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                 curthread->td_locks++;
 278         }
 279         return (rval);
 280 }
 281
 282 void
 283 _rw_wunlock(struct rwlock *rw, const char *file, int line)
 284 {
 285
 286         if (SCHEDULER_STOPPED())
 287                 return;
 288         KASSERT(rw->rw_lock != RW_DESTROYED,
 289             ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
 290         _rw_assert(rw, RA_WLOCKED, file, line);
 291         curthread->td_locks--;
 292         WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 293         LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
 294             line);
 295         if (!rw_recursed(rw))
 296                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_WUNLOCK_RELEASE, rw);
 297         __rw_wunlock(rw, curthread, file, line);
 298 }
 299 /*
 300  * Determines whether a new reader can acquire a lock.  Succeeds if the
 301  * reader already owns a read lock and the lock is locked for read to
 302  * prevent deadlock from reader recursion.  Also succeeds if the lock
 303  * is unlocked and has no writer waiters or spinners.  Failing otherwise
 304  * prioritizes writers before readers.
 305  */
 306 #define RW_CAN_READ(_rw)                                                \
 307     ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) &      \
 308     (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) ==  \
 309     RW_LOCK_READ)
 310
 311 void
 312 _rw_rlock(struct rwlock *rw, const char *file, int line)
 313 {
 314         struct turnstile *ts;
 315 #ifdef ADAPTIVE_RWLOCKS
 316         volatile struct thread *owner;
 317         int spintries = 0;
 318         int i;
 319 #endif
 320 #ifdef LOCK_PROFILING
 321         uint64_t waittime = 0;
 322         int contested = 0;
 323 #endif
 324         uintptr_t v;
 325 #ifdef KDTRACE_HOOKS
 326         uint64_t spin_cnt = 0;
 327         uint64_t sleep_cnt = 0;
 328         int64_t sleep_time = 0;
 329 #endif
 330
 331         if (SCHEDULER_STOPPED())
 332                 return;
 333
 334         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 335             ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d",
 336             curthread, rw->lock_object.lo_name, file, line));
 337         KASSERT(rw->rw_lock != RW_DESTROYED,
 338             ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
 339         KASSERT(rw_wowner(rw) != curthread,
 340             ("rw_rlock: wlock already held for %s @ %s:%d",
 341             rw->lock_object.lo_name, file, line));
 342         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL);
 343
 344         for (;;) {
 345 #ifdef KDTRACE_HOOKS
 346                 spin_cnt++;
 347 #endif
 348                 /*
 349                  * Handle the easy case.  If no other thread has a write
 350                  * lock, then try to bump up the count of read locks.  Note
 351                  * that we have to preserve the current state of the
 352                  * RW_LOCK_WRITE_WAITERS flag.  If we fail to acquire a
 353                  * read lock, then rw_lock must have changed, so restart
 354                  * the loop.  Note that this handles the case of a
 355                  * completely unlocked rwlock since such a lock is encoded
 356                  * as a read lock with no waiters.
 357                  */
 358                 v = rw->rw_lock;
 359                 if (RW_CAN_READ(v)) {
 360                         /*
 361                          * The RW_LOCK_READ_WAITERS flag should only be set
 362                          * if the lock has been unlocked and write waiters
 363                          * were present.
 364                          */
 365                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v,
 366                             v + RW_ONE_READER)) {
 367                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 368                                         CTR4(KTR_LOCK,
 369                                             "%s: %p succeed %p -> %p", __func__,
 370                                             rw, (void *)v,
 371                                             (void *)(v + RW_ONE_READER));
 372                                 break;
 373                         }
 374                         continue;
 375                 }
 376 #ifdef HWPMC_HOOKS
 377                 PMC_SOFT_CALL( , , lock, failed);
 378 #endif
 379                 lock_profile_obtain_lock_failed(&rw->lock_object,
 380                     &contested, &waittime);
 381
 382 #ifdef ADAPTIVE_RWLOCKS
 383                 /*
 384                  * If the owner is running on another CPU, spin until
 385                  * the owner stops running or the state of the lock
 386                  * changes.
 387                  */
 388                 if ((v & RW_LOCK_READ) == 0) {
 389                         owner = (struct thread *)RW_OWNER(v);
 390                         if (TD_IS_RUNNING(owner)) {
 391                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 392                                         CTR3(KTR_LOCK,
 393                                             "%s: spinning on %p held by %p",
 394                                             __func__, rw, owner);
 395                                 while ((struct thread*)RW_OWNER(rw->rw_lock) ==
 396                                     owner && TD_IS_RUNNING(owner)) {
 397                                         cpu_spinwait();
 398 #ifdef KDTRACE_HOOKS
 399                                         spin_cnt++;
 400 #endif
 401                                 }
 402                                 continue;
 403                         }
 404                 } else if (spintries < ROWNER_RETRIES) {
 405                         spintries++;
 406                         for (i = 0; i < ROWNER_LOOPS; i++) {
 407                                 v = rw->rw_lock;
 408                                 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v))
 409                                         break;
 410                                 cpu_spinwait();
 411                         }
 412                         if (i != ROWNER_LOOPS)
 413                                 continue;
 414                 }
 415 #endif
 416
 417                 /*
 418                  * Okay, now it's the hard case.  Some other thread already
 419                  * has a write lock or there are write waiters present,
 420                  * acquire the turnstile lock so we can begin the process
 421                  * of blocking.
 422                  */
 423                 ts = turnstile_trywait(&rw->lock_object);
 424
 425                 /*
 426                  * The lock might have been released while we spun, so
 427                  * recheck its state and restart the loop if needed.
 428                  */
 429                 v = rw->rw_lock;
 430                 if (RW_CAN_READ(v)) {
 431                         turnstile_cancel(ts);
 432                         continue;
 433                 }
 434
 435 #ifdef ADAPTIVE_RWLOCKS
 436                 /*
 437                  * The current lock owner might have started executing
 438                  * on another CPU (or the lock could have changed
 439                  * owners) while we were waiting on the turnstile
 440                  * chain lock.  If so, drop the turnstile lock and try
 441                  * again.
 442                  */
 443                 if ((v & RW_LOCK_READ) == 0) {
 444                         owner = (struct thread *)RW_OWNER(v);
 445                         if (TD_IS_RUNNING(owner)) {
 446                                 turnstile_cancel(ts);
 447                                 continue;
 448                         }
 449                 }
 450 #endif
 451
 452                 /*
 453                  * The lock is held in write mode or it already has waiters.
 454                  */
 455                 MPASS(!RW_CAN_READ(v));
 456
 457                 /*
 458                  * If the RW_LOCK_READ_WAITERS flag is already set, then
 459                  * we can go ahead and block.  If it is not set then try
 460                  * to set it.  If we fail to set it drop the turnstile
 461                  * lock and restart the loop.
 462                  */
 463                 if (!(v & RW_LOCK_READ_WAITERS)) {
 464                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 465                             v | RW_LOCK_READ_WAITERS)) {
 466                                 turnstile_cancel(ts);
 467                                 continue;
 468                         }
 469                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 470                                 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
 471                                     __func__, rw);
 472                 }
 473
 474                 /*
 475                  * We were unable to acquire the lock and the read waiters
 476                  * flag is set, so we must block on the turnstile.
 477                  */
 478                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 479                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 480                             rw);
 481 #ifdef KDTRACE_HOOKS
 482                 sleep_time -= lockstat_nsecs();
 483 #endif
 484                 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE);
 485 #ifdef KDTRACE_HOOKS
 486                 sleep_time += lockstat_nsecs();
 487                 sleep_cnt++;
 488 #endif
 489                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 490                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 491                             __func__, rw);
 492         }
 493
 494         /*
 495          * TODO: acquire "owner of record" here.  Here be turnstile dragons
 496          * however.  turnstiles don't like owners changing between calls to
 497          * turnstile_wait() currently.
 498          */
 499         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_RLOCK_ACQUIRE, rw, contested,
 500             waittime, file, line);
 501         LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
 502         WITNESS_LOCK(&rw->lock_object, 0, file, line);
 503         curthread->td_locks++;
 504         curthread->td_rw_rlocks++;
 505 #ifdef KDTRACE_HOOKS
 506         if (sleep_time)
 507                 LOCKSTAT_RECORD1(LS_RW_RLOCK_BLOCK, rw, sleep_time);
 508
 509         /*
 510          * Record only the loops spinning and not sleeping.
 511          */
 512         if (spin_cnt > sleep_cnt)
 513                 LOCKSTAT_RECORD1(LS_RW_RLOCK_SPIN, rw, (spin_cnt - sleep_cnt));
 514 #endif
 515 }
 516
 517 int
 518 _rw_try_rlock(struct rwlock *rw, const char *file, int line)
 519 {
 520         uintptr_t x;
 521
 522         if (SCHEDULER_STOPPED())
 523                 return (1);
 524
 525         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 526             ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d",
 527             curthread, rw->lock_object.lo_name, file, line));
 528
 529         for (;;) {
 530                 x = rw->rw_lock;
 531                 KASSERT(rw->rw_lock != RW_DESTROYED,
 532                     ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
 533                 if (!(x & RW_LOCK_READ))
 534                         break;
 535                 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) {
 536                         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
 537                             line);
 538                         WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
 539                         curthread->td_locks++;
 540                         curthread->td_rw_rlocks++;
 541                         return (1);
 542                 }
 543         }
 544
 545         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
 546         return (0);
 547 }
 548
 549 void
 550 _rw_runlock(struct rwlock *rw, const char *file, int line)
 551 {
 552         struct turnstile *ts;
 553         uintptr_t x, v, queue;
 554
 555         if (SCHEDULER_STOPPED())
 556                 return;
 557
 558         KASSERT(rw->rw_lock != RW_DESTROYED,
 559             ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
 560         _rw_assert(rw, RA_RLOCKED, file, line);
 561         curthread->td_locks--;
 562         curthread->td_rw_rlocks--;
 563         WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
 564         LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
 565
 566         /* TODO: drop "owner of record" here. */
 567
 568         for (;;) {
 569                 /*
 570                  * See if there is more than one read lock held.  If so,
 571                  * just drop one and return.
 572                  */
 573                 x = rw->rw_lock;
 574                 if (RW_READERS(x) > 1) {
 575                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
 576                             x - RW_ONE_READER)) {
 577                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 578                                         CTR4(KTR_LOCK,
 579                                             "%s: %p succeeded %p -> %p",
 580                                             __func__, rw, (void *)x,
 581                                             (void *)(x - RW_ONE_READER));
 582                                 break;
 583                         }
 584                         continue;
 585                 }
 586                 /*
 587                  * If there aren't any waiters for a write lock, then try
 588                  * to drop it quickly.
 589                  */
 590                 if (!(x & RW_LOCK_WAITERS)) {
 591                         MPASS((x & ~RW_LOCK_WRITE_SPINNER) ==
 592                             RW_READERS_LOCK(1));
 593                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
 594                             RW_UNLOCKED)) {
 595                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 596                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 597                                             __func__, rw);
 598                                 break;
 599                         }
 600                         continue;
 601                 }
 602                 /*
 603                  * Ok, we know we have waiters and we think we are the
 604                  * last reader, so grab the turnstile lock.
 605                  */
 606                 turnstile_chain_lock(&rw->lock_object);
 607                 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 608                 MPASS(v & RW_LOCK_WAITERS);
 609
 610                 /*
 611                  * Try to drop our lock leaving the lock in a unlocked
 612                  * state.
 613                  *
 614                  * If you wanted to do explicit lock handoff you'd have to
 615                  * do it here.  You'd also want to use turnstile_signal()
 616                  * and you'd have to handle the race where a higher
 617                  * priority thread blocks on the write lock before the
 618                  * thread you wakeup actually runs and have the new thread
 619                  * "steal" the lock.  For now it's a lot simpler to just
 620                  * wakeup all of the waiters.
 621                  *
 622                  * As above, if we fail, then another thread might have
 623                  * acquired a read lock, so drop the turnstile lock and
 624                  * restart.
 625                  */
 626                 x = RW_UNLOCKED;
 627                 if (v & RW_LOCK_WRITE_WAITERS) {
 628                         queue = TS_EXCLUSIVE_QUEUE;
 629                         x |= (v & RW_LOCK_READ_WAITERS);
 630                 } else
 631                         queue = TS_SHARED_QUEUE;
 632                 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
 633                     x)) {
 634                         turnstile_chain_unlock(&rw->lock_object);
 635                         continue;
 636                 }
 637                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 638                         CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
 639                             __func__, rw);
 640
 641                 /*
 642                  * Ok.  The lock is released and all that's left is to
 643                  * wake up the waiters.  Note that the lock might not be
 644                  * free anymore, but in that case the writers will just
 645                  * block again if they run before the new lock holder(s)
 646                  * release the lock.
 647                  */
 648                 ts = turnstile_lookup(&rw->lock_object);
 649                 MPASS(ts != NULL);
 650                 turnstile_broadcast(ts, queue);
 651                 turnstile_unpend(ts, TS_SHARED_LOCK);
 652                 turnstile_chain_unlock(&rw->lock_object);
 653                 break;
 654         }
 655         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_RUNLOCK_RELEASE, rw);
 656 }
 657
 658 /*
 659  * This function is called when we are unable to obtain a write lock on the
 660  * first try.  This means that at least one other thread holds either a
 661  * read or write lock.
 662  */
 663 void
 664 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
 665 {
 666         struct turnstile *ts;
 667 #ifdef ADAPTIVE_RWLOCKS
 668         volatile struct thread *owner;
 669         int spintries = 0;
 670         int i;
 671 #endif
 672         uintptr_t v, x;
 673 #ifdef LOCK_PROFILING
 674         uint64_t waittime = 0;
 675         int contested = 0;
 676 #endif
 677 #ifdef KDTRACE_HOOKS
 678         uint64_t spin_cnt = 0;
 679         uint64_t sleep_cnt = 0;
 680         int64_t sleep_time = 0;
 681 #endif
 682
 683         if (SCHEDULER_STOPPED())
 684                 return;
 685
 686         if (rw_wlocked(rw)) {
 687                 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE,
 688                     ("%s: recursing but non-recursive rw %s @ %s:%d\n",
 689                     __func__, rw->lock_object.lo_name, file, line));
 690                 rw->rw_recurse++;
 691                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 692                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
 693                 return;
 694         }
 695
 696         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 697                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 698                     rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
 699
 700         while (!_rw_write_lock(rw, tid)) {
 701 #ifdef KDTRACE_HOOKS
 702                 spin_cnt++;
 703 #endif
 704 #ifdef HWPMC_HOOKS
 705                 PMC_SOFT_CALL( , , lock, failed);
 706 #endif
 707                 lock_profile_obtain_lock_failed(&rw->lock_object,
 708                     &contested, &waittime);
 709 #ifdef ADAPTIVE_RWLOCKS
 710                 /*
 711                  * If the lock is write locked and the owner is
 712                  * running on another CPU, spin until the owner stops
 713                  * running or the state of the lock changes.
 714                  */
 715                 v = rw->rw_lock;
 716                 owner = (struct thread *)RW_OWNER(v);
 717                 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
 718                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 719                                 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
 720                                     __func__, rw, owner);
 721                         while ((struct thread*)RW_OWNER(rw->rw_lock) == owner &&
 722                             TD_IS_RUNNING(owner)) {
 723                                 cpu_spinwait();
 724 #ifdef KDTRACE_HOOKS
 725                                 spin_cnt++;
 726 #endif
 727                         }
 728                         continue;
 729                 }
 730                 if ((v & RW_LOCK_READ) && RW_READERS(v) &&
 731                     spintries < ROWNER_RETRIES) {
 732                         if (!(v & RW_LOCK_WRITE_SPINNER)) {
 733                                 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 734                                     v | RW_LOCK_WRITE_SPINNER)) {
 735                                         continue;
 736                                 }
 737                         }
 738                         spintries++;
 739                         for (i = 0; i < ROWNER_LOOPS; i++) {
 740                                 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
 741                                         break;
 742                                 cpu_spinwait();
 743                         }
 744 #ifdef KDTRACE_HOOKS
 745                         spin_cnt += ROWNER_LOOPS - i;
 746 #endif
 747                         if (i != ROWNER_LOOPS)
 748                                 continue;
 749                 }
 750 #endif
 751                 ts = turnstile_trywait(&rw->lock_object);
 752                 v = rw->rw_lock;
 753
 754 #ifdef ADAPTIVE_RWLOCKS
 755                 /*
 756                  * The current lock owner might have started executing
 757                  * on another CPU (or the lock could have changed
 758                  * owners) while we were waiting on the turnstile
 759                  * chain lock.  If so, drop the turnstile lock and try
 760                  * again.
 761                  */
 762                 if (!(v & RW_LOCK_READ)) {
 763                         owner = (struct thread *)RW_OWNER(v);
 764                         if (TD_IS_RUNNING(owner)) {
 765                                 turnstile_cancel(ts);
 766                                 continue;
 767                         }
 768                 }
 769 #endif
 770                 /*
 771                  * Check for the waiters flags about this rwlock.
 772                  * If the lock was released, without maintain any pending
 773                  * waiters queue, simply try to acquire it.
 774                  * If a pending waiters queue is present, claim the lock
 775                  * ownership and maintain the pending queue.
 776                  */
 777                 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 778                 if ((v & ~x) == RW_UNLOCKED) {
 779                         x &= ~RW_LOCK_WRITE_SPINNER;
 780                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) {
 781                                 if (x)
 782                                         turnstile_claim(ts);
 783                                 else
 784                                         turnstile_cancel(ts);
 785                                 break;
 786                         }
 787                         turnstile_cancel(ts);
 788                         continue;
 789                 }
 790                 /*
 791                  * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
 792                  * set it.  If we fail to set it, then loop back and try
 793                  * again.
 794                  */
 795                 if (!(v & RW_LOCK_WRITE_WAITERS)) {
 796                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 797                             v | RW_LOCK_WRITE_WAITERS)) {
 798                                 turnstile_cancel(ts);
 799                                 continue;
 800                         }
 801                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 802                                 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
 803                                     __func__, rw);
 804                 }
 805                 /*
 806                  * We were unable to acquire the lock and the write waiters
 807                  * flag is set, so we must block on the turnstile.
 808                  */
 809                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 810                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 811                             rw);
 812 #ifdef KDTRACE_HOOKS
 813                 sleep_time -= lockstat_nsecs();
 814 #endif
 815                 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
 816 #ifdef KDTRACE_HOOKS
 817                 sleep_time += lockstat_nsecs();
 818                 sleep_cnt++;
 819 #endif
 820                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 821                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 822                             __func__, rw);
 823 #ifdef ADAPTIVE_RWLOCKS
 824                 spintries = 0;
 825 #endif
 826         }
 827         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_WLOCK_ACQUIRE, rw, contested,
 828             waittime, file, line);
 829 #ifdef KDTRACE_HOOKS
 830         if (sleep_time)
 831                 LOCKSTAT_RECORD1(LS_RW_WLOCK_BLOCK, rw, sleep_time);
 832
 833         /*
 834          * Record only the loops spinning and not sleeping.
 835          */
 836         if (spin_cnt > sleep_cnt)
 837                 LOCKSTAT_RECORD1(LS_RW_WLOCK_SPIN, rw, (spin_cnt - sleep_cnt));
 838 #endif
 839 }
 840
 841 /*
 842  * This function is called if the first try at releasing a write lock failed.
 843  * This means that one of the 2 waiter bits must be set indicating that at
 844  * least one thread is waiting on this lock.
 845  */
 846 void
 847 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
 848 {
 849         struct turnstile *ts;
 850         uintptr_t v;
 851         int queue;
 852
 853         if (SCHEDULER_STOPPED())
 854                 return;
 855
 856         if (rw_wlocked(rw) && rw_recursed(rw)) {
 857                 rw->rw_recurse--;
 858                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 859                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
 860                 return;
 861         }
 862
 863         KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
 864             ("%s: neither of the waiter flags are set", __func__));
 865
 866         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 867                 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
 868
 869         turnstile_chain_lock(&rw->lock_object);
 870         ts = turnstile_lookup(&rw->lock_object);
 871         MPASS(ts != NULL);
 872
 873         /*
 874          * Use the same algo as sx locks for now.  Prefer waking up shared
 875          * waiters if we have any over writers.  This is probably not ideal.
 876          *
 877          * 'v' is the value we are going to write back to rw_lock.  If we
 878          * have waiters on both queues, we need to preserve the state of
 879          * the waiter flag for the queue we don't wake up.  For now this is
 880          * hardcoded for the algorithm mentioned above.
 881          *
 882          * In the case of both readers and writers waiting we wakeup the
 883          * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
 884          * new writer comes in before a reader it will claim the lock up
 885          * above.  There is probably a potential priority inversion in
 886          * there that could be worked around either by waking both queues
 887          * of waiters or doing some complicated lock handoff gymnastics.
 888          */
 889         v = RW_UNLOCKED;
 890         if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
 891                 queue = TS_EXCLUSIVE_QUEUE;
 892                 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
 893         } else
 894                 queue = TS_SHARED_QUEUE;
 895
 896         /* Wake up all waiters for the specific queue. */
 897         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 898                 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
 899                     queue == TS_SHARED_QUEUE ? "read" : "write");
 900         turnstile_broadcast(ts, queue);
 901         atomic_store_rel_ptr(&rw->rw_lock, v);
 902         turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
 903         turnstile_chain_unlock(&rw->lock_object);
 904 }
 905
 906 /*
 907  * Attempt to do a non-blocking upgrade from a read lock to a write
 908  * lock.  This will only succeed if this thread holds a single read
 909  * lock.  Returns true if the upgrade succeeded and false otherwise.
 910  */
 911 int
 912 _rw_try_upgrade(struct rwlock *rw, const char *file, int line)
 913 {
 914         uintptr_t v, x, tid;
 915         struct turnstile *ts;
 916         int success;
 917
 918         if (SCHEDULER_STOPPED())
 919                 return (1);
 920
 921         KASSERT(rw->rw_lock != RW_DESTROYED,
 922             ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
 923         _rw_assert(rw, RA_RLOCKED, file, line);
 924
 925         /*
 926          * Attempt to switch from one reader to a writer.  If there
 927          * are any write waiters, then we will have to lock the
 928          * turnstile first to prevent races with another writer
 929          * calling turnstile_wait() before we have claimed this
 930          * turnstile.  So, do the simple case of no waiters first.
 931          */
 932         tid = (uintptr_t)curthread;
 933         success = 0;
 934         for (;;) {
 935                 v = rw->rw_lock;
 936                 if (RW_READERS(v) > 1)
 937                         break;
 938                 if (!(v & RW_LOCK_WAITERS)) {
 939                         success = atomic_cmpset_ptr(&rw->rw_lock, v, tid);
 940                         if (!success)
 941                                 continue;
 942                         break;
 943                 }
 944
 945                 /*
 946                  * Ok, we think we have waiters, so lock the turnstile.
 947                  */
 948                 ts = turnstile_trywait(&rw->lock_object);
 949                 v = rw->rw_lock;
 950                 if (RW_READERS(v) > 1) {
 951                         turnstile_cancel(ts);
 952                         break;
 953                 }
 954                 /*
 955                  * Try to switch from one reader to a writer again.  This time
 956                  * we honor the current state of the waiters flags.
 957                  * If we obtain the lock with the flags set, then claim
 958                  * ownership of the turnstile.
 959                  */
 960                 x = rw->rw_lock & RW_LOCK_WAITERS;
 961                 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
 962                 if (success) {
 963                         if (x)
 964                                 turnstile_claim(ts);
 965                         else
 966                                 turnstile_cancel(ts);
 967                         break;
 968                 }
 969                 turnstile_cancel(ts);
 970         }
 971         LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
 972         if (success) {
 973                 curthread->td_rw_rlocks--;
 974                 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 975                     file, line);
 976                 LOCKSTAT_RECORD0(LS_RW_TRYUPGRADE_UPGRADE, rw);
 977         }
 978         return (success);
 979 }
 980
 981 /*
 982  * Downgrade a write lock into a single read lock.
 983  */
 984 void
 985 _rw_downgrade(struct rwlock *rw, const char *file, int line)
 986 {
 987         struct turnstile *ts;
 988         uintptr_t tid, v;
 989         int rwait, wwait;
 990
 991         if (SCHEDULER_STOPPED())
 992                 return;
 993
 994         KASSERT(rw->rw_lock != RW_DESTROYED,
 995             ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
 996         _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line);
 997 #ifndef INVARIANTS
 998         if (rw_recursed(rw))
 999                 panic("downgrade of a recursed lock");
1000 #endif
1001
1002         WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
1003
1004         /*
1005          * Convert from a writer to a single reader.  First we handle
1006          * the easy case with no waiters.  If there are any waiters, we
1007          * lock the turnstile and "disown" the lock.
1008          */
1009         tid = (uintptr_t)curthread;
1010         if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
1011                 goto out;
1012
1013         /*
1014          * Ok, we think we have waiters, so lock the turnstile so we can
1015          * read the waiter flags without any races.
1016          */
1017         turnstile_chain_lock(&rw->lock_object);
1018         v = rw->rw_lock & RW_LOCK_WAITERS;
1019         rwait = v & RW_LOCK_READ_WAITERS;
1020         wwait = v & RW_LOCK_WRITE_WAITERS;
1021         MPASS(rwait | wwait);
1022
1023         /*
1024          * Downgrade from a write lock while preserving waiters flag
1025          * and give up ownership of the turnstile.
1026          */
1027         ts = turnstile_lookup(&rw->lock_object);
1028         MPASS(ts != NULL);
1029         if (!wwait)
1030                 v &= ~RW_LOCK_READ_WAITERS;
1031         atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
1032         /*
1033          * Wake other readers if there are no writers pending.  Otherwise they
1034          * won't be able to acquire the lock anyway.
1035          */
1036         if (rwait && !wwait) {
1037                 turnstile_broadcast(ts, TS_SHARED_QUEUE);
1038                 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
1039         } else
1040                 turnstile_disown(ts);
1041         turnstile_chain_unlock(&rw->lock_object);
1042 out:
1043         curthread->td_rw_rlocks++;
1044         LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
1045         LOCKSTAT_RECORD0(LS_RW_DOWNGRADE_DOWNGRADE, rw);
1046 }
1047
1048 #ifdef INVARIANT_SUPPORT
1049 #ifndef INVARIANTS
1050 #undef _rw_assert
1051 #endif
1052
1053 /*
1054  * In the non-WITNESS case, rw_assert() can only detect that at least
1055  * *some* thread owns an rlock, but it cannot guarantee that *this*
1056  * thread owns an rlock.
1057  */
1058 void
1059 _rw_assert(struct rwlock *rw, int what, const char *file, int line)
1060 {
1061
1062         if (panicstr != NULL)
1063                 return;
1064         switch (what) {
1065         case RA_LOCKED:
1066         case RA_LOCKED | RA_RECURSED:
1067         case RA_LOCKED | RA_NOTRECURSED:
1068         case RA_RLOCKED:
1069         case RA_RLOCKED | RA_RECURSED:
1070         case RA_RLOCKED | RA_NOTRECURSED:
1071 #ifdef WITNESS
1072                 witness_assert(&rw->lock_object, what, file, line);
1073 #else
1074                 /*
1075                  * If some other thread has a write lock or we have one
1076                  * and are asserting a read lock, fail.  Also, if no one
1077                  * has a lock at all, fail.
1078                  */
1079                 if (rw->rw_lock == RW_UNLOCKED ||
1080                     (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED ||
1081                     rw_wowner(rw) != curthread)))
1082                         panic("Lock %s not %slocked @ %s:%d\n",
1083                             rw->lock_object.lo_name, (what & RA_RLOCKED) ?
1084                             "read " : "", file, line);
1085
1086                 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
1087                         if (rw_recursed(rw)) {
1088                                 if (what & RA_NOTRECURSED)
1089                                         panic("Lock %s recursed @ %s:%d\n",
1090                                             rw->lock_object.lo_name, file,
1091                                             line);
1092                         } else if (what & RA_RECURSED)
1093                                 panic("Lock %s not recursed @ %s:%d\n",
1094                                     rw->lock_object.lo_name, file, line);
1095                 }
1096 #endif
1097                 break;
1098         case RA_WLOCKED:
1099         case RA_WLOCKED | RA_RECURSED:
1100         case RA_WLOCKED | RA_NOTRECURSED:
1101                 if (rw_wowner(rw) != curthread)
1102                         panic("Lock %s not exclusively locked @ %s:%d\n",
1103                             rw->lock_object.lo_name, file, line);
1104                 if (rw_recursed(rw)) {
1105                         if (what & RA_NOTRECURSED)
1106                                 panic("Lock %s recursed @ %s:%d\n",
1107                                     rw->lock_object.lo_name, file, line);
1108                 } else if (what & RA_RECURSED)
1109                         panic("Lock %s not recursed @ %s:%d\n",
1110                             rw->lock_object.lo_name, file, line);
1111                 break;
1112         case RA_UNLOCKED:
1113 #ifdef WITNESS
1114                 witness_assert(&rw->lock_object, what, file, line);
1115 #else
1116                 /*
1117                  * If we hold a write lock fail.  We can't reliably check
1118                  * to see if we hold a read lock or not.
1119                  */
1120                 if (rw_wowner(rw) == curthread)
1121                         panic("Lock %s exclusively locked @ %s:%d\n",
1122                             rw->lock_object.lo_name, file, line);
1123 #endif
1124                 break;
1125         default:
1126                 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
1127                     line);
1128         }
1129 }
1130 #endif /* INVARIANT_SUPPORT */
1131
1132 #ifdef DDB
1133 void
1134 db_show_rwlock(struct lock_object *lock)
1135 {
1136         struct rwlock *rw;
1137         struct thread *td;
1138
1139         rw = (struct rwlock *)lock;
1140
1141         db_printf(" state: ");
1142         if (rw->rw_lock == RW_UNLOCKED)
1143                 db_printf("UNLOCKED\n");
1144         else if (rw->rw_lock == RW_DESTROYED) {
1145                 db_printf("DESTROYED\n");
1146                 return;
1147         } else if (rw->rw_lock & RW_LOCK_READ)
1148                 db_printf("RLOCK: %ju locks\n",
1149                     (uintmax_t)(RW_READERS(rw->rw_lock)));
1150         else {
1151                 td = rw_wowner(rw);
1152                 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1153                     td->td_tid, td->td_proc->p_pid, td->td_name);
1154                 if (rw_recursed(rw))
1155                         db_printf(" recursed: %u\n", rw->rw_recurse);
1156         }
1157         db_printf(" waiters: ");
1158         switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
1159         case RW_LOCK_READ_WAITERS:
1160                 db_printf("readers\n");
1161                 break;
1162         case RW_LOCK_WRITE_WAITERS:
1163                 db_printf("writers\n");
1164                 break;
1165         case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
1166                 db_printf("readers and writers\n");
1167                 break;
1168         default:
1169                 db_printf("none\n");
1170                 break;
1171         }
1172 }
1173
1174 #endif