rw, args->ra_desc, args->ra_flags);
 223 }
 224
 225 int
 226 rw_wowned(struct rwlock *rw)
 227 {
 228
 229         return (rw_wowner(rw) == curthread);
 230 }
 231
 232 void
 233 _rw_wlock(struct rwlock *rw, const char *file, int line)
 234 {
 235
 236         if (SCHEDULER_STOPPED())
 237                 return;
 238         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 239             ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d",
 240             curthread, rw->lock_object.lo_name, file, line));
 241         KASSERT(rw->rw_lock != RW_DESTROYED,
 242             ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
 243         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 244             line, NULL);
 245         __rw_wlock(rw, curthread, file, line);
 246         LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
 247         WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 248         curthread->td_locks++;
 249 }
 250
 251 int
 252 _rw_try_wlock(struct rwlock *rw, const char *file, int line)
 253 {
 254         int rval;
 255
 256         if (SCHEDULER_STOPPED())
 257                 return (1);
 258
 259         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 260             ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d",
 261             curthread, rw->lock_object.lo_name, file, line));
 262         KASSERT(rw->rw_lock != RW_DESTROYED,
 263             ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
 264
 265         if (rw_wlocked(rw) &&
 266             (rw->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 267                 rw->rw_recurse++;
 268                 rval = 1;
 269         } else
 270                 rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED,
 271                     (uintptr_t)curthread);
 272
 273         LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
 274         if (rval) {
 275                 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 276                     file, line);
 277                 if (!rw_recursed(rw))
 278                         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_WLOCK_ACQUIRE,
 279                             rw, 0, 0, file, line);
 280                 curthread->td_locks++;
 281         }
 282         return (rval);
 283 }
 284
 285 void
 286 _rw_wunlock(struct rwlock *rw, const char *file, int line)
 287 {
 288
 289         if (SCHEDULER_STOPPED())
 290                 return;
 291         KASSERT(rw->rw_lock != RW_DESTROYED,
 292             ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
 293         _rw_assert(rw, RA_WLOCKED, file, line);
 294         curthread->td_locks--;
 295         WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 296         LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
 297             line);
 298         if (!rw_recursed(rw))
 299                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_WUNLOCK_RELEASE, rw);
 300         __rw_wunlock(rw, curthread, file, line);
 301 }
 302 /*
 303  * Determines whether a new reader can acquire a lock.  Succeeds if the
 304  * reader already owns a read lock and the lock is locked for read to
 305  * prevent deadlock from reader recursion.  Also succeeds if the lock
 306  * is unlocked and has no writer waiters or spinners.  Failing otherwise
 307  * prioritizes writers before readers.
 308  */
 309 #define RW_CAN_READ(_rw)                                                \
 310     ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) &      \
 311     (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) ==  \
 312     RW_LOCK_READ)
 313
 314 void
 315 _rw_rlock(struct rwlock *rw, const char *file, int line)
 316 {
 317         struct turnstile *ts;
 318 #ifdef ADAPTIVE_RWLOCKS
 319         volatile struct thread *owner;
 320         int spintries = 0;
 321         int i;
 322 #endif
 323 #ifdef LOCK_PROFILING
 324         uint64_t waittime = 0;
 325         int contested = 0;
 326 #endif
 327         uintptr_t v;
 328 #ifdef KDTRACE_HOOKS
 329         uintptr_t state;
 330         uint64_t spin_cnt = 0;
 331         uint64_t sleep_cnt = 0;
 332         int64_t sleep_time = 0;
 333         int64_t all_time = 0;
 334 #endif
 335
 336         if (SCHEDULER_STOPPED())
 337                 return;
 338
 339         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 340             ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d",
 341             curthread, rw->lock_object.lo_name, file, line));
 342         KASSERT(rw->rw_lock != RW_DESTROYED,
 343             ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
 344         KASSERT(rw_wowner(rw) != curthread,
 345             ("rw_rlock: wlock already held for %s @ %s:%d",
 346             rw->lock_object.lo_name, file, line));
 347         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL);
 348
 349 #ifdef KDTRACE_HOOKS
 350         all_time -= lockstat_nsecs();
 351         state = rw->rw_lock;
 352 #endif
 353         for (;;) {
 354 #ifdef KDTRACE_HOOKS
 355                 spin_cnt++;
 356 #endif
 357                 /*
 358                  * Handle the easy case.  If no other thread has a write
 359                  * lock, then try to bump up the count of read locks.  Note
 360                  * that we have to preserve the current state of the
 361                  * RW_LOCK_WRITE_WAITERS flag.  If we fail to acquire a
 362                  * read lock, then rw_lock must have changed, so restart
 363                  * the loop.  Note that this handles the case of a
 364                  * completely unlocked rwlock since such a lock is encoded
 365                  * as a read lock with no waiters.
 366                  */
 367                 v = rw->rw_lock;
 368                 if (RW_CAN_READ(v)) {
 369                         /*
 370                          * The RW_LOCK_READ_WAITERS flag should only be set
 371                          * if the lock has been unlocked and write waiters
 372                          * were present.
 373                          */
 374                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v,
 375                             v + RW_ONE_READER)) {
 376                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 377                                         CTR4(KTR_LOCK,
 378                                             "%s: %p succeed %p -> %p", __func__,
 379                                             rw, (void *)v,
 380                                             (void *)(v + RW_ONE_READER));
 381                                 break;
 382                         }
 383                         continue;
 384                 }
 385 #ifdef HWPMC_HOOKS
 386                 PMC_SOFT_CALL( , , lock, failed);
 387 #endif
 388                 lock_profile_obtain_lock_failed(&rw->lock_object,
 389                     &contested, &waittime);
 390
 391 #ifdef ADAPTIVE_RWLOCKS
 392                 /*
 393                  * If the owner is running on another CPU, spin until
 394                  * the owner stops running or the state of the lock
 395                  * changes.
 396                  */
 397                 if ((v & RW_LOCK_READ) == 0) {
 398                         owner = (struct thread *)RW_OWNER(v);
 399                         if (TD_IS_RUNNING(owner)) {
 400                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 401                                         CTR3(KTR_LOCK,
 402                                             "%s: spinning on %p held by %p",
 403                                             __func__, rw, owner);
 404                                 while ((struct thread*)RW_OWNER(rw->rw_lock) ==
 405                                     owner && TD_IS_RUNNING(owner)) {
 406                                         cpu_spinwait();
 407 #ifdef KDTRACE_HOOKS
 408                                         spin_cnt++;
 409 #endif
 410                                 }
 411                                 continue;
 412                         }
 413                 } else if (spintries < ROWNER_RETRIES) {
 414                         spintries++;
 415                         for (i = 0; i < ROWNER_LOOPS; i++) {
 416                                 v = rw->rw_lock;
 417                                 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v))
 418                                         break;
 419                                 cpu_spinwait();
 420                         }
 421                         if (i != ROWNER_LOOPS)
 422                                 continue;
 423                 }
 424 #endif
 425
 426                 /*
 427                  * Okay, now it's the hard case.  Some other thread already
 428                  * has a write lock or there are write waiters present,
 429                  * acquire the turnstile lock so we can begin the process
 430                  * of blocking.
 431                  */
 432                 ts = turnstile_trywait(&rw->lock_object);
 433
 434                 /*
 435                  * The lock might have been released while we spun, so
 436                  * recheck its state and restart the loop if needed.
 437                  */
 438                 v = rw->rw_lock;
 439                 if (RW_CAN_READ(v)) {
 440                         turnstile_cancel(ts);
 441                         continue;
 442                 }
 443
 444 #ifdef ADAPTIVE_RWLOCKS
 445                 /*
 446                  * The current lock owner might have started executing
 447                  * on another CPU (or the lock could have changed
 448                  * owners) while we were waiting on the turnstile
 449                  * chain lock.  If so, drop the turnstile lock and try
 450                  * again.
 451                  */
 452                 if ((v & RW_LOCK_READ) == 0) {
 453                         owner = (struct thread *)RW_OWNER(v);
 454                         if (TD_IS_RUNNING(owner)) {
 455                                 turnstile_cancel(ts);
 456                                 continue;
 457                         }
 458                 }
 459 #endif
 460
 461                 /*
 462                  * The lock is held in write mode or it already has waiters.
 463                  */
 464                 MPASS(!RW_CAN_READ(v));
 465
 466                 /*
 467                  * If the RW_LOCK_READ_WAITERS flag is already set, then
 468                  * we can go ahead and block.  If it is not set then try
 469                  * to set it.  If we fail to set it drop the turnstile
 470                  * lock and restart the loop.
 471                  */
 472                 if (!(v & RW_LOCK_READ_WAITERS)) {
 473                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 474                             v | RW_LOCK_READ_WAITERS)) {
 475                                 turnstile_cancel(ts);
 476                                 continue;
 477                         }
 478                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 479                                 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
 480                                     __func__, rw);
 481                 }
 482
 483                 /*
 484                  * We were unable to acquire the lock and the read waiters
 485                  * flag is set, so we must block on the turnstile.
 486                  */
 487                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 488                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 489                             rw);
 490 #ifdef KDTRACE_HOOKS
 491                 sleep_time -= lockstat_nsecs();
 492 #endif
 493                 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE);
 494 #ifdef KDTRACE_HOOKS
 495                 sleep_time += lockstat_nsecs();
 496                 sleep_cnt++;
 497 #endif
 498                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 499                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 500                             __func__, rw);
 501         }
 502 #ifdef KDTRACE_HOOKS
 503         all_time += lockstat_nsecs();
 504         if (sleep_time)
 505                 LOCKSTAT_RECORD4(LS_RW_RLOCK_BLOCK, rw, sleep_time,
 506                     LOCKSTAT_READER, (state & RW_LOCK_READ) == 0,
 507                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 508
 509         /* Record only the loops spinning and not sleeping. */
 510         if (spin_cnt > sleep_cnt)
 511                 LOCKSTAT_RECORD4(LS_RW_RLOCK_SPIN, rw, all_time - sleep_time,
 512                     LOCKSTAT_READER, (state & RW_LOCK_READ) == 0,
 513                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 514 #endif
 515         /*
 516          * TODO: acquire "owner of record" here.  Here be turnstile dragons
 517          * however.  turnstiles don't like owners changing between calls to
 518          * turnstile_wait() currently.
 519          */
 520         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_RLOCK_ACQUIRE, rw, contested,
 521             waittime, file, line);
 522         LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
 523         WITNESS_LOCK(&rw->lock_object, 0, file, line);
 524         curthread->td_locks++;
 525         curthread->td_rw_rlocks++;
 526 }
 527
 528 int
 529 _rw_try_rlock(struct rwlock *rw, const char *file, int line)
 530 {
 531         uintptr_t x;
 532
 533         if (SCHEDULER_STOPPED())
 534                 return (1);
 535
 536         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 537             ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d",
 538             curthread, rw->lock_object.lo_name, file, line));
 539
 540         for (;;) {
 541                 x = rw->rw_lock;
 542                 KASSERT(rw->rw_lock != RW_DESTROYED,
 543                     ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
 544                 if (!(x & RW_LOCK_READ))
 545                         break;
 546                 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) {
 547                         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
 548                             line);
 549                         WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
 550                         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_RLOCK_ACQUIRE,
 551                             rw, 0, 0, file, line);
 552                         curthread->td_locks++;
 553                         curthread->td_rw_rlocks++;
 554                         return (1);
 555                 }
 556         }
 557
 558         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
 559         return (0);
 560 }
 561
 562 void
 563 _rw_runlock(struct rwlock *rw, const char *file, int line)
 564 {
 565         struct turnstile *ts;
 566         uintptr_t x, v, queue;
 567
 568         if (SCHEDULER_STOPPED())
 569                 return;
 570
 571         KASSERT(rw->rw_lock != RW_DESTROYED,
 572             ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
 573         _rw_assert(rw, RA_RLOCKED, file, line);
 574         curthread->td_locks--;
 575         curthread->td_rw_rlocks--;
 576         WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
 577         LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
 578
 579         /* TODO: drop "owner of record" here. */
 580
 581         for (;;) {
 582                 /*
 583                  * See if there is more than one read lock held.  If so,
 584                  * just drop one and return.
 585                  */
 586                 x = rw->rw_lock;
 587                 if (RW_READERS(x) > 1) {
 588                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
 589                             x - RW_ONE_READER)) {
 590                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 591                                         CTR4(KTR_LOCK,
 592                                             "%s: %p succeeded %p -> %p",
 593                                             __func__, rw, (void *)x,
 594                                             (void *)(x - RW_ONE_READER));
 595                                 break;
 596                         }
 597                         continue;
 598                 }
 599                 /*
 600                  * If there aren't any waiters for a write lock, then try
 601                  * to drop it quickly.
 602                  */
 603                 if (!(x & RW_LOCK_WAITERS)) {
 604                         MPASS((x & ~RW_LOCK_WRITE_SPINNER) ==
 605                             RW_READERS_LOCK(1));
 606                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
 607                             RW_UNLOCKED)) {
 608                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 609                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 610                                             __func__, rw);
 611                                 break;
 612                         }
 613                         continue;
 614                 }
 615                 /*
 616                  * Ok, we know we have waiters and we think we are the
 617                  * last reader, so grab the turnstile lock.
 618                  */
 619                 turnstile_chain_lock(&rw->lock_object);
 620                 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 621                 MPASS(v & RW_LOCK_WAITERS);
 622
 623                 /*
 624                  * Try to drop our lock leaving the lock in a unlocked
 625                  * state.
 626                  *
 627                  * If you wanted to do explicit lock handoff you'd have to
 628                  * do it here.  You'd also want to use turnstile_signal()
 629                  * and you'd have to handle the race where a higher
 630                  * priority thread blocks on the write lock before the
 631                  * thread you wakeup actually runs and have the new thread
 632                  * "steal" the lock.  For now it's a lot simpler to just
 633                  * wakeup all of the waiters.
 634                  *
 635                  * As above, if we fail, then another thread might have
 636                  * acquired a read lock, so drop the turnstile lock and
 637                  * restart.
 638                  */
 639                 x = RW_UNLOCKED;
 640                 if (v & RW_LOCK_WRITE_WAITERS) {
 641                         queue = TS_EXCLUSIVE_QUEUE;
 642                         x |= (v & RW_LOCK_READ_WAITERS);
 643                 } else
 644                         queue = TS_SHARED_QUEUE;
 645                 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
 646                     x)) {
 647                         turnstile_chain_unlock(&rw->lock_object);
 648                         continue;
 649                 }
 650                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 651                         CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
 652                             __func__, rw);
 653
 654                 /*
 655                  * Ok.  The lock is released and all that's left is to
 656                  * wake up the waiters.  Note that the lock might not be
 657                  * free anymore, but in that case the writers will just
 658                  * block again if they run before the new lock holder(s)
 659                  * release the lock.
 660                  */
 661                 ts = turnstile_lookup(&rw->lock_object);
 662                 MPASS(ts != NULL);
 663                 turnstile_broadcast(ts, queue);
 664                 turnstile_unpend(ts, TS_SHARED_LOCK);
 665                 turnstile_chain_unlock(&rw->lock_object);
 666                 break;
 667         }
 668         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_RUNLOCK_RELEASE, rw);
 669 }
 670
 671 /*
 672  * This function is called when we are unable to obtain a write lock on the
 673  * first try.  This means that at least one other thread holds either a
 674  * read or write lock.
 675  */
 676 void
 677 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
 678 {
 679         struct turnstile *ts;
 680 #ifdef ADAPTIVE_RWLOCKS
 681         volatile struct thread *owner;
 682         int spintries = 0;
 683         int i;
 684 #endif
 685         uintptr_t v, x;
 686 #ifdef LOCK_PROFILING
 687         uint64_t waittime = 0;
 688         int contested = 0;
 689 #endif
 690 #ifdef KDTRACE_HOOKS
 691         uintptr_t state;
 692         uint64_t spin_cnt = 0;
 693         uint64_t sleep_cnt = 0;
 694         int64_t sleep_time = 0;
 695         int64_t all_time = 0;
 696 #endif
 697
 698         if (SCHEDULER_STOPPED())
 699                 return;
 700
 701         if (rw_wlocked(rw)) {
 702                 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE,
 703                     ("%s: recursing but non-recursive rw %s @ %s:%d\n",
 704                     __func__, rw->lock_object.lo_name, file, line));
 705                 rw->rw_recurse++;
 706                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 707                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
 708                 return;
 709         }
 710
 711         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 712                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 713                     rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
 714
 715 #ifdef KDTRACE_HOOKS
 716         all_time -= lockstat_nsecs();
 717         state = rw->rw_lock;
 718 #endif
 719         while (!_rw_write_lock(rw, tid)) {
 720 #ifdef KDTRACE_HOOKS
 721                 spin_cnt++;
 722 #endif
 723 #ifdef HWPMC_HOOKS
 724                 PMC_SOFT_CALL( , , lock, failed);
 725 #endif
 726                 lock_profile_obtain_lock_failed(&rw->lock_object,
 727                     &contested, &waittime);
 728 #ifdef ADAPTIVE_RWLOCKS
 729                 /*
 730                  * If the lock is write locked and the owner is
 731                  * running on another CPU, spin until the owner stops
 732                  * running or the state of the lock changes.
 733                  */
 734                 v = rw->rw_lock;
 735                 owner = (struct thread *)RW_OWNER(v);
 736                 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
 737                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 738                                 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
 739                                     __func__, rw, owner);
 740                         while ((struct thread*)RW_OWNER(rw->rw_lock) == owner &&
 741                             TD_IS_RUNNING(owner)) {
 742                                 cpu_spinwait();
 743 #ifdef KDTRACE_HOOKS
 744                                 spin_cnt++;
 745 #endif
 746                         }
 747                         continue;
 748                 }
 749                 if ((v & RW_LOCK_READ) && RW_READERS(v) &&
 750                     spintries < ROWNER_RETRIES) {
 751                         if (!(v & RW_LOCK_WRITE_SPINNER)) {
 752                                 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 753                                     v | RW_LOCK_WRITE_SPINNER)) {
 754                                         continue;
 755                                 }
 756                         }
 757                         spintries++;
 758                         for (i = 0; i < ROWNER_LOOPS; i++) {
 759                                 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
 760                                         break;
 761                                 cpu_spinwait();
 762                         }
 763 #ifdef KDTRACE_HOOKS
 764                         spin_cnt += ROWNER_LOOPS - i;
 765 #endif
 766                         if (i != ROWNER_LOOPS)
 767                                 continue;
 768                 }
 769 #endif
 770                 ts = turnstile_trywait(&rw->lock_object);
 771                 v = rw->rw_lock;
 772
 773 #ifdef ADAPTIVE_RWLOCKS
 774                 /*
 775                  * The current lock owner might have started executing
 776                  * on another CPU (or the lock could have changed
 777                  * owners) while we were waiting on the turnstile
 778                  * chain lock.  If so, drop the turnstile lock and try
 779                  * again.
 780                  */
 781                 if (!(v & RW_LOCK_READ)) {
 782                         owner = (struct thread *)RW_OWNER(v);
 783                         if (TD_IS_RUNNING(owner)) {
 784                                 turnstile_cancel(ts);
 785                                 continue;
 786                         }
 787                 }
 788 #endif
 789                 /*
 790                  * Check for the waiters flags about this rwlock.
 791                  * If the lock was released, without maintain any pending
 792                  * waiters queue, simply try to acquire it.
 793                  * If a pending waiters queue is present, claim the lock
 794                  * ownership and maintain the pending queue.
 795                  */
 796                 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 797                 if ((v & ~x) == RW_UNLOCKED) {
 798                         x &= ~RW_LOCK_WRITE_SPINNER;
 799                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) {
 800                                 if (x)
 801                                         turnstile_claim(ts);
 802                                 else
 803                                         turnstile_cancel(ts);
 804                                 break;
 805                         }
 806                         turnstile_cancel(ts);
 807                         continue;
 808                 }
 809                 /*
 810                  * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
 811                  * set it.  If we fail to set it, then loop back and try
 812                  * again.
 813                  */
 814                 if (!(v & RW_LOCK_WRITE_WAITERS)) {
 815                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 816                             v | RW_LOCK_WRITE_WAITERS)) {
 817                                 turnstile_cancel(ts);
 818                                 continue;
 819                         }
 820                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 821                                 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
 822                                     __func__, rw);
 823                 }
 824                 /*
 825                  * We were unable to acquire the lock and the write waiters
 826                  * flag is set, so we must block on the turnstile.
 827                  */
 828                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 829                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 830                             rw);
 831 #ifdef KDTRACE_HOOKS
 832                 sleep_time -= lockstat_nsecs();
 833 #endif
 834                 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
 835 #ifdef KDTRACE_HOOKS
 836                 sleep_time += lockstat_nsecs();
 837                 sleep_cnt++;
 838 #endif
 839                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 840                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 841                             __func__, rw);
 842 #ifdef ADAPTIVE_RWLOCKS
 843                 spintries = 0;
 844 #endif
 845         }
 846 #ifdef KDTRACE_HOOKS
 847         all_time += lockstat_nsecs();
 848         if (sleep_time)
 849                 LOCKSTAT_RECORD4(LS_RW_WLOCK_BLOCK, rw, sleep_time,
 850                     LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0,
 851                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 852
 853         /* Record only the loops spinning and not sleeping. */
 854         if (spin_cnt > sleep_cnt)
 855                 LOCKSTAT_RECORD4(LS_RW_WLOCK_SPIN, rw, all_time - sleep_time,
 856                     LOCKSTAT_READER, (state & RW_LOCK_READ) == 0,
 857                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 858 #endif
 859         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_WLOCK_ACQUIRE, rw, contested,
 860             waittime, file, line);
 861 }
 862
 863 /*
 864  * This function is called if the first try at releasing a write lock failed.
 865  * This means that one of the 2 waiter bits must be set indicating that at
 866  * least one thread is waiting on this lock.
 867  */
 868 void
 869 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
 870 {
 871         struct turnstile *ts;
 872         uintptr_t v;
 873         int queue;
 874
 875         if (SCHEDULER_STOPPED())
 876                 return;
 877
 878         if (rw_wlocked(rw) && rw_recursed(rw)) {
 879                 rw->rw_recurse--;
 880                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 881                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
 882                 return;
 883         }
 884
 885         KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
 886             ("%s: neither of the waiter flags are set", __func__));
 887
 888         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 889                 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
 890
 891         turnstile_chain_lock(&rw->lock_object);
 892         ts = turnstile_lookup(&rw->lock_object);
 893         MPASS(ts != NULL);
 894
 895         /*
 896          * Use the same algo as sx locks for now.  Prefer waking up shared
 897          * waiters if we have any over writers.  This is probably not ideal.
 898          *
 899          * 'v' is the value we are going to write back to rw_lock.  If we
 900          * have waiters on both queues, we need to preserve the state of
 901          * the waiter flag for the queue we don't wake up.  For now this is
 902          * hardcoded for the algorithm mentioned above.
 903          *
 904          * In the case of both readers and writers waiting we wakeup the
 905          * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
 906          * new writer comes in before a reader it will claim the lock up
 907          * above.  There is probably a potential priority inversion in
 908          * there that could be worked around either by waking both queues
 909          * of waiters or doing some complicated lock handoff gymnastics.
 910          */
 911         v = RW_UNLOCKED;
 912         if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
 913                 queue = TS_EXCLUSIVE_QUEUE;
 914                 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
 915         } else
 916                 queue = TS_SHARED_QUEUE;
 917
 918         /* Wake up all waiters for the specific queue. */
 919         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 920                 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
 921                     queue == TS_SHARED_QUEUE ? "read" : "write");
 922         turnstile_broadcast(ts, queue);
 923         atomic_store_rel_ptr(&rw->rw_lock, v);
 924         turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
 925         turnstile_chain_unlock(&rw->lock_object);
 926 }
 927
 928 /*
 929  * Attempt to do a non-blocking upgrade from a read lock to a write
 930  * lock.  This will only succeed if this thread holds a single read
 931  * lock.  Returns true if the upgrade succeeded and false otherwise.
 932  */
 933 int
 934 _rw_try_upgrade(struct rwlock *rw, const char *file, int line)
 935 {
 936         uintptr_t v, x, tid;
 937         struct turnstile *ts;
 938         int success;
 939
 940         if (SCHEDULER_STOPPED())
 941                 return (1);
 942
 943         KASSERT(rw->rw_lock != RW_DESTROYED,
 944             ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
 945         _rw_assert(rw, RA_RLOCKED, file, line);
 946
 947         /*
 948          * Attempt to switch from one reader to a writer.  If there
 949          * are any write waiters, then we will have to lock the
 950          * turnstile first to prevent races with another writer
 951          * calling turnstile_wait() before we have claimed this
 952          * turnstile.  So, do the simple case of no waiters first.
 953          */
 954         tid = (uintptr_t)curthread;
 955         success = 0;
 956         for (;;) {
 957                 v = rw->rw_lock;
 958                 if (RW_READERS(v) > 1)
 959                         break;
 960                 if (!(v & RW_LOCK_WAITERS)) {
 961                         success = atomic_cmpset_ptr(&rw->rw_lock, v, tid);
 962                         if (!success)
 963                                 continue;
 964                         break;
 965                 }
 966
 967                 /*
 968                  * Ok, we think we have waiters, so lock the turnstile.
 969                  */
 970                 ts = turnstile_trywait(&rw->lock_object);
 971                 v = rw->rw_lock;
 972                 if (RW_READERS(v) > 1) {
 973                         turnstile_cancel(ts);
 974                         break;
 975                 }
 976                 /*
 977                  * Try to switch from one reader to a writer again.  This time
 978                  * we honor the current state of the waiters flags.
 979                  * If we obtain the lock with the flags set, then claim
 980                  * ownership of the turnstile.
 981                  */
 982                 x = rw->rw_lock & RW_LOCK_WAITERS;
 983                 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
 984                 if (success) {
 985                         if (x)
 986                                 turnstile_claim(ts);
 987                         else
 988                                 turnstile_cancel(ts);
 989                         break;
 990                 }
 991                 turnstile_cancel(ts);
 992         }
 993         LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
 994         if (success) {
 995                 curthread->td_rw_rlocks--;
 996                 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 997                     file, line);
 998                 LOCKSTAT_RECORD0(LS_RW_TRYUPGRADE_UPGRADE, rw);
 999         }
1000         return (success);
1001 }
1002
1003 /*
1004  * Downgrade a write lock into a single read lock.
1005  */
1006 void
1007 _rw_downgrade(struct rwlock *rw, const char *file, int line)
1008 {
1009         struct turnstile *ts;
1010         uintptr_t tid, v;
1011         int rwait, wwait;
1012
1013         if (SCHEDULER_STOPPED())
1014                 return;
1015
1016         KASSERT(rw->rw_lock != RW_DESTROYED,
1017             ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
1018         _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line);
1019 #ifndef INVARIANTS
1020         if (rw_recursed(rw))
1021                 panic("downgrade of a recursed lock");
1022 #endif
1023
1024         WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
1025
1026         /*
1027          * Convert from a writer to a single reader.  First we handle
1028          * the easy case with no waiters.  If there are any waiters, we
1029          * lock the turnstile and "disown" the lock.
1030          */
1031         tid = (uintptr_t)curthread;
1032         if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
1033                 goto out;
1034
1035         /*
1036          * Ok, we think we have waiters, so lock the turnstile so we can
1037          * read the waiter flags without any races.
1038          */
1039         turnstile_chain_lock(&rw->lock_object);
1040         v = rw->rw_lock & RW_LOCK_WAITERS;
1041         rwait = v & RW_LOCK_READ_WAITERS;
1042         wwait = v & RW_LOCK_WRITE_WAITERS;
1043         MPASS(rwait | wwait);
1044
1045         /*
1046          * Downgrade from a write lock while preserving waiters flag
1047          * and give up ownership of the turnstile.
1048          */
1049         ts = turnstile_lookup(&rw->lock_object);
1050         MPASS(ts != NULL);
1051         if (!wwait)
1052                 v &= ~RW_LOCK_READ_WAITERS;
1053         atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
1054         /*
1055          * Wake other readers if there are no writers pending.  Otherwise they
1056          * won't be able to acquire the lock anyway.
1057          */
1058         if (rwait && !wwait) {
1059                 turnstile_broadcast(ts, TS_SHARED_QUEUE);
1060                 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
1061         } else
1062                 turnstile_disown(ts);
1063         turnstile_chain_unlock(&rw->lock_object);
1064 out:
1065         curthread->td_rw_rlocks++;
1066         LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
1067         LOCKSTAT_RECORD0(LS_RW_DOWNGRADE_DOWNGRADE, rw);
1068 }
1069
1070 #ifdef INVARIANT_SUPPORT
1071 #ifndef INVARIANTS
1072 #undef _rw_assert
1073 #endif
1074
1075 /*
1076  * In the non-WITNESS case, rw_assert() can only detect that at least
1077  * *some* thread owns an rlock, but it cannot guarantee that *this*
1078  * thread owns an rlock.
1079  */
1080 void
1081 _rw_assert(struct rwlock *rw, int what, const char *file, int line)
1082 {
1083
1084         if (panicstr != NULL)
1085                 return;
1086         switch (what) {
1087         case RA_LOCKED:
1088         case RA_LOCKED | RA_RECURSED:
1089         case RA_LOCKED | RA_NOTRECURSED:
1090         case RA_RLOCKED:
1091         case RA_RLOCKED | RA_RECURSED:
1092         case RA_RLOCKED | RA_NOTRECURSED:
1093 #ifdef WITNESS
1094                 witness_assert(&rw->lock_object, what, file, line);
1095 #else
1096                 /*
1097                  * If some other thread has a write lock or we have one
1098                  * and are asserting a read lock, fail.  Also, if no one
1099                  * has a lock at all, fail.
1100                  */
1101                 if (rw->rw_lock == RW_UNLOCKED ||
1102                     (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED ||
1103                     rw_wowner(rw) != curthread)))
1104                         panic("Lock %s not %slocked @ %s:%d\n",
1105                             rw->lock_object.lo_name, (what & RA_RLOCKED) ?
1106                             "read " : "", file, line);
1107
1108                 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
1109                         if (rw_recursed(rw)) {
1110                                 if (what & RA_NOTRECURSED)
1111                                         panic("Lock %s recursed @ %s:%d\n",
1112                                             rw->lock_object.lo_name, file,
1113                                             line);
1114                         } else if (what & RA_RECURSED)
1115                                 panic("Lock %s not recursed @ %s:%d\n",
1116                                     rw->lock_object.lo_name, file, line);
1117                 }
1118 #endif
1119                 break;
1120         case RA_WLOCKED:
1121         case RA_WLOCKED | RA_RECURSED:
1122         case RA_WLOCKED | RA_NOTRECURSED:
1123                 if (rw_wowner(rw) != curthread)
1124                         panic("Lock %s not exclusively locked @ %s:%d\n",
1125                             rw->lock_object.lo_name, file, line);
1126                 if (rw_recursed(rw)) {
1127                         if (what & RA_NOTRECURSED)
1128                                 panic("Lock %s recursed @ %s:%d\n",
1129                                     rw->lock_object.lo_name, file, line);
1130                 } else if (what & RA_RECURSED)
1131                         panic("Lock %s not recursed @ %s:%d\n",
1132                             rw->lock_object.lo_name, file, line);
1133                 break;
1134         case RA_UNLOCKED:
1135 #ifdef WITNESS
1136                 witness_assert(&rw->lock_object, what, file, line);
1137 #else
1138                 /*
1139                  * If we hold a write lock fail.  We can't reliably check
1140                  * to see if we hold a read lock or not.
1141                  */
1142                 if (rw_wowner(rw) == curthread)
1143                         panic("Lock %s exclusively locked @ %s:%d\n",
1144                             rw->lock_object.lo_name, file, line);
1145 #endif
1146                 break;
1147         default:
1148                 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
1149                     line);
1150         }
1151 }
1152 #endif /* INVARIANT_SUPPORT */
1153
1154 #ifdef DDB
1155 void
1156 db_show_rwlock(struct lock_object *lock)
1157 {
1158         struct rwlock *rw;
1159         struct thread *td;
1160
1161         rw = (struct rwlock *)lock;
1162
1163         db_printf(" state: ");
1164         if (rw->rw_lock == RW_UNLOCKED)
1165                 db_printf("UNLOCKED\n");
1166         else if (rw->rw_lock == RW_DESTROYED) {
1167                 db_printf("DESTROYED\n");
1168                 return;
1169         } else if (rw->rw_lock & RW_LOCK_READ)
1170                 db_printf("RLOCK: %ju locks\n",
1171                     (uintmax_t)(RW_READERS(rw->rw_lock)));
1172         else {
1173                 td = rw_wowner(rw);
1174                 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1175                     td->td_tid, td->td_proc->p_pid, td->td_name);
1176                 if (rw_recursed(rw))
1177                         db_printf(" recursed: %u\n", rw->rw_recurse);
1178         }
1179         db_printf(" waiters: ");
1180         switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
1181         case RW_LOCK_READ_WAITERS:
1182                 db_printf("readers\n");
1183                 break;
1184         case RW_LOCK_WRITE_WAITERS:
1185                 db_printf("writers\n");
1186                 break;
1187         case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
1188                 db_printf("readers and writers\n");
1189                 break;
1190         default:
1191                 db_printf("none\n");
1192                 break;
1193         }
1194 }
1195
1196 #endif