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 __read_frequently rowner_retries = 10;
  97 static int __read_frequently 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_frequently 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 || SCHEDULER_STOPPED() ||
 273             !TD_IS_IDLETHREAD(curthread),
 274             ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d",
 275             curthread, rw->lock_object.lo_name, file, line));
 276         KASSERT(rw->rw_lock != RW_DESTROYED,
 277             ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
 278         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
 279             line, NULL);
 280         tid = (uintptr_t)curthread;
 281         v = RW_UNLOCKED;
 282         if (!_rw_write_lock_fetch(rw, &v, tid))
 283                 _rw_wlock_hard(rw, v, tid, file, line);
 284         else
 285                 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw,
 286                     0, 0, file, line, LOCKSTAT_WRITER);
 287
 288         LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
 289         WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 290         TD_LOCKS_INC(curthread);
 291 }
 292
 293 int
 294 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line)
 295 {
 296         struct rwlock *rw;
 297         struct thread *td;
 298         uintptr_t tid, v;
 299         int rval;
 300         bool recursed;
 301
 302         td = curthread;
 303         tid = (uintptr_t)td;
 304         if (SCHEDULER_STOPPED_TD(td))
 305                 return (1);
 306
 307         rw = rwlock2rw(c);
 308
 309         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td),
 310             ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d",
 311             curthread, rw->lock_object.lo_name, file, line));
 312         KASSERT(rw->rw_lock != RW_DESTROYED,
 313             ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
 314
 315         rval = 1;
 316         recursed = false;
 317         v = RW_UNLOCKED;
 318         for (;;) {
 319                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid))
 320                         break;
 321                 if (v == RW_UNLOCKED)
 322                         continue;
 323                 if (v == tid && (rw->lock_object.lo_flags & LO_RECURSABLE)) {
 324                         rw->rw_recurse++;
 325                         atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
 326                         break;
 327                 }
 328                 rval = 0;
 329                 break;
 330         }
 331
 332         LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
 333         if (rval) {
 334                 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 335                     file, line);
 336                 if (!recursed)
 337                         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire,
 338                             rw, 0, 0, file, line, LOCKSTAT_WRITER);
 339                 TD_LOCKS_INC(curthread);
 340         }
 341         return (rval);
 342 }
 343
 344 void
 345 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line)
 346 {
 347         struct rwlock *rw;
 348
 349         rw = rwlock2rw(c);
 350
 351         KASSERT(rw->rw_lock != RW_DESTROYED,
 352             ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
 353         __rw_assert(c, RA_WLOCKED, file, line);
 354         WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
 355         LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
 356             line);
 357
 358 #ifdef LOCK_PROFILING
 359         _rw_wunlock_hard(rw, (uintptr_t)curthread, file, line);
 360 #else
 361         __rw_wunlock(rw, curthread, file, line);
 362 #endif
 363
 364         TD_LOCKS_DEC(curthread);
 365 }
 366
 367 /*
 368  * Determines whether a new reader can acquire a lock.  Succeeds if the
 369  * reader already owns a read lock and the lock is locked for read to
 370  * prevent deadlock from reader recursion.  Also succeeds if the lock
 371  * is unlocked and has no writer waiters or spinners.  Failing otherwise
 372  * prioritizes writers before readers.
 373  */
 374 #define RW_CAN_READ(td, _rw)                                            \
 375     (((_rw) & (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) ==\
 376     RW_LOCK_READ || ((td)->td_rw_rlocks && (_rw) & 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, n;
 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 += n) {
 492                                 n = RW_READERS(v);
 493                                 lock_delay_spin(n);
 494                                 v = RW_READ_VALUE(rw);
 495                                 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(td, v))
 496                                         break;
 497                         }
 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 || SCHEDULER_STOPPED_TD(td) ||
 619             !TD_IS_IDLETHREAD(td),
 620             ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d",
 621             td, rw->lock_object.lo_name, file, line));
 622         KASSERT(rw->rw_lock != RW_DESTROYED,
 623             ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
 624         KASSERT(rw_wowner(rw) != td,
 625             ("rw_rlock: wlock already held for %s @ %s:%d",
 626             rw->lock_object.lo_name, file, line));
 627         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL);
 628
 629         v = RW_READ_VALUE(rw);
 630         if (__predict_false(LOCKSTAT_OOL_PROFILE_ENABLED(rw__acquire) ||
 631             !__rw_rlock_try(rw, td, &v, file, line)))
 632                 __rw_rlock_hard(c, td, v, file, line);
 633
 634         LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
 635         WITNESS_LOCK(&rw->lock_object, 0, file, line);
 636         TD_LOCKS_INC(curthread);
 637 }
 638
 639 int
 640 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line)
 641 {
 642         struct rwlock *rw;
 643         uintptr_t x;
 644
 645         if (SCHEDULER_STOPPED())
 646                 return (1);
 647
 648         rw = rwlock2rw(c);
 649
 650         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
 651             ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d",
 652             curthread, rw->lock_object.lo_name, file, line));
 653
 654         x = rw->rw_lock;
 655         for (;;) {
 656                 KASSERT(rw->rw_lock != RW_DESTROYED,
 657                     ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
 658                 if (!(x & RW_LOCK_READ))
 659                         break;
 660                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &x, x + RW_ONE_READER)) {
 661                         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
 662                             line);
 663                         WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
 664                         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire,
 665                             rw, 0, 0, file, line, LOCKSTAT_READER);
 666                         TD_LOCKS_INC(curthread);
 667                         curthread->td_rw_rlocks++;
 668                         return (1);
 669                 }
 670         }
 671
 672         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
 673         return (0);
 674 }
 675
 676 static bool __always_inline
 677 __rw_runlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp)
 678 {
 679
 680         for (;;) {
 681                 /*
 682                  * See if there is more than one read lock held.  If so,
 683                  * just drop one and return.
 684                  */
 685                 if (RW_READERS(*vp) > 1) {
 686                         if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp,
 687                             *vp - RW_ONE_READER)) {
 688                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 689                                         CTR4(KTR_LOCK,
 690                                             "%s: %p succeeded %p -> %p",
 691                                             __func__, rw, (void *)*vp,
 692                                             (void *)(*vp - RW_ONE_READER));
 693                                 td->td_rw_rlocks--;
 694                                 return (true);
 695                         }
 696                         continue;
 697                 }
 698                 /*
 699                  * If there aren't any waiters for a write lock, then try
 700                  * to drop it quickly.
 701                  */
 702                 if (!(*vp & RW_LOCK_WAITERS)) {
 703                         MPASS((*vp & ~RW_LOCK_WRITE_SPINNER) ==
 704                             RW_READERS_LOCK(1));
 705                         if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp,
 706                             RW_UNLOCKED)) {
 707                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 708                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
 709                                             __func__, rw);
 710                                 td->td_rw_rlocks--;
 711                                 return (true);
 712                         }
 713                         continue;
 714                 }
 715                 break;
 716         }
 717         return (false);
 718 }
 719
 720 static void __noinline
 721 __rw_runlock_hard(volatile uintptr_t *c, struct thread *td, uintptr_t v,
 722     const char *file, int line)
 723 {
 724         struct rwlock *rw;
 725         struct turnstile *ts;
 726         uintptr_t x, queue;
 727
 728         if (SCHEDULER_STOPPED())
 729                 return;
 730
 731         rw = rwlock2rw(c);
 732
 733         for (;;) {
 734                 if (__rw_runlock_try(rw, td, &v))
 735                         break;
 736
 737                 /*
 738                  * Ok, we know we have waiters and we think we are the
 739                  * last reader, so grab the turnstile lock.
 740                  */
 741                 turnstile_chain_lock(&rw->lock_object);
 742                 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 743                 MPASS(v & RW_LOCK_WAITERS);
 744
 745                 /*
 746                  * Try to drop our lock leaving the lock in a unlocked
 747                  * state.
 748                  *
 749                  * If you wanted to do explicit lock handoff you'd have to
 750                  * do it here.  You'd also want to use turnstile_signal()
 751                  * and you'd have to handle the race where a higher
 752                  * priority thread blocks on the write lock before the
 753                  * thread you wakeup actually runs and have the new thread
 754                  * "steal" the lock.  For now it's a lot simpler to just
 755                  * wakeup all of the waiters.
 756                  *
 757                  * As above, if we fail, then another thread might have
 758                  * acquired a read lock, so drop the turnstile lock and
 759                  * restart.
 760                  */
 761                 x = RW_UNLOCKED;
 762                 if (v & RW_LOCK_WRITE_WAITERS) {
 763                         queue = TS_EXCLUSIVE_QUEUE;
 764                         x |= (v & RW_LOCK_READ_WAITERS);
 765                 } else
 766                         queue = TS_SHARED_QUEUE;
 767                 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
 768                     x)) {
 769                         turnstile_chain_unlock(&rw->lock_object);
 770                         v = RW_READ_VALUE(rw);
 771                         continue;
 772                 }
 773                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 774                         CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
 775                             __func__, rw);
 776
 777                 /*
 778                  * Ok.  The lock is released and all that's left is to
 779                  * wake up the waiters.  Note that the lock might not be
 780                  * free anymore, but in that case the writers will just
 781                  * block again if they run before the new lock holder(s)
 782                  * release the lock.
 783                  */
 784                 ts = turnstile_lookup(&rw->lock_object);
 785                 MPASS(ts != NULL);
 786                 turnstile_broadcast(ts, queue);
 787                 turnstile_unpend(ts, TS_SHARED_LOCK);
 788                 turnstile_chain_unlock(&rw->lock_object);
 789                 td->td_rw_rlocks--;
 790                 break;
 791         }
 792         LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_READER);
 793 }
 794
 795 void
 796 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line)
 797 {
 798         struct rwlock *rw;
 799         struct thread *td;
 800         uintptr_t v;
 801
 802         rw = rwlock2rw(c);
 803
 804         KASSERT(rw->rw_lock != RW_DESTROYED,
 805             ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
 806         __rw_assert(c, RA_RLOCKED, file, line);
 807         WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
 808         LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
 809
 810         td = curthread;
 811         v = RW_READ_VALUE(rw);
 812
 813         if (__predict_false(LOCKSTAT_OOL_PROFILE_ENABLED(rw__release) ||
 814             !__rw_runlock_try(rw, td, &v)))
 815                 __rw_runlock_hard(c, td, v, file, line);
 816
 817         TD_LOCKS_DEC(curthread);
 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, n;
 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 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
 851         int doing_lockprof;
 852 #endif
 853
 854         if (SCHEDULER_STOPPED())
 855                 return;
 856
 857 #if defined(ADAPTIVE_RWLOCKS)
 858         lock_delay_arg_init(&lda, &rw_delay);
 859 #elif defined(KDTRACE_HOOKS)
 860         lock_delay_arg_init(&lda, NULL);
 861 #endif
 862         rw = rwlock2rw(c);
 863         if (__predict_false(v == RW_UNLOCKED))
 864                 v = RW_READ_VALUE(rw);
 865
 866         if (__predict_false(lv_rw_wowner(v) == (struct thread *)tid)) {
 867                 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE,
 868                     ("%s: recursing but non-recursive rw %s @ %s:%d\n",
 869                     __func__, rw->lock_object.lo_name, file, line));
 870                 rw->rw_recurse++;
 871                 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
 872                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 873                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
 874                 return;
 875         }
 876
 877         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 878                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
 879                     rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
 880
 881 #ifdef LOCK_PROFILING
 882         doing_lockprof = 1;
 883         state = v;
 884 #elif defined(KDTRACE_HOOKS)
 885         doing_lockprof = lockstat_enabled;
 886         if (__predict_false(doing_lockprof)) {
 887                 all_time -= lockstat_nsecs(&rw->lock_object);
 888                 state = v;
 889         }
 890 #endif
 891
 892         for (;;) {
 893                 if (v == RW_UNLOCKED) {
 894                         if (_rw_write_lock_fetch(rw, &v, tid))
 895                                 break;
 896                         continue;
 897                 }
 898 #ifdef KDTRACE_HOOKS
 899                 lda.spin_cnt++;
 900 #endif
 901 #ifdef HWPMC_HOOKS
 902                 PMC_SOFT_CALL( , , lock, failed);
 903 #endif
 904                 lock_profile_obtain_lock_failed(&rw->lock_object,
 905                     &contested, &waittime);
 906 #ifdef ADAPTIVE_RWLOCKS
 907                 /*
 908                  * If the lock is write locked and the owner is
 909                  * running on another CPU, spin until the owner stops
 910                  * running or the state of the lock changes.
 911                  */
 912                 owner = lv_rw_wowner(v);
 913                 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
 914                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 915                                 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
 916                                     __func__, rw, owner);
 917                         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
 918                             "spinning", "lockname:\"%s\"",
 919                             rw->lock_object.lo_name);
 920                         do {
 921                                 lock_delay(&lda);
 922                                 v = RW_READ_VALUE(rw);
 923                                 owner = lv_rw_wowner(v);
 924                         } while (owner != NULL && TD_IS_RUNNING(owner));
 925                         KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 926                             "running");
 927                         continue;
 928                 }
 929                 if ((v & RW_LOCK_READ) && RW_READERS(v) &&
 930                     spintries < rowner_retries) {
 931                         if (!(v & RW_LOCK_WRITE_SPINNER)) {
 932                                 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
 933                                     v | RW_LOCK_WRITE_SPINNER)) {
 934                                         v = RW_READ_VALUE(rw);
 935                                         continue;
 936                                 }
 937                         }
 938                         spintries++;
 939                         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
 940                             "spinning", "lockname:\"%s\"",
 941                             rw->lock_object.lo_name);
 942                         for (i = 0; i < rowner_loops; i += n) {
 943                                 n = RW_READERS(v);
 944                                 lock_delay_spin(n);
 945                                 v = RW_READ_VALUE(rw);
 946                                 if ((v & RW_LOCK_WRITE_SPINNER) == 0)
 947                                         break;
 948                         }
 949                         KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 950                             "running");
 951 #ifdef KDTRACE_HOOKS
 952                         lda.spin_cnt += rowner_loops - i;
 953 #endif
 954                         if (i != rowner_loops)
 955                                 continue;
 956                 }
 957 #endif
 958                 ts = turnstile_trywait(&rw->lock_object);
 959                 v = RW_READ_VALUE(rw);
 960
 961 #ifdef ADAPTIVE_RWLOCKS
 962                 /*
 963                  * The current lock owner might have started executing
 964                  * on another CPU (or the lock could have changed
 965                  * owners) while we were waiting on the turnstile
 966                  * chain lock.  If so, drop the turnstile lock and try
 967                  * again.
 968                  */
 969                 if (!(v & RW_LOCK_READ)) {
 970                         owner = (struct thread *)RW_OWNER(v);
 971                         if (TD_IS_RUNNING(owner)) {
 972                                 turnstile_cancel(ts);
 973                                 continue;
 974                         }
 975                 }
 976 #endif
 977                 /*
 978                  * Check for the waiters flags about this rwlock.
 979                  * If the lock was released, without maintain any pending
 980                  * waiters queue, simply try to acquire it.
 981                  * If a pending waiters queue is present, claim the lock
 982                  * ownership and maintain the pending queue.
 983                  */
 984                 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 985                 if ((v & ~x) == RW_UNLOCKED) {
 986                         x &= ~RW_LOCK_WRITE_SPINNER;
 987                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) {
 988                                 if (x)
 989                                         turnstile_claim(ts);
 990                                 else
 991                                         turnstile_cancel(ts);
 992                                 break;
 993                         }
 994                         turnstile_cancel(ts);
 995                         v = RW_READ_VALUE(rw);
 996                         continue;
 997                 }
 998                 /*
 999                  * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
1000                  * set it.  If we fail to set it, then loop back and try
1001                  * again.
1002                  */
1003                 if (!(v & RW_LOCK_WRITE_WAITERS)) {
1004                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
1005                             v | RW_LOCK_WRITE_WAITERS)) {
1006                                 turnstile_cancel(ts);
1007                                 v = RW_READ_VALUE(rw);
1008                                 continue;
1009                         }
1010                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
1011                                 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
1012                                     __func__, rw);
1013                 }
1014                 /*
1015                  * We were unable to acquire the lock and the write waiters
1016                  * flag is set, so we must block on the turnstile.
1017                  */
1018                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
1019                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
1020                             rw);
1021 #ifdef KDTRACE_HOOKS
1022                 sleep_time -= lockstat_nsecs(&rw->lock_object);
1023 #endif
1024                 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
1025 #ifdef KDTRACE_HOOKS
1026                 sleep_time += lockstat_nsecs(&rw->lock_object);
1027                 sleep_cnt++;
1028 #endif
1029                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
1030                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
1031                             __func__, rw);
1032 #ifdef ADAPTIVE_RWLOCKS
1033                 spintries = 0;
1034 #endif
1035                 v = RW_READ_VALUE(rw);
1036         }
1037 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
1038         if (__predict_true(!doing_lockprof))
1039                 return;
1040 #endif
1041 #ifdef KDTRACE_HOOKS
1042         all_time += lockstat_nsecs(&rw->lock_object);
1043         if (sleep_time)
1044                 LOCKSTAT_RECORD4(rw__block, rw, sleep_time,
1045                     LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0,
1046                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
1047
1048         /* Record only the loops spinning and not sleeping. */
1049         if (lda.spin_cnt > sleep_cnt)
1050                 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time,
1051                     LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0,
1052                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
1053 #endif
1054         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested,
1055             waittime, file, line, LOCKSTAT_WRITER);
1056 }
1057
1058 /*
1059  * This function is called if lockstat is active or the first try at releasing
1060  * a write lock failed.  The latter means that the lock is recursed or one of
1061  * the 2 waiter bits must be set indicating that at least one thread is waiting
1062  * on this lock.
1063  */
1064 void
1065 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t tid, const char *file,
1066     int line)
1067 {
1068         struct rwlock *rw;
1069         struct turnstile *ts;
1070         uintptr_t v;
1071         int queue;
1072
1073         if (SCHEDULER_STOPPED())
1074                 return;
1075
1076         rw = rwlock2rw(c);
1077         v = RW_READ_VALUE(rw);
1078         if (v & RW_LOCK_WRITER_RECURSED) {
1079                 if (--(rw->rw_recurse) == 0)
1080                         atomic_clear_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
1081                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
1082                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
1083                 return;
1084         }
1085
1086         LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_WRITER);
1087         if (v == tid && _rw_write_unlock(rw, tid))
1088                 return;
1089
1090         KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
1091             ("%s: neither of the waiter flags are set", __func__));
1092
1093         if (LOCK_LOG_TEST(&rw->lock_object, 0))
1094                 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
1095
1096         turnstile_chain_lock(&rw->lock_object);
1097         ts = turnstile_lookup(&rw->lock_object);
1098         MPASS(ts != NULL);
1099
1100         /*
1101          * Use the same algo as sx locks for now.  Prefer waking up shared
1102          * waiters if we have any over writers.  This is probably not ideal.
1103          *
1104          * 'v' is the value we are going to write back to rw_lock.  If we
1105          * have waiters on both queues, we need to preserve the state of
1106          * the waiter flag for the queue we don't wake up.  For now this is
1107          * hardcoded for the algorithm mentioned above.
1108          *
1109          * In the case of both readers and writers waiting we wakeup the
1110          * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
1111          * new writer comes in before a reader it will claim the lock up
1112          * above.  There is probably a potential priority inversion in
1113          * there that could be worked around either by waking both queues
1114          * of waiters or doing some complicated lock handoff gymnastics.
1115          */
1116         v = RW_UNLOCKED;
1117         if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
1118                 queue = TS_EXCLUSIVE_QUEUE;
1119                 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
1120         } else
1121                 queue = TS_SHARED_QUEUE;
1122
1123         /* Wake up all waiters for the specific queue. */
1124         if (LOCK_LOG_TEST(&rw->lock_object, 0))
1125                 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
1126                     queue == TS_SHARED_QUEUE ? "read" : "write");
1127         turnstile_broadcast(ts, queue);
1128         atomic_store_rel_ptr(&rw->rw_lock, v);
1129         turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
1130         turnstile_chain_unlock(&rw->lock_object);
1131 }
1132
1133 /*
1134  * Attempt to do a non-blocking upgrade from a read lock to a write
1135  * lock.  This will only succeed if this thread holds a single read
1136  * lock.  Returns true if the upgrade succeeded and false otherwise.
1137  */
1138 int
1139 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line)
1140 {
1141         struct rwlock *rw;
1142         uintptr_t v, x, tid;
1143         struct turnstile *ts;
1144         int success;
1145
1146         if (SCHEDULER_STOPPED())
1147                 return (1);
1148
1149         rw = rwlock2rw(c);
1150
1151         KASSERT(rw->rw_lock != RW_DESTROYED,
1152             ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
1153         __rw_assert(c, RA_RLOCKED, file, line);
1154
1155         /*
1156          * Attempt to switch from one reader to a writer.  If there
1157          * are any write waiters, then we will have to lock the
1158          * turnstile first to prevent races with another writer
1159          * calling turnstile_wait() before we have claimed this
1160          * turnstile.  So, do the simple case of no waiters first.
1161          */
1162         tid = (uintptr_t)curthread;
1163         success = 0;
1164         for (;;) {
1165                 v = rw->rw_lock;
1166                 if (RW_READERS(v) > 1)
1167                         break;
1168                 if (!(v & RW_LOCK_WAITERS)) {
1169                         success = atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid);
1170                         if (!success)
1171                                 continue;
1172                         break;
1173                 }
1174
1175                 /*
1176                  * Ok, we think we have waiters, so lock the turnstile.
1177                  */
1178                 ts = turnstile_trywait(&rw->lock_object);
1179                 v = rw->rw_lock;
1180                 if (RW_READERS(v) > 1) {
1181                         turnstile_cancel(ts);
1182                         break;
1183                 }
1184                 /*
1185                  * Try to switch from one reader to a writer again.  This time
1186                  * we honor the current state of the waiters flags.
1187                  * If we obtain the lock with the flags set, then claim
1188                  * ownership of the turnstile.
1189                  */
1190                 x = rw->rw_lock & RW_LOCK_WAITERS;
1191                 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
1192                 if (success) {
1193                         if (x)
1194                                 turnstile_claim(ts);
1195                         else
1196                                 turnstile_cancel(ts);
1197                         break;
1198                 }
1199                 turnstile_cancel(ts);
1200         }
1201         LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
1202         if (success) {
1203                 curthread->td_rw_rlocks--;
1204                 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
1205                     file, line);
1206                 LOCKSTAT_RECORD0(rw__upgrade, rw);
1207         }
1208         return (success);
1209 }
1210
1211 /*
1212  * Downgrade a write lock into a single read lock.
1213  */
1214 void
1215 __rw_downgrade(volatile uintptr_t *c, const char *file, int line)
1216 {
1217         struct rwlock *rw;
1218         struct turnstile *ts;
1219         uintptr_t tid, v;
1220         int rwait, wwait;
1221
1222         if (SCHEDULER_STOPPED())
1223                 return;
1224
1225         rw = rwlock2rw(c);
1226
1227         KASSERT(rw->rw_lock != RW_DESTROYED,
1228             ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
1229         __rw_assert(c, RA_WLOCKED | RA_NOTRECURSED, file, line);
1230 #ifndef INVARIANTS
1231         if (rw_recursed(rw))
1232                 panic("downgrade of a recursed lock");
1233 #endif
1234
1235         WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
1236
1237         /*
1238          * Convert from a writer to a single reader.  First we handle
1239          * the easy case with no waiters.  If there are any waiters, we
1240          * lock the turnstile and "disown" the lock.
1241          */
1242         tid = (uintptr_t)curthread;
1243         if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
1244                 goto out;
1245
1246         /*
1247          * Ok, we think we have waiters, so lock the turnstile so we can
1248          * read the waiter flags without any races.
1249          */
1250         turnstile_chain_lock(&rw->lock_object);
1251         v = rw->rw_lock & RW_LOCK_WAITERS;
1252         rwait = v & RW_LOCK_READ_WAITERS;
1253         wwait = v & RW_LOCK_WRITE_WAITERS;
1254         MPASS(rwait | wwait);
1255
1256         /*
1257          * Downgrade from a write lock while preserving waiters flag
1258          * and give up ownership of the turnstile.
1259          */
1260         ts = turnstile_lookup(&rw->lock_object);
1261         MPASS(ts != NULL);
1262         if (!wwait)
1263                 v &= ~RW_LOCK_READ_WAITERS;
1264         atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
1265         /*
1266          * Wake other readers if there are no writers pending.  Otherwise they
1267          * won't be able to acquire the lock anyway.
1268          */
1269         if (rwait && !wwait) {
1270                 turnstile_broadcast(ts, TS_SHARED_QUEUE);
1271                 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
1272         } else
1273                 turnstile_disown(ts);
1274         turnstile_chain_unlock(&rw->lock_object);
1275 out:
1276         curthread->td_rw_rlocks++;
1277         LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
1278         LOCKSTAT_RECORD0(rw__downgrade, rw);
1279 }
1280
1281 #ifdef INVARIANT_SUPPORT
1282 #ifndef INVARIANTS
1283 #undef __rw_assert
1284 #endif
1285
1286 /*
1287  * In the non-WITNESS case, rw_assert() can only detect that at least
1288  * *some* thread owns an rlock, but it cannot guarantee that *this*
1289  * thread owns an rlock.
1290  */
1291 void
1292 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line)
1293 {
1294         const struct rwlock *rw;
1295
1296         if (panicstr != NULL)
1297                 return;
1298
1299         rw = rwlock2rw(c);
1300
1301         switch (what) {
1302         case RA_LOCKED:
1303         case RA_LOCKED | RA_RECURSED:
1304         case RA_LOCKED | RA_NOTRECURSED:
1305         case RA_RLOCKED:
1306         case RA_RLOCKED | RA_RECURSED:
1307         case RA_RLOCKED | RA_NOTRECURSED:
1308 #ifdef WITNESS
1309                 witness_assert(&rw->lock_object, what, file, line);
1310 #else
1311                 /*
1312                  * If some other thread has a write lock or we have one
1313                  * and are asserting a read lock, fail.  Also, if no one
1314                  * has a lock at all, fail.
1315                  */
1316                 if (rw->rw_lock == RW_UNLOCKED ||
1317                     (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED ||
1318                     rw_wowner(rw) != curthread)))
1319                         panic("Lock %s not %slocked @ %s:%d\n",
1320                             rw->lock_object.lo_name, (what & RA_RLOCKED) ?
1321                             "read " : "", file, line);
1322
1323                 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
1324                         if (rw_recursed(rw)) {
1325                                 if (what & RA_NOTRECURSED)
1326                                         panic("Lock %s recursed @ %s:%d\n",
1327                                             rw->lock_object.lo_name, file,
1328                                             line);
1329                         } else if (what & RA_RECURSED)
1330                                 panic("Lock %s not recursed @ %s:%d\n",
1331                                     rw->lock_object.lo_name, file, line);
1332                 }
1333 #endif
1334                 break;
1335         case RA_WLOCKED:
1336         case RA_WLOCKED | RA_RECURSED:
1337         case RA_WLOCKED | RA_NOTRECURSED:
1338                 if (rw_wowner(rw) != curthread)
1339                         panic("Lock %s not exclusively locked @ %s:%d\n",
1340                             rw->lock_object.lo_name, file, line);
1341                 if (rw_recursed(rw)) {
1342                         if (what & RA_NOTRECURSED)
1343                                 panic("Lock %s recursed @ %s:%d\n",
1344                                     rw->lock_object.lo_name, file, line);
1345                 } else if (what & RA_RECURSED)
1346                         panic("Lock %s not recursed @ %s:%d\n",
1347                             rw->lock_object.lo_name, file, line);
1348                 break;
1349         case RA_UNLOCKED:
1350 #ifdef WITNESS
1351                 witness_assert(&rw->lock_object, what, file, line);
1352 #else
1353                 /*
1354                  * If we hold a write lock fail.  We can't reliably check
1355                  * to see if we hold a read lock or not.
1356                  */
1357                 if (rw_wowner(rw) == curthread)
1358                         panic("Lock %s exclusively locked @ %s:%d\n",
1359                             rw->lock_object.lo_name, file, line);
1360 #endif
1361                 break;
1362         default:
1363                 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
1364                     line);
1365         }
1366 }
1367 #endif /* INVARIANT_SUPPORT */
1368
1369 #ifdef DDB
1370 void
1371 db_show_rwlock(const struct lock_object *lock)
1372 {
1373         const struct rwlock *rw;
1374         struct thread *td;
1375
1376         rw = (const struct rwlock *)lock;
1377
1378         db_printf(" state: ");
1379         if (rw->rw_lock == RW_UNLOCKED)
1380                 db_printf("UNLOCKED\n");
1381         else if (rw->rw_lock == RW_DESTROYED) {
1382                 db_printf("DESTROYED\n");
1383                 return;
1384         } else if (rw->rw_lock & RW_LOCK_READ)
1385                 db_printf("RLOCK: %ju locks\n",
1386                     (uintmax_t)(RW_READERS(rw->rw_lock)));
1387         else {
1388                 td = rw_wowner(rw);
1389                 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
1390                     td->td_tid, td->td_proc->p_pid, td->td_name);
1391                 if (rw_recursed(rw))
1392                         db_printf(" recursed: %u\n", rw->rw_recurse);
1393         }
1394         db_printf(" waiters: ");
1395         switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
1396         case RW_LOCK_READ_WAITERS:
1397                 db_printf("readers\n");
1398                 break;
1399         case RW_LOCK_WRITE_WAITERS:
1400                 db_printf("writers\n");
1401                 break;
1402         case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
1403                 db_printf("readers and writers\n");
1404                 break;
1405         default:
1406                 db_printf("none\n");
1407                 break;
1408         }
1409 }
1410
1411 #endif