1 /*- 2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD 3 * 4 * Copyright (c) 2006 John Baldwin <jhb@FreeBSD.org> 5 * All rights reserved. 6 * 7 * Redistribution and use in source and binary forms, with or without 8 * modification, are permitted provided that the following conditions 9 * are met: 10 * 1. Redistributions of source code must retain the above copyright 11 * notice, this list of conditions and the following disclaimer. 12 * 2. Redistributions in binary form must reproduce the above copyright 13 * notice, this list of conditions and the following disclaimer in the 14 * documentation and/or other materials provided with the distribution. 15 * 16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 * SUCH DAMAGE. 27 */ 28 29 /* 30 * Machine independent bits of reader/writer lock implementation. 31 */ 32 33 #include <sys/cdefs.h> 34 __FBSDID("$FreeBSD$"); 35 36 #include "opt_ddb.h" 37 #include "opt_hwpmc_hooks.h" 38 #include "opt_no_adaptive_rwlocks.h" 39 40 #include <sys/param.h> 41 #include <sys/kdb.h> 42 #include <sys/ktr.h> 43 #include <sys/kernel.h> 44 #include <sys/lock.h> 45 #include <sys/mutex.h> 46 #include <sys/proc.h> 47 #include <sys/rwlock.h> 48 #include <sys/sched.h> 49 #include <sys/smp.h> 50 #include <sys/sysctl.h> 51 #include <sys/systm.h> 52 #include <sys/turnstile.h> 53 54 #include <machine/cpu.h> 55 56 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS) 57 #define ADAPTIVE_RWLOCKS 58 #endif 59 60 #ifdef HWPMC_HOOKS 61 #include <sys/pmckern.h> 62 PMC_SOFT_DECLARE( , , lock, failed); 63 #endif 64 65 /* 66 * Return the rwlock address when the lock cookie address is provided. 67 * This functionality assumes that struct rwlock* have a member named rw_lock. 68 */ 69 #define rwlock2rw(c) (__containerof(c, struct rwlock, rw_lock)) 70 71 #ifdef DDB 72 #include <ddb/ddb.h> 73 74 static void db_show_rwlock(const struct lock_object *lock); 75 #endif 76 static void assert_rw(const struct lock_object *lock, int what); 77 static void lock_rw(struct lock_object *lock, uintptr_t how); 78 #ifdef KDTRACE_HOOKS 79 static int owner_rw(const struct lock_object *lock, struct thread **owner); 80 #endif 81 static uintptr_t unlock_rw(struct lock_object *lock); 82 83 struct lock_class lock_class_rw = { 84 .lc_name = "rw", 85 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 86 .lc_assert = assert_rw, 87 #ifdef DDB 88 .lc_ddb_show = db_show_rwlock, 89 #endif 90 .lc_lock = lock_rw, 91 .lc_unlock = unlock_rw, 92 #ifdef KDTRACE_HOOKS 93 .lc_owner = owner_rw, 94 #endif 95 }; 96 97 #ifdef ADAPTIVE_RWLOCKS 98 static int __read_frequently rowner_retries; 99 static int __read_frequently rowner_loops; 100 static SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL, 101 "rwlock debugging"); 102 SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, ""); 103 SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, ""); 104 105 static struct lock_delay_config __read_frequently rw_delay; 106 107 SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_base, CTLFLAG_RW, &rw_delay.base, 108 0, ""); 109 SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_max, CTLFLAG_RW, &rw_delay.max, 110 0, ""); 111 112 static void 113 rw_lock_delay_init(void *arg __unused) 114 { 115 116 lock_delay_default_init(&rw_delay); 117 rowner_retries = 10; 118 rowner_loops = max(10000, rw_delay.max); 119 } 120 LOCK_DELAY_SYSINIT(rw_lock_delay_init); 121 #endif 122 123 /* 124 * Return a pointer to the owning thread if the lock is write-locked or 125 * NULL if the lock is unlocked or read-locked. 126 */ 127 128 #define lv_rw_wowner(v) \ 129 ((v) & RW_LOCK_READ ? NULL : \ 130 (struct thread *)RW_OWNER((v))) 131 132 #define rw_wowner(rw) lv_rw_wowner(RW_READ_VALUE(rw)) 133 134 /* 135 * Returns if a write owner is recursed. Write ownership is not assured 136 * here and should be previously checked. 137 */ 138 #define rw_recursed(rw) ((rw)->rw_recurse != 0) 139 140 /* 141 * Return true if curthread helds the lock. 142 */ 143 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 144 145 /* 146 * Return a pointer to the owning thread for this lock who should receive 147 * any priority lent by threads that block on this lock. Currently this 148 * is identical to rw_wowner(). 149 */ 150 #define rw_owner(rw) rw_wowner(rw) 151 152 #ifndef INVARIANTS 153 #define __rw_assert(c, what, file, line) 154 #endif 155 156 void 157 assert_rw(const struct lock_object *lock, int what) 158 { 159 160 rw_assert((const struct rwlock *)lock, what); 161 } 162 163 void 164 lock_rw(struct lock_object *lock, uintptr_t how) 165 { 166 struct rwlock *rw; 167 168 rw = (struct rwlock *)lock; 169 if (how) 170 rw_rlock(rw); 171 else 172 rw_wlock(rw); 173 } 174 175 uintptr_t 176 unlock_rw(struct lock_object *lock) 177 { 178 struct rwlock *rw; 179 180 rw = (struct rwlock *)lock; 181 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED); 182 if (rw->rw_lock & RW_LOCK_READ) { 183 rw_runlock(rw); 184 return (1); 185 } else { 186 rw_wunlock(rw); 187 return (0); 188 } 189 } 190 191 #ifdef KDTRACE_HOOKS 192 int 193 owner_rw(const struct lock_object *lock, struct thread **owner) 194 { 195 const struct rwlock *rw = (const struct rwlock *)lock; 196 uintptr_t x = rw->rw_lock; 197 198 *owner = rw_wowner(rw); 199 return ((x & RW_LOCK_READ) != 0 ? (RW_READERS(x) != 0) : 200 (*owner != NULL)); 201 } 202 #endif 203 204 void 205 _rw_init_flags(volatile uintptr_t *c, const char *name, int opts) 206 { 207 struct rwlock *rw; 208 int flags; 209 210 rw = rwlock2rw(c); 211 212 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 213 RW_RECURSE | RW_NEW)) == 0); 214 ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock, 215 ("%s: rw_lock not aligned for %s: %p", __func__, name, 216 &rw->rw_lock)); 217 218 flags = LO_UPGRADABLE; 219 if (opts & RW_DUPOK) 220 flags |= LO_DUPOK; 221 if (opts & RW_NOPROFILE) 222 flags |= LO_NOPROFILE; 223 if (!(opts & RW_NOWITNESS)) 224 flags |= LO_WITNESS; 225 if (opts & RW_RECURSE) 226 flags |= LO_RECURSABLE; 227 if (opts & RW_QUIET) 228 flags |= LO_QUIET; 229 if (opts & RW_NEW) 230 flags |= LO_NEW; 231 232 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 233 rw->rw_lock = RW_UNLOCKED; 234 rw->rw_recurse = 0; 235 } 236 237 void 238 _rw_destroy(volatile uintptr_t *c) 239 { 240 struct rwlock *rw; 241 242 rw = rwlock2rw(c); 243 244 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw)); 245 KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw)); 246 rw->rw_lock = RW_DESTROYED; 247 lock_destroy(&rw->lock_object); 248 } 249 250 void 251 rw_sysinit(void *arg) 252 { 253 struct rw_args *args; 254 255 args = arg; 256 rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc, 257 args->ra_flags); 258 } 259 260 int 261 _rw_wowned(const volatile uintptr_t *c) 262 { 263 264 return (rw_wowner(rwlock2rw(c)) == curthread); 265 } 266 267 void 268 _rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line) 269 { 270 struct rwlock *rw; 271 uintptr_t tid, v; 272 273 rw = rwlock2rw(c); 274 275 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED() || 276 !TD_IS_IDLETHREAD(curthread), 277 ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d", 278 curthread, rw->lock_object.lo_name, file, line)); 279 KASSERT(rw->rw_lock != RW_DESTROYED, 280 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 281 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 282 line, NULL); 283 tid = (uintptr_t)curthread; 284 v = RW_UNLOCKED; 285 if (!_rw_write_lock_fetch(rw, &v, tid)) 286 _rw_wlock_hard(rw, v, file, line); 287 else 288 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, 289 0, 0, file, line, LOCKSTAT_WRITER); 290 291 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 292 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 293 TD_LOCKS_INC(curthread); 294 } 295 296 int 297 __rw_try_wlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 298 { 299 struct thread *td; 300 uintptr_t tid, v; 301 int rval; 302 bool recursed; 303 304 td = curthread; 305 tid = (uintptr_t)td; 306 if (SCHEDULER_STOPPED_TD(td)) 307 return (1); 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 int 345 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line) 346 { 347 struct rwlock *rw; 348 349 rw = rwlock2rw(c); 350 return (__rw_try_wlock_int(rw LOCK_FILE_LINE_ARG)); 351 } 352 353 void 354 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line) 355 { 356 struct rwlock *rw; 357 358 rw = rwlock2rw(c); 359 360 KASSERT(rw->rw_lock != RW_DESTROYED, 361 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 362 __rw_assert(c, RA_WLOCKED, file, line); 363 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 364 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 365 line); 366 367 #ifdef LOCK_PROFILING 368 _rw_wunlock_hard(rw, (uintptr_t)curthread, file, line); 369 #else 370 __rw_wunlock(rw, curthread, file, line); 371 #endif 372 373 TD_LOCKS_DEC(curthread); 374 } 375 376 /* 377 * Determines whether a new reader can acquire a lock. Succeeds if the 378 * reader already owns a read lock and the lock is locked for read to 379 * prevent deadlock from reader recursion. Also succeeds if the lock 380 * is unlocked and has no writer waiters or spinners. Failing otherwise 381 * prioritizes writers before readers. 382 */ 383 static bool __always_inline 384 __rw_can_read(struct thread *td, uintptr_t v, bool fp) 385 { 386 387 if ((v & (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) 388 == RW_LOCK_READ) 389 return (true); 390 if (!fp && td->td_rw_rlocks && (v & RW_LOCK_READ)) 391 return (true); 392 return (false); 393 } 394 395 static bool __always_inline 396 __rw_rlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp, bool fp 397 LOCK_FILE_LINE_ARG_DEF) 398 { 399 400 /* 401 * Handle the easy case. If no other thread has a write 402 * lock, then try to bump up the count of read locks. Note 403 * that we have to preserve the current state of the 404 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 405 * read lock, then rw_lock must have changed, so restart 406 * the loop. Note that this handles the case of a 407 * completely unlocked rwlock since such a lock is encoded 408 * as a read lock with no waiters. 409 */ 410 while (__rw_can_read(td, *vp, fp)) { 411 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, vp, 412 *vp + RW_ONE_READER)) { 413 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 414 CTR4(KTR_LOCK, 415 "%s: %p succeed %p -> %p", __func__, 416 rw, (void *)*vp, 417 (void *)(*vp + RW_ONE_READER)); 418 td->td_rw_rlocks++; 419 return (true); 420 } 421 } 422 return (false); 423 } 424 425 static void __noinline 426 __rw_rlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v 427 LOCK_FILE_LINE_ARG_DEF) 428 { 429 struct turnstile *ts; 430 struct thread *owner; 431 #ifdef ADAPTIVE_RWLOCKS 432 int spintries = 0; 433 int i, n; 434 #endif 435 #ifdef LOCK_PROFILING 436 uint64_t waittime = 0; 437 int contested = 0; 438 #endif 439 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS) 440 struct lock_delay_arg lda; 441 #endif 442 #ifdef KDTRACE_HOOKS 443 u_int sleep_cnt = 0; 444 int64_t sleep_time = 0; 445 int64_t all_time = 0; 446 #endif 447 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 448 uintptr_t state; 449 int doing_lockprof = 0; 450 #endif 451 452 #ifdef KDTRACE_HOOKS 453 if (LOCKSTAT_PROFILE_ENABLED(rw__acquire)) { 454 if (__rw_rlock_try(rw, td, &v, false LOCK_FILE_LINE_ARG)) 455 goto out_lockstat; 456 doing_lockprof = 1; 457 all_time -= lockstat_nsecs(&rw->lock_object); 458 state = v; 459 } 460 #endif 461 #ifdef LOCK_PROFILING 462 doing_lockprof = 1; 463 state = v; 464 #endif 465 466 if (SCHEDULER_STOPPED()) 467 return; 468 469 #if defined(ADAPTIVE_RWLOCKS) 470 lock_delay_arg_init(&lda, &rw_delay); 471 #elif defined(KDTRACE_HOOKS) 472 lock_delay_arg_init(&lda, NULL); 473 #endif 474 475 #ifdef HWPMC_HOOKS 476 PMC_SOFT_CALL( , , lock, failed); 477 #endif 478 lock_profile_obtain_lock_failed(&rw->lock_object, 479 &contested, &waittime); 480 481 for (;;) { 482 if (__rw_rlock_try(rw, td, &v, false LOCK_FILE_LINE_ARG)) 483 break; 484 #ifdef KDTRACE_HOOKS 485 lda.spin_cnt++; 486 #endif 487 488 #ifdef ADAPTIVE_RWLOCKS 489 /* 490 * If the owner is running on another CPU, spin until 491 * the owner stops running or the state of the lock 492 * changes. 493 */ 494 if ((v & RW_LOCK_READ) == 0) { 495 owner = (struct thread *)RW_OWNER(v); 496 if (TD_IS_RUNNING(owner)) { 497 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 498 CTR3(KTR_LOCK, 499 "%s: spinning on %p held by %p", 500 __func__, rw, owner); 501 KTR_STATE1(KTR_SCHED, "thread", 502 sched_tdname(curthread), "spinning", 503 "lockname:\"%s\"", rw->lock_object.lo_name); 504 do { 505 lock_delay(&lda); 506 v = RW_READ_VALUE(rw); 507 owner = lv_rw_wowner(v); 508 } while (owner != NULL && TD_IS_RUNNING(owner)); 509 KTR_STATE0(KTR_SCHED, "thread", 510 sched_tdname(curthread), "running"); 511 continue; 512 } 513 } else if (spintries < rowner_retries) { 514 spintries++; 515 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 516 "spinning", "lockname:\"%s\"", 517 rw->lock_object.lo_name); 518 for (i = 0; i < rowner_loops; i += n) { 519 n = RW_READERS(v); 520 lock_delay_spin(n); 521 v = RW_READ_VALUE(rw); 522 if ((v & RW_LOCK_READ) == 0 || __rw_can_read(td, v, false)) 523 break; 524 } 525 #ifdef KDTRACE_HOOKS 526 lda.spin_cnt += rowner_loops - i; 527 #endif 528 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 529 "running"); 530 if (i < rowner_loops) 531 continue; 532 } 533 #endif 534 535 /* 536 * Okay, now it's the hard case. Some other thread already 537 * has a write lock or there are write waiters present, 538 * acquire the turnstile lock so we can begin the process 539 * of blocking. 540 */ 541 ts = turnstile_trywait(&rw->lock_object); 542 543 /* 544 * The lock might have been released while we spun, so 545 * recheck its state and restart the loop if needed. 546 */ 547 v = RW_READ_VALUE(rw); 548 retry_ts: 549 if (__rw_can_read(td, v, false)) { 550 turnstile_cancel(ts); 551 continue; 552 } 553 554 owner = lv_rw_wowner(v); 555 556 #ifdef ADAPTIVE_RWLOCKS 557 /* 558 * The current lock owner might have started executing 559 * on another CPU (or the lock could have changed 560 * owners) while we were waiting on the turnstile 561 * chain lock. If so, drop the turnstile lock and try 562 * again. 563 */ 564 if (owner != NULL) { 565 if (TD_IS_RUNNING(owner)) { 566 turnstile_cancel(ts); 567 continue; 568 } 569 } 570 #endif 571 572 /* 573 * The lock is held in write mode or it already has waiters. 574 */ 575 MPASS(!__rw_can_read(td, v, false)); 576 577 /* 578 * If the RW_LOCK_READ_WAITERS flag is already set, then 579 * we can go ahead and block. If it is not set then try 580 * to set it. If we fail to set it drop the turnstile 581 * lock and restart the loop. 582 */ 583 if (!(v & RW_LOCK_READ_WAITERS)) { 584 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 585 v | RW_LOCK_READ_WAITERS)) 586 goto retry_ts; 587 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 588 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 589 __func__, rw); 590 } 591 592 /* 593 * We were unable to acquire the lock and the read waiters 594 * flag is set, so we must block on the turnstile. 595 */ 596 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 597 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 598 rw); 599 #ifdef KDTRACE_HOOKS 600 sleep_time -= lockstat_nsecs(&rw->lock_object); 601 #endif 602 MPASS(owner == rw_owner(rw)); 603 turnstile_wait(ts, owner, TS_SHARED_QUEUE); 604 #ifdef KDTRACE_HOOKS 605 sleep_time += lockstat_nsecs(&rw->lock_object); 606 sleep_cnt++; 607 #endif 608 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 609 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 610 __func__, rw); 611 v = RW_READ_VALUE(rw); 612 } 613 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 614 if (__predict_true(!doing_lockprof)) 615 return; 616 #endif 617 #ifdef KDTRACE_HOOKS 618 all_time += lockstat_nsecs(&rw->lock_object); 619 if (sleep_time) 620 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 621 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 622 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 623 624 /* Record only the loops spinning and not sleeping. */ 625 if (lda.spin_cnt > sleep_cnt) 626 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 627 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 628 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 629 out_lockstat: 630 #endif 631 /* 632 * TODO: acquire "owner of record" here. Here be turnstile dragons 633 * however. turnstiles don't like owners changing between calls to 634 * turnstile_wait() currently. 635 */ 636 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 637 waittime, file, line, LOCKSTAT_READER); 638 } 639 640 void 641 __rw_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 642 { 643 struct thread *td; 644 uintptr_t v; 645 646 td = curthread; 647 648 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED_TD(td) || 649 !TD_IS_IDLETHREAD(td), 650 ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d", 651 td, rw->lock_object.lo_name, file, line)); 652 KASSERT(rw->rw_lock != RW_DESTROYED, 653 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 654 KASSERT(rw_wowner(rw) != td, 655 ("rw_rlock: wlock already held for %s @ %s:%d", 656 rw->lock_object.lo_name, file, line)); 657 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL); 658 659 v = RW_READ_VALUE(rw); 660 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__acquire) || 661 !__rw_rlock_try(rw, td, &v, true LOCK_FILE_LINE_ARG))) 662 __rw_rlock_hard(rw, td, v LOCK_FILE_LINE_ARG); 663 else 664 lock_profile_obtain_lock_success(&rw->lock_object, 0, 0, 665 file, line); 666 667 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 668 WITNESS_LOCK(&rw->lock_object, 0, file, line); 669 TD_LOCKS_INC(curthread); 670 } 671 672 void 673 __rw_rlock(volatile uintptr_t *c, const char *file, int line) 674 { 675 struct rwlock *rw; 676 677 rw = rwlock2rw(c); 678 __rw_rlock_int(rw LOCK_FILE_LINE_ARG); 679 } 680 681 int 682 __rw_try_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 683 { 684 uintptr_t x; 685 686 if (SCHEDULER_STOPPED()) 687 return (1); 688 689 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 690 ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d", 691 curthread, rw->lock_object.lo_name, file, line)); 692 693 x = rw->rw_lock; 694 for (;;) { 695 KASSERT(rw->rw_lock != RW_DESTROYED, 696 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line)); 697 if (!(x & RW_LOCK_READ)) 698 break; 699 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &x, x + RW_ONE_READER)) { 700 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file, 701 line); 702 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line); 703 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, 704 rw, 0, 0, file, line, LOCKSTAT_READER); 705 TD_LOCKS_INC(curthread); 706 curthread->td_rw_rlocks++; 707 return (1); 708 } 709 } 710 711 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line); 712 return (0); 713 } 714 715 int 716 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line) 717 { 718 struct rwlock *rw; 719 720 rw = rwlock2rw(c); 721 return (__rw_try_rlock_int(rw LOCK_FILE_LINE_ARG)); 722 } 723 724 static bool __always_inline 725 __rw_runlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp) 726 { 727 728 for (;;) { 729 /* 730 * See if there is more than one read lock held. If so, 731 * just drop one and return. 732 */ 733 if (RW_READERS(*vp) > 1) { 734 if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp, 735 *vp - RW_ONE_READER)) { 736 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 737 CTR4(KTR_LOCK, 738 "%s: %p succeeded %p -> %p", 739 __func__, rw, (void *)*vp, 740 (void *)(*vp - RW_ONE_READER)); 741 td->td_rw_rlocks--; 742 return (true); 743 } 744 continue; 745 } 746 /* 747 * If there aren't any waiters for a write lock, then try 748 * to drop it quickly. 749 */ 750 if (!(*vp & RW_LOCK_WAITERS)) { 751 MPASS((*vp & ~RW_LOCK_WRITE_SPINNER) == 752 RW_READERS_LOCK(1)); 753 if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp, 754 RW_UNLOCKED)) { 755 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 756 CTR2(KTR_LOCK, "%s: %p last succeeded", 757 __func__, rw); 758 td->td_rw_rlocks--; 759 return (true); 760 } 761 continue; 762 } 763 break; 764 } 765 return (false); 766 } 767 768 static void __noinline 769 __rw_runlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v 770 LOCK_FILE_LINE_ARG_DEF) 771 { 772 struct turnstile *ts; 773 uintptr_t setv, queue; 774 775 if (SCHEDULER_STOPPED()) 776 return; 777 778 if (__rw_runlock_try(rw, td, &v)) 779 goto out_lockstat; 780 781 /* 782 * Ok, we know we have waiters and we think we are the 783 * last reader, so grab the turnstile lock. 784 */ 785 turnstile_chain_lock(&rw->lock_object); 786 v = RW_READ_VALUE(rw); 787 for (;;) { 788 if (__rw_runlock_try(rw, td, &v)) 789 break; 790 791 v &= (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 792 MPASS(v & RW_LOCK_WAITERS); 793 794 /* 795 * Try to drop our lock leaving the lock in a unlocked 796 * state. 797 * 798 * If you wanted to do explicit lock handoff you'd have to 799 * do it here. You'd also want to use turnstile_signal() 800 * and you'd have to handle the race where a higher 801 * priority thread blocks on the write lock before the 802 * thread you wakeup actually runs and have the new thread 803 * "steal" the lock. For now it's a lot simpler to just 804 * wakeup all of the waiters. 805 * 806 * As above, if we fail, then another thread might have 807 * acquired a read lock, so drop the turnstile lock and 808 * restart. 809 */ 810 setv = RW_UNLOCKED; 811 queue = TS_SHARED_QUEUE; 812 if (v & RW_LOCK_WRITE_WAITERS) { 813 queue = TS_EXCLUSIVE_QUEUE; 814 setv |= (v & RW_LOCK_READ_WAITERS); 815 } 816 v |= RW_READERS_LOCK(1); 817 if (!atomic_fcmpset_rel_ptr(&rw->rw_lock, &v, setv)) 818 continue; 819 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 820 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 821 __func__, rw); 822 823 /* 824 * Ok. The lock is released and all that's left is to 825 * wake up the waiters. Note that the lock might not be 826 * free anymore, but in that case the writers will just 827 * block again if they run before the new lock holder(s) 828 * release the lock. 829 */ 830 ts = turnstile_lookup(&rw->lock_object); 831 MPASS(ts != NULL); 832 turnstile_broadcast(ts, queue); 833 turnstile_unpend(ts, TS_SHARED_LOCK); 834 td->td_rw_rlocks--; 835 break; 836 } 837 turnstile_chain_unlock(&rw->lock_object); 838 out_lockstat: 839 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_READER); 840 } 841 842 void 843 _rw_runlock_cookie_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 844 { 845 struct thread *td; 846 uintptr_t v; 847 848 KASSERT(rw->rw_lock != RW_DESTROYED, 849 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 850 __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line); 851 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 852 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 853 854 td = curthread; 855 v = RW_READ_VALUE(rw); 856 857 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__release) || 858 !__rw_runlock_try(rw, td, &v))) 859 __rw_runlock_hard(rw, td, v LOCK_FILE_LINE_ARG); 860 else 861 lock_profile_release_lock(&rw->lock_object); 862 863 TD_LOCKS_DEC(curthread); 864 } 865 866 void 867 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line) 868 { 869 struct rwlock *rw; 870 871 rw = rwlock2rw(c); 872 _rw_runlock_cookie_int(rw LOCK_FILE_LINE_ARG); 873 } 874 875 /* 876 * This function is called when we are unable to obtain a write lock on the 877 * first try. This means that at least one other thread holds either a 878 * read or write lock. 879 */ 880 void 881 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) 882 { 883 uintptr_t tid; 884 struct rwlock *rw; 885 struct turnstile *ts; 886 struct thread *owner; 887 #ifdef ADAPTIVE_RWLOCKS 888 int spintries = 0; 889 int i, n; 890 enum { READERS, WRITER } sleep_reason; 891 #endif 892 uintptr_t x; 893 #ifdef LOCK_PROFILING 894 uint64_t waittime = 0; 895 int contested = 0; 896 #endif 897 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS) 898 struct lock_delay_arg lda; 899 #endif 900 #ifdef KDTRACE_HOOKS 901 u_int sleep_cnt = 0; 902 int64_t sleep_time = 0; 903 int64_t all_time = 0; 904 #endif 905 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 906 uintptr_t state; 907 int doing_lockprof = 0; 908 #endif 909 910 tid = (uintptr_t)curthread; 911 rw = rwlock2rw(c); 912 913 #ifdef KDTRACE_HOOKS 914 if (LOCKSTAT_PROFILE_ENABLED(rw__acquire)) { 915 while (v == RW_UNLOCKED) { 916 if (_rw_write_lock_fetch(rw, &v, tid)) 917 goto out_lockstat; 918 } 919 doing_lockprof = 1; 920 all_time -= lockstat_nsecs(&rw->lock_object); 921 state = v; 922 } 923 #endif 924 #ifdef LOCK_PROFILING 925 doing_lockprof = 1; 926 state = v; 927 #endif 928 929 if (SCHEDULER_STOPPED()) 930 return; 931 932 #if defined(ADAPTIVE_RWLOCKS) 933 lock_delay_arg_init(&lda, &rw_delay); 934 #elif defined(KDTRACE_HOOKS) 935 lock_delay_arg_init(&lda, NULL); 936 #endif 937 if (__predict_false(v == RW_UNLOCKED)) 938 v = RW_READ_VALUE(rw); 939 940 if (__predict_false(lv_rw_wowner(v) == (struct thread *)tid)) { 941 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE, 942 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 943 __func__, rw->lock_object.lo_name, file, line)); 944 rw->rw_recurse++; 945 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 946 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 947 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 948 return; 949 } 950 951 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 952 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 953 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 954 955 #ifdef HWPMC_HOOKS 956 PMC_SOFT_CALL( , , lock, failed); 957 #endif 958 lock_profile_obtain_lock_failed(&rw->lock_object, 959 &contested, &waittime); 960 961 for (;;) { 962 if (v == RW_UNLOCKED) { 963 if (_rw_write_lock_fetch(rw, &v, tid)) 964 break; 965 continue; 966 } 967 #ifdef KDTRACE_HOOKS 968 lda.spin_cnt++; 969 #endif 970 971 #ifdef ADAPTIVE_RWLOCKS 972 /* 973 * If the lock is write locked and the owner is 974 * running on another CPU, spin until the owner stops 975 * running or the state of the lock changes. 976 */ 977 if (!(v & RW_LOCK_READ)) { 978 sleep_reason = WRITER; 979 owner = lv_rw_wowner(v); 980 if (!TD_IS_RUNNING(owner)) 981 goto ts; 982 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 983 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 984 __func__, rw, owner); 985 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 986 "spinning", "lockname:\"%s\"", 987 rw->lock_object.lo_name); 988 do { 989 lock_delay(&lda); 990 v = RW_READ_VALUE(rw); 991 owner = lv_rw_wowner(v); 992 } while (owner != NULL && TD_IS_RUNNING(owner)); 993 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 994 "running"); 995 continue; 996 } else if (RW_READERS(v) > 0) { 997 sleep_reason = READERS; 998 if (spintries == rowner_retries) 999 goto ts; 1000 if (!(v & RW_LOCK_WRITE_SPINNER)) { 1001 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 1002 v | RW_LOCK_WRITE_SPINNER)) { 1003 continue; 1004 } 1005 } 1006 spintries++; 1007 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 1008 "spinning", "lockname:\"%s\"", 1009 rw->lock_object.lo_name); 1010 for (i = 0; i < rowner_loops; i += n) { 1011 n = RW_READERS(v); 1012 lock_delay_spin(n); 1013 v = RW_READ_VALUE(rw); 1014 if ((v & RW_LOCK_WRITE_SPINNER) == 0) 1015 break; 1016 } 1017 #ifdef KDTRACE_HOOKS 1018 lda.spin_cnt += i; 1019 #endif 1020 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 1021 "running"); 1022 if (i < rowner_loops) 1023 continue; 1024 } 1025 ts: 1026 #endif 1027 ts = turnstile_trywait(&rw->lock_object); 1028 v = RW_READ_VALUE(rw); 1029 retry_ts: 1030 owner = lv_rw_wowner(v); 1031 1032 #ifdef ADAPTIVE_RWLOCKS 1033 /* 1034 * The current lock owner might have started executing 1035 * on another CPU (or the lock could have changed 1036 * owners) while we were waiting on the turnstile 1037 * chain lock. If so, drop the turnstile lock and try 1038 * again. 1039 */ 1040 if (owner != NULL) { 1041 if (TD_IS_RUNNING(owner)) { 1042 turnstile_cancel(ts); 1043 continue; 1044 } 1045 } else if (RW_READERS(v) > 0 && sleep_reason == WRITER) { 1046 turnstile_cancel(ts); 1047 continue; 1048 } 1049 #endif 1050 /* 1051 * Check for the waiters flags about this rwlock. 1052 * If the lock was released, without maintain any pending 1053 * waiters queue, simply try to acquire it. 1054 * If a pending waiters queue is present, claim the lock 1055 * ownership and maintain the pending queue. 1056 */ 1057 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 1058 if ((v & ~x) == RW_UNLOCKED) { 1059 x &= ~RW_LOCK_WRITE_SPINNER; 1060 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid | x)) { 1061 if (x) 1062 turnstile_claim(ts); 1063 else 1064 turnstile_cancel(ts); 1065 break; 1066 } 1067 goto retry_ts; 1068 } 1069 /* 1070 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 1071 * set it. If we fail to set it, then loop back and try 1072 * again. 1073 */ 1074 if (!(v & RW_LOCK_WRITE_WAITERS)) { 1075 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 1076 v | RW_LOCK_WRITE_WAITERS)) 1077 goto retry_ts; 1078 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1079 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 1080 __func__, rw); 1081 } 1082 /* 1083 * We were unable to acquire the lock and the write waiters 1084 * flag is set, so we must block on the turnstile. 1085 */ 1086 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1087 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 1088 rw); 1089 #ifdef KDTRACE_HOOKS 1090 sleep_time -= lockstat_nsecs(&rw->lock_object); 1091 #endif 1092 MPASS(owner == rw_owner(rw)); 1093 turnstile_wait(ts, owner, TS_EXCLUSIVE_QUEUE); 1094 #ifdef KDTRACE_HOOKS 1095 sleep_time += lockstat_nsecs(&rw->lock_object); 1096 sleep_cnt++; 1097 #endif 1098 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1099 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 1100 __func__, rw); 1101 #ifdef ADAPTIVE_RWLOCKS 1102 spintries = 0; 1103 #endif 1104 v = RW_READ_VALUE(rw); 1105 } 1106 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 1107 if (__predict_true(!doing_lockprof)) 1108 return; 1109 #endif 1110 #ifdef KDTRACE_HOOKS 1111 all_time += lockstat_nsecs(&rw->lock_object); 1112 if (sleep_time) 1113 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 1114 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1115 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1116 1117 /* Record only the loops spinning and not sleeping. */ 1118 if (lda.spin_cnt > sleep_cnt) 1119 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 1120 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1121 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1122 out_lockstat: 1123 #endif 1124 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 1125 waittime, file, line, LOCKSTAT_WRITER); 1126 } 1127 1128 /* 1129 * This function is called if lockstat is active or the first try at releasing 1130 * a write lock failed. The latter means that the lock is recursed or one of 1131 * the 2 waiter bits must be set indicating that at least one thread is waiting 1132 * on this lock. 1133 */ 1134 void 1135 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) 1136 { 1137 struct rwlock *rw; 1138 struct turnstile *ts; 1139 uintptr_t tid, setv; 1140 int queue; 1141 1142 tid = (uintptr_t)curthread; 1143 if (SCHEDULER_STOPPED()) 1144 return; 1145 1146 rw = rwlock2rw(c); 1147 if (__predict_false(v == tid)) 1148 v = RW_READ_VALUE(rw); 1149 1150 if (v & RW_LOCK_WRITER_RECURSED) { 1151 if (--(rw->rw_recurse) == 0) 1152 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 1153 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1154 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 1155 return; 1156 } 1157 1158 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_WRITER); 1159 if (v == tid && _rw_write_unlock(rw, tid)) 1160 return; 1161 1162 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 1163 ("%s: neither of the waiter flags are set", __func__)); 1164 1165 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1166 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 1167 1168 turnstile_chain_lock(&rw->lock_object); 1169 1170 /* 1171 * Use the same algo as sx locks for now. Prefer waking up shared 1172 * waiters if we have any over writers. This is probably not ideal. 1173 * 1174 * 'v' is the value we are going to write back to rw_lock. If we 1175 * have waiters on both queues, we need to preserve the state of 1176 * the waiter flag for the queue we don't wake up. For now this is 1177 * hardcoded for the algorithm mentioned above. 1178 * 1179 * In the case of both readers and writers waiting we wakeup the 1180 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 1181 * new writer comes in before a reader it will claim the lock up 1182 * above. There is probably a potential priority inversion in 1183 * there that could be worked around either by waking both queues 1184 * of waiters or doing some complicated lock handoff gymnastics. 1185 */ 1186 setv = RW_UNLOCKED; 1187 v = RW_READ_VALUE(rw); 1188 queue = TS_SHARED_QUEUE; 1189 if (v & RW_LOCK_WRITE_WAITERS) { 1190 queue = TS_EXCLUSIVE_QUEUE; 1191 setv |= (v & RW_LOCK_READ_WAITERS); 1192 } 1193 atomic_store_rel_ptr(&rw->rw_lock, setv); 1194 1195 /* Wake up all waiters for the specific queue. */ 1196 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1197 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 1198 queue == TS_SHARED_QUEUE ? "read" : "write"); 1199 1200 ts = turnstile_lookup(&rw->lock_object); 1201 MPASS(ts != NULL); 1202 turnstile_broadcast(ts, queue); 1203 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1204 turnstile_chain_unlock(&rw->lock_object); 1205 } 1206 1207 /* 1208 * Attempt to do a non-blocking upgrade from a read lock to a write 1209 * lock. This will only succeed if this thread holds a single read 1210 * lock. Returns true if the upgrade succeeded and false otherwise. 1211 */ 1212 int 1213 __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 1214 { 1215 uintptr_t v, setv, tid; 1216 struct turnstile *ts; 1217 int success; 1218 1219 if (SCHEDULER_STOPPED()) 1220 return (1); 1221 1222 KASSERT(rw->rw_lock != RW_DESTROYED, 1223 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 1224 __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line); 1225 1226 /* 1227 * Attempt to switch from one reader to a writer. If there 1228 * are any write waiters, then we will have to lock the 1229 * turnstile first to prevent races with another writer 1230 * calling turnstile_wait() before we have claimed this 1231 * turnstile. So, do the simple case of no waiters first. 1232 */ 1233 tid = (uintptr_t)curthread; 1234 success = 0; 1235 v = RW_READ_VALUE(rw); 1236 for (;;) { 1237 if (RW_READERS(v) > 1) 1238 break; 1239 if (!(v & RW_LOCK_WAITERS)) { 1240 success = atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid); 1241 if (!success) 1242 continue; 1243 break; 1244 } 1245 1246 /* 1247 * Ok, we think we have waiters, so lock the turnstile. 1248 */ 1249 ts = turnstile_trywait(&rw->lock_object); 1250 v = RW_READ_VALUE(rw); 1251 retry_ts: 1252 if (RW_READERS(v) > 1) { 1253 turnstile_cancel(ts); 1254 break; 1255 } 1256 /* 1257 * Try to switch from one reader to a writer again. This time 1258 * we honor the current state of the waiters flags. 1259 * If we obtain the lock with the flags set, then claim 1260 * ownership of the turnstile. 1261 */ 1262 setv = tid | (v & RW_LOCK_WAITERS); 1263 success = atomic_fcmpset_ptr(&rw->rw_lock, &v, setv); 1264 if (success) { 1265 if (v & RW_LOCK_WAITERS) 1266 turnstile_claim(ts); 1267 else 1268 turnstile_cancel(ts); 1269 break; 1270 } 1271 goto retry_ts; 1272 } 1273 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 1274 if (success) { 1275 curthread->td_rw_rlocks--; 1276 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 1277 file, line); 1278 LOCKSTAT_RECORD0(rw__upgrade, rw); 1279 } 1280 return (success); 1281 } 1282 1283 int 1284 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line) 1285 { 1286 struct rwlock *rw; 1287 1288 rw = rwlock2rw(c); 1289 return (__rw_try_upgrade_int(rw LOCK_FILE_LINE_ARG)); 1290 } 1291 1292 /* 1293 * Downgrade a write lock into a single read lock. 1294 */ 1295 void 1296 __rw_downgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 1297 { 1298 struct turnstile *ts; 1299 uintptr_t tid, v; 1300 int rwait, wwait; 1301 1302 if (SCHEDULER_STOPPED()) 1303 return; 1304 1305 KASSERT(rw->rw_lock != RW_DESTROYED, 1306 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 1307 __rw_assert(&rw->rw_lock, RA_WLOCKED | RA_NOTRECURSED, file, line); 1308 #ifndef INVARIANTS 1309 if (rw_recursed(rw)) 1310 panic("downgrade of a recursed lock"); 1311 #endif 1312 1313 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 1314 1315 /* 1316 * Convert from a writer to a single reader. First we handle 1317 * the easy case with no waiters. If there are any waiters, we 1318 * lock the turnstile and "disown" the lock. 1319 */ 1320 tid = (uintptr_t)curthread; 1321 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 1322 goto out; 1323 1324 /* 1325 * Ok, we think we have waiters, so lock the turnstile so we can 1326 * read the waiter flags without any races. 1327 */ 1328 turnstile_chain_lock(&rw->lock_object); 1329 v = rw->rw_lock & RW_LOCK_WAITERS; 1330 rwait = v & RW_LOCK_READ_WAITERS; 1331 wwait = v & RW_LOCK_WRITE_WAITERS; 1332 MPASS(rwait | wwait); 1333 1334 /* 1335 * Downgrade from a write lock while preserving waiters flag 1336 * and give up ownership of the turnstile. 1337 */ 1338 ts = turnstile_lookup(&rw->lock_object); 1339 MPASS(ts != NULL); 1340 if (!wwait) 1341 v &= ~RW_LOCK_READ_WAITERS; 1342 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v); 1343 /* 1344 * Wake other readers if there are no writers pending. Otherwise they 1345 * won't be able to acquire the lock anyway. 1346 */ 1347 if (rwait && !wwait) { 1348 turnstile_broadcast(ts, TS_SHARED_QUEUE); 1349 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1350 } else 1351 turnstile_disown(ts); 1352 turnstile_chain_unlock(&rw->lock_object); 1353 out: 1354 curthread->td_rw_rlocks++; 1355 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 1356 LOCKSTAT_RECORD0(rw__downgrade, rw); 1357 } 1358 1359 void 1360 __rw_downgrade(volatile uintptr_t *c, const char *file, int line) 1361 { 1362 struct rwlock *rw; 1363 1364 rw = rwlock2rw(c); 1365 __rw_downgrade_int(rw LOCK_FILE_LINE_ARG); 1366 } 1367 1368 #ifdef INVARIANT_SUPPORT 1369 #ifndef INVARIANTS 1370 #undef __rw_assert 1371 #endif 1372 1373 /* 1374 * In the non-WITNESS case, rw_assert() can only detect that at least 1375 * *some* thread owns an rlock, but it cannot guarantee that *this* 1376 * thread owns an rlock. 1377 */ 1378 void 1379 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line) 1380 { 1381 const struct rwlock *rw; 1382 1383 if (panicstr != NULL) 1384 return; 1385 1386 rw = rwlock2rw(c); 1387 1388 switch (what) { 1389 case RA_LOCKED: 1390 case RA_LOCKED | RA_RECURSED: 1391 case RA_LOCKED | RA_NOTRECURSED: 1392 case RA_RLOCKED: 1393 case RA_RLOCKED | RA_RECURSED: 1394 case RA_RLOCKED | RA_NOTRECURSED: 1395 #ifdef WITNESS 1396 witness_assert(&rw->lock_object, what, file, line); 1397 #else 1398 /* 1399 * If some other thread has a write lock or we have one 1400 * and are asserting a read lock, fail. Also, if no one 1401 * has a lock at all, fail. 1402 */ 1403 if (rw->rw_lock == RW_UNLOCKED || 1404 (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED || 1405 rw_wowner(rw) != curthread))) 1406 panic("Lock %s not %slocked @ %s:%d\n", 1407 rw->lock_object.lo_name, (what & RA_RLOCKED) ? 1408 "read " : "", file, line); 1409 1410 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) { 1411 if (rw_recursed(rw)) { 1412 if (what & RA_NOTRECURSED) 1413 panic("Lock %s recursed @ %s:%d\n", 1414 rw->lock_object.lo_name, file, 1415 line); 1416 } else if (what & RA_RECURSED) 1417 panic("Lock %s not recursed @ %s:%d\n", 1418 rw->lock_object.lo_name, file, line); 1419 } 1420 #endif 1421 break; 1422 case RA_WLOCKED: 1423 case RA_WLOCKED | RA_RECURSED: 1424 case RA_WLOCKED | RA_NOTRECURSED: 1425 if (rw_wowner(rw) != curthread) 1426 panic("Lock %s not exclusively locked @ %s:%d\n", 1427 rw->lock_object.lo_name, file, line); 1428 if (rw_recursed(rw)) { 1429 if (what & RA_NOTRECURSED) 1430 panic("Lock %s recursed @ %s:%d\n", 1431 rw->lock_object.lo_name, file, line); 1432 } else if (what & RA_RECURSED) 1433 panic("Lock %s not recursed @ %s:%d\n", 1434 rw->lock_object.lo_name, file, line); 1435 break; 1436 case RA_UNLOCKED: 1437 #ifdef WITNESS 1438 witness_assert(&rw->lock_object, what, file, line); 1439 #else 1440 /* 1441 * If we hold a write lock fail. We can't reliably check 1442 * to see if we hold a read lock or not. 1443 */ 1444 if (rw_wowner(rw) == curthread) 1445 panic("Lock %s exclusively locked @ %s:%d\n", 1446 rw->lock_object.lo_name, file, line); 1447 #endif 1448 break; 1449 default: 1450 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 1451 line); 1452 } 1453 } 1454 #endif /* INVARIANT_SUPPORT */ 1455 1456 #ifdef DDB 1457 void 1458 db_show_rwlock(const struct lock_object *lock) 1459 { 1460 const struct rwlock *rw; 1461 struct thread *td; 1462 1463 rw = (const struct rwlock *)lock; 1464 1465 db_printf(" state: "); 1466 if (rw->rw_lock == RW_UNLOCKED) 1467 db_printf("UNLOCKED\n"); 1468 else if (rw->rw_lock == RW_DESTROYED) { 1469 db_printf("DESTROYED\n"); 1470 return; 1471 } else if (rw->rw_lock & RW_LOCK_READ) 1472 db_printf("RLOCK: %ju locks\n", 1473 (uintmax_t)(RW_READERS(rw->rw_lock))); 1474 else { 1475 td = rw_wowner(rw); 1476 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 1477 td->td_tid, td->td_proc->p_pid, td->td_name); 1478 if (rw_recursed(rw)) 1479 db_printf(" recursed: %u\n", rw->rw_recurse); 1480 } 1481 db_printf(" waiters: "); 1482 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 1483 case RW_LOCK_READ_WAITERS: 1484 db_printf("readers\n"); 1485 break; 1486 case RW_LOCK_WRITE_WAITERS: 1487 db_printf("writers\n"); 1488 break; 1489 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 1490 db_printf("readers and writers\n"); 1491 break; 1492 default: 1493 db_printf("none\n"); 1494 break; 1495 } 1496 } 1497 1498 #endif 1499