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 * 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: releng/11.2/sys/kern/kern_rwlock.c 334437 2018-05-31 15:58:28Z mjg $"); 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; 97 static int __read_frequently rowner_loops; 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 static void 111 rw_lock_delay_init(void *arg __unused) 112 { 113 114 lock_delay_default_init(&rw_delay); 115 rowner_retries = 10; 116 rowner_loops = max(10000, rw_delay.max); 117 } 118 LOCK_DELAY_SYSINIT(rw_lock_delay_init); 119 #endif 120 121 /* 122 * Return a pointer to the owning thread if the lock is write-locked or 123 * NULL if the lock is unlocked or read-locked. 124 */ 125 126 #define lv_rw_wowner(v) \ 127 ((v) & RW_LOCK_READ ? NULL : \ 128 (struct thread *)RW_OWNER((v))) 129 130 #define rw_wowner(rw) lv_rw_wowner(RW_READ_VALUE(rw)) 131 132 /* 133 * Returns if a write owner is recursed. Write ownership is not assured 134 * here and should be previously checked. 135 */ 136 #define rw_recursed(rw) ((rw)->rw_recurse != 0) 137 138 /* 139 * Return true if curthread helds the lock. 140 */ 141 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 142 143 /* 144 * Return a pointer to the owning thread for this lock who should receive 145 * any priority lent by threads that block on this lock. Currently this 146 * is identical to rw_wowner(). 147 */ 148 #define rw_owner(rw) rw_wowner(rw) 149 150 #ifndef INVARIANTS 151 #define __rw_assert(c, what, file, line) 152 #endif 153 154 void 155 assert_rw(const struct lock_object *lock, int what) 156 { 157 158 rw_assert((const struct rwlock *)lock, what); 159 } 160 161 void 162 lock_rw(struct lock_object *lock, uintptr_t how) 163 { 164 struct rwlock *rw; 165 166 rw = (struct rwlock *)lock; 167 if (how) 168 rw_rlock(rw); 169 else 170 rw_wlock(rw); 171 } 172 173 uintptr_t 174 unlock_rw(struct lock_object *lock) 175 { 176 struct rwlock *rw; 177 178 rw = (struct rwlock *)lock; 179 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED); 180 if (rw->rw_lock & RW_LOCK_READ) { 181 rw_runlock(rw); 182 return (1); 183 } else { 184 rw_wunlock(rw); 185 return (0); 186 } 187 } 188 189 #ifdef KDTRACE_HOOKS 190 int 191 owner_rw(const struct lock_object *lock, struct thread **owner) 192 { 193 const struct rwlock *rw = (const struct rwlock *)lock; 194 uintptr_t x = rw->rw_lock; 195 196 *owner = rw_wowner(rw); 197 return ((x & RW_LOCK_READ) != 0 ? (RW_READERS(x) != 0) : 198 (*owner != NULL)); 199 } 200 #endif 201 202 void 203 _rw_init_flags(volatile uintptr_t *c, const char *name, int opts) 204 { 205 struct rwlock *rw; 206 int flags; 207 208 rw = rwlock2rw(c); 209 210 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 211 RW_RECURSE | RW_NEW)) == 0); 212 ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock, 213 ("%s: rw_lock not aligned for %s: %p", __func__, name, 214 &rw->rw_lock)); 215 216 flags = LO_UPGRADABLE; 217 if (opts & RW_DUPOK) 218 flags |= LO_DUPOK; 219 if (opts & RW_NOPROFILE) 220 flags |= LO_NOPROFILE; 221 if (!(opts & RW_NOWITNESS)) 222 flags |= LO_WITNESS; 223 if (opts & RW_RECURSE) 224 flags |= LO_RECURSABLE; 225 if (opts & RW_QUIET) 226 flags |= LO_QUIET; 227 if (opts & RW_NEW) 228 flags |= LO_NEW; 229 230 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 231 rw->rw_lock = RW_UNLOCKED; 232 rw->rw_recurse = 0; 233 } 234 235 void 236 _rw_destroy(volatile uintptr_t *c) 237 { 238 struct rwlock *rw; 239 240 rw = rwlock2rw(c); 241 242 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw)); 243 KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw)); 244 rw->rw_lock = RW_DESTROYED; 245 lock_destroy(&rw->lock_object); 246 } 247 248 void 249 rw_sysinit(void *arg) 250 { 251 struct rw_args *args; 252 253 args = arg; 254 rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc, 255 args->ra_flags); 256 } 257 258 int 259 _rw_wowned(const volatile uintptr_t *c) 260 { 261 262 return (rw_wowner(rwlock2rw(c)) == curthread); 263 } 264 265 void 266 _rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line) 267 { 268 struct rwlock *rw; 269 uintptr_t tid, v; 270 271 rw = rwlock2rw(c); 272 273 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED() || 274 !TD_IS_IDLETHREAD(curthread), 275 ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d", 276 curthread, rw->lock_object.lo_name, file, line)); 277 KASSERT(rw->rw_lock != RW_DESTROYED, 278 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 279 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 280 line, NULL); 281 tid = (uintptr_t)curthread; 282 v = RW_UNLOCKED; 283 if (!_rw_write_lock_fetch(rw, &v, tid)) 284 _rw_wlock_hard(rw, v, file, line); 285 else 286 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, 287 0, 0, file, line, LOCKSTAT_WRITER); 288 289 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 290 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 291 TD_LOCKS_INC(curthread); 292 } 293 294 int 295 __rw_try_wlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 296 { 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 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td), 308 ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d", 309 curthread, rw->lock_object.lo_name, file, line)); 310 KASSERT(rw->rw_lock != RW_DESTROYED, 311 ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line)); 312 313 rval = 1; 314 recursed = false; 315 v = RW_UNLOCKED; 316 for (;;) { 317 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid)) 318 break; 319 if (v == RW_UNLOCKED) 320 continue; 321 if (v == tid && (rw->lock_object.lo_flags & LO_RECURSABLE)) { 322 rw->rw_recurse++; 323 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 324 break; 325 } 326 rval = 0; 327 break; 328 } 329 330 LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line); 331 if (rval) { 332 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 333 file, line); 334 if (!recursed) 335 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, 336 rw, 0, 0, file, line, LOCKSTAT_WRITER); 337 TD_LOCKS_INC(curthread); 338 } 339 return (rval); 340 } 341 342 int 343 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line) 344 { 345 struct rwlock *rw; 346 347 rw = rwlock2rw(c); 348 return (__rw_try_wlock_int(rw LOCK_FILE_LINE_ARG)); 349 } 350 351 void 352 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line) 353 { 354 struct rwlock *rw; 355 356 rw = rwlock2rw(c); 357 358 KASSERT(rw->rw_lock != RW_DESTROYED, 359 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 360 __rw_assert(c, RA_WLOCKED, file, line); 361 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 362 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 363 line); 364 365 #ifdef LOCK_PROFILING 366 _rw_wunlock_hard(rw, (uintptr_t)curthread, file, line); 367 #else 368 __rw_wunlock(rw, curthread, file, line); 369 #endif 370 371 TD_LOCKS_DEC(curthread); 372 } 373 374 /* 375 * Determines whether a new reader can acquire a lock. Succeeds if the 376 * reader already owns a read lock and the lock is locked for read to 377 * prevent deadlock from reader recursion. Also succeeds if the lock 378 * is unlocked and has no writer waiters or spinners. Failing otherwise 379 * prioritizes writers before readers. 380 */ 381 static bool __always_inline 382 __rw_can_read(struct thread *td, uintptr_t v, bool fp) 383 { 384 385 if ((v & (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) 386 == RW_LOCK_READ) 387 return (true); 388 if (!fp && td->td_rw_rlocks && (v & RW_LOCK_READ)) 389 return (true); 390 return (false); 391 } 392 393 static bool __always_inline 394 __rw_rlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp, bool fp 395 LOCK_FILE_LINE_ARG_DEF) 396 { 397 398 /* 399 * Handle the easy case. If no other thread has a write 400 * lock, then try to bump up the count of read locks. Note 401 * that we have to preserve the current state of the 402 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 403 * read lock, then rw_lock must have changed, so restart 404 * the loop. Note that this handles the case of a 405 * completely unlocked rwlock since such a lock is encoded 406 * as a read lock with no waiters. 407 */ 408 while (__rw_can_read(td, *vp, fp)) { 409 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, vp, 410 *vp + RW_ONE_READER)) { 411 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 412 CTR4(KTR_LOCK, 413 "%s: %p succeed %p -> %p", __func__, 414 rw, (void *)*vp, 415 (void *)(*vp + RW_ONE_READER)); 416 td->td_rw_rlocks++; 417 return (true); 418 } 419 } 420 return (false); 421 } 422 423 static void __noinline 424 __rw_rlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v 425 LOCK_FILE_LINE_ARG_DEF) 426 { 427 struct turnstile *ts; 428 struct thread *owner; 429 #ifdef ADAPTIVE_RWLOCKS 430 int spintries = 0; 431 int i, n; 432 #endif 433 #ifdef LOCK_PROFILING 434 uint64_t waittime = 0; 435 int contested = 0; 436 #endif 437 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS) 438 struct lock_delay_arg lda; 439 #endif 440 #ifdef KDTRACE_HOOKS 441 u_int sleep_cnt = 0; 442 int64_t sleep_time = 0; 443 int64_t all_time = 0; 444 #endif 445 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 446 uintptr_t state; 447 int doing_lockprof = 0; 448 #endif 449 450 #ifdef KDTRACE_HOOKS 451 if (LOCKSTAT_PROFILE_ENABLED(rw__acquire)) { 452 if (__rw_rlock_try(rw, td, &v, false LOCK_FILE_LINE_ARG)) 453 goto out_lockstat; 454 doing_lockprof = 1; 455 all_time -= lockstat_nsecs(&rw->lock_object); 456 state = v; 457 } 458 #endif 459 #ifdef LOCK_PROFILING 460 doing_lockprof = 1; 461 state = v; 462 #endif 463 464 if (SCHEDULER_STOPPED()) 465 return; 466 467 #if defined(ADAPTIVE_RWLOCKS) 468 lock_delay_arg_init(&lda, &rw_delay); 469 #elif defined(KDTRACE_HOOKS) 470 lock_delay_arg_init(&lda, NULL); 471 #endif 472 473 #ifdef HWPMC_HOOKS 474 PMC_SOFT_CALL( , , lock, failed); 475 #endif 476 lock_profile_obtain_lock_failed(&rw->lock_object, 477 &contested, &waittime); 478 479 for (;;) { 480 if (__rw_rlock_try(rw, td, &v, false LOCK_FILE_LINE_ARG)) 481 break; 482 #ifdef KDTRACE_HOOKS 483 lda.spin_cnt++; 484 #endif 485 486 #ifdef ADAPTIVE_RWLOCKS 487 /* 488 * If the owner is running on another CPU, spin until 489 * the owner stops running or the state of the lock 490 * changes. 491 */ 492 if ((v & RW_LOCK_READ) == 0) { 493 owner = (struct thread *)RW_OWNER(v); 494 if (TD_IS_RUNNING(owner)) { 495 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 496 CTR3(KTR_LOCK, 497 "%s: spinning on %p held by %p", 498 __func__, rw, owner); 499 KTR_STATE1(KTR_SCHED, "thread", 500 sched_tdname(curthread), "spinning", 501 "lockname:\"%s\"", rw->lock_object.lo_name); 502 do { 503 lock_delay(&lda); 504 v = RW_READ_VALUE(rw); 505 owner = lv_rw_wowner(v); 506 } while (owner != NULL && TD_IS_RUNNING(owner)); 507 KTR_STATE0(KTR_SCHED, "thread", 508 sched_tdname(curthread), "running"); 509 continue; 510 } 511 } else if (spintries < rowner_retries) { 512 spintries++; 513 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 514 "spinning", "lockname:\"%s\"", 515 rw->lock_object.lo_name); 516 for (i = 0; i < rowner_loops; i += n) { 517 n = RW_READERS(v); 518 lock_delay_spin(n); 519 v = RW_READ_VALUE(rw); 520 if ((v & RW_LOCK_READ) == 0 || __rw_can_read(td, v, false)) 521 break; 522 } 523 #ifdef KDTRACE_HOOKS 524 lda.spin_cnt += rowner_loops - i; 525 #endif 526 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 527 "running"); 528 if (i < rowner_loops) 529 continue; 530 } 531 #endif 532 533 /* 534 * Okay, now it's the hard case. Some other thread already 535 * has a write lock or there are write waiters present, 536 * acquire the turnstile lock so we can begin the process 537 * of blocking. 538 */ 539 ts = turnstile_trywait(&rw->lock_object); 540 541 /* 542 * The lock might have been released while we spun, so 543 * recheck its state and restart the loop if needed. 544 */ 545 v = RW_READ_VALUE(rw); 546 retry_ts: 547 if (__rw_can_read(td, v, false)) { 548 turnstile_cancel(ts); 549 continue; 550 } 551 552 owner = lv_rw_wowner(v); 553 554 #ifdef ADAPTIVE_RWLOCKS 555 /* 556 * The current lock owner might have started executing 557 * on another CPU (or the lock could have changed 558 * owners) while we were waiting on the turnstile 559 * chain lock. If so, drop the turnstile lock and try 560 * again. 561 */ 562 if (owner != NULL) { 563 if (TD_IS_RUNNING(owner)) { 564 turnstile_cancel(ts); 565 continue; 566 } 567 } 568 #endif 569 570 /* 571 * The lock is held in write mode or it already has waiters. 572 */ 573 MPASS(!__rw_can_read(td, v, false)); 574 575 /* 576 * If the RW_LOCK_READ_WAITERS flag is already set, then 577 * we can go ahead and block. If it is not set then try 578 * to set it. If we fail to set it drop the turnstile 579 * lock and restart the loop. 580 */ 581 if (!(v & RW_LOCK_READ_WAITERS)) { 582 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 583 v | RW_LOCK_READ_WAITERS)) 584 goto retry_ts; 585 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 586 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 587 __func__, rw); 588 } 589 590 /* 591 * We were unable to acquire the lock and the read waiters 592 * flag is set, so we must block on the turnstile. 593 */ 594 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 595 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 596 rw); 597 #ifdef KDTRACE_HOOKS 598 sleep_time -= lockstat_nsecs(&rw->lock_object); 599 #endif 600 MPASS(owner == rw_owner(rw)); 601 turnstile_wait(ts, owner, TS_SHARED_QUEUE); 602 #ifdef KDTRACE_HOOKS 603 sleep_time += lockstat_nsecs(&rw->lock_object); 604 sleep_cnt++; 605 #endif 606 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 607 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 608 __func__, rw); 609 v = RW_READ_VALUE(rw); 610 } 611 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 612 if (__predict_true(!doing_lockprof)) 613 return; 614 #endif 615 #ifdef KDTRACE_HOOKS 616 all_time += lockstat_nsecs(&rw->lock_object); 617 if (sleep_time) 618 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 619 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 620 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 621 622 /* Record only the loops spinning and not sleeping. */ 623 if (lda.spin_cnt > sleep_cnt) 624 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 625 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 626 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 627 out_lockstat: 628 #endif 629 /* 630 * TODO: acquire "owner of record" here. Here be turnstile dragons 631 * however. turnstiles don't like owners changing between calls to 632 * turnstile_wait() currently. 633 */ 634 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 635 waittime, file, line, LOCKSTAT_READER); 636 } 637 638 void 639 __rw_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 640 { 641 struct thread *td; 642 uintptr_t v; 643 644 td = curthread; 645 646 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED_TD(td) || 647 !TD_IS_IDLETHREAD(td), 648 ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d", 649 td, rw->lock_object.lo_name, file, line)); 650 KASSERT(rw->rw_lock != RW_DESTROYED, 651 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 652 KASSERT(rw_wowner(rw) != td, 653 ("rw_rlock: wlock already held for %s @ %s:%d", 654 rw->lock_object.lo_name, file, line)); 655 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL); 656 657 v = RW_READ_VALUE(rw); 658 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__acquire) || 659 !__rw_rlock_try(rw, td, &v, true LOCK_FILE_LINE_ARG))) 660 __rw_rlock_hard(rw, td, v LOCK_FILE_LINE_ARG); 661 else 662 lock_profile_obtain_lock_success(&rw->lock_object, 0, 0, 663 file, line); 664 665 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 666 WITNESS_LOCK(&rw->lock_object, 0, file, line); 667 TD_LOCKS_INC(curthread); 668 } 669 670 void 671 __rw_rlock(volatile uintptr_t *c, const char *file, int line) 672 { 673 struct rwlock *rw; 674 675 rw = rwlock2rw(c); 676 __rw_rlock_int(rw LOCK_FILE_LINE_ARG); 677 } 678 679 int 680 __rw_try_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 681 { 682 uintptr_t x; 683 684 if (SCHEDULER_STOPPED()) 685 return (1); 686 687 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 688 ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d", 689 curthread, rw->lock_object.lo_name, file, line)); 690 691 x = rw->rw_lock; 692 for (;;) { 693 KASSERT(rw->rw_lock != RW_DESTROYED, 694 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line)); 695 if (!(x & RW_LOCK_READ)) 696 break; 697 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &x, x + RW_ONE_READER)) { 698 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file, 699 line); 700 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line); 701 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, 702 rw, 0, 0, file, line, LOCKSTAT_READER); 703 TD_LOCKS_INC(curthread); 704 curthread->td_rw_rlocks++; 705 return (1); 706 } 707 } 708 709 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line); 710 return (0); 711 } 712 713 int 714 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line) 715 { 716 struct rwlock *rw; 717 718 rw = rwlock2rw(c); 719 return (__rw_try_rlock_int(rw LOCK_FILE_LINE_ARG)); 720 } 721 722 static bool __always_inline 723 __rw_runlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp) 724 { 725 726 for (;;) { 727 /* 728 * See if there is more than one read lock held. If so, 729 * just drop one and return. 730 */ 731 if (RW_READERS(*vp) > 1) { 732 if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp, 733 *vp - RW_ONE_READER)) { 734 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 735 CTR4(KTR_LOCK, 736 "%s: %p succeeded %p -> %p", 737 __func__, rw, (void *)*vp, 738 (void *)(*vp - RW_ONE_READER)); 739 td->td_rw_rlocks--; 740 return (true); 741 } 742 continue; 743 } 744 /* 745 * If there aren't any waiters for a write lock, then try 746 * to drop it quickly. 747 */ 748 if (!(*vp & RW_LOCK_WAITERS)) { 749 MPASS((*vp & ~RW_LOCK_WRITE_SPINNER) == 750 RW_READERS_LOCK(1)); 751 if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp, 752 RW_UNLOCKED)) { 753 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 754 CTR2(KTR_LOCK, "%s: %p last succeeded", 755 __func__, rw); 756 td->td_rw_rlocks--; 757 return (true); 758 } 759 continue; 760 } 761 break; 762 } 763 return (false); 764 } 765 766 static void __noinline 767 __rw_runlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v 768 LOCK_FILE_LINE_ARG_DEF) 769 { 770 struct turnstile *ts; 771 uintptr_t setv, queue; 772 773 if (SCHEDULER_STOPPED()) 774 return; 775 776 if (__rw_runlock_try(rw, td, &v)) 777 goto out_lockstat; 778 779 /* 780 * Ok, we know we have waiters and we think we are the 781 * last reader, so grab the turnstile lock. 782 */ 783 turnstile_chain_lock(&rw->lock_object); 784 v = RW_READ_VALUE(rw); 785 for (;;) { 786 if (__rw_runlock_try(rw, td, &v)) 787 break; 788 789 v &= (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 790 MPASS(v & RW_LOCK_WAITERS); 791 792 /* 793 * Try to drop our lock leaving the lock in a unlocked 794 * state. 795 * 796 * If you wanted to do explicit lock handoff you'd have to 797 * do it here. You'd also want to use turnstile_signal() 798 * and you'd have to handle the race where a higher 799 * priority thread blocks on the write lock before the 800 * thread you wakeup actually runs and have the new thread 801 * "steal" the lock. For now it's a lot simpler to just 802 * wakeup all of the waiters. 803 * 804 * As above, if we fail, then another thread might have 805 * acquired a read lock, so drop the turnstile lock and 806 * restart. 807 */ 808 setv = RW_UNLOCKED; 809 queue = TS_SHARED_QUEUE; 810 if (v & RW_LOCK_WRITE_WAITERS) { 811 queue = TS_EXCLUSIVE_QUEUE; 812 setv |= (v & RW_LOCK_READ_WAITERS); 813 } 814 v |= RW_READERS_LOCK(1); 815 if (!atomic_fcmpset_rel_ptr(&rw->rw_lock, &v, setv)) 816 continue; 817 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 818 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 819 __func__, rw); 820 821 /* 822 * Ok. The lock is released and all that's left is to 823 * wake up the waiters. Note that the lock might not be 824 * free anymore, but in that case the writers will just 825 * block again if they run before the new lock holder(s) 826 * release the lock. 827 */ 828 ts = turnstile_lookup(&rw->lock_object); 829 MPASS(ts != NULL); 830 turnstile_broadcast(ts, queue); 831 turnstile_unpend(ts, TS_SHARED_LOCK); 832 td->td_rw_rlocks--; 833 break; 834 } 835 turnstile_chain_unlock(&rw->lock_object); 836 out_lockstat: 837 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_READER); 838 } 839 840 void 841 _rw_runlock_cookie_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 842 { 843 struct thread *td; 844 uintptr_t v; 845 846 KASSERT(rw->rw_lock != RW_DESTROYED, 847 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 848 __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line); 849 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 850 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 851 852 td = curthread; 853 v = RW_READ_VALUE(rw); 854 855 if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__release) || 856 !__rw_runlock_try(rw, td, &v))) 857 __rw_runlock_hard(rw, td, v LOCK_FILE_LINE_ARG); 858 else 859 lock_profile_release_lock(&rw->lock_object); 860 861 TD_LOCKS_DEC(curthread); 862 } 863 864 void 865 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line) 866 { 867 struct rwlock *rw; 868 869 rw = rwlock2rw(c); 870 _rw_runlock_cookie_int(rw LOCK_FILE_LINE_ARG); 871 } 872 873 /* 874 * This function is called when we are unable to obtain a write lock on the 875 * first try. This means that at least one other thread holds either a 876 * read or write lock. 877 */ 878 void 879 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) 880 { 881 uintptr_t tid; 882 struct rwlock *rw; 883 struct turnstile *ts; 884 struct thread *owner; 885 #ifdef ADAPTIVE_RWLOCKS 886 int spintries = 0; 887 int i, n; 888 enum { READERS, WRITER } sleep_reason = READERS; 889 #endif 890 uintptr_t x; 891 #ifdef LOCK_PROFILING 892 uint64_t waittime = 0; 893 int contested = 0; 894 #endif 895 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS) 896 struct lock_delay_arg lda; 897 #endif 898 #ifdef KDTRACE_HOOKS 899 u_int sleep_cnt = 0; 900 int64_t sleep_time = 0; 901 int64_t all_time = 0; 902 #endif 903 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 904 uintptr_t state; 905 int doing_lockprof = 0; 906 #endif 907 908 tid = (uintptr_t)curthread; 909 rw = rwlock2rw(c); 910 911 #ifdef KDTRACE_HOOKS 912 if (LOCKSTAT_PROFILE_ENABLED(rw__acquire)) { 913 while (v == RW_UNLOCKED) { 914 if (_rw_write_lock_fetch(rw, &v, tid)) 915 goto out_lockstat; 916 } 917 doing_lockprof = 1; 918 all_time -= lockstat_nsecs(&rw->lock_object); 919 state = v; 920 } 921 #endif 922 #ifdef LOCK_PROFILING 923 doing_lockprof = 1; 924 state = v; 925 #endif 926 927 if (SCHEDULER_STOPPED()) 928 return; 929 930 #if defined(ADAPTIVE_RWLOCKS) 931 lock_delay_arg_init(&lda, &rw_delay); 932 #elif defined(KDTRACE_HOOKS) 933 lock_delay_arg_init(&lda, NULL); 934 #endif 935 if (__predict_false(v == RW_UNLOCKED)) 936 v = RW_READ_VALUE(rw); 937 938 if (__predict_false(lv_rw_wowner(v) == (struct thread *)tid)) { 939 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE, 940 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 941 __func__, rw->lock_object.lo_name, file, line)); 942 rw->rw_recurse++; 943 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 944 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 945 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 946 return; 947 } 948 949 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 950 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 951 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 952 953 #ifdef HWPMC_HOOKS 954 PMC_SOFT_CALL( , , lock, failed); 955 #endif 956 lock_profile_obtain_lock_failed(&rw->lock_object, 957 &contested, &waittime); 958 959 for (;;) { 960 if (v == RW_UNLOCKED) { 961 if (_rw_write_lock_fetch(rw, &v, tid)) 962 break; 963 continue; 964 } 965 #ifdef KDTRACE_HOOKS 966 lda.spin_cnt++; 967 #endif 968 969 #ifdef ADAPTIVE_RWLOCKS 970 /* 971 * If the lock is write locked and the owner is 972 * running on another CPU, spin until the owner stops 973 * running or the state of the lock changes. 974 */ 975 if (!(v & RW_LOCK_READ)) { 976 sleep_reason = WRITER; 977 owner = lv_rw_wowner(v); 978 if (!TD_IS_RUNNING(owner)) 979 goto ts; 980 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 981 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 982 __func__, rw, owner); 983 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 984 "spinning", "lockname:\"%s\"", 985 rw->lock_object.lo_name); 986 do { 987 lock_delay(&lda); 988 v = RW_READ_VALUE(rw); 989 owner = lv_rw_wowner(v); 990 } while (owner != NULL && TD_IS_RUNNING(owner)); 991 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 992 "running"); 993 continue; 994 } else if (RW_READERS(v) > 0) { 995 sleep_reason = READERS; 996 if (spintries == rowner_retries) 997 goto ts; 998 if (!(v & RW_LOCK_WRITE_SPINNER)) { 999 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 1000 v | RW_LOCK_WRITE_SPINNER)) { 1001 continue; 1002 } 1003 } 1004 spintries++; 1005 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 1006 "spinning", "lockname:\"%s\"", 1007 rw->lock_object.lo_name); 1008 for (i = 0; i < rowner_loops; i += n) { 1009 n = RW_READERS(v); 1010 lock_delay_spin(n); 1011 v = RW_READ_VALUE(rw); 1012 if ((v & RW_LOCK_WRITE_SPINNER) == 0) 1013 break; 1014 } 1015 #ifdef KDTRACE_HOOKS 1016 lda.spin_cnt += i; 1017 #endif 1018 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 1019 "running"); 1020 if (i < rowner_loops) 1021 continue; 1022 } 1023 ts: 1024 #endif 1025 ts = turnstile_trywait(&rw->lock_object); 1026 v = RW_READ_VALUE(rw); 1027 retry_ts: 1028 owner = lv_rw_wowner(v); 1029 1030 #ifdef ADAPTIVE_RWLOCKS 1031 /* 1032 * The current lock owner might have started executing 1033 * on another CPU (or the lock could have changed 1034 * owners) while we were waiting on the turnstile 1035 * chain lock. If so, drop the turnstile lock and try 1036 * again. 1037 */ 1038 if (owner != NULL) { 1039 if (TD_IS_RUNNING(owner)) { 1040 turnstile_cancel(ts); 1041 continue; 1042 } 1043 } else if (RW_READERS(v) > 0 && sleep_reason == WRITER) { 1044 turnstile_cancel(ts); 1045 continue; 1046 } 1047 #endif 1048 /* 1049 * Check for the waiters flags about this rwlock. 1050 * If the lock was released, without maintain any pending 1051 * waiters queue, simply try to acquire it. 1052 * If a pending waiters queue is present, claim the lock 1053 * ownership and maintain the pending queue. 1054 */ 1055 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 1056 if ((v & ~x) == RW_UNLOCKED) { 1057 x &= ~RW_LOCK_WRITE_SPINNER; 1058 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid | x)) { 1059 if (x) 1060 turnstile_claim(ts); 1061 else 1062 turnstile_cancel(ts); 1063 break; 1064 } 1065 goto retry_ts; 1066 } 1067 /* 1068 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 1069 * set it. If we fail to set it, then loop back and try 1070 * again. 1071 */ 1072 if (!(v & RW_LOCK_WRITE_WAITERS)) { 1073 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, 1074 v | RW_LOCK_WRITE_WAITERS)) 1075 goto retry_ts; 1076 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1077 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 1078 __func__, rw); 1079 } 1080 /* 1081 * We were unable to acquire the lock and the write waiters 1082 * flag is set, so we must block on the turnstile. 1083 */ 1084 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1085 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 1086 rw); 1087 #ifdef KDTRACE_HOOKS 1088 sleep_time -= lockstat_nsecs(&rw->lock_object); 1089 #endif 1090 MPASS(owner == rw_owner(rw)); 1091 turnstile_wait(ts, owner, TS_EXCLUSIVE_QUEUE); 1092 #ifdef KDTRACE_HOOKS 1093 sleep_time += lockstat_nsecs(&rw->lock_object); 1094 sleep_cnt++; 1095 #endif 1096 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1097 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 1098 __func__, rw); 1099 #ifdef ADAPTIVE_RWLOCKS 1100 spintries = 0; 1101 #endif 1102 v = RW_READ_VALUE(rw); 1103 } 1104 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING) 1105 if (__predict_true(!doing_lockprof)) 1106 return; 1107 #endif 1108 #ifdef KDTRACE_HOOKS 1109 all_time += lockstat_nsecs(&rw->lock_object); 1110 if (sleep_time) 1111 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 1112 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1113 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1114 1115 /* Record only the loops spinning and not sleeping. */ 1116 if (lda.spin_cnt > sleep_cnt) 1117 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 1118 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1119 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1120 out_lockstat: 1121 #endif 1122 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 1123 waittime, file, line, LOCKSTAT_WRITER); 1124 } 1125 1126 /* 1127 * This function is called if lockstat is active or the first try at releasing 1128 * a write lock failed. The latter means that the lock is recursed or one of 1129 * the 2 waiter bits must be set indicating that at least one thread is waiting 1130 * on this lock. 1131 */ 1132 void 1133 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF) 1134 { 1135 struct rwlock *rw; 1136 struct turnstile *ts; 1137 uintptr_t tid, setv; 1138 int queue; 1139 1140 tid = (uintptr_t)curthread; 1141 if (SCHEDULER_STOPPED()) 1142 return; 1143 1144 rw = rwlock2rw(c); 1145 if (__predict_false(v == tid)) 1146 v = RW_READ_VALUE(rw); 1147 1148 if (v & RW_LOCK_WRITER_RECURSED) { 1149 if (--(rw->rw_recurse) == 0) 1150 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 1151 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1152 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 1153 return; 1154 } 1155 1156 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_WRITER); 1157 if (v == tid && _rw_write_unlock(rw, tid)) 1158 return; 1159 1160 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 1161 ("%s: neither of the waiter flags are set", __func__)); 1162 1163 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1164 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 1165 1166 turnstile_chain_lock(&rw->lock_object); 1167 1168 /* 1169 * Use the same algo as sx locks for now. Prefer waking up shared 1170 * waiters if we have any over writers. This is probably not ideal. 1171 * 1172 * 'v' is the value we are going to write back to rw_lock. If we 1173 * have waiters on both queues, we need to preserve the state of 1174 * the waiter flag for the queue we don't wake up. For now this is 1175 * hardcoded for the algorithm mentioned above. 1176 * 1177 * In the case of both readers and writers waiting we wakeup the 1178 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 1179 * new writer comes in before a reader it will claim the lock up 1180 * above. There is probably a potential priority inversion in 1181 * there that could be worked around either by waking both queues 1182 * of waiters or doing some complicated lock handoff gymnastics. 1183 */ 1184 setv = RW_UNLOCKED; 1185 v = RW_READ_VALUE(rw); 1186 queue = TS_SHARED_QUEUE; 1187 if (v & RW_LOCK_WRITE_WAITERS) { 1188 queue = TS_EXCLUSIVE_QUEUE; 1189 setv |= (v & RW_LOCK_READ_WAITERS); 1190 } 1191 atomic_store_rel_ptr(&rw->rw_lock, setv); 1192 1193 /* Wake up all waiters for the specific queue. */ 1194 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1195 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 1196 queue == TS_SHARED_QUEUE ? "read" : "write"); 1197 1198 ts = turnstile_lookup(&rw->lock_object); 1199 MPASS(ts != NULL); 1200 turnstile_broadcast(ts, queue); 1201 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1202 turnstile_chain_unlock(&rw->lock_object); 1203 } 1204 1205 /* 1206 * Attempt to do a non-blocking upgrade from a read lock to a write 1207 * lock. This will only succeed if this thread holds a single read 1208 * lock. Returns true if the upgrade succeeded and false otherwise. 1209 */ 1210 int 1211 __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 1212 { 1213 uintptr_t v, setv, tid; 1214 struct turnstile *ts; 1215 int success; 1216 1217 if (SCHEDULER_STOPPED()) 1218 return (1); 1219 1220 KASSERT(rw->rw_lock != RW_DESTROYED, 1221 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 1222 __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line); 1223 1224 /* 1225 * Attempt to switch from one reader to a writer. If there 1226 * are any write waiters, then we will have to lock the 1227 * turnstile first to prevent races with another writer 1228 * calling turnstile_wait() before we have claimed this 1229 * turnstile. So, do the simple case of no waiters first. 1230 */ 1231 tid = (uintptr_t)curthread; 1232 success = 0; 1233 v = RW_READ_VALUE(rw); 1234 for (;;) { 1235 if (RW_READERS(v) > 1) 1236 break; 1237 if (!(v & RW_LOCK_WAITERS)) { 1238 success = atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid); 1239 if (!success) 1240 continue; 1241 break; 1242 } 1243 1244 /* 1245 * Ok, we think we have waiters, so lock the turnstile. 1246 */ 1247 ts = turnstile_trywait(&rw->lock_object); 1248 v = RW_READ_VALUE(rw); 1249 retry_ts: 1250 if (RW_READERS(v) > 1) { 1251 turnstile_cancel(ts); 1252 break; 1253 } 1254 /* 1255 * Try to switch from one reader to a writer again. This time 1256 * we honor the current state of the waiters flags. 1257 * If we obtain the lock with the flags set, then claim 1258 * ownership of the turnstile. 1259 */ 1260 setv = tid | (v & RW_LOCK_WAITERS); 1261 success = atomic_fcmpset_ptr(&rw->rw_lock, &v, setv); 1262 if (success) { 1263 if (v & RW_LOCK_WAITERS) 1264 turnstile_claim(ts); 1265 else 1266 turnstile_cancel(ts); 1267 break; 1268 } 1269 goto retry_ts; 1270 } 1271 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 1272 if (success) { 1273 curthread->td_rw_rlocks--; 1274 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 1275 file, line); 1276 LOCKSTAT_RECORD0(rw__upgrade, rw); 1277 } 1278 return (success); 1279 } 1280 1281 int 1282 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line) 1283 { 1284 struct rwlock *rw; 1285 1286 rw = rwlock2rw(c); 1287 return (__rw_try_upgrade_int(rw LOCK_FILE_LINE_ARG)); 1288 } 1289 1290 /* 1291 * Downgrade a write lock into a single read lock. 1292 */ 1293 void 1294 __rw_downgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF) 1295 { 1296 struct turnstile *ts; 1297 uintptr_t tid, v; 1298 int rwait, wwait; 1299 1300 if (SCHEDULER_STOPPED()) 1301 return; 1302 1303 KASSERT(rw->rw_lock != RW_DESTROYED, 1304 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 1305 __rw_assert(&rw->rw_lock, RA_WLOCKED | RA_NOTRECURSED, file, line); 1306 #ifndef INVARIANTS 1307 if (rw_recursed(rw)) 1308 panic("downgrade of a recursed lock"); 1309 #endif 1310 1311 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 1312 1313 /* 1314 * Convert from a writer to a single reader. First we handle 1315 * the easy case with no waiters. If there are any waiters, we 1316 * lock the turnstile and "disown" the lock. 1317 */ 1318 tid = (uintptr_t)curthread; 1319 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 1320 goto out; 1321 1322 /* 1323 * Ok, we think we have waiters, so lock the turnstile so we can 1324 * read the waiter flags without any races. 1325 */ 1326 turnstile_chain_lock(&rw->lock_object); 1327 v = rw->rw_lock & RW_LOCK_WAITERS; 1328 rwait = v & RW_LOCK_READ_WAITERS; 1329 wwait = v & RW_LOCK_WRITE_WAITERS; 1330 MPASS(rwait | wwait); 1331 1332 /* 1333 * Downgrade from a write lock while preserving waiters flag 1334 * and give up ownership of the turnstile. 1335 */ 1336 ts = turnstile_lookup(&rw->lock_object); 1337 MPASS(ts != NULL); 1338 if (!wwait) 1339 v &= ~RW_LOCK_READ_WAITERS; 1340 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v); 1341 /* 1342 * Wake other readers if there are no writers pending. Otherwise they 1343 * won't be able to acquire the lock anyway. 1344 */ 1345 if (rwait && !wwait) { 1346 turnstile_broadcast(ts, TS_SHARED_QUEUE); 1347 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1348 } else 1349 turnstile_disown(ts); 1350 turnstile_chain_unlock(&rw->lock_object); 1351 out: 1352 curthread->td_rw_rlocks++; 1353 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 1354 LOCKSTAT_RECORD0(rw__downgrade, rw); 1355 } 1356 1357 void 1358 __rw_downgrade(volatile uintptr_t *c, const char *file, int line) 1359 { 1360 struct rwlock *rw; 1361 1362 rw = rwlock2rw(c); 1363 __rw_downgrade_int(rw LOCK_FILE_LINE_ARG); 1364 } 1365 1366 #ifdef INVARIANT_SUPPORT 1367 #ifndef INVARIANTS 1368 #undef __rw_assert 1369 #endif 1370 1371 /* 1372 * In the non-WITNESS case, rw_assert() can only detect that at least 1373 * *some* thread owns an rlock, but it cannot guarantee that *this* 1374 * thread owns an rlock. 1375 */ 1376 void 1377 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line) 1378 { 1379 const struct rwlock *rw; 1380 1381 if (panicstr != NULL) 1382 return; 1383 1384 rw = rwlock2rw(c); 1385 1386 switch (what) { 1387 case RA_LOCKED: 1388 case RA_LOCKED | RA_RECURSED: 1389 case RA_LOCKED | RA_NOTRECURSED: 1390 case RA_RLOCKED: 1391 case RA_RLOCKED | RA_RECURSED: 1392 case RA_RLOCKED | RA_NOTRECURSED: 1393 #ifdef WITNESS 1394 witness_assert(&rw->lock_object, what, file, line); 1395 #else 1396 /* 1397 * If some other thread has a write lock or we have one 1398 * and are asserting a read lock, fail. Also, if no one 1399 * has a lock at all, fail. 1400 */ 1401 if (rw->rw_lock == RW_UNLOCKED || 1402 (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED || 1403 rw_wowner(rw) != curthread))) 1404 panic("Lock %s not %slocked @ %s:%d\n", 1405 rw->lock_object.lo_name, (what & RA_RLOCKED) ? 1406 "read " : "", file, line); 1407 1408 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) { 1409 if (rw_recursed(rw)) { 1410 if (what & RA_NOTRECURSED) 1411 panic("Lock %s recursed @ %s:%d\n", 1412 rw->lock_object.lo_name, file, 1413 line); 1414 } else if (what & RA_RECURSED) 1415 panic("Lock %s not recursed @ %s:%d\n", 1416 rw->lock_object.lo_name, file, line); 1417 } 1418 #endif 1419 break; 1420 case RA_WLOCKED: 1421 case RA_WLOCKED | RA_RECURSED: 1422 case RA_WLOCKED | RA_NOTRECURSED: 1423 if (rw_wowner(rw) != curthread) 1424 panic("Lock %s not exclusively locked @ %s:%d\n", 1425 rw->lock_object.lo_name, file, line); 1426 if (rw_recursed(rw)) { 1427 if (what & RA_NOTRECURSED) 1428 panic("Lock %s recursed @ %s:%d\n", 1429 rw->lock_object.lo_name, file, line); 1430 } else if (what & RA_RECURSED) 1431 panic("Lock %s not recursed @ %s:%d\n", 1432 rw->lock_object.lo_name, file, line); 1433 break; 1434 case RA_UNLOCKED: 1435 #ifdef WITNESS 1436 witness_assert(&rw->lock_object, what, file, line); 1437 #else 1438 /* 1439 * If we hold a write lock fail. We can't reliably check 1440 * to see if we hold a read lock or not. 1441 */ 1442 if (rw_wowner(rw) == curthread) 1443 panic("Lock %s exclusively locked @ %s:%d\n", 1444 rw->lock_object.lo_name, file, line); 1445 #endif 1446 break; 1447 default: 1448 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 1449 line); 1450 } 1451 } 1452 #endif /* INVARIANT_SUPPORT */ 1453 1454 #ifdef DDB 1455 void 1456 db_show_rwlock(const struct lock_object *lock) 1457 { 1458 const struct rwlock *rw; 1459 struct thread *td; 1460 1461 rw = (const struct rwlock *)lock; 1462 1463 db_printf(" state: "); 1464 if (rw->rw_lock == RW_UNLOCKED) 1465 db_printf("UNLOCKED\n"); 1466 else if (rw->rw_lock == RW_DESTROYED) { 1467 db_printf("DESTROYED\n"); 1468 return; 1469 } else if (rw->rw_lock & RW_LOCK_READ) 1470 db_printf("RLOCK: %ju locks\n", 1471 (uintmax_t)(RW_READERS(rw->rw_lock))); 1472 else { 1473 td = rw_wowner(rw); 1474 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 1475 td->td_tid, td->td_proc->p_pid, td->td_name); 1476 if (rw_recursed(rw)) 1477 db_printf(" recursed: %u\n", rw->rw_recurse); 1478 } 1479 db_printf(" waiters: "); 1480 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 1481 case RW_LOCK_READ_WAITERS: 1482 db_printf("readers\n"); 1483 break; 1484 case RW_LOCK_WRITE_WAITERS: 1485 db_printf("writers\n"); 1486 break; 1487 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 1488 db_printf("readers and writers\n"); 1489 break; 1490 default: 1491 db_printf("none\n"); 1492 break; 1493 } 1494 } 1495 1496 #endif
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