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  * 3. Neither the name of the author nor the names of any co-contributors
  14  *    may be used to endorse or promote products derived from this software
  15  *    without specific prior written permission.
  16  *
  17  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  18  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  19  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  20  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  21  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  22  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  23  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  27  * SUCH DAMAGE.
  28  */
  29
  30 /*
  31  * Machine independent bits of reader/writer lock implementation.
  32  */
  33
  34 #include <sys/cdefs.h>
  35 __FBSDID("$FreeBSD$");
  36
  37 #include "opt_ddb.h"
  38 #include "opt_hwpmc_hooks.h"
  39 #include "opt_kdtrace.h"
  40 #include "opt_no_adaptive_rwlocks.h"
  41
  42 #include <sys/param.h>
  43 #include <sys/kdb.h>
  44 #include <sys/ktr.h>
  45 #include <sys/kernel.h>
  46 #include <sys/lock.h>
  47 #include <sys/mutex.h>
  48 #include <sys/proc.h>
  49 #include <sys/rwlock.h>
  50 #include <sys/sysctl.h>
  51 #include <sys/systm.h>
  52 #include <sys/turnstile.h>
  53
  54 #include <machine/cpu.h>
  55
  56 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS)
  57 #define ADAPTIVE_RWLOCKS
  58 #endif
  59
  60 #ifdef HWPMC_HOOKS
  61 #include <sys/pmckern.h>
  62 PMC_SOFT_DECLARE( , , lock, failed);
  63 #endif
  64
  65 /*
  66  * Return the rwlock address when the lock cookie address is provided.
  67  * This functionality assumes that struct rwlock* have a member named rw_lock.
  68  */
  69 #define rwlock2rw(c)    (__containerof(c, struct rwlock, rw_lock))
  70
  71 #ifdef ADAPTIVE_RWLOCKS
  72 static int rowner_retries = 10;
  73 static int rowner_loops = 10000;
  74 static SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL,
  75     "rwlock debugging");
  76 SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, "");
  77 SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, "");
  78 #endif
  79
  80 #ifdef DDB
  81 #include <ddb/ddb.h>
  82
  83 static void     db_show_rwlock(const struct lock_object *lock);
  84 #endif
  85 static void     assert_rw(const struct lock_object *lock, int what);
  86 static void     lock_rw(struct lock_object *lock, uintptr_t how);
  87 #ifdef KDTRACE_HOOKS
  88 static int      owner_rw(const struct lock_object *lock, struct thread **owner);
  89 #endif
  90 static uintptr_t unlock_rw(struct lock_object *lock);
  91
  92 struct lock_class lock_class_rw = {
  93         .lc_name = "rw",
  94         .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
  95         .lc_assert = assert_rw,
  96 #ifdef DDB
  97         .lc_ddb_show = db_show_rwlock,
  98 #endif
  99         .lc_lock = lock_rw,
 100         .lc_unlock = unlock_rw,
 101 #ifdef KDTRACE_HOOKS
 102         .lc_owner = owner_rw,
 103 #endif
 104 };
 105
 106 /*
 107  * Return a pointer to the owning thread if the lock is write-locked or
 108  * NULL if the lock is unlocked or read-locked.
 109  */
 110 #define rw_wowner(rw)                                                   \
 111         ((rw)->rw_lock & RW_LOCK_READ ? NULL :                          \
 112             (struct thread *)RW_OWNER((rw)->rw_lock))
 113
 114 /*
 115  * Returns if a write owner is recursed.  Write ownership is not assured
 116  * here and should be previously checked.
 117  */
 118 #define rw_recursed(rw)         ((rw)->rw_recurse != 0)
 119
 120 /*
 121  * Return true if curthread helds the lock.
 122  */
 123 #define rw_wlocked(rw)          (rw_wowner((rw)) == curthread)
 124
 125 /*
 126  * Return a pointer to the owning thread for this lock who should receive
 127  * any priority lent by threads that block on this lock.  Currently this
 128  * is identical to rw_wowner().
 129  */
 130 #define rw_owner(rw)            rw_wowner(rw)
 131
 132 #ifndef INVARIANTS
 133 #define __rw_assert(c, what, file, line)
 134 #endif
 135
 136 void
 137 assert_rw(const struct lock_object *lock, int what)
 138 {
 139
 140         rw_assert((const struct rwlock *)lock, what);
 141 }
 142
 143 void
 144 lock_rw(struct lock_object *lock, uintptr_t how)
 145 {
 146         struct rwlock *rw;
 147
 148         rw = (struct rwlock *)lock;
 149         if (how)
 150                 rw_rlock(rw);
 151         else
 152                 rw_wlock(rw);
 153 }
 154
 155 uintptr_t
 156 unlock_rw(struct lock_object *lock)
 157 {
 158         struct rwlock *rw;
 159
 160         rw = (struct rwlock *)lock;
 161         rw_assert(rw, RA_LOCKED | LA_NOTRECURSED);
 162         if (rw->rw_lock & RW_LOCK_READ) {
 163                 rw_runlock(rw);
 164                 return (1);
 165         } else {
 166                 rw_wunlock(rw);
 167                 return (0);
 168         }
 169 }
 170
 171 #ifdef KDTRACE_HOOKS
 172 int
 173 owner_rw(const struct lock_object *lock, struct thread **owner)
 174 {
 175         const struct rwlock *rw = (const struct rwlock *)lock;
 176         uintptr_t x = rw->rw_lock;
 177
 178         *owner = rw_wowner(rw);
 179         return ((x & RW_LOCK_READ) != 0 ?  (RW_READERS(x) != 0) :
 180             (*owner != NULL));
 181 }
 182 #endif
 183
 184 void
 185 _rw_init_flags(volatile uintptr_t *c, const char *name, int opts)
 186 {
 187         struct rwlock *rw;
 188         int flags;
 189
 190         rw = rwlock2rw(c);
 191
 192         MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
 193             RW_RECURSE)) == 0);
 194         ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock,
 195             ("%s: rw_lock not aligned for %s: %p", __func__, name,
 196             &rw->rw_lock));
 197
 198         flags = LO_UPGRADABLE;
 199         if (opts & RW_DUPOK)
 200                 flags |= LO_DUPOK;
 201         if (opts & RW_NOPROFILE)
 202                 flags |= LO_NOPROFILE;
 203         if (!(opts & RW_NOWITNESS))
 204                 flags |= LO_WITNESS;
 205         if (opts & RW_RECURSE)
 206                 flags |= LO_RECURSABLE;
 207         if (opts & RW_QUIET)
 208                 flags |= LO_QUIET;
 209
 210         lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
 211         rw->rw_lock = RW_UNLOCKED;
 212         rw->rw_recurse = 0;
 213 }
 214
 215 void
 216 _rw_destroy(volatile uintptr_t *c)
 217 {
 218         struct rwlock *rw;
 219
 220         rw = rwlock2rw(c);
 221
 222         KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw));
 223         KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw));
 224         rw->rw_lock = RW_DESTROYED;
 225         lock_destroy(&rw->lock_object);
 226 }
 227
 228 void
 229 rw_sysinit(void *arg)
 230 {
 231         struct rw_args *args = arg;
 232
 233         rw_init((struct rwlock *)args->ra_rw, args->ra_desc);
 234 }
 235
 236 void
 237 rw_sysinit_flags(void *arg)
 238 {
 239         struct rw_args_flags *args = arg;
 240
 241         rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc,
 242             args->ra_flags);
 243 }
 244
 245 int
 246 _rw_wowned(const volatile uintptr_t *c)
 247 {
 248
 249         return (rw_wowner(rwlock2rw(c)) == curthread);
 250 }
 251
 252 void
 253 _rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line)
 254 {
 255         struct rwlock *rw;
 256
 257         if (SCHEDULER_STOPPED())
 258                 return;
 259
 260         rw = rwlock2rw(c);
 261
 262         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 263             ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d",
 264             curthread, rw->lock_object.lo_name, file, line));
 265         KASSERT(rw->rw_lock != RW_DESTROYED,
 266             ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
 267         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 268             line, NULL);
 269         __rw_wlock(rw, curthread, file, line);
 270         LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
 271         WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 272         curthread->td_locks++;
 273 }
 274
 275 int
 276 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line)
 277 {
 278         struct rwlock *rw;
 279         int rval;
 280
 281         if (SCHEDULER_STOPPED())
 282                 return (1);
 283
 284         rw = rwlock2rw(c);
 285
 286         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 287             ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d",
 288             curthread, rw->lock_object.lo_name, file, line));
 289         KASSERT(rw->rw_lock != RW_DESTROYED,
 290             ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
 291
 292         if (rw_wlocked(rw) &&
 293             (rw->lock_object.lo_flags & LO_RECURSABLE) != 0) {
 294                 rw->rw_recurse++;
 295                 rval = 1;
 296         } else
 297                 rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED,
 298                     (uintptr_t)curthread);
 299
 300         LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
 301         if (rval) {
 302                 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 303                     file, line);
 304                 curthread->td_locks++;
 305         }
 306         return (rval);
 307 }
 308
 309 void
 310 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line)
 311 {
 312         struct rwlock *rw;
 313
 314         if (SCHEDULER_STOPPED())
 315                 return;
 316
 317         rw = rwlock2rw(c);
 318
 319         KASSERT(rw->rw_lock != RW_DESTROYED,
 320             ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
 321         __rw_assert(c, RA_WLOCKED, file, line);
 322         WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 323         LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
 324             line);
 325         if (!rw_recursed(rw))
 326                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_WUNLOCK_RELEASE, rw);
 327         __rw_wunlock(rw, curthread, file, line);
 328         curthread->td_locks--;
 329 }
 330 /*
 331  * Determines whether a new reader can acquire a lock.  Succeeds if the
 332  * reader already owns a read lock and the lock is locked for read to
 333  * prevent deadlock from reader recursion.  Also succeeds if the lock
 334  * is unlocked and has no writer waiters or spinners.  Failing otherwise
 335  * prioritizes writers before readers.
 336  */
 337 #define RW_CAN_READ(_rw)                                                \
 338     ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) &      \
 339     (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) ==  \
 340     RW_LOCK_READ)
 341
 342 void
 343 __rw_rlock(volatile uintptr_t *c, const char *file, int line)
 344 {
 345         struct rwlock *rw;
 346         struct turnstile *ts;
 347 #ifdef ADAPTIVE_RWLOCKS
 348         volatile struct thread *owner;
 349         int spintries = 0;
 350         int i;
 351 #endif
 352 #ifdef LOCK_PROFILING
 353         uint64_t waittime = 0;
 354         int contested = 0;
 355 #endif
 356         uintptr_t v;
 357 #ifdef KDTRACE_HOOKS
 358         uint64_t spin_cnt = 0;
 359         uint64_t sleep_cnt = 0;
 360         int64_t sleep_time = 0;
 361 #endif
 362
 363         if (SCHEDULER_STOPPED())
 364                 return;
 365
 366         rw = rwlock2rw(c);
 367
 368         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 369             ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d",
 370             curthread, rw->lock_object.lo_name, file, line));
 371         KASSERT(rw->rw_lock != RW_DESTROYED,
 372             ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
 373         KASSERT(rw_wowner(rw) != curthread,
 374             ("rw_rlock: wlock already held for %s @ %s:%d",
 375             rw->lock_object.lo_name, file, line));
 376         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL);
 377
 378         for (;;) {
 379 #ifdef KDTRACE_HOOKS
 380                 spin_cnt++;
 381 #endif
 382                 /*
 383                  * Handle the easy case.  If no other thread has a write
 384                  * lock, then try to bump up the count of read locks.  Note
 385                  * that we have to preserve the current state of the
 386                  * RW_LOCK_WRITE_WAITERS flag.  If we fail to acquire a
 387                  * read lock, then rw_lock must have changed, so restart
 388                  * the loop.  Note that this handles the case of a
 389                  * completely unlocked rwlock since such a lock is encoded
 390                  * as a read lock with no waiters.
 391                  */
 392                 v = rw->rw_lock;
 393                 if (RW_CAN_READ(v)) {
 394                         /*
 395                          * The RW_LOCK_READ_WAITERS flag should only be set
 396                          * if the lock has been unlocked and write waiters
 397                          * were present.
 398                          */
 399                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v,
 400                             v + RW_ONE_READER)) {
 401                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 402                                         CTR4(KTR_LOCK,
 403                                             "%s: %p succeed %p -> %p", __func__,
 404                                             rw, (void *)v,
 405                                             (void *)(v + RW_ONE_READER));
 406                                 break;
 407                         }
 408                         continue;
 409                 }
 410 #ifdef HWPMC_HOOKS
 411                 PMC_SOFT_CALL( , , lock, failed);
 412 #endif
 413                 lock_profile_obtain_lock_failed(&rw->lock_object,
 414                     &contested, &waittime);
 415
 416 #ifdef ADAPTIVE_RWLOCKS
 417                 /*
 418                  * If the owner is running on another CPU, spin until
 419                  * the owner stops running or the state of the lock
 420                  * changes.
 421                  */
 422                 if ((v & RW_LOCK_READ) == 0) {
 423                         owner = (struct thread *)RW_OWNER(v);
 424                         if (TD_IS_RUNNING(owner)) {
 425                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 426                                         CTR3(KTR_LOCK,
 427                                             "%s: spinning on %p held by %p",
 428                                             __func__, rw, owner);
 429                                 while ((struct thread*)RW_OWNER(rw->rw_lock) ==
 430                                     owner && TD_IS_RUNNING(owner)) {
 431                                         cpu_spinwait();
 432 #ifdef KDTRACE_HOOKS
 433                                         spin_cnt++;
 434 #endif
 435                                 }
 436                                 continue;
 437                         }
 438                 } else if (spintries < rowner_retries) {
 439                         spintries++;
 440                         for (i = 0; i < rowner_loops; i++) {
 441                                 v = rw->rw_lock;
 442                                 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v))
 443                                         break;
 444                                 cpu_spinwait();
 445                         }
 446                         if (i != rowner_loops)
 447                                 continue;
 448                 }
 449 #endif
 450
 451                 /*
 452                  * Okay, now it's the hard case.  Some other thread already
 453                  * has a write lock or there are write waiters present,
 454                  * acquire the turnstile lock so we can begin the process
 455                  * of blocking.
 456                  */
 457                 ts = turnstile_trywait(&rw->lock_object);
 458
 459                 /*
 460                  * The lock might have been released while we spun, so
 461                  * recheck its state and restart the loop if needed.
 462                  */
 463                 v = rw->rw_lock;
 464                 if (RW_CAN_READ(v)) {
 465                         turnstile_cancel(ts);
 466                         continue;
 467                 }
 468
 469 #ifdef ADAPTIVE_RWLOCKS
 470                 /*
 471                  * The current lock owner might have started executing
 472                  * on another CPU (or the lock could have changed
 473                  * owners) while we were waiting on the turnstile
 474                  * chain lock.  If so, drop the turnstile lock and try
 475                  * again.
 476                  */
 477                 if ((v & RW_LOCK_READ) == 0) {
 478                         owner = (struct thread *)RW_OWNER(v);
 479                         if (TD_IS_RUNNING(owner)) {
 480                                 turnstile_cancel(ts);
 481                                 continue;
 482                         }
 483                 }
 484 #endif
 485
 486                 /*
 487                  * The lock is held in write mode or it already has waiters.
 488                  */
 489                 MPASS(!RW_CAN_READ(v));
 490
 491                 /*
 492                  * If the RW_LOCK_READ_WAITERS flag is already set, then
 493                  * we can go ahead and block.  If it is not set then try
 494                  * to set it.  If we fail to set it drop the turnstile
 495                  * lock and restart the loop.
 496                  */
 497                 if (!(v & RW_LOCK_READ_WAITERS)) {
 498                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 499                             v | RW_LOCK_READ_WAITERS)) {
 500                                 turnstile_cancel(ts);
 501                                 continue;
 502                         }
 503                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 504                                 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
 505                                     __func__, rw);
 506                 }
 507
 508                 /*
 509                  * We were unable to acquire the lock and the read waiters
 510                  * flag is set, so we must block on the turnstile.
 511                  */
 512                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 513                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 514                             rw);
 515 #ifdef KDTRACE_HOOKS
 516                 sleep_time -= lockstat_nsecs();
 517 #endif
 518                 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE);
 519 #ifdef KDTRACE_HOOKS
 520                 sleep_time += lockstat_nsecs();
 521                 sleep_cnt++;
 522 #endif
 523                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 524                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 525                             __func__, rw);
 526         }
 527
 528         /*
 529          * TODO: acquire "owner of record" here.  Here be turnstile dragons
 530          * however.  turnstiles don't like owners changing between calls to
 531          * turnstile_wait() currently.
 532          */
 533         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_RLOCK_ACQUIRE, rw, contested,
 534             waittime, file, line);
 535         LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
 536         WITNESS_LOCK(&rw->lock_object, 0, file, line);
 537         curthread->td_locks++;
 538         curthread->td_rw_rlocks++;
 539 #ifdef KDTRACE_HOOKS
 540         if (sleep_time)
 541                 LOCKSTAT_RECORD1(LS_RW_RLOCK_BLOCK, rw, sleep_time);
 542
 543         /*
 544          * Record only the loops spinning and not sleeping.
 545          */
 546         if (spin_cnt > sleep_cnt)
 547                 LOCKSTAT_RECORD1(LS_RW_RLOCK_SPIN, rw, (spin_cnt - sleep_cnt));
 548 #endif
 549 }
 550
 551 int
 552 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line)
 553 {
 554         struct rwlock *rw;
 555         uintptr_t x;
 556
 557         if (SCHEDULER_STOPPED())
 558                 return (1);
 559
 560         rw = rwlock2rw(c);
 561
 562         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 563             ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d",
 564             curthread, rw->lock_object.lo_name, file, line));
 565
 566         for (;;) {
 567                 x = rw->rw_lock;
 568                 KASSERT(rw->rw_lock != RW_DESTROYED,
 569                     ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
 570                 if (!(x & RW_LOCK_READ))
 571                         break;
 572                 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) {
 573                         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
 574                             line);
 575                         WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
 576                         curthread->td_locks++;
 577                         curthread->td_rw_rlocks++;
 578                         return (1);
 579                 }
 580         }
 581
 582         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
 583         return (0);
 584 }
 585
 586 void
 587 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line)
 588 {
 589         struct rwlock *rw;
 590         struct turnstile *ts;
 591         uintptr_t x, v, queue;
 592
 593         if (SCHEDULER_STOPPED())
 594                 return;
 595
 596         rw = rwlock2rw(c);
 597
 598         KASSERT(rw->rw_lock != RW_DESTROYED,
 599             ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
 600         __rw_assert(c, RA_RLOCKED, file, line);
 601         WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
 602         LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
 603
 604         /* TODO: drop "owner of record" here. */
 605
 606         for (;;) {
 607                 /*
 608                  * See if there is more than one read lock held.  If so,
 609                  * just drop one and return.
 610                  */
 611                 x = rw->rw_lock;
 612                 if (RW_READERS(x) > 1) {
 613                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
 614                             x - RW_ONE_READER)) {
 615                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 616                                         CTR4(KTR_LOCK,
 617                                             "%s: %p succeeded %p -> %p",
 618                                             __func__, rw, (void *)x,
 619                                             (void *)(x - RW_ONE_READER));
 620                                 break;
 621                         }
 622                         continue;
 623                 }
 624                 /*
 625                  * If there aren't any waiters for a write lock, then try
 626                  * to drop it quickly.
 627                  */
 628                 if (!(x & RW_LOCK_WAITERS)) {
 629                         MPASS((x & ~RW_LOCK_WRITE_SPINNER) ==
 630                             RW_READERS_LOCK(1));
 631                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
 632                             RW_UNLOCKED)) {
 633                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 634                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 635                                             __func__, rw);
 636                                 break;
 637                         }
 638                         continue;
 639                 }
 640                 /*
 641                  * Ok, we know we have waiters and we think we are the
 642                  * last reader, so grab the turnstile lock.
 643                  */
 644                 turnstile_chain_lock(&rw->lock_object);
 645                 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 646                 MPASS(v & RW_LOCK_WAITERS);
 647
 648                 /*
 649                  * Try to drop our lock leaving the lock in a unlocked
 650                  * state.
 651                  *
 652                  * If you wanted to do explicit lock handoff you'd have to
 653                  * do it here.  You'd also want to use turnstile_signal()
 654                  * and you'd have to handle the race where a higher
 655                  * priority thread blocks on the write lock before the
 656                  * thread you wakeup actually runs and have the new thread
 657                  * "steal" the lock.  For now it's a lot simpler to just
 658                  * wakeup all of the waiters.
 659                  *
 660                  * As above, if we fail, then another thread might have
 661                  * acquired a read lock, so drop the turnstile lock and
 662                  * restart.
 663                  */
 664                 x = RW_UNLOCKED;
 665                 if (v & RW_LOCK_WRITE_WAITERS) {
 666                         queue = TS_EXCLUSIVE_QUEUE;
 667                         x |= (v & RW_LOCK_READ_WAITERS);
 668                 } else
 669                         queue = TS_SHARED_QUEUE;
 670                 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
 671                     x)) {
 672                         turnstile_chain_unlock(&rw->lock_object);
 673                         continue;
 674                 }
 675                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 676                         CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
 677                             __func__, rw);
 678
 679                 /*
 680                  * Ok.  The lock is released and all that's left is to
 681                  * wake up the waiters.  Note that the lock might not be
 682                  * free anymore, but in that case the writers will just
 683                  * block again if they run before the new lock holder(s)
 684                  * release the lock.
 685                  */
 686                 ts = turnstile_lookup(&rw->lock_object);
 687                 MPASS(ts != NULL);
 688                 turnstile_broadcast(ts, queue);
 689                 turnstile_unpend(ts, TS_SHARED_LOCK);
 690                 turnstile_chain_unlock(&rw->lock_object);
 691                 break;
 692         }
 693         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_RUNLOCK_RELEASE, rw);
 694         curthread->td_locks--;
 695         curthread->td_rw_rlocks--;
 696 }
 697
 698 /*
 699  * This function is called when we are unable to obtain a write lock on the
 700  * first try.  This means that at least one other thread holds either a
 701  * read or write lock.
 702  */
 703 void
 704 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t tid, const char *file,
 705     int line)
 706 {
 707         struct rwlock *rw;
 708         struct turnstile *ts;
 709 #ifdef ADAPTIVE_RWLOCKS
 710         volatile struct thread *owner;
 711         int spintries = 0;
 712         int i;
 713 #endif
 714         uintptr_t v, x;
 715 #ifdef LOCK_PROFILING
 716         uint64_t waittime = 0;
 717         int contested = 0;
 718 #endif
 719 #ifdef KDTRACE_HOOKS
 720         uint64_t spin_cnt = 0;
 721         uint64_t sleep_cnt = 0;
 722         int64_t sleep_time = 0;
 723 #endif
 724
 725         if (SCHEDULER_STOPPED())
 726                 return;
 727
 728         rw = rwlock2rw(c);
 729
 730         if (rw_wlocked(rw)) {
 731                 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE,
 732                     ("%s: recursing but non-recursive rw %s @ %s:%d\n",
 733                     __func__, rw->lock_object.lo_name, file, line));
 734                 rw->rw_recurse++;
 735                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 736                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
 737                 return;
 738         }
 739
 740         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 741                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 742                     rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
 743
 744         while (!_rw_write_lock(rw, tid)) {
 745 #ifdef KDTRACE_HOOKS
 746                 spin_cnt++;
 747 #endif
 748 #ifdef HWPMC_HOOKS
 749                 PMC_SOFT_CALL( , , lock, failed);
 750 #endif
 751                 lock_profile_obtain_lock_failed(&rw->lock_object,
 752                     &contested, &waittime);
 753 #ifdef ADAPTIVE_RWLOCKS
 754                 /*
 755                  * If the lock is write locked and the owner is
 756                  * running on another CPU, spin until the owner stops
 757                  * running or the state of the lock changes.
 758                  */
 759                 v = rw->rw_lock;
 760                 owner = (struct thread *)RW_OWNER(v);
 761                 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
 762                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 763                                 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
 764                                     __func__, rw, owner);
 765                         while ((struct thread*)RW_OWNER(rw->rw_lock) == owner &&
 766                             TD_IS_RUNNING(owner)) {
 767                                 cpu_spinwait();
 768 #ifdef KDTRACE_HOOKS
 769                                 spin_cnt++;
 770 #endif
 771                         }
 772                         continue;
 773                 }
 774                 if ((v & RW_LOCK_READ) && RW_READERS(v) &&
 775                     spintries < rowner_retries) {
 776                         if (!(v & RW_LOCK_WRITE_SPINNER)) {
 777                                 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 778                                     v | RW_LOCK_WRITE_SPINNER)) {
 779                                         continue;
 780                                 }
 781                         }
 782                         spintries++;
 783                         for (i = 0; i < rowner_loops; i++) {
 784                                 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
 785                                         break;
 786                                 cpu_spinwait();
 787                         }
 788 #ifdef KDTRACE_HOOKS
 789                         spin_cnt += rowner_loops - i;
 790 #endif
 791                         if (i != rowner_loops)
 792                                 continue;
 793                 }
 794 #endif
 795                 ts = turnstile_trywait(&rw->lock_object);
 796                 v = rw->rw_lock;
 797
 798 #ifdef ADAPTIVE_RWLOCKS
 799                 /*
 800                  * The current lock owner might have started executing
 801                  * on another CPU (or the lock could have changed
 802                  * owners) while we were waiting on the turnstile
 803                  * chain lock.  If so, drop the turnstile lock and try
 804                  * again.
 805                  */
 806                 if (!(v & RW_LOCK_READ)) {
 807                         owner = (struct thread *)RW_OWNER(v);
 808                         if (TD_IS_RUNNING(owner)) {
 809                                 turnstile_cancel(ts);
 810                                 continue;
 811                         }
 812                 }
 813 #endif
 814                 /*
 815                  * Check for the waiters flags about this rwlock.
 816                  * If the lock was released, without maintain any pending
 817                  * waiters queue, simply try to acquire it.
 818                  * If a pending waiters queue is present, claim the lock
 819                  * ownership and maintain the pending queue.
 820                  */
 821                 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 822                 if ((v & ~x) == RW_UNLOCKED) {
 823                         x &= ~RW_LOCK_WRITE_SPINNER;
 824                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) {
 825                                 if (x)
 826                                         turnstile_claim(ts);
 827                                 else
 828                                         turnstile_cancel(ts);
 829                                 break;
 830                         }
 831                         turnstile_cancel(ts);
 832                         continue;
 833                 }
 834                 /*
 835                  * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
 836                  * set it.  If we fail to set it, then loop back and try
 837                  * again.
 838                  */
 839                 if (!(v & RW_LOCK_WRITE_WAITERS)) {
 840                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 841                             v | RW_LOCK_WRITE_WAITERS)) {
 842                                 turnstile_cancel(ts);
 843                                 continue;
 844                         }
 845                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 846                                 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
 847                                     __func__, rw);
 848                 }
 849                 /*
 850                  * We were unable to acquire the lock and the write waiters
 851                  * flag is set, so we must block on the turnstile.
 852                  */
 853                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 854                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 855                             rw);
 856 #ifdef KDTRACE_HOOKS
 857                 sleep_time -= lockstat_nsecs();
 858 #endif
 859                 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
 860 #ifdef KDTRACE_HOOKS
 861                 sleep_time += lockstat_nsecs();
 862                 sleep_cnt++;
 863 #endif
 864                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 865                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 866                             __func__, rw);
 867 #ifdef ADAPTIVE_RWLOCKS
 868                 spintries = 0;
 869 #endif
 870         }
 871         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_WLOCK_ACQUIRE, rw, contested,
 872             waittime, file, line);
 873 #ifdef KDTRACE_HOOKS
 874         if (sleep_time)
 875                 LOCKSTAT_RECORD1(LS_RW_WLOCK_BLOCK, rw, sleep_time);
 876
 877         /*
 878          * Record only the loops spinning and not sleeping.
 879          */
 880         if (spin_cnt > sleep_cnt)
 881                 LOCKSTAT_RECORD1(LS_RW_WLOCK_SPIN, rw, (spin_cnt - sleep_cnt));
 882 #endif
 883 }
 884
 885 /*
 886  * This function is called if the first try at releasing a write lock failed.
 887  * This means that one of the 2 waiter bits must be set indicating that at
 888  * least one thread is waiting on this lock.
 889  */
 890 void
 891 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t tid, const char *file,
 892     int line)
 893 {
 894         struct rwlock *rw;
 895         struct turnstile *ts;
 896         uintptr_t v;
 897         int queue;
 898
 899         if (SCHEDULER_STOPPED())
 900                 return;
 901
 902         rw = rwlock2rw(c);
 903
 904         if (rw_wlocked(rw) && rw_recursed(rw)) {
 905                 rw->rw_recurse--;
 906                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 907                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
 908                 return;
 909         }
 910
 911         KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
 912             ("%s: neither of the waiter flags are set", __func__));
 913
 914         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 915                 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
 916
 917         turnstile_chain_lock(&rw->lock_object);
 918         ts = turnstile_lookup(&rw->lock_object);
 919         MPASS(ts != NULL);
 920
 921         /*
 922          * Use the same algo as sx locks for now.  Prefer waking up shared
 923          * waiters if we have any over writers.  This is probably not ideal.
 924          *
 925          * 'v' is the value we are going to write back to rw_lock.  If we
 926          * have waiters on both queues, we need to preserve the state of
 927          * the waiter flag for the queue we don't wake up.  For now this is
 928          * hardcoded for the algorithm mentioned above.
 929          *
 930          * In the case of both readers and writers waiting we wakeup the
 931          * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
 932          * new writer comes in before a reader it will claim the lock up
 933          * above.  There is probably a potential priority inversion in
 934          * there that could be worked around either by waking both queues
 935          * of waiters or doing some complicated lock handoff gymnastics.
 936          */
 937         v = RW_UNLOCKED;
 938         if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
 939                 queue = TS_EXCLUSIVE_QUEUE;
 940                 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
 941         } else
 942                 queue = TS_SHARED_QUEUE;
 943
 944         /* Wake up all waiters for the specific queue. */
 945         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 946                 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
 947                     queue == TS_SHARED_QUEUE ? "read" : "write");
 948         turnstile_broadcast(ts, queue);
 949         atomic_store_rel_ptr(&rw->rw_lock, v);
 950         turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
 951         turnstile_chain_unlock(&rw->lock_object);
 952 }
 953
 954 /*
 955  * Attempt to do a non-blocking upgrade from a read lock to a write
 956  * lock.  This will only succeed if this thread holds a single read
 957  * lock.  Returns true if the upgrade succeeded and false otherwise.
 958  */
 959 int
 960 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line)
 961 {
 962         struct rwlock *rw;
 963         uintptr_t v, x, tid;
 964         struct turnstile *ts;
 965         int success;
 966
 967         if (SCHEDULER_STOPPED())
 968                 return (1);
 969
 970         rw = rwlock2rw(c);
 971
 972         KASSERT(rw->rw_lock != RW_DESTROYED,
 973             ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
 974         __rw_assert(c, RA_RLOCKED, file, line);
 975
 976         /*
 977          * Attempt to switch from one reader to a writer.  If there
 978          * are any write waiters, then we will have to lock the
 979          * turnstile first to prevent races with another writer
 980          * calling turnstile_wait() before we have claimed this
 981          * turnstile.  So, do the simple case of no waiters first.
 982          */
 983         tid = (uintptr_t)curthread;
 984         success = 0;
 985         for (;;) {
 986                 v = rw->rw_lock;
 987                 if (RW_READERS(v) > 1)
 988                         break;
 989                 if (!(v & RW_LOCK_WAITERS)) {
 990                         success = atomic_cmpset_ptr(&rw->rw_lock, v, tid);
 991                         if (!success)
 992                                 continue;
 993                         break;
 994                 }
 995
 996                 /*
 997                  * Ok, we think we have waiters, so lock the turnstile.
 998                  */
 999                 ts = turnstile_trywait(&rw->lock_object);
1000                 v = rw->rw_lock;
1001                 if (RW_READERS(v) > 1) {
1002                         turnstile_cancel(ts);
1003                         break;
1004                 }
1005                 /*
1006                  * Try to switch from one reader to a writer again.  This time
1007                  * we honor the current state of the waiters flags.
1008                  * If we obtain the lock with the flags set, then claim
1009                  * ownership of the turnstile.
1010                  */
1011                 x = rw->rw_lock & RW_LOCK_WAITERS;
1012                 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
1013                 if (success) {
1014                         if (x)
1015                                 turnstile_claim(ts);
1016                         else
1017                                 turnstile_cancel(ts);
1018                         break;
1019                 }
1020                 turnstile_cancel(ts);
1021         }
1022         LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
1023         if (success) {
1024                 curthread->td_rw_rlocks--;
1025                 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
1026                     file, line);
1027                 LOCKSTAT_RECORD0(LS_RW_TRYUPGRADE_UPGRADE, rw);
1028         }
1029         return (success);
1030 }
1031
1032 /*
1033  * Downgrade a write lock into a single read lock.
1034  */
1035 void
1036 __rw_downgrade(volatile uintptr_t *c, const char *file, int line)
1037 {
1038         struct rwlock *rw;
1039         struct turnstile *ts;
1040         uintptr_t tid, v;
1041         int rwait, wwait;
1042
1043         if (SCHEDULER_STOPPED())
1044                 return;
1045
1046         rw = rwlock2rw(c);
1047
1048         KASSERT(rw->rw_lock != RW_DESTROYED,
1049             ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
1050         __rw_assert(c, RA_WLOCKED | RA_NOTRECURSED, file, line);
1051 #ifndef INVARIANTS
1052         if (rw_recursed(rw))
1053                 panic("downgrade of a recursed lock");
1054 #endif
1055
1056         WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
1057
1058         /*
1059          * Convert from a writer to a single reader.  First we handle
1060          * the easy case with no waiters.  If there are any waiters, we
1061          * lock the turnstile and "disown" the lock.
1062          */
1063         tid = (uintptr_t)curthread;
1064         if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
1065                 goto out;
1066
1067         /*
1068          * Ok, we think we have waiters, so lock the turnstile so we can
1069          * read the waiter flags without any races.
1070          */
1071         turnstile_chain_lock(&rw->lock_object);
1072         v = rw->rw_lock & RW_LOCK_WAITERS;
1073         rwait = v & RW_LOCK_READ_WAITERS;
1074         wwait = v & RW_LOCK_WRITE_WAITERS;
1075         MPASS(rwait | wwait);
1076
1077         /*
1078          * Downgrade from a write lock while preserving waiters flag
1079          * and give up ownership of the turnstile.
1080          */
1081         ts = turnstile_lookup(&rw->lock_object);
1082         MPASS(ts != NULL);
1083         if (!wwait)
1084                 v &= ~RW_LOCK_READ_WAITERS;
1085         atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
1086         /*
1087          * Wake other readers if there are no writers pending.  Otherwise they
1088          * won't be able to acquire the lock anyway.
1089          */
1090         if (rwait && !wwait) {
1091                 turnstile_broadcast(ts, TS_SHARED_QUEUE);
1092                 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
1093         } else
1094                 turnstile_disown(ts);
1095         turnstile_chain_unlock(&rw->lock_object);
1096 out:
1097         curthread->td_rw_rlocks++;
1098         LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
1099         LOCKSTAT_RECORD0(LS_RW_DOWNGRADE_DOWNGRADE, rw);
1100 }
1101
1102 #ifdef INVARIANT_SUPPORT
1103 #ifndef INVARIANTS
1104 #undef __rw_assert
1105 #endif
1106
1107 /*
1108  * In the non-WITNESS case, rw_assert() can only detect that at least
1109  * *some* thread owns an rlock, but it cannot guarantee that *this*
1110  * thread owns an rlock.
1111  */
1112 void
1113 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line)
1114 {
1115         const struct rwlock *rw;
1116
1117         if (panicstr != NULL)
1118                 return;
1119
1120         rw = rwlock2rw(c);
1121
1122         switch (what) {
1123         case RA_LOCKED:
1124         case RA_LOCKED | RA_RECURSED:
1125         case RA_LOCKED | RA_NOTRECURSED:
1126         case RA_RLOCKED:
1127         case RA_RLOCKED | RA_RECURSED:
1128         case RA_RLOCKED | RA_NOTRECURSED:
1129 #ifdef WITNESS
1130                 witness_assert(&rw->lock_object, what, file, line);
1131 #else
1132                 /*
1133                  * If some other thread has a write lock or we have one
1134                  * and are asserting a read lock, fail.  Also, if no one
1135                  * has a lock at all, fail.
1136                  */
1137                 if (rw->rw_lock == RW_UNLOCKED ||
1138                     (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED ||
1139                     rw_wowner(rw) != curthread)))
1140                         panic("Lock %s not %slocked @ %s:%d\n",
1141                             rw->lock_object.lo_name, (what & RA_RLOCKED) ?
1142                             "read " : "", file, line);
1143
1144                 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
1145                         if (rw_recursed(rw)) {
1146                                 if (what & RA_NOTRECURSED)
1147                                         panic("Lock %s recursed @ %s:%d\n",
1148                                             rw->lock_object.lo_name, file,
1149                                             line);
1150                         } else if (what & RA_RECURSED)
1151                                 panic("Lock %s not recursed @ %s:%d\n",
1152                                     rw->lock_object.lo_name, file, line);
1153                 }
1154 #endif
1155                 break;
1156         case RA_WLOCKED:
1157         case RA_WLOCKED | RA_RECURSED:
1158         case RA_WLOCKED | RA_NOTRECURSED:
1159                 if (rw_wowner(rw) != curthread)
1160                         panic("Lock %s not exclusively locked @ %s:%d\n",
1161                             rw->lock_object.lo_name, file, line);
1162                 if (rw_recursed(rw)) {
1163                         if (what & RA_NOTRECURSED)
1164                                 panic("Lock %s recursed @ %s:%d\n",
1165                                     rw->lock_object.lo_name, file, line);
1166                 } else if (what & RA_RECURSED)
1167                         panic("Lock %s not recursed @ %s:%d\n",
1168                             rw->lock_object.lo_name, file, line);
1169                 break;
1170         case RA_UNLOCKED:
1171 #ifdef WITNESS
1172                 witness_assert(&rw->lock_object, what, file, line);
1173 #else
1174                 /*
1175                  * If we hold a write lock fail.  We can't reliably check
1176                  * to see if we hold a read lock or not.
1177                  */
1178                 if (rw_wowner(rw) == curthread)
1179                         panic("Lock %s exclusively locked @ %s:%d\n",
1180                             rw->lock_object.lo_name, file, line);
1181 #endif
1182                 break;
1183         default:
1184                 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
1185                     line);
1186         }
1187 }
1188 #endif /* INVARIANT_SUPPORT */
1189
1190 #ifdef DDB
1191 void
1192 db_show_rwlock(const struct lock_object *lock)
1193 {
1194         const struct rwlock *rw;
1195         struct thread *td;
1196
1197         rw = (const struct rwlock *)lock;
1198
1199         db_printf(" state: ");
1200         if (rw->rw_lock == RW_UNLOCKED)
1201                 db_printf("UNLOCKED\n");
1202         else if (rw->rw_lock == RW_DESTROYED) {
1203                 db_printf("DESTROYED\n");
1204                 return;
1205         } else if (rw->rw_lock & RW_LOCK_READ)
1206                 db_printf("RLOCK: %ju locks\n",
1207                     (uintmax_t)(RW_READERS(rw->rw_lock)));
1208         else {
1209                 td = rw_wowner(rw);
1210                 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1211                     td->td_tid, td->td_proc->p_pid, td->td_name);
1212                 if (rw_recursed(rw))
1213                         db_printf(" recursed: %u\n", rw->rw_recurse);
1214         }
1215         db_printf(" waiters: ");
1216         switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
1217         case RW_LOCK_READ_WAITERS:
1218                 db_printf("readers\n");
1219                 break;
1220         case RW_LOCK_WRITE_WAITERS:
1221                 db_printf("writers\n");
1222                 break;
1223         case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
1224                 db_printf("readers and writers\n");
1225                 break;
1226         default:
1227                 db_printf("none\n");
1228                 break;
1229         }
1230 }
1231
1232 #endif