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_no_adaptive_rwlocks.h"
  37
  38 #include <sys/param.h>
  39 #include <sys/kdb.h>
  40 #include <sys/ktr.h>
  41 #include <sys/kernel.h>
  42 #include <sys/lock.h>
  43 #include <sys/mutex.h>
  44 #include <sys/proc.h>
  45 #include <sys/rwlock.h>
  46 #include <sys/sched.h>
  47 #include <sys/smp.h>
  48 #include <sys/sysctl.h>
  49 #include <sys/systm.h>
  50 #include <sys/turnstile.h>
  51
  52 #include <machine/cpu.h>
  53
  54 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS)
  55 #define ADAPTIVE_RWLOCKS
  56 #endif
  57
  58 #ifdef HWPMC_HOOKS
  59 #include <sys/pmckern.h>
  60 PMC_SOFT_DECLARE( , , lock, failed);
  61 #endif
  62
  63 /*
  64  * Return the rwlock address when the lock cookie address is provided.
  65  * This functionality assumes that struct rwlock* have a member named rw_lock.
  66  */
  67 #define rwlock2rw(c)    (__containerof(c, struct rwlock, rw_lock))
  68
  69 #ifdef DDB
  70 #include <ddb/ddb.h>
  71
  72 static void     db_show_rwlock(const struct lock_object *lock);
  73 #endif
  74 static void     assert_rw(const struct lock_object *lock, int what);
  75 static void     lock_rw(struct lock_object *lock, uintptr_t how);
  76 #ifdef KDTRACE_HOOKS
  77 static int      owner_rw(const struct lock_object *lock, struct thread **owner);
  78 #endif
  79 static uintptr_t unlock_rw(struct lock_object *lock);
  80
  81 struct lock_class lock_class_rw = {
  82         .lc_name = "rw",
  83         .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
  84         .lc_assert = assert_rw,
  85 #ifdef DDB
  86         .lc_ddb_show = db_show_rwlock,
  87 #endif
  88         .lc_lock = lock_rw,
  89         .lc_unlock = unlock_rw,
  90 #ifdef KDTRACE_HOOKS
  91         .lc_owner = owner_rw,
  92 #endif
  93 };
  94
  95 #ifdef ADAPTIVE_RWLOCKS
  96 static int rowner_retries = 10;
  97 static int rowner_loops = 10000;
  98 static SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL,
  99     "rwlock debugging");
 100 SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, "");
 101 SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, "");
 102
 103 static struct lock_delay_config __read_mostly rw_delay;
 104
 105 SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_base, CTLFLAG_RW, &rw_delay.base,
 106     0, "");
 107 SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_max, CTLFLAG_RW, &rw_delay.max,
 108     0, "");
 109
 110 LOCK_DELAY_SYSINIT_DEFAULT(rw_delay);
 111 #endif
 112
 113 /*
 114  * Return a pointer to the owning thread if the lock is write-locked or
 115  * NULL if the lock is unlocked or read-locked.
 116  */
 117
 118 #define lv_rw_wowner(v)                                                 \
 119         ((v) & RW_LOCK_READ ? NULL :                                    \
 120          (struct thread *)RW_OWNER((v)))
 121
 122 #define rw_wowner(rw)   lv_rw_wowner(RW_READ_VALUE(rw))
 123
 124 /*
 125  * Returns if a write owner is recursed.  Write ownership is not assured
 126  * here and should be previously checked.
 127  */
 128 #define rw_recursed(rw)         ((rw)->rw_recurse != 0)
 129
 130 /*
 131  * Return true if curthread helds the lock.
 132  */
 133 #define rw_wlocked(rw)          (rw_wowner((rw)) == curthread)
 134
 135 /*
 136  * Return a pointer to the owning thread for this lock who should receive
 137  * any priority lent by threads that block on this lock.  Currently this
 138  * is identical to rw_wowner().
 139  */
 140 #define rw_owner(rw)            rw_wowner(rw)
 141
 142 #ifndef INVARIANTS
 143 #define __rw_assert(c, what, file, line)
 144 #endif
 145
 146 void
 147 assert_rw(const struct lock_object *lock, int what)
 148 {
 149
 150         rw_assert((const struct rwlock *)lock, what);
 151 }
 152
 153 void
 154 lock_rw(struct lock_object *lock, uintptr_t how)
 155 {
 156         struct rwlock *rw;
 157
 158         rw = (struct rwlock *)lock;
 159         if (how)
 160                 rw_rlock(rw);
 161         else
 162                 rw_wlock(rw);
 163 }
 164
 165 uintptr_t
 166 unlock_rw(struct lock_object *lock)
 167 {
 168         struct rwlock *rw;
 169
 170         rw = (struct rwlock *)lock;
 171         rw_assert(rw, RA_LOCKED | LA_NOTRECURSED);
 172         if (rw->rw_lock & RW_LOCK_READ) {
 173                 rw_runlock(rw);
 174                 return (1);
 175         } else {
 176                 rw_wunlock(rw);
 177                 return (0);
 178         }
 179 }
 180
 181 #ifdef KDTRACE_HOOKS
 182 int
 183 owner_rw(const struct lock_object *lock, struct thread **owner)
 184 {
 185         const struct rwlock *rw = (const struct rwlock *)lock;
 186         uintptr_t x = rw->rw_lock;
 187
 188         *owner = rw_wowner(rw);
 189         return ((x & RW_LOCK_READ) != 0 ?  (RW_READERS(x) != 0) :
 190             (*owner != NULL));
 191 }
 192 #endif
 193
 194 void
 195 _rw_init_flags(volatile uintptr_t *c, const char *name, int opts)
 196 {
 197         struct rwlock *rw;
 198         int flags;
 199
 200         rw = rwlock2rw(c);
 201
 202         MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
 203             RW_RECURSE | RW_NEW)) == 0);
 204         ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock,
 205             ("%s: rw_lock not aligned for %s: %p", __func__, name,
 206             &rw->rw_lock));
 207
 208         flags = LO_UPGRADABLE;
 209         if (opts & RW_DUPOK)
 210                 flags |= LO_DUPOK;
 211         if (opts & RW_NOPROFILE)
 212                 flags |= LO_NOPROFILE;
 213         if (!(opts & RW_NOWITNESS))
 214                 flags |= LO_WITNESS;
 215         if (opts & RW_RECURSE)
 216                 flags |= LO_RECURSABLE;
 217         if (opts & RW_QUIET)
 218                 flags |= LO_QUIET;
 219         if (opts & RW_NEW)
 220                 flags |= LO_NEW;
 221
 222         lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
 223         rw->rw_lock = RW_UNLOCKED;
 224         rw->rw_recurse = 0;
 225 }
 226
 227 void
 228 _rw_destroy(volatile uintptr_t *c)
 229 {
 230         struct rwlock *rw;
 231
 232         rw = rwlock2rw(c);
 233
 234         KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw));
 235         KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw));
 236         rw->rw_lock = RW_DESTROYED;
 237         lock_destroy(&rw->lock_object);
 238 }
 239
 240 void
 241 rw_sysinit(void *arg)
 242 {
 243         struct rw_args *args = arg;
 244
 245         rw_init((struct rwlock *)args->ra_rw, args->ra_desc);
 246 }
 247
 248 void
 249 rw_sysinit_flags(void *arg)
 250 {
 251         struct rw_args_flags *args = arg;
 252
 253         rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc,
 254             args->ra_flags);
 255 }
 256
 257 int
 258 _rw_wowned(const volatile uintptr_t *c)
 259 {
 260
 261         return (rw_wowner(rwlock2rw(c)) == curthread);
 262 }
 263
 264 void
 265 _rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line)
 266 {
 267         struct rwlock *rw;
 268         uintptr_t tid, v;
 269
 270         rw = rwlock2rw(c);
 271
 272         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 273             ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d",
 274             curthread, rw->lock_object.lo_name, file, line));
 275         KASSERT(rw->rw_lock != RW_DESTROYED,
 276             ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
 277         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 278             line, NULL);
 279         tid = (uintptr_t)curthread;
 280         v = RW_UNLOCKED;
 281         if (!_rw_write_lock_fetch(rw, &v, tid))
 282                 _rw_wlock_hard(rw, v, tid, file, line);
 283         else
 284                 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw,
 285                     0, 0, file, line, LOCKSTAT_WRITER);
 286
 287         LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
 288         WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 289         TD_LOCKS_INC(curthread);
 290 }
 291
 292 int
 293 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line)
 294 {
 295         struct rwlock *rw;
 296         struct thread *td;
 297         uintptr_t tid, v;
 298         int rval;
 299         bool recursed;
 300
 301         td = curthread;
 302         tid = (uintptr_t)td;
 303         if (SCHEDULER_STOPPED_TD(td))
 304                 return (1);
 305
 306         rw = rwlock2rw(c);
 307
 308         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 309             ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d",
 310             curthread, rw->lock_object.lo_name, file, line));
 311         KASSERT(rw->rw_lock != RW_DESTROYED,
 312             ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
 313
 314         rval = 1;
 315         recursed = false;
 316         v = RW_UNLOCKED;
 317         for (;;) {
 318                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid))
 319                         break;
 320                 if (v == RW_UNLOCKED)
 321                         continue;
 322                 if (v == tid && (rw->lock_object.lo_flags & LO_RECURSABLE)) {
 323                         rw->rw_recurse++;
 324                         atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
 325                         break;
 326                 }
 327                 rval = 0;
 328                 break;
 329         }
 330
 331         LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
 332         if (rval) {
 333                 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 334                     file, line);
 335                 if (!recursed)
 336                         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire,
 337                             rw, 0, 0, file, line, LOCKSTAT_WRITER);
 338                 TD_LOCKS_INC(curthread);
 339         }
 340         return (rval);
 341 }
 342
 343 void
 344 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line)
 345 {
 346         struct rwlock *rw;
 347
 348         rw = rwlock2rw(c);
 349
 350         KASSERT(rw->rw_lock != RW_DESTROYED,
 351             ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
 352         __rw_assert(c, RA_WLOCKED, file, line);
 353         WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 354         LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
 355             line);
 356
 357 #ifdef LOCK_PROFILING
 358         _rw_wunlock_hard(rw, (uintptr_t)curthread, file, line);
 359 #else
 360         __rw_wunlock(rw, curthread, file, line);
 361 #endif
 362
 363         TD_LOCKS_DEC(curthread);
 364 }
 365
 366 /*
 367  * Determines whether a new reader can acquire a lock.  Succeeds if the
 368  * reader already owns a read lock and the lock is locked for read to
 369  * prevent deadlock from reader recursion.  Also succeeds if the lock
 370  * is unlocked and has no writer waiters or spinners.  Failing otherwise
 371  * prioritizes writers before readers.
 372  */
 373 #define RW_CAN_READ(td, _rw)                                            \
 374     (((td)->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) &   \
 375     (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) ==  \
 376     RW_LOCK_READ)
 377
 378 static bool __always_inline
 379 __rw_rlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp,
 380     const char *file, int line)
 381 {
 382
 383         /*
 384          * Handle the easy case.  If no other thread has a write
 385          * lock, then try to bump up the count of read locks.  Note
 386          * that we have to preserve the current state of the
 387          * RW_LOCK_WRITE_WAITERS flag.  If we fail to acquire a
 388          * read lock, then rw_lock must have changed, so restart
 389          * the loop.  Note that this handles the case of a
 390          * completely unlocked rwlock since such a lock is encoded
 391          * as a read lock with no waiters.
 392          */
 393         while (RW_CAN_READ(td, *vp)) {
 394                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, vp,
 395                         *vp + RW_ONE_READER)) {
 396                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 397                                 CTR4(KTR_LOCK,
 398                                     "%s: %p succeed %p -> %p", __func__,
 399                                     rw, (void *)*vp,
 400                                     (void *)(*vp + RW_ONE_READER));
 401                         td->td_rw_rlocks++;
 402                         return (true);
 403                 }
 404         }
 405         return (false);
 406 }
 407
 408 static void __noinline
 409 __rw_rlock_hard(volatile uintptr_t *c, struct thread *td, uintptr_t v,
 410     const char *file, int line)
 411 {
 412         struct rwlock *rw;
 413         struct turnstile *ts;
 414 #ifdef ADAPTIVE_RWLOCKS
 415         volatile struct thread *owner;
 416         int spintries = 0;
 417         int i;
 418 #endif
 419 #ifdef LOCK_PROFILING
 420         uint64_t waittime = 0;
 421         int contested = 0;
 422 #endif
 423 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS)
 424         struct lock_delay_arg lda;
 425 #endif
 426 #ifdef KDTRACE_HOOKS
 427         uintptr_t state;
 428         u_int sleep_cnt = 0;
 429         int64_t sleep_time = 0;
 430         int64_t all_time = 0;
 431 #endif
 432
 433         if (SCHEDULER_STOPPED())
 434                 return;
 435
 436 #if defined(ADAPTIVE_RWLOCKS)
 437         lock_delay_arg_init(&lda, &rw_delay);
 438 #elif defined(KDTRACE_HOOKS)
 439         lock_delay_arg_init(&lda, NULL);
 440 #endif
 441         rw = rwlock2rw(c);
 442
 443 #ifdef KDTRACE_HOOKS
 444         all_time -= lockstat_nsecs(&rw->lock_object);
 445 #endif
 446 #ifdef KDTRACE_HOOKS
 447         state = v;
 448 #endif
 449         for (;;) {
 450                 if (__rw_rlock_try(rw, td, &v, file, line))
 451                         break;
 452 #ifdef KDTRACE_HOOKS
 453                 lda.spin_cnt++;
 454 #endif
 455 #ifdef HWPMC_HOOKS
 456                 PMC_SOFT_CALL( , , lock, failed);
 457 #endif
 458                 lock_profile_obtain_lock_failed(&rw->lock_object,
 459                     &contested, &waittime);
 460
 461 #ifdef ADAPTIVE_RWLOCKS
 462                 /*
 463                  * If the owner is running on another CPU, spin until
 464                  * the owner stops running or the state of the lock
 465                  * changes.
 466                  */
 467                 if ((v & RW_LOCK_READ) == 0) {
 468                         owner = (struct thread *)RW_OWNER(v);
 469                         if (TD_IS_RUNNING(owner)) {
 470                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 471                                         CTR3(KTR_LOCK,
 472                                             "%s: spinning on %p held by %p",
 473                                             __func__, rw, owner);
 474                                 KTR_STATE1(KTR_SCHED, "thread",
 475                                     sched_tdname(curthread), "spinning",
 476                                     "lockname:\"%s\"", rw->lock_object.lo_name);
 477                                 do {
 478                                         lock_delay(&lda);
 479                                         v = RW_READ_VALUE(rw);
 480                                         owner = lv_rw_wowner(v);
 481                                 } while (owner != NULL && TD_IS_RUNNING(owner));
 482                                 KTR_STATE0(KTR_SCHED, "thread",
 483                                     sched_tdname(curthread), "running");
 484                                 continue;
 485                         }
 486                 } else if (spintries < rowner_retries) {
 487                         spintries++;
 488                         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
 489                             "spinning", "lockname:\"%s\"",
 490                             rw->lock_object.lo_name);
 491                         for (i = 0; i < rowner_loops; i++) {
 492                                 v = RW_READ_VALUE(rw);
 493                                 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(td, v))
 494                                         break;
 495                                 cpu_spinwait();
 496                         }
 497                         v = RW_READ_VALUE(rw);
 498 #ifdef KDTRACE_HOOKS
 499                         lda.spin_cnt += rowner_loops - i;
 500 #endif
 501                         KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 502                             "running");
 503                         if (i != rowner_loops)
 504                                 continue;
 505                 }
 506 #endif
 507
 508                 /*
 509                  * Okay, now it's the hard case.  Some other thread already
 510                  * has a write lock or there are write waiters present,
 511                  * acquire the turnstile lock so we can begin the process
 512                  * of blocking.
 513                  */
 514                 ts = turnstile_trywait(&rw->lock_object);
 515
 516                 /*
 517                  * The lock might have been released while we spun, so
 518                  * recheck its state and restart the loop if needed.
 519                  */
 520                 v = RW_READ_VALUE(rw);
 521                 if (RW_CAN_READ(td, v)) {
 522                         turnstile_cancel(ts);
 523                         continue;
 524                 }
 525
 526 #ifdef ADAPTIVE_RWLOCKS
 527                 /*
 528                  * The current lock owner might have started executing
 529                  * on another CPU (or the lock could have changed
 530                  * owners) while we were waiting on the turnstile
 531                  * chain lock.  If so, drop the turnstile lock and try
 532                  * again.
 533                  */
 534                 if ((v & RW_LOCK_READ) == 0) {
 535                         owner = (struct thread *)RW_OWNER(v);
 536                         if (TD_IS_RUNNING(owner)) {
 537                                 turnstile_cancel(ts);
 538                                 continue;
 539                         }
 540                 }
 541 #endif
 542
 543                 /*
 544                  * The lock is held in write mode or it already has waiters.
 545                  */
 546                 MPASS(!RW_CAN_READ(td, v));
 547
 548                 /*
 549                  * If the RW_LOCK_READ_WAITERS flag is already set, then
 550                  * we can go ahead and block.  If it is not set then try
 551                  * to set it.  If we fail to set it drop the turnstile
 552                  * lock and restart the loop.
 553                  */
 554                 if (!(v & RW_LOCK_READ_WAITERS)) {
 555                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 556                             v | RW_LOCK_READ_WAITERS)) {
 557                                 turnstile_cancel(ts);
 558                                 v = RW_READ_VALUE(rw);
 559                                 continue;
 560                         }
 561                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 562                                 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
 563                                     __func__, rw);
 564                 }
 565
 566                 /*
 567                  * We were unable to acquire the lock and the read waiters
 568                  * flag is set, so we must block on the turnstile.
 569                  */
 570                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 571                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 572                             rw);
 573 #ifdef KDTRACE_HOOKS
 574                 sleep_time -= lockstat_nsecs(&rw->lock_object);
 575 #endif
 576                 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE);
 577 #ifdef KDTRACE_HOOKS
 578                 sleep_time += lockstat_nsecs(&rw->lock_object);
 579                 sleep_cnt++;
 580 #endif
 581                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 582                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 583                             __func__, rw);
 584                 v = RW_READ_VALUE(rw);
 585         }
 586 #ifdef KDTRACE_HOOKS
 587         all_time += lockstat_nsecs(&rw->lock_object);
 588         if (sleep_time)
 589                 LOCKSTAT_RECORD4(rw__block, rw, sleep_time,
 590                     LOCKSTAT_READER, (state & RW_LOCK_READ) == 0,
 591                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 592
 593         /* Record only the loops spinning and not sleeping. */
 594         if (lda.spin_cnt > sleep_cnt)
 595                 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time,
 596                     LOCKSTAT_READER, (state & RW_LOCK_READ) == 0,
 597                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 598 #endif
 599         /*
 600          * TODO: acquire "owner of record" here.  Here be turnstile dragons
 601          * however.  turnstiles don't like owners changing between calls to
 602          * turnstile_wait() currently.
 603          */
 604         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested,
 605             waittime, file, line, LOCKSTAT_READER);
 606 }
 607
 608 void
 609 __rw_rlock(volatile uintptr_t *c, const char *file, int line)
 610 {
 611         struct rwlock *rw;
 612         struct thread *td;
 613         uintptr_t v;
 614
 615         td = curthread;
 616         rw = rwlock2rw(c);
 617
 618         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td),
 619             ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d",
 620             td, rw->lock_object.lo_name, file, line));
 621         KASSERT(rw->rw_lock != RW_DESTROYED,
 622             ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
 623         KASSERT(rw_wowner(rw) != td,
 624             ("rw_rlock: wlock already held for %s @ %s:%d",
 625             rw->lock_object.lo_name, file, line));
 626         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL);
 627
 628         v = RW_READ_VALUE(rw);
 629         if (__predict_false(LOCKSTAT_OOL_PROFILE_ENABLED(rw__acquire) ||
 630             !__rw_rlock_try(rw, td, &v, file, line)))
 631                 __rw_rlock_hard(c, td, v, file, line);
 632
 633         LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
 634         WITNESS_LOCK(&rw->lock_object, 0, file, line);
 635         TD_LOCKS_INC(curthread);
 636 }
 637
 638 int
 639 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line)
 640 {
 641         struct rwlock *rw;
 642         uintptr_t x;
 643
 644         if (SCHEDULER_STOPPED())
 645                 return (1);
 646
 647         rw = rwlock2rw(c);
 648
 649         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 650             ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d",
 651             curthread, rw->lock_object.lo_name, file, line));
 652
 653         x = rw->rw_lock;
 654         for (;;) {
 655                 KASSERT(rw->rw_lock != RW_DESTROYED,
 656                     ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
 657                 if (!(x & RW_LOCK_READ))
 658                         break;
 659                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &x, x + RW_ONE_READER)) {
 660                         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
 661                             line);
 662                         WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
 663                         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire,
 664                             rw, 0, 0, file, line, LOCKSTAT_READER);
 665                         TD_LOCKS_INC(curthread);
 666                         curthread->td_rw_rlocks++;
 667                         return (1);
 668                 }
 669         }
 670
 671         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
 672         return (0);
 673 }
 674
 675 static bool __always_inline
 676 __rw_runlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp)
 677 {
 678
 679         for (;;) {
 680                 /*
 681                  * See if there is more than one read lock held.  If so,
 682                  * just drop one and return.
 683                  */
 684                 if (RW_READERS(*vp) > 1) {
 685                         if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp,
 686                             *vp - RW_ONE_READER)) {
 687                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 688                                         CTR4(KTR_LOCK,
 689                                             "%s: %p succeeded %p -> %p",
 690                                             __func__, rw, (void *)*vp,
 691                                             (void *)(*vp - RW_ONE_READER));
 692                                 td->td_rw_rlocks--;
 693                                 return (true);
 694                         }
 695                         continue;
 696                 }
 697                 /*
 698                  * If there aren't any waiters for a write lock, then try
 699                  * to drop it quickly.
 700                  */
 701                 if (!(*vp & RW_LOCK_WAITERS)) {
 702                         MPASS((*vp & ~RW_LOCK_WRITE_SPINNER) ==
 703                             RW_READERS_LOCK(1));
 704                         if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp,
 705                             RW_UNLOCKED)) {
 706                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 707                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 708                                             __func__, rw);
 709                                 td->td_rw_rlocks--;
 710                                 return (true);
 711                         }
 712                         continue;
 713                 }
 714                 break;
 715         }
 716         return (false);
 717 }
 718
 719 static void __noinline
 720 __rw_runlock_hard(volatile uintptr_t *c, struct thread *td, uintptr_t v,
 721     const char *file, int line)
 722 {
 723         struct rwlock *rw;
 724         struct turnstile *ts;
 725         uintptr_t x, queue;
 726
 727         if (SCHEDULER_STOPPED())
 728                 return;
 729
 730         rw = rwlock2rw(c);
 731
 732         for (;;) {
 733                 if (__rw_runlock_try(rw, td, &v))
 734                         break;
 735
 736                 /*
 737                  * Ok, we know we have waiters and we think we are the
 738                  * last reader, so grab the turnstile lock.
 739                  */
 740                 turnstile_chain_lock(&rw->lock_object);
 741                 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 742                 MPASS(v & RW_LOCK_WAITERS);
 743
 744                 /*
 745                  * Try to drop our lock leaving the lock in a unlocked
 746                  * state.
 747                  *
 748                  * If you wanted to do explicit lock handoff you'd have to
 749                  * do it here.  You'd also want to use turnstile_signal()
 750                  * and you'd have to handle the race where a higher
 751                  * priority thread blocks on the write lock before the
 752                  * thread you wakeup actually runs and have the new thread
 753                  * "steal" the lock.  For now it's a lot simpler to just
 754                  * wakeup all of the waiters.
 755                  *
 756                  * As above, if we fail, then another thread might have
 757                  * acquired a read lock, so drop the turnstile lock and
 758                  * restart.
 759                  */
 760                 x = RW_UNLOCKED;
 761                 if (v & RW_LOCK_WRITE_WAITERS) {
 762                         queue = TS_EXCLUSIVE_QUEUE;
 763                         x |= (v & RW_LOCK_READ_WAITERS);
 764                 } else
 765                         queue = TS_SHARED_QUEUE;
 766                 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
 767                     x)) {
 768                         turnstile_chain_unlock(&rw->lock_object);
 769                         v = RW_READ_VALUE(rw);
 770                         continue;
 771                 }
 772                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 773                         CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
 774                             __func__, rw);
 775
 776                 /*
 777                  * Ok.  The lock is released and all that's left is to
 778                  * wake up the waiters.  Note that the lock might not be
 779                  * free anymore, but in that case the writers will just
 780                  * block again if they run before the new lock holder(s)
 781                  * release the lock.
 782                  */
 783                 ts = turnstile_lookup(&rw->lock_object);
 784                 MPASS(ts != NULL);
 785                 turnstile_broadcast(ts, queue);
 786                 turnstile_unpend(ts, TS_SHARED_LOCK);
 787                 turnstile_chain_unlock(&rw->lock_object);
 788                 td->td_rw_rlocks--;
 789                 break;
 790         }
 791         LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_READER);
 792 }
 793
 794 void
 795 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line)
 796 {
 797         struct rwlock *rw;
 798         struct thread *td;
 799         uintptr_t v;
 800
 801         rw = rwlock2rw(c);
 802
 803         KASSERT(rw->rw_lock != RW_DESTROYED,
 804             ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
 805         __rw_assert(c, RA_RLOCKED, file, line);
 806         WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
 807         LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
 808
 809         td = curthread;
 810         v = RW_READ_VALUE(rw);
 811
 812         if (__predict_false(LOCKSTAT_OOL_PROFILE_ENABLED(rw__release) ||
 813             !__rw_runlock_try(rw, td, &v)))
 814                 __rw_runlock_hard(c, td, v, file, line);
 815
 816         TD_LOCKS_DEC(curthread);
 817 }
 818
 819
 820 /*
 821  * This function is called when we are unable to obtain a write lock on the
 822  * first try.  This means that at least one other thread holds either a
 823  * read or write lock.
 824  */
 825 void
 826 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v, uintptr_t tid,
 827     const char *file, int line)
 828 {
 829         struct rwlock *rw;
 830         struct turnstile *ts;
 831 #ifdef ADAPTIVE_RWLOCKS
 832         volatile struct thread *owner;
 833         int spintries = 0;
 834         int i;
 835 #endif
 836         uintptr_t x;
 837 #ifdef LOCK_PROFILING
 838         uint64_t waittime = 0;
 839         int contested = 0;
 840 #endif
 841 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS)
 842         struct lock_delay_arg lda;
 843 #endif
 844 #ifdef KDTRACE_HOOKS
 845         uintptr_t state;
 846         u_int sleep_cnt = 0;
 847         int64_t sleep_time = 0;
 848         int64_t all_time = 0;
 849 #endif
 850
 851         if (SCHEDULER_STOPPED())
 852                 return;
 853
 854 #if defined(ADAPTIVE_RWLOCKS)
 855         lock_delay_arg_init(&lda, &rw_delay);
 856 #elif defined(KDTRACE_HOOKS)
 857         lock_delay_arg_init(&lda, NULL);
 858 #endif
 859         rw = rwlock2rw(c);
 860         if (__predict_false(v == RW_UNLOCKED))
 861                 v = RW_READ_VALUE(rw);
 862
 863         if (__predict_false(lv_rw_wowner(v) == (struct thread *)tid)) {
 864                 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE,
 865                     ("%s: recursing but non-recursive rw %s @ %s:%d\n",
 866                     __func__, rw->lock_object.lo_name, file, line));
 867                 rw->rw_recurse++;
 868                 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
 869                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 870                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
 871                 return;
 872         }
 873
 874         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 875                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 876                     rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
 877
 878 #ifdef KDTRACE_HOOKS
 879         all_time -= lockstat_nsecs(&rw->lock_object);
 880         state = v;
 881 #endif
 882         for (;;) {
 883                 if (v == RW_UNLOCKED) {
 884                         if (_rw_write_lock_fetch(rw, &v, tid))
 885                                 break;
 886                         continue;
 887                 }
 888 #ifdef KDTRACE_HOOKS
 889                 lda.spin_cnt++;
 890 #endif
 891 #ifdef HWPMC_HOOKS
 892                 PMC_SOFT_CALL( , , lock, failed);
 893 #endif
 894                 lock_profile_obtain_lock_failed(&rw->lock_object,
 895                     &contested, &waittime);
 896 #ifdef ADAPTIVE_RWLOCKS
 897                 /*
 898                  * If the lock is write locked and the owner is
 899                  * running on another CPU, spin until the owner stops
 900                  * running or the state of the lock changes.
 901                  */
 902                 owner = lv_rw_wowner(v);
 903                 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
 904                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 905                                 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
 906                                     __func__, rw, owner);
 907                         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
 908                             "spinning", "lockname:\"%s\"",
 909                             rw->lock_object.lo_name);
 910                         do {
 911                                 lock_delay(&lda);
 912                                 v = RW_READ_VALUE(rw);
 913                                 owner = lv_rw_wowner(v);
 914                         } while (owner != NULL && TD_IS_RUNNING(owner));
 915                         KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 916                             "running");
 917                         continue;
 918                 }
 919                 if ((v & RW_LOCK_READ) && RW_READERS(v) &&
 920                     spintries < rowner_retries) {
 921                         if (!(v & RW_LOCK_WRITE_SPINNER)) {
 922                                 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 923                                     v | RW_LOCK_WRITE_SPINNER)) {
 924                                         v = RW_READ_VALUE(rw);
 925                                         continue;
 926                                 }
 927                         }
 928                         spintries++;
 929                         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
 930                             "spinning", "lockname:\"%s\"",
 931                             rw->lock_object.lo_name);
 932                         for (i = 0; i < rowner_loops; i++) {
 933                                 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
 934                                         break;
 935                                 cpu_spinwait();
 936                         }
 937                         KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 938                             "running");
 939                         v = RW_READ_VALUE(rw);
 940 #ifdef KDTRACE_HOOKS
 941                         lda.spin_cnt += rowner_loops - i;
 942 #endif
 943                         if (i != rowner_loops)
 944                                 continue;
 945                 }
 946 #endif
 947                 ts = turnstile_trywait(&rw->lock_object);
 948                 v = RW_READ_VALUE(rw);
 949
 950 #ifdef ADAPTIVE_RWLOCKS
 951                 /*
 952                  * The current lock owner might have started executing
 953                  * on another CPU (or the lock could have changed
 954                  * owners) while we were waiting on the turnstile
 955                  * chain lock.  If so, drop the turnstile lock and try
 956                  * again.
 957                  */
 958                 if (!(v & RW_LOCK_READ)) {
 959                         owner = (struct thread *)RW_OWNER(v);
 960                         if (TD_IS_RUNNING(owner)) {
 961                                 turnstile_cancel(ts);
 962                                 continue;
 963                         }
 964                 }
 965 #endif
 966                 /*
 967                  * Check for the waiters flags about this rwlock.
 968                  * If the lock was released, without maintain any pending
 969                  * waiters queue, simply try to acquire it.
 970                  * If a pending waiters queue is present, claim the lock
 971                  * ownership and maintain the pending queue.
 972                  */
 973                 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 974                 if ((v & ~x) == RW_UNLOCKED) {
 975                         x &= ~RW_LOCK_WRITE_SPINNER;
 976                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) {
 977                                 if (x)
 978                                         turnstile_claim(ts);
 979                                 else
 980                                         turnstile_cancel(ts);
 981                                 break;
 982                         }
 983                         turnstile_cancel(ts);
 984                         v = RW_READ_VALUE(rw);
 985                         continue;
 986                 }
 987                 /*
 988                  * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
 989                  * set it.  If we fail to set it, then loop back and try
 990                  * again.
 991                  */
 992                 if (!(v & RW_LOCK_WRITE_WAITERS)) {
 993                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 994                             v | RW_LOCK_WRITE_WAITERS)) {
 995                                 turnstile_cancel(ts);
 996                                 v = RW_READ_VALUE(rw);
 997                                 continue;
 998                         }
 999                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
1000                                 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
1001                                     __func__, rw);
1002                 }
1003                 /*
1004                  * We were unable to acquire the lock and the write waiters
1005                  * flag is set, so we must block on the turnstile.
1006                  */
1007                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
1008                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
1009                             rw);
1010 #ifdef KDTRACE_HOOKS
1011                 sleep_time -= lockstat_nsecs(&rw->lock_object);
1012 #endif
1013                 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
1014 #ifdef KDTRACE_HOOKS
1015                 sleep_time += lockstat_nsecs(&rw->lock_object);
1016                 sleep_cnt++;
1017 #endif
1018                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
1019                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
1020                             __func__, rw);
1021 #ifdef ADAPTIVE_RWLOCKS
1022                 spintries = 0;
1023 #endif
1024                 v = RW_READ_VALUE(rw);
1025         }
1026 #ifdef KDTRACE_HOOKS
1027         all_time += lockstat_nsecs(&rw->lock_object);
1028         if (sleep_time)
1029                 LOCKSTAT_RECORD4(rw__block, rw, sleep_time,
1030                     LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0,
1031                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
1032
1033         /* Record only the loops spinning and not sleeping. */
1034         if (lda.spin_cnt > sleep_cnt)
1035                 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time,
1036                     LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0,
1037                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
1038 #endif
1039         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested,
1040             waittime, file, line, LOCKSTAT_WRITER);
1041 }
1042
1043 /*
1044  * This function is called if lockstat is active or the first try at releasing
1045  * a write lock failed.  The latter means that the lock is recursed or one of
1046  * the 2 waiter bits must be set indicating that at least one thread is waiting
1047  * on this lock.
1048  */
1049 void
1050 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t tid, const char *file,
1051     int line)
1052 {
1053         struct rwlock *rw;
1054         struct turnstile *ts;
1055         uintptr_t v;
1056         int queue;
1057
1058         if (SCHEDULER_STOPPED())
1059                 return;
1060
1061         rw = rwlock2rw(c);
1062         v = RW_READ_VALUE(rw);
1063         if (v & RW_LOCK_WRITER_RECURSED) {
1064                 if (--(rw->rw_recurse) == 0)
1065                         atomic_clear_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
1066                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
1067                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
1068                 return;
1069         }
1070
1071         LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_WRITER);
1072         if (v == tid && _rw_write_unlock(rw, tid))
1073                 return;
1074
1075         KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
1076             ("%s: neither of the waiter flags are set", __func__));
1077
1078         if (LOCK_LOG_TEST(&rw->lock_object, 0))
1079                 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
1080
1081         turnstile_chain_lock(&rw->lock_object);
1082         ts = turnstile_lookup(&rw->lock_object);
1083         MPASS(ts != NULL);
1084
1085         /*
1086          * Use the same algo as sx locks for now.  Prefer waking up shared
1087          * waiters if we have any over writers.  This is probably not ideal.
1088          *
1089          * 'v' is the value we are going to write back to rw_lock.  If we
1090          * have waiters on both queues, we need to preserve the state of
1091          * the waiter flag for the queue we don't wake up.  For now this is
1092          * hardcoded for the algorithm mentioned above.
1093          *
1094          * In the case of both readers and writers waiting we wakeup the
1095          * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
1096          * new writer comes in before a reader it will claim the lock up
1097          * above.  There is probably a potential priority inversion in
1098          * there that could be worked around either by waking both queues
1099          * of waiters or doing some complicated lock handoff gymnastics.
1100          */
1101         v = RW_UNLOCKED;
1102         if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
1103                 queue = TS_EXCLUSIVE_QUEUE;
1104                 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
1105         } else
1106                 queue = TS_SHARED_QUEUE;
1107
1108         /* Wake up all waiters for the specific queue. */
1109         if (LOCK_LOG_TEST(&rw->lock_object, 0))
1110                 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
1111                     queue == TS_SHARED_QUEUE ? "read" : "write");
1112         turnstile_broadcast(ts, queue);
1113         atomic_store_rel_ptr(&rw->rw_lock, v);
1114         turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
1115         turnstile_chain_unlock(&rw->lock_object);
1116 }
1117
1118 /*
1119  * Attempt to do a non-blocking upgrade from a read lock to a write
1120  * lock.  This will only succeed if this thread holds a single read
1121  * lock.  Returns true if the upgrade succeeded and false otherwise.
1122  */
1123 int
1124 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line)
1125 {
1126         struct rwlock *rw;
1127         uintptr_t v, x, tid;
1128         struct turnstile *ts;
1129         int success;
1130
1131         if (SCHEDULER_STOPPED())
1132                 return (1);
1133
1134         rw = rwlock2rw(c);
1135
1136         KASSERT(rw->rw_lock != RW_DESTROYED,
1137             ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
1138         __rw_assert(c, RA_RLOCKED, file, line);
1139
1140         /*
1141          * Attempt to switch from one reader to a writer.  If there
1142          * are any write waiters, then we will have to lock the
1143          * turnstile first to prevent races with another writer
1144          * calling turnstile_wait() before we have claimed this
1145          * turnstile.  So, do the simple case of no waiters first.
1146          */
1147         tid = (uintptr_t)curthread;
1148         success = 0;
1149         for (;;) {
1150                 v = rw->rw_lock;
1151                 if (RW_READERS(v) > 1)
1152                         break;
1153                 if (!(v & RW_LOCK_WAITERS)) {
1154                         success = atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid);
1155                         if (!success)
1156                                 continue;
1157                         break;
1158                 }
1159
1160                 /*
1161                  * Ok, we think we have waiters, so lock the turnstile.
1162                  */
1163                 ts = turnstile_trywait(&rw->lock_object);
1164                 v = rw->rw_lock;
1165                 if (RW_READERS(v) > 1) {
1166                         turnstile_cancel(ts);
1167                         break;
1168                 }
1169                 /*
1170                  * Try to switch from one reader to a writer again.  This time
1171                  * we honor the current state of the waiters flags.
1172                  * If we obtain the lock with the flags set, then claim
1173                  * ownership of the turnstile.
1174                  */
1175                 x = rw->rw_lock & RW_LOCK_WAITERS;
1176                 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
1177                 if (success) {
1178                         if (x)
1179                                 turnstile_claim(ts);
1180                         else
1181                                 turnstile_cancel(ts);
1182                         break;
1183                 }
1184                 turnstile_cancel(ts);
1185         }
1186         LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
1187         if (success) {
1188                 curthread->td_rw_rlocks--;
1189                 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
1190                     file, line);
1191                 LOCKSTAT_RECORD0(rw__upgrade, rw);
1192         }
1193         return (success);
1194 }
1195
1196 /*
1197  * Downgrade a write lock into a single read lock.
1198  */
1199 void
1200 __rw_downgrade(volatile uintptr_t *c, const char *file, int line)
1201 {
1202         struct rwlock *rw;
1203         struct turnstile *ts;
1204         uintptr_t tid, v;
1205         int rwait, wwait;
1206
1207         if (SCHEDULER_STOPPED())
1208                 return;
1209
1210         rw = rwlock2rw(c);
1211
1212         KASSERT(rw->rw_lock != RW_DESTROYED,
1213             ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
1214         __rw_assert(c, RA_WLOCKED | RA_NOTRECURSED, file, line);
1215 #ifndef INVARIANTS
1216         if (rw_recursed(rw))
1217                 panic("downgrade of a recursed lock");
1218 #endif
1219
1220         WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
1221
1222         /*
1223          * Convert from a writer to a single reader.  First we handle
1224          * the easy case with no waiters.  If there are any waiters, we
1225          * lock the turnstile and "disown" the lock.
1226          */
1227         tid = (uintptr_t)curthread;
1228         if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
1229                 goto out;
1230
1231         /*
1232          * Ok, we think we have waiters, so lock the turnstile so we can
1233          * read the waiter flags without any races.
1234          */
1235         turnstile_chain_lock(&rw->lock_object);
1236         v = rw->rw_lock & RW_LOCK_WAITERS;
1237         rwait = v & RW_LOCK_READ_WAITERS;
1238         wwait = v & RW_LOCK_WRITE_WAITERS;
1239         MPASS(rwait | wwait);
1240
1241         /*
1242          * Downgrade from a write lock while preserving waiters flag
1243          * and give up ownership of the turnstile.
1244          */
1245         ts = turnstile_lookup(&rw->lock_object);
1246         MPASS(ts != NULL);
1247         if (!wwait)
1248                 v &= ~RW_LOCK_READ_WAITERS;
1249         atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
1250         /*
1251          * Wake other readers if there are no writers pending.  Otherwise they
1252          * won't be able to acquire the lock anyway.
1253          */
1254         if (rwait && !wwait) {
1255                 turnstile_broadcast(ts, TS_SHARED_QUEUE);
1256                 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
1257         } else
1258                 turnstile_disown(ts);
1259         turnstile_chain_unlock(&rw->lock_object);
1260 out:
1261         curthread->td_rw_rlocks++;
1262         LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
1263         LOCKSTAT_RECORD0(rw__downgrade, rw);
1264 }
1265
1266 #ifdef INVARIANT_SUPPORT
1267 #ifndef INVARIANTS
1268 #undef __rw_assert
1269 #endif
1270
1271 /*
1272  * In the non-WITNESS case, rw_assert() can only detect that at least
1273  * *some* thread owns an rlock, but it cannot guarantee that *this*
1274  * thread owns an rlock.
1275  */
1276 void
1277 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line)
1278 {
1279         const struct rwlock *rw;
1280
1281         if (panicstr != NULL)
1282                 return;
1283
1284         rw = rwlock2rw(c);
1285
1286         switch (what) {
1287         case RA_LOCKED:
1288         case RA_LOCKED | RA_RECURSED:
1289         case RA_LOCKED | RA_NOTRECURSED:
1290         case RA_RLOCKED:
1291         case RA_RLOCKED | RA_RECURSED:
1292         case RA_RLOCKED | RA_NOTRECURSED:
1293 #ifdef WITNESS
1294                 witness_assert(&rw->lock_object, what, file, line);
1295 #else
1296                 /*
1297                  * If some other thread has a write lock or we have one
1298                  * and are asserting a read lock, fail.  Also, if no one
1299                  * has a lock at all, fail.
1300                  */
1301                 if (rw->rw_lock == RW_UNLOCKED ||
1302                     (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED ||
1303                     rw_wowner(rw) != curthread)))
1304                         panic("Lock %s not %slocked @ %s:%d\n",
1305                             rw->lock_object.lo_name, (what & RA_RLOCKED) ?
1306                             "read " : "", file, line);
1307
1308                 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
1309                         if (rw_recursed(rw)) {
1310                                 if (what & RA_NOTRECURSED)
1311                                         panic("Lock %s recursed @ %s:%d\n",
1312                                             rw->lock_object.lo_name, file,
1313                                             line);
1314                         } else if (what & RA_RECURSED)
1315                                 panic("Lock %s not recursed @ %s:%d\n",
1316                                     rw->lock_object.lo_name, file, line);
1317                 }
1318 #endif
1319                 break;
1320         case RA_WLOCKED:
1321         case RA_WLOCKED | RA_RECURSED:
1322         case RA_WLOCKED | RA_NOTRECURSED:
1323                 if (rw_wowner(rw) != curthread)
1324                         panic("Lock %s not exclusively locked @ %s:%d\n",
1325                             rw->lock_object.lo_name, file, line);
1326                 if (rw_recursed(rw)) {
1327                         if (what & RA_NOTRECURSED)
1328                                 panic("Lock %s recursed @ %s:%d\n",
1329                                     rw->lock_object.lo_name, file, line);
1330                 } else if (what & RA_RECURSED)
1331                         panic("Lock %s not recursed @ %s:%d\n",
1332                             rw->lock_object.lo_name, file, line);
1333                 break;
1334         case RA_UNLOCKED:
1335 #ifdef WITNESS
1336                 witness_assert(&rw->lock_object, what, file, line);
1337 #else
1338                 /*
1339                  * If we hold a write lock fail.  We can't reliably check
1340                  * to see if we hold a read lock or not.
1341                  */
1342                 if (rw_wowner(rw) == curthread)
1343                         panic("Lock %s exclusively locked @ %s:%d\n",
1344                             rw->lock_object.lo_name, file, line);
1345 #endif
1346                 break;
1347         default:
1348                 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
1349                     line);
1350         }
1351 }
1352 #endif /* INVARIANT_SUPPORT */
1353
1354 #ifdef DDB
1355 void
1356 db_show_rwlock(const struct lock_object *lock)
1357 {
1358         const struct rwlock *rw;
1359         struct thread *td;
1360
1361         rw = (const struct rwlock *)lock;
1362
1363         db_printf(" state: ");
1364         if (rw->rw_lock == RW_UNLOCKED)
1365                 db_printf("UNLOCKED\n");
1366         else if (rw->rw_lock == RW_DESTROYED) {
1367                 db_printf("DESTROYED\n");
1368                 return;
1369         } else if (rw->rw_lock & RW_LOCK_READ)
1370                 db_printf("RLOCK: %ju locks\n",
1371                     (uintmax_t)(RW_READERS(rw->rw_lock)));
1372         else {
1373                 td = rw_wowner(rw);
1374                 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1375                     td->td_tid, td->td_proc->p_pid, td->td_name);
1376                 if (rw_recursed(rw))
1377                         db_printf(" recursed: %u\n", rw->rw_recurse);
1378         }
1379         db_printf(" waiters: ");
1380         switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
1381         case RW_LOCK_READ_WAITERS:
1382                 db_printf("readers\n");
1383                 break;
1384         case RW_LOCK_WRITE_WAITERS:
1385                 db_printf("writers\n");
1386                 break;
1387         case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
1388                 db_printf("readers and writers\n");
1389                 break;
1390         default:
1391                 db_printf("none\n");
1392                 break;
1393         }
1394 }
1395
1396 #endif