Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)
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sys/kern/kern_rwlock.c
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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: releng/12.0/sys/kern/kern_rwlock.c 334546 2018-06-02 22:37:53Z mjg $"); 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 { 514 if ((v & RW_LOCK_WRITE_SPINNER) && RW_READERS(v) == 0) { 515 MPASS(!__rw_can_read(td, v, false)); 516 lock_delay_spin(2); 517 v = RW_READ_VALUE(rw); 518 continue; 519 } 520 if (spintries < rowner_retries) { 521 spintries++; 522 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 523 "spinning", "lockname:\"%s\"", 524 rw->lock_object.lo_name); 525 n = RW_READERS(v); 526 for (i = 0; i < rowner_loops; i += n) { 527 lock_delay_spin(n); 528 v = RW_READ_VALUE(rw); 529 if (!(v & RW_LOCK_READ)) 530 break; 531 n = RW_READERS(v); 532 if (n == 0) 533 break; 534 if (__rw_can_read(td, v, false)) 535 break; 536 } 537 #ifdef KDTRACE_HOOKS 538 lda.spin_cnt += rowner_loops - i; 539 #endif 540 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 541 "running"); 542 if (i < rowner_loops) 543 continue; 544 } 545 } 546 #endif 547 548 /* 549 * Okay, now it's the hard case. Some other thread already 550 * has a write lock or there are write waiters present, 551 * acquire the turnstile lock so we can begin the process 552 * of blocking. 553 */ 554 ts = turnstile_trywait(&rw->lock_object); 555 556 /* 557 * The lock might have been released while we spun, so 558 * recheck its state and restart the loop if needed. 559 */ 560 v = RW_READ_VALUE(rw); 561 retry_ts: 562 if (((v & RW_LOCK_WRITE_SPINNER) && RW_READERS(v) == 0) || 563 __rw_can_read(td, v, false)) { 564 turnstile_cancel(ts); 565 continue; 566 } 567 568 owner = lv_rw_wowner(v); 569 570 #ifdef ADAPTIVE_RWLOCKS 571 /* 572 * The current lock owner might have started executing 573 * on another CPU (or the lock could have changed 574 * owners) while we were waiting on the turnstile 575 * chain lock. If so, drop the turnstile lock and try 576 * again. 577 */ 578 if (owner != NULL) { 579 if (TD_IS_RUNNING(owner)) { 580 turnstile_cancel(ts); 581 continue; 582 } 583 } 584 #endif 585 586 /* 587 * The lock is held in write mode or it already has waiters. 588 */ 589 MPASS(!__rw_can_read(td, v, false)); 590 591 /* 592 * If the RW_LOCK_READ_WAITERS flag is already set, then 593 * we can go ahead and block. If it is not set then try 594 * to set it. If we fail to set it drop the turnstile 595 * lock and restart the loop. 596 */ 597 if (!(v & RW_LOCK_READ_WAITERS)) { 598 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 599 v | RW_LOCK_READ_WAITERS)) 600 goto retry_ts; 601 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 602 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 603 __func__, rw); 604 } 605 606 /* 607 * We were unable to acquire the lock and the read waiters 608 * flag is set, so we must block on the turnstile. 609 */ 610 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 611 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 612 rw); 613 #ifdef KDTRACE_HOOKS 614 sleep_time -= lockstat_nsecs(&rw->lock_object); 615 #endif 616 MPASS(owner == rw_owner(rw)); 617 turnstile_wait(ts, owner, TS_SHARED_QUEUE); 618 #ifdef KDTRACE_HOOKS 619 sleep_time += lockstat_nsecs(&rw->lock_object); 620 sleep_cnt++; 621 #endif 622 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 623 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 624 __func__, rw); 625 v = RW_READ_VALUE(rw); 626 } 627 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 628 if (__predict_true(!doing_lockprof)) 629 return; 630 #endif 631 #ifdef KDTRACE_HOOKS 632 all_time += lockstat_nsecs(&rw->lock_object); 633 if (sleep_time) 634 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 635 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 636 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 637 638 /* Record only the loops spinning and not sleeping. */ 639 if (lda.spin_cnt > sleep_cnt) 640 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 641 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 642 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 643 out_lockstat: 644 #endif 645 /* 646 * TODO: acquire "owner of record" here. Here be turnstile dragons 647 * however. turnstiles don't like owners changing between calls to 648 * turnstile_wait() currently. 649 */ 650 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 651 waittime, file, line, LOCKSTAT_READER); 652 } 653 654 void 655 __rw_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 656 { 657 struct thread *td; 658 uintptr_t v; 659 660 td = curthread; 661 662 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED_TD(td) || 663 !TD_IS_IDLETHREAD(td), 664 ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d", 665 td, rw->lock_object.lo_name, file, line)); 666 KASSERT(rw->rw_lock != RW_DESTROYED, 667 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 668 KASSERT(rw_wowner(rw) != td, 669 ("rw_rlock: wlock already held for %s @ %s:%d", 670 rw->lock_object.lo_name, file, line)); 671 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL); 672 673 v = RW_READ_VALUE(rw); 674 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__acquire) || 675 !__rw_rlock_try(rw, td, &v, true LOCK_FILE_LINE_ARG))) 676 __rw_rlock_hard(rw, td, v LOCK_FILE_LINE_ARG); 677 else 678 lock_profile_obtain_lock_success(&rw->lock_object, 0, 0, 679 file, line); 680 681 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 682 WITNESS_LOCK(&rw->lock_object, 0, file, line); 683 TD_LOCKS_INC(curthread); 684 } 685 686 void 687 __rw_rlock(volatile uintptr_t *c, const char *file, int line) 688 { 689 struct rwlock *rw; 690 691 rw = rwlock2rw(c); 692 __rw_rlock_int(rw LOCK_FILE_LINE_ARG); 693 } 694 695 int 696 __rw_try_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 697 { 698 uintptr_t x; 699 700 if (SCHEDULER_STOPPED()) 701 return (1); 702 703 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 704 ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d", 705 curthread, rw->lock_object.lo_name, file, line)); 706 707 x = rw->rw_lock; 708 for (;;) { 709 KASSERT(rw->rw_lock != RW_DESTROYED, 710 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line)); 711 if (!(x & RW_LOCK_READ)) 712 break; 713 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &x, x + RW_ONE_READER)) { 714 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file, 715 line); 716 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line); 717 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, 718 rw, 0, 0, file, line, LOCKSTAT_READER); 719 TD_LOCKS_INC(curthread); 720 curthread->td_rw_rlocks++; 721 return (1); 722 } 723 } 724 725 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line); 726 return (0); 727 } 728 729 int 730 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line) 731 { 732 struct rwlock *rw; 733 734 rw = rwlock2rw(c); 735 return (__rw_try_rlock_int(rw LOCK_FILE_LINE_ARG)); 736 } 737 738 static bool __always_inline 739 __rw_runlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp) 740 { 741 742 for (;;) { 743 if (RW_READERS(*vp) > 1 || !(*vp & RW_LOCK_WAITERS)) { 744 if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp, 745 *vp - RW_ONE_READER)) { 746 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 747 CTR4(KTR_LOCK, 748 "%s: %p succeeded %p -> %p", 749 __func__, rw, (void *)*vp, 750 (void *)(*vp - RW_ONE_READER)); 751 td->td_rw_rlocks--; 752 return (true); 753 } 754 continue; 755 } 756 break; 757 } 758 return (false); 759 } 760 761 static void __noinline 762 __rw_runlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v 763 LOCK_FILE_LINE_ARG_DEF) 764 { 765 struct turnstile *ts; 766 uintptr_t setv, queue; 767 768 if (SCHEDULER_STOPPED()) 769 return; 770 771 if (__rw_runlock_try(rw, td, &v)) 772 goto out_lockstat; 773 774 /* 775 * Ok, we know we have waiters and we think we are the 776 * last reader, so grab the turnstile lock. 777 */ 778 turnstile_chain_lock(&rw->lock_object); 779 v = RW_READ_VALUE(rw); 780 for (;;) { 781 if (__rw_runlock_try(rw, td, &v)) 782 break; 783 784 MPASS(v & RW_LOCK_WAITERS); 785 786 /* 787 * Try to drop our lock leaving the lock in a unlocked 788 * state. 789 * 790 * If you wanted to do explicit lock handoff you'd have to 791 * do it here. You'd also want to use turnstile_signal() 792 * and you'd have to handle the race where a higher 793 * priority thread blocks on the write lock before the 794 * thread you wakeup actually runs and have the new thread 795 * "steal" the lock. For now it's a lot simpler to just 796 * wakeup all of the waiters. 797 * 798 * As above, if we fail, then another thread might have 799 * acquired a read lock, so drop the turnstile lock and 800 * restart. 801 */ 802 setv = RW_UNLOCKED; 803 queue = TS_SHARED_QUEUE; 804 if (v & RW_LOCK_WRITE_WAITERS) { 805 queue = TS_EXCLUSIVE_QUEUE; 806 setv |= (v & RW_LOCK_READ_WAITERS); 807 } 808 setv |= (v & RW_LOCK_WRITE_SPINNER); 809 if (!atomic_fcmpset_rel_ptr(&rw->rw_lock, &v, setv)) 810 continue; 811 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 812 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 813 __func__, rw); 814 815 /* 816 * Ok. The lock is released and all that's left is to 817 * wake up the waiters. Note that the lock might not be 818 * free anymore, but in that case the writers will just 819 * block again if they run before the new lock holder(s) 820 * release the lock. 821 */ 822 ts = turnstile_lookup(&rw->lock_object); 823 MPASS(ts != NULL); 824 turnstile_broadcast(ts, queue); 825 turnstile_unpend(ts); 826 td->td_rw_rlocks--; 827 break; 828 } 829 turnstile_chain_unlock(&rw->lock_object); 830 out_lockstat: 831 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_READER); 832 } 833 834 void 835 _rw_runlock_cookie_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 836 { 837 struct thread *td; 838 uintptr_t v; 839 840 KASSERT(rw->rw_lock != RW_DESTROYED, 841 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 842 __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line); 843 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 844 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 845 846 td = curthread; 847 v = RW_READ_VALUE(rw); 848 849 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__release) || 850 !__rw_runlock_try(rw, td, &v))) 851 __rw_runlock_hard(rw, td, v LOCK_FILE_LINE_ARG); 852 else 853 lock_profile_release_lock(&rw->lock_object); 854 855 TD_LOCKS_DEC(curthread); 856 } 857 858 void 859 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line) 860 { 861 struct rwlock *rw; 862 863 rw = rwlock2rw(c); 864 _rw_runlock_cookie_int(rw LOCK_FILE_LINE_ARG); 865 } 866 867 #ifdef ADAPTIVE_RWLOCKS 868 static inline void 869 rw_drop_critical(uintptr_t v, bool *in_critical, int *extra_work) 870 { 871 872 if (v & RW_LOCK_WRITE_SPINNER) 873 return; 874 if (*in_critical) { 875 critical_exit(); 876 *in_critical = false; 877 (*extra_work)--; 878 } 879 } 880 #else 881 #define rw_drop_critical(v, in_critical, extra_work) do { } while (0) 882 #endif 883 884 /* 885 * This function is called when we are unable to obtain a write lock on the 886 * first try. This means that at least one other thread holds either a 887 * read or write lock. 888 */ 889 void 890 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) 891 { 892 uintptr_t tid; 893 struct rwlock *rw; 894 struct turnstile *ts; 895 struct thread *owner; 896 #ifdef ADAPTIVE_RWLOCKS 897 int spintries = 0; 898 int i, n; 899 enum { READERS, WRITER } sleep_reason = READERS; 900 bool in_critical = false; 901 #endif 902 uintptr_t setv; 903 #ifdef LOCK_PROFILING 904 uint64_t waittime = 0; 905 int contested = 0; 906 #endif 907 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS) 908 struct lock_delay_arg lda; 909 #endif 910 #ifdef KDTRACE_HOOKS 911 u_int sleep_cnt = 0; 912 int64_t sleep_time = 0; 913 int64_t all_time = 0; 914 #endif 915 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 916 uintptr_t state; 917 int doing_lockprof = 0; 918 #endif 919 int extra_work = 0; 920 921 tid = (uintptr_t)curthread; 922 rw = rwlock2rw(c); 923 924 #ifdef KDTRACE_HOOKS 925 if (LOCKSTAT_PROFILE_ENABLED(rw__acquire)) { 926 while (v == RW_UNLOCKED) { 927 if (_rw_write_lock_fetch(rw, &v, tid)) 928 goto out_lockstat; 929 } 930 extra_work = 1; 931 doing_lockprof = 1; 932 all_time -= lockstat_nsecs(&rw->lock_object); 933 state = v; 934 } 935 #endif 936 #ifdef LOCK_PROFILING 937 extra_work = 1; 938 doing_lockprof = 1; 939 state = v; 940 #endif 941 942 if (SCHEDULER_STOPPED()) 943 return; 944 945 #if defined(ADAPTIVE_RWLOCKS) 946 lock_delay_arg_init(&lda, &rw_delay); 947 #elif defined(KDTRACE_HOOKS) 948 lock_delay_arg_init(&lda, NULL); 949 #endif 950 if (__predict_false(v == RW_UNLOCKED)) 951 v = RW_READ_VALUE(rw); 952 953 if (__predict_false(lv_rw_wowner(v) == (struct thread *)tid)) { 954 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE, 955 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 956 __func__, rw->lock_object.lo_name, file, line)); 957 rw->rw_recurse++; 958 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 959 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 960 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 961 return; 962 } 963 964 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 965 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 966 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 967 968 #ifdef HWPMC_HOOKS 969 PMC_SOFT_CALL( , , lock, failed); 970 #endif 971 lock_profile_obtain_lock_failed(&rw->lock_object, 972 &contested, &waittime); 973 974 for (;;) { 975 if (v == RW_UNLOCKED) { 976 if (_rw_write_lock_fetch(rw, &v, tid)) 977 break; 978 continue; 979 } 980 #ifdef KDTRACE_HOOKS 981 lda.spin_cnt++; 982 #endif 983 984 #ifdef ADAPTIVE_RWLOCKS 985 if (v == (RW_LOCK_READ | RW_LOCK_WRITE_SPINNER)) { 986 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid)) 987 break; 988 continue; 989 } 990 991 /* 992 * If the lock is write locked and the owner is 993 * running on another CPU, spin until the owner stops 994 * running or the state of the lock changes. 995 */ 996 if (!(v & RW_LOCK_READ)) { 997 rw_drop_critical(v, &in_critical, &extra_work); 998 sleep_reason = WRITER; 999 owner = lv_rw_wowner(v); 1000 if (!TD_IS_RUNNING(owner)) 1001 goto ts; 1002 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1003 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 1004 __func__, rw, owner); 1005 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 1006 "spinning", "lockname:\"%s\"", 1007 rw->lock_object.lo_name); 1008 do { 1009 lock_delay(&lda); 1010 v = RW_READ_VALUE(rw); 1011 owner = lv_rw_wowner(v); 1012 } while (owner != NULL && TD_IS_RUNNING(owner)); 1013 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 1014 "running"); 1015 continue; 1016 } else if (RW_READERS(v) > 0) { 1017 sleep_reason = READERS; 1018 if (spintries == rowner_retries) 1019 goto ts; 1020 if (!(v & RW_LOCK_WRITE_SPINNER)) { 1021 if (!in_critical) { 1022 critical_enter(); 1023 in_critical = true; 1024 extra_work++; 1025 } 1026 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 1027 v | RW_LOCK_WRITE_SPINNER)) { 1028 critical_exit(); 1029 in_critical = false; 1030 extra_work--; 1031 continue; 1032 } 1033 } 1034 spintries++; 1035 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 1036 "spinning", "lockname:\"%s\"", 1037 rw->lock_object.lo_name); 1038 n = RW_READERS(v); 1039 for (i = 0; i < rowner_loops; i += n) { 1040 lock_delay_spin(n); 1041 v = RW_READ_VALUE(rw); 1042 if (!(v & RW_LOCK_WRITE_SPINNER)) 1043 break; 1044 if (!(v & RW_LOCK_READ)) 1045 break; 1046 n = RW_READERS(v); 1047 if (n == 0) 1048 break; 1049 } 1050 #ifdef KDTRACE_HOOKS 1051 lda.spin_cnt += i; 1052 #endif 1053 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 1054 "running"); 1055 if (i < rowner_loops) 1056 continue; 1057 } 1058 ts: 1059 #endif 1060 ts = turnstile_trywait(&rw->lock_object); 1061 v = RW_READ_VALUE(rw); 1062 retry_ts: 1063 owner = lv_rw_wowner(v); 1064 1065 #ifdef ADAPTIVE_RWLOCKS 1066 /* 1067 * The current lock owner might have started executing 1068 * on another CPU (or the lock could have changed 1069 * owners) while we were waiting on the turnstile 1070 * chain lock. If so, drop the turnstile lock and try 1071 * again. 1072 */ 1073 if (owner != NULL) { 1074 if (TD_IS_RUNNING(owner)) { 1075 turnstile_cancel(ts); 1076 rw_drop_critical(v, &in_critical, &extra_work); 1077 continue; 1078 } 1079 } else if (RW_READERS(v) > 0 && sleep_reason == WRITER) { 1080 turnstile_cancel(ts); 1081 rw_drop_critical(v, &in_critical, &extra_work); 1082 continue; 1083 } 1084 #endif 1085 /* 1086 * Check for the waiters flags about this rwlock. 1087 * If the lock was released, without maintain any pending 1088 * waiters queue, simply try to acquire it. 1089 * If a pending waiters queue is present, claim the lock 1090 * ownership and maintain the pending queue. 1091 */ 1092 setv = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 1093 if ((v & ~setv) == RW_UNLOCKED) { 1094 setv &= ~RW_LOCK_WRITE_SPINNER; 1095 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid | setv)) { 1096 if (setv) 1097 turnstile_claim(ts); 1098 else 1099 turnstile_cancel(ts); 1100 break; 1101 } 1102 goto retry_ts; 1103 } 1104 1105 #ifdef ADAPTIVE_RWLOCKS 1106 if (in_critical) { 1107 if ((v & RW_LOCK_WRITE_SPINNER) || 1108 !((v & RW_LOCK_WRITE_WAITERS))) { 1109 setv = v & ~RW_LOCK_WRITE_SPINNER; 1110 setv |= RW_LOCK_WRITE_WAITERS; 1111 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, setv)) 1112 goto retry_ts; 1113 } 1114 critical_exit(); 1115 in_critical = false; 1116 extra_work--; 1117 } else { 1118 #endif 1119 /* 1120 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 1121 * set it. If we fail to set it, then loop back and try 1122 * again. 1123 */ 1124 if (!(v & RW_LOCK_WRITE_WAITERS)) { 1125 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 1126 v | RW_LOCK_WRITE_WAITERS)) 1127 goto retry_ts; 1128 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1129 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 1130 __func__, rw); 1131 } 1132 #ifdef ADAPTIVE_RWLOCKS 1133 } 1134 #endif 1135 /* 1136 * We were unable to acquire the lock and the write waiters 1137 * flag is set, so we must block on the turnstile. 1138 */ 1139 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1140 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 1141 rw); 1142 #ifdef KDTRACE_HOOKS 1143 sleep_time -= lockstat_nsecs(&rw->lock_object); 1144 #endif 1145 MPASS(owner == rw_owner(rw)); 1146 turnstile_wait(ts, owner, TS_EXCLUSIVE_QUEUE); 1147 #ifdef KDTRACE_HOOKS 1148 sleep_time += lockstat_nsecs(&rw->lock_object); 1149 sleep_cnt++; 1150 #endif 1151 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1152 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 1153 __func__, rw); 1154 #ifdef ADAPTIVE_RWLOCKS 1155 spintries = 0; 1156 #endif 1157 v = RW_READ_VALUE(rw); 1158 } 1159 if (__predict_true(!extra_work)) 1160 return; 1161 #ifdef ADAPTIVE_RWLOCKS 1162 if (in_critical) 1163 critical_exit(); 1164 #endif 1165 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 1166 if (__predict_true(!doing_lockprof)) 1167 return; 1168 #endif 1169 #ifdef KDTRACE_HOOKS 1170 all_time += lockstat_nsecs(&rw->lock_object); 1171 if (sleep_time) 1172 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 1173 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1174 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1175 1176 /* Record only the loops spinning and not sleeping. */ 1177 if (lda.spin_cnt > sleep_cnt) 1178 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 1179 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1180 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1181 out_lockstat: 1182 #endif 1183 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 1184 waittime, file, line, LOCKSTAT_WRITER); 1185 } 1186 1187 /* 1188 * This function is called if lockstat is active or the first try at releasing 1189 * a write lock failed. The latter means that the lock is recursed or one of 1190 * the 2 waiter bits must be set indicating that at least one thread is waiting 1191 * on this lock. 1192 */ 1193 void 1194 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) 1195 { 1196 struct rwlock *rw; 1197 struct turnstile *ts; 1198 uintptr_t tid, setv; 1199 int queue; 1200 1201 tid = (uintptr_t)curthread; 1202 if (SCHEDULER_STOPPED()) 1203 return; 1204 1205 rw = rwlock2rw(c); 1206 if (__predict_false(v == tid)) 1207 v = RW_READ_VALUE(rw); 1208 1209 if (v & RW_LOCK_WRITER_RECURSED) { 1210 if (--(rw->rw_recurse) == 0) 1211 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 1212 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1213 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 1214 return; 1215 } 1216 1217 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_WRITER); 1218 if (v == tid && _rw_write_unlock(rw, tid)) 1219 return; 1220 1221 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 1222 ("%s: neither of the waiter flags are set", __func__)); 1223 1224 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1225 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 1226 1227 turnstile_chain_lock(&rw->lock_object); 1228 1229 /* 1230 * Use the same algo as sx locks for now. Prefer waking up shared 1231 * waiters if we have any over writers. This is probably not ideal. 1232 * 1233 * 'v' is the value we are going to write back to rw_lock. If we 1234 * have waiters on both queues, we need to preserve the state of 1235 * the waiter flag for the queue we don't wake up. For now this is 1236 * hardcoded for the algorithm mentioned above. 1237 * 1238 * In the case of both readers and writers waiting we wakeup the 1239 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 1240 * new writer comes in before a reader it will claim the lock up 1241 * above. There is probably a potential priority inversion in 1242 * there that could be worked around either by waking both queues 1243 * of waiters or doing some complicated lock handoff gymnastics. 1244 */ 1245 setv = RW_UNLOCKED; 1246 v = RW_READ_VALUE(rw); 1247 queue = TS_SHARED_QUEUE; 1248 if (v & RW_LOCK_WRITE_WAITERS) { 1249 queue = TS_EXCLUSIVE_QUEUE; 1250 setv |= (v & RW_LOCK_READ_WAITERS); 1251 } 1252 atomic_store_rel_ptr(&rw->rw_lock, setv); 1253 1254 /* Wake up all waiters for the specific queue. */ 1255 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1256 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 1257 queue == TS_SHARED_QUEUE ? "read" : "write"); 1258 1259 ts = turnstile_lookup(&rw->lock_object); 1260 MPASS(ts != NULL); 1261 turnstile_broadcast(ts, queue); 1262 turnstile_unpend(ts); 1263 turnstile_chain_unlock(&rw->lock_object); 1264 } 1265 1266 /* 1267 * Attempt to do a non-blocking upgrade from a read lock to a write 1268 * lock. This will only succeed if this thread holds a single read 1269 * lock. Returns true if the upgrade succeeded and false otherwise. 1270 */ 1271 int 1272 __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 1273 { 1274 uintptr_t v, setv, tid; 1275 struct turnstile *ts; 1276 int success; 1277 1278 if (SCHEDULER_STOPPED()) 1279 return (1); 1280 1281 KASSERT(rw->rw_lock != RW_DESTROYED, 1282 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 1283 __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line); 1284 1285 /* 1286 * Attempt to switch from one reader to a writer. If there 1287 * are any write waiters, then we will have to lock the 1288 * turnstile first to prevent races with another writer 1289 * calling turnstile_wait() before we have claimed this 1290 * turnstile. So, do the simple case of no waiters first. 1291 */ 1292 tid = (uintptr_t)curthread; 1293 success = 0; 1294 v = RW_READ_VALUE(rw); 1295 for (;;) { 1296 if (RW_READERS(v) > 1) 1297 break; 1298 if (!(v & RW_LOCK_WAITERS)) { 1299 success = atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid); 1300 if (!success) 1301 continue; 1302 break; 1303 } 1304 1305 /* 1306 * Ok, we think we have waiters, so lock the turnstile. 1307 */ 1308 ts = turnstile_trywait(&rw->lock_object); 1309 v = RW_READ_VALUE(rw); 1310 retry_ts: 1311 if (RW_READERS(v) > 1) { 1312 turnstile_cancel(ts); 1313 break; 1314 } 1315 /* 1316 * Try to switch from one reader to a writer again. This time 1317 * we honor the current state of the waiters flags. 1318 * If we obtain the lock with the flags set, then claim 1319 * ownership of the turnstile. 1320 */ 1321 setv = tid | (v & RW_LOCK_WAITERS); 1322 success = atomic_fcmpset_ptr(&rw->rw_lock, &v, setv); 1323 if (success) { 1324 if (v & RW_LOCK_WAITERS) 1325 turnstile_claim(ts); 1326 else 1327 turnstile_cancel(ts); 1328 break; 1329 } 1330 goto retry_ts; 1331 } 1332 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 1333 if (success) { 1334 curthread->td_rw_rlocks--; 1335 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 1336 file, line); 1337 LOCKSTAT_RECORD0(rw__upgrade, rw); 1338 } 1339 return (success); 1340 } 1341 1342 int 1343 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line) 1344 { 1345 struct rwlock *rw; 1346 1347 rw = rwlock2rw(c); 1348 return (__rw_try_upgrade_int(rw LOCK_FILE_LINE_ARG)); 1349 } 1350 1351 /* 1352 * Downgrade a write lock into a single read lock. 1353 */ 1354 void 1355 __rw_downgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 1356 { 1357 struct turnstile *ts; 1358 uintptr_t tid, v; 1359 int rwait, wwait; 1360 1361 if (SCHEDULER_STOPPED()) 1362 return; 1363 1364 KASSERT(rw->rw_lock != RW_DESTROYED, 1365 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 1366 __rw_assert(&rw->rw_lock, RA_WLOCKED | RA_NOTRECURSED, file, line); 1367 #ifndef INVARIANTS 1368 if (rw_recursed(rw)) 1369 panic("downgrade of a recursed lock"); 1370 #endif 1371 1372 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 1373 1374 /* 1375 * Convert from a writer to a single reader. First we handle 1376 * the easy case with no waiters. If there are any waiters, we 1377 * lock the turnstile and "disown" the lock. 1378 */ 1379 tid = (uintptr_t)curthread; 1380 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 1381 goto out; 1382 1383 /* 1384 * Ok, we think we have waiters, so lock the turnstile so we can 1385 * read the waiter flags without any races. 1386 */ 1387 turnstile_chain_lock(&rw->lock_object); 1388 v = rw->rw_lock & RW_LOCK_WAITERS; 1389 rwait = v & RW_LOCK_READ_WAITERS; 1390 wwait = v & RW_LOCK_WRITE_WAITERS; 1391 MPASS(rwait | wwait); 1392 1393 /* 1394 * Downgrade from a write lock while preserving waiters flag 1395 * and give up ownership of the turnstile. 1396 */ 1397 ts = turnstile_lookup(&rw->lock_object); 1398 MPASS(ts != NULL); 1399 if (!wwait) 1400 v &= ~RW_LOCK_READ_WAITERS; 1401 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v); 1402 /* 1403 * Wake other readers if there are no writers pending. Otherwise they 1404 * won't be able to acquire the lock anyway. 1405 */ 1406 if (rwait && !wwait) { 1407 turnstile_broadcast(ts, TS_SHARED_QUEUE); 1408 turnstile_unpend(ts); 1409 } else 1410 turnstile_disown(ts); 1411 turnstile_chain_unlock(&rw->lock_object); 1412 out: 1413 curthread->td_rw_rlocks++; 1414 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 1415 LOCKSTAT_RECORD0(rw__downgrade, rw); 1416 } 1417 1418 void 1419 __rw_downgrade(volatile uintptr_t *c, const char *file, int line) 1420 { 1421 struct rwlock *rw; 1422 1423 rw = rwlock2rw(c); 1424 __rw_downgrade_int(rw LOCK_FILE_LINE_ARG); 1425 } 1426 1427 #ifdef INVARIANT_SUPPORT 1428 #ifndef INVARIANTS 1429 #undef __rw_assert 1430 #endif 1431 1432 /* 1433 * In the non-WITNESS case, rw_assert() can only detect that at least 1434 * *some* thread owns an rlock, but it cannot guarantee that *this* 1435 * thread owns an rlock. 1436 */ 1437 void 1438 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line) 1439 { 1440 const struct rwlock *rw; 1441 1442 if (panicstr != NULL) 1443 return; 1444 1445 rw = rwlock2rw(c); 1446 1447 switch (what) { 1448 case RA_LOCKED: 1449 case RA_LOCKED | RA_RECURSED: 1450 case RA_LOCKED | RA_NOTRECURSED: 1451 case RA_RLOCKED: 1452 case RA_RLOCKED | RA_RECURSED: 1453 case RA_RLOCKED | RA_NOTRECURSED: 1454 #ifdef WITNESS 1455 witness_assert(&rw->lock_object, what, file, line); 1456 #else 1457 /* 1458 * If some other thread has a write lock or we have one 1459 * and are asserting a read lock, fail. Also, if no one 1460 * has a lock at all, fail. 1461 */ 1462 if (rw->rw_lock == RW_UNLOCKED || 1463 (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED || 1464 rw_wowner(rw) != curthread))) 1465 panic("Lock %s not %slocked @ %s:%d\n", 1466 rw->lock_object.lo_name, (what & RA_RLOCKED) ? 1467 "read " : "", file, line); 1468 1469 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) { 1470 if (rw_recursed(rw)) { 1471 if (what & RA_NOTRECURSED) 1472 panic("Lock %s recursed @ %s:%d\n", 1473 rw->lock_object.lo_name, file, 1474 line); 1475 } else if (what & RA_RECURSED) 1476 panic("Lock %s not recursed @ %s:%d\n", 1477 rw->lock_object.lo_name, file, line); 1478 } 1479 #endif 1480 break; 1481 case RA_WLOCKED: 1482 case RA_WLOCKED | RA_RECURSED: 1483 case RA_WLOCKED | RA_NOTRECURSED: 1484 if (rw_wowner(rw) != curthread) 1485 panic("Lock %s not exclusively locked @ %s:%d\n", 1486 rw->lock_object.lo_name, file, line); 1487 if (rw_recursed(rw)) { 1488 if (what & RA_NOTRECURSED) 1489 panic("Lock %s recursed @ %s:%d\n", 1490 rw->lock_object.lo_name, file, line); 1491 } else if (what & RA_RECURSED) 1492 panic("Lock %s not recursed @ %s:%d\n", 1493 rw->lock_object.lo_name, file, line); 1494 break; 1495 case RA_UNLOCKED: 1496 #ifdef WITNESS 1497 witness_assert(&rw->lock_object, what, file, line); 1498 #else 1499 /* 1500 * If we hold a write lock fail. We can't reliably check 1501 * to see if we hold a read lock or not. 1502 */ 1503 if (rw_wowner(rw) == curthread) 1504 panic("Lock %s exclusively locked @ %s:%d\n", 1505 rw->lock_object.lo_name, file, line); 1506 #endif 1507 break; 1508 default: 1509 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 1510 line); 1511 } 1512 } 1513 #endif /* INVARIANT_SUPPORT */ 1514 1515 #ifdef DDB 1516 void 1517 db_show_rwlock(const struct lock_object *lock) 1518 { 1519 const struct rwlock *rw; 1520 struct thread *td; 1521 1522 rw = (const struct rwlock *)lock; 1523 1524 db_printf(" state: "); 1525 if (rw->rw_lock == RW_UNLOCKED) 1526 db_printf("UNLOCKED\n"); 1527 else if (rw->rw_lock == RW_DESTROYED) { 1528 db_printf("DESTROYED\n"); 1529 return; 1530 } else if (rw->rw_lock & RW_LOCK_READ) 1531 db_printf("RLOCK: %ju locks\n", 1532 (uintmax_t)(RW_READERS(rw->rw_lock))); 1533 else { 1534 td = rw_wowner(rw); 1535 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 1536 td->td_tid, td->td_proc->p_pid, td->td_name); 1537 if (rw_recursed(rw)) 1538 db_printf(" recursed: %u\n", rw->rw_recurse); 1539 } 1540 db_printf(" waiters: "); 1541 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 1542 case RW_LOCK_READ_WAITERS: 1543 db_printf("readers\n"); 1544 break; 1545 case RW_LOCK_WRITE_WAITERS: 1546 db_printf("writers\n"); 1547 break; 1548 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 1549 db_printf("readers and writers\n"); 1550 break; 1551 default: 1552 db_printf("none\n"); 1553 break; 1554 } 1555 } 1556 1557 #endif
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