Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)
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FreeBSD/Linux Kernel Cross Reference
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/11.1/sys/kern/kern_rwlock.c 320241 2017-06-22 18:40:34Z markj $"); 33 34 #include "opt_ddb.h" 35 #include "opt_hwpmc_hooks.h" 36 #include "opt_no_adaptive_rwlocks.h" 37 38 #include <sys/param.h> 39 #include <sys/kdb.h> 40 #include <sys/ktr.h> 41 #include <sys/kernel.h> 42 #include <sys/lock.h> 43 #include <sys/mutex.h> 44 #include <sys/proc.h> 45 #include <sys/rwlock.h> 46 #include <sys/sched.h> 47 #include <sys/smp.h> 48 #include <sys/sysctl.h> 49 #include <sys/systm.h> 50 #include <sys/turnstile.h> 51 52 #include <machine/cpu.h> 53 54 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS) 55 #define ADAPTIVE_RWLOCKS 56 #endif 57 58 #ifdef HWPMC_HOOKS 59 #include <sys/pmckern.h> 60 PMC_SOFT_DECLARE( , , lock, failed); 61 #endif 62 63 /* 64 * Return the rwlock address when the lock cookie address is provided. 65 * This functionality assumes that struct rwlock* have a member named rw_lock. 66 */ 67 #define rwlock2rw(c) (__containerof(c, struct rwlock, rw_lock)) 68 69 #ifdef DDB 70 #include <ddb/ddb.h> 71 72 static void db_show_rwlock(const struct lock_object *lock); 73 #endif 74 static void assert_rw(const struct lock_object *lock, int what); 75 static void lock_rw(struct lock_object *lock, uintptr_t how); 76 #ifdef KDTRACE_HOOKS 77 static int owner_rw(const struct lock_object *lock, struct thread **owner); 78 #endif 79 static uintptr_t unlock_rw(struct lock_object *lock); 80 81 struct lock_class lock_class_rw = { 82 .lc_name = "rw", 83 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 84 .lc_assert = assert_rw, 85 #ifdef DDB 86 .lc_ddb_show = db_show_rwlock, 87 #endif 88 .lc_lock = lock_rw, 89 .lc_unlock = unlock_rw, 90 #ifdef KDTRACE_HOOKS 91 .lc_owner = owner_rw, 92 #endif 93 }; 94 95 #ifdef ADAPTIVE_RWLOCKS 96 static int rowner_retries = 10; 97 static int rowner_loops = 10000; 98 static SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL, 99 "rwlock debugging"); 100 SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, ""); 101 SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, ""); 102 103 static struct lock_delay_config __read_mostly rw_delay; 104 105 SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_base, CTLFLAG_RW, &rw_delay.base, 106 0, ""); 107 SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_max, CTLFLAG_RW, &rw_delay.max, 108 0, ""); 109 110 LOCK_DELAY_SYSINIT_DEFAULT(rw_delay); 111 #endif 112 113 /* 114 * Return a pointer to the owning thread if the lock is write-locked or 115 * NULL if the lock is unlocked or read-locked. 116 */ 117 118 #define lv_rw_wowner(v) \ 119 ((v) & RW_LOCK_READ ? NULL : \ 120 (struct thread *)RW_OWNER((v))) 121 122 #define rw_wowner(rw) lv_rw_wowner(RW_READ_VALUE(rw)) 123 124 /* 125 * Returns if a write owner is recursed. Write ownership is not assured 126 * here and should be previously checked. 127 */ 128 #define rw_recursed(rw) ((rw)->rw_recurse != 0) 129 130 /* 131 * Return true if curthread helds the lock. 132 */ 133 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 134 135 /* 136 * Return a pointer to the owning thread for this lock who should receive 137 * any priority lent by threads that block on this lock. Currently this 138 * is identical to rw_wowner(). 139 */ 140 #define rw_owner(rw) rw_wowner(rw) 141 142 #ifndef INVARIANTS 143 #define __rw_assert(c, what, file, line) 144 #endif 145 146 void 147 assert_rw(const struct lock_object *lock, int what) 148 { 149 150 rw_assert((const struct rwlock *)lock, what); 151 } 152 153 void 154 lock_rw(struct lock_object *lock, uintptr_t how) 155 { 156 struct rwlock *rw; 157 158 rw = (struct rwlock *)lock; 159 if (how) 160 rw_rlock(rw); 161 else 162 rw_wlock(rw); 163 } 164 165 uintptr_t 166 unlock_rw(struct lock_object *lock) 167 { 168 struct rwlock *rw; 169 170 rw = (struct rwlock *)lock; 171 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED); 172 if (rw->rw_lock & RW_LOCK_READ) { 173 rw_runlock(rw); 174 return (1); 175 } else { 176 rw_wunlock(rw); 177 return (0); 178 } 179 } 180 181 #ifdef KDTRACE_HOOKS 182 int 183 owner_rw(const struct lock_object *lock, struct thread **owner) 184 { 185 const struct rwlock *rw = (const struct rwlock *)lock; 186 uintptr_t x = rw->rw_lock; 187 188 *owner = rw_wowner(rw); 189 return ((x & RW_LOCK_READ) != 0 ? (RW_READERS(x) != 0) : 190 (*owner != NULL)); 191 } 192 #endif 193 194 void 195 _rw_init_flags(volatile uintptr_t *c, const char *name, int opts) 196 { 197 struct rwlock *rw; 198 int flags; 199 200 rw = rwlock2rw(c); 201 202 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 203 RW_RECURSE | RW_NEW)) == 0); 204 ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock, 205 ("%s: rw_lock not aligned for %s: %p", __func__, name, 206 &rw->rw_lock)); 207 208 flags = LO_UPGRADABLE; 209 if (opts & RW_DUPOK) 210 flags |= LO_DUPOK; 211 if (opts & RW_NOPROFILE) 212 flags |= LO_NOPROFILE; 213 if (!(opts & RW_NOWITNESS)) 214 flags |= LO_WITNESS; 215 if (opts & RW_RECURSE) 216 flags |= LO_RECURSABLE; 217 if (opts & RW_QUIET) 218 flags |= LO_QUIET; 219 if (opts & RW_NEW) 220 flags |= LO_NEW; 221 222 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 223 rw->rw_lock = RW_UNLOCKED; 224 rw->rw_recurse = 0; 225 } 226 227 void 228 _rw_destroy(volatile uintptr_t *c) 229 { 230 struct rwlock *rw; 231 232 rw = rwlock2rw(c); 233 234 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw)); 235 KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw)); 236 rw->rw_lock = RW_DESTROYED; 237 lock_destroy(&rw->lock_object); 238 } 239 240 void 241 rw_sysinit(void *arg) 242 { 243 struct rw_args *args = arg; 244 245 rw_init((struct rwlock *)args->ra_rw, args->ra_desc); 246 } 247 248 void 249 rw_sysinit_flags(void *arg) 250 { 251 struct rw_args_flags *args = arg; 252 253 rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc, 254 args->ra_flags); 255 } 256 257 int 258 _rw_wowned(const volatile uintptr_t *c) 259 { 260 261 return (rw_wowner(rwlock2rw(c)) == curthread); 262 } 263 264 void 265 _rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line) 266 { 267 struct rwlock *rw; 268 uintptr_t tid, v; 269 270 rw = rwlock2rw(c); 271 272 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED() || 273 !TD_IS_IDLETHREAD(curthread), 274 ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d", 275 curthread, rw->lock_object.lo_name, file, line)); 276 KASSERT(rw->rw_lock != RW_DESTROYED, 277 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 278 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 279 line, NULL); 280 tid = (uintptr_t)curthread; 281 v = RW_UNLOCKED; 282 if (!_rw_write_lock_fetch(rw, &v, tid)) 283 _rw_wlock_hard(rw, v, tid, file, line); 284 else 285 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, 286 0, 0, file, line, LOCKSTAT_WRITER); 287 288 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 289 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 290 TD_LOCKS_INC(curthread); 291 } 292 293 int 294 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line) 295 { 296 struct rwlock *rw; 297 struct thread *td; 298 uintptr_t tid, v; 299 int rval; 300 bool recursed; 301 302 td = curthread; 303 tid = (uintptr_t)td; 304 if (SCHEDULER_STOPPED_TD(td)) 305 return (1); 306 307 rw = rwlock2rw(c); 308 309 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td), 310 ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d", 311 curthread, rw->lock_object.lo_name, file, line)); 312 KASSERT(rw->rw_lock != RW_DESTROYED, 313 ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line)); 314 315 rval = 1; 316 recursed = false; 317 v = RW_UNLOCKED; 318 for (;;) { 319 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid)) 320 break; 321 if (v == RW_UNLOCKED) 322 continue; 323 if (v == tid && (rw->lock_object.lo_flags & LO_RECURSABLE)) { 324 rw->rw_recurse++; 325 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 326 break; 327 } 328 rval = 0; 329 break; 330 } 331 332 LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line); 333 if (rval) { 334 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 335 file, line); 336 if (!recursed) 337 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, 338 rw, 0, 0, file, line, LOCKSTAT_WRITER); 339 TD_LOCKS_INC(curthread); 340 } 341 return (rval); 342 } 343 344 void 345 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line) 346 { 347 struct rwlock *rw; 348 349 rw = rwlock2rw(c); 350 351 KASSERT(rw->rw_lock != RW_DESTROYED, 352 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 353 __rw_assert(c, RA_WLOCKED, file, line); 354 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 355 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 356 line); 357 358 #ifdef LOCK_PROFILING 359 _rw_wunlock_hard(rw, (uintptr_t)curthread, file, line); 360 #else 361 __rw_wunlock(rw, curthread, file, line); 362 #endif 363 364 TD_LOCKS_DEC(curthread); 365 } 366 367 /* 368 * Determines whether a new reader can acquire a lock. Succeeds if the 369 * reader already owns a read lock and the lock is locked for read to 370 * prevent deadlock from reader recursion. Also succeeds if the lock 371 * is unlocked and has no writer waiters or spinners. Failing otherwise 372 * prioritizes writers before readers. 373 */ 374 #define RW_CAN_READ(td, _rw) \ 375 (((td)->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) & \ 376 (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) == \ 377 RW_LOCK_READ) 378 379 static bool __always_inline 380 __rw_rlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp, 381 const char *file, int line) 382 { 383 384 /* 385 * Handle the easy case. If no other thread has a write 386 * lock, then try to bump up the count of read locks. Note 387 * that we have to preserve the current state of the 388 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 389 * read lock, then rw_lock must have changed, so restart 390 * the loop. Note that this handles the case of a 391 * completely unlocked rwlock since such a lock is encoded 392 * as a read lock with no waiters. 393 */ 394 while (RW_CAN_READ(td, *vp)) { 395 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, vp, 396 *vp + RW_ONE_READER)) { 397 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 398 CTR4(KTR_LOCK, 399 "%s: %p succeed %p -> %p", __func__, 400 rw, (void *)*vp, 401 (void *)(*vp + RW_ONE_READER)); 402 td->td_rw_rlocks++; 403 return (true); 404 } 405 } 406 return (false); 407 } 408 409 static void __noinline 410 __rw_rlock_hard(volatile uintptr_t *c, struct thread *td, uintptr_t v, 411 const char *file, int line) 412 { 413 struct rwlock *rw; 414 struct turnstile *ts; 415 #ifdef ADAPTIVE_RWLOCKS 416 volatile struct thread *owner; 417 int spintries = 0; 418 int i; 419 #endif 420 #ifdef LOCK_PROFILING 421 uint64_t waittime = 0; 422 int contested = 0; 423 #endif 424 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS) 425 struct lock_delay_arg lda; 426 #endif 427 #ifdef KDTRACE_HOOKS 428 uintptr_t state; 429 u_int sleep_cnt = 0; 430 int64_t sleep_time = 0; 431 int64_t all_time = 0; 432 #endif 433 434 if (SCHEDULER_STOPPED()) 435 return; 436 437 #if defined(ADAPTIVE_RWLOCKS) 438 lock_delay_arg_init(&lda, &rw_delay); 439 #elif defined(KDTRACE_HOOKS) 440 lock_delay_arg_init(&lda, NULL); 441 #endif 442 rw = rwlock2rw(c); 443 444 #ifdef KDTRACE_HOOKS 445 all_time -= lockstat_nsecs(&rw->lock_object); 446 #endif 447 #ifdef KDTRACE_HOOKS 448 state = v; 449 #endif 450 for (;;) { 451 if (__rw_rlock_try(rw, td, &v, file, line)) 452 break; 453 #ifdef KDTRACE_HOOKS 454 lda.spin_cnt++; 455 #endif 456 #ifdef HWPMC_HOOKS 457 PMC_SOFT_CALL( , , lock, failed); 458 #endif 459 lock_profile_obtain_lock_failed(&rw->lock_object, 460 &contested, &waittime); 461 462 #ifdef ADAPTIVE_RWLOCKS 463 /* 464 * If the owner is running on another CPU, spin until 465 * the owner stops running or the state of the lock 466 * changes. 467 */ 468 if ((v & RW_LOCK_READ) == 0) { 469 owner = (struct thread *)RW_OWNER(v); 470 if (TD_IS_RUNNING(owner)) { 471 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 472 CTR3(KTR_LOCK, 473 "%s: spinning on %p held by %p", 474 __func__, rw, owner); 475 KTR_STATE1(KTR_SCHED, "thread", 476 sched_tdname(curthread), "spinning", 477 "lockname:\"%s\"", rw->lock_object.lo_name); 478 do { 479 lock_delay(&lda); 480 v = RW_READ_VALUE(rw); 481 owner = lv_rw_wowner(v); 482 } while (owner != NULL && TD_IS_RUNNING(owner)); 483 KTR_STATE0(KTR_SCHED, "thread", 484 sched_tdname(curthread), "running"); 485 continue; 486 } 487 } else if (spintries < rowner_retries) { 488 spintries++; 489 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 490 "spinning", "lockname:\"%s\"", 491 rw->lock_object.lo_name); 492 for (i = 0; i < rowner_loops; i++) { 493 v = RW_READ_VALUE(rw); 494 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(td, v)) 495 break; 496 cpu_spinwait(); 497 } 498 v = RW_READ_VALUE(rw); 499 #ifdef KDTRACE_HOOKS 500 lda.spin_cnt += rowner_loops - i; 501 #endif 502 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 503 "running"); 504 if (i != rowner_loops) 505 continue; 506 } 507 #endif 508 509 /* 510 * Okay, now it's the hard case. Some other thread already 511 * has a write lock or there are write waiters present, 512 * acquire the turnstile lock so we can begin the process 513 * of blocking. 514 */ 515 ts = turnstile_trywait(&rw->lock_object); 516 517 /* 518 * The lock might have been released while we spun, so 519 * recheck its state and restart the loop if needed. 520 */ 521 v = RW_READ_VALUE(rw); 522 if (RW_CAN_READ(td, v)) { 523 turnstile_cancel(ts); 524 continue; 525 } 526 527 #ifdef ADAPTIVE_RWLOCKS 528 /* 529 * The current lock owner might have started executing 530 * on another CPU (or the lock could have changed 531 * owners) while we were waiting on the turnstile 532 * chain lock. If so, drop the turnstile lock and try 533 * again. 534 */ 535 if ((v & RW_LOCK_READ) == 0) { 536 owner = (struct thread *)RW_OWNER(v); 537 if (TD_IS_RUNNING(owner)) { 538 turnstile_cancel(ts); 539 continue; 540 } 541 } 542 #endif 543 544 /* 545 * The lock is held in write mode or it already has waiters. 546 */ 547 MPASS(!RW_CAN_READ(td, v)); 548 549 /* 550 * If the RW_LOCK_READ_WAITERS flag is already set, then 551 * we can go ahead and block. If it is not set then try 552 * to set it. If we fail to set it drop the turnstile 553 * lock and restart the loop. 554 */ 555 if (!(v & RW_LOCK_READ_WAITERS)) { 556 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 557 v | RW_LOCK_READ_WAITERS)) { 558 turnstile_cancel(ts); 559 v = RW_READ_VALUE(rw); 560 continue; 561 } 562 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 563 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 564 __func__, rw); 565 } 566 567 /* 568 * We were unable to acquire the lock and the read waiters 569 * flag is set, so we must block on the turnstile. 570 */ 571 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 572 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 573 rw); 574 #ifdef KDTRACE_HOOKS 575 sleep_time -= lockstat_nsecs(&rw->lock_object); 576 #endif 577 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE); 578 #ifdef KDTRACE_HOOKS 579 sleep_time += lockstat_nsecs(&rw->lock_object); 580 sleep_cnt++; 581 #endif 582 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 583 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 584 __func__, rw); 585 v = RW_READ_VALUE(rw); 586 } 587 #ifdef KDTRACE_HOOKS 588 all_time += lockstat_nsecs(&rw->lock_object); 589 if (sleep_time) 590 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 591 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 592 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 593 594 /* Record only the loops spinning and not sleeping. */ 595 if (lda.spin_cnt > sleep_cnt) 596 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 597 LOCKSTAT_READER, (state & RW_LOCK_READ) == 0, 598 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 599 #endif 600 /* 601 * TODO: acquire "owner of record" here. Here be turnstile dragons 602 * however. turnstiles don't like owners changing between calls to 603 * turnstile_wait() currently. 604 */ 605 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 606 waittime, file, line, LOCKSTAT_READER); 607 } 608 609 void 610 __rw_rlock(volatile uintptr_t *c, const char *file, int line) 611 { 612 struct rwlock *rw; 613 struct thread *td; 614 uintptr_t v; 615 616 td = curthread; 617 rw = rwlock2rw(c); 618 619 KASSERT(kdb_active != 0 || SCHEDULER_STOPPED_TD(td) || 620 !TD_IS_IDLETHREAD(td), 621 ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d", 622 td, rw->lock_object.lo_name, file, line)); 623 KASSERT(rw->rw_lock != RW_DESTROYED, 624 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 625 KASSERT(rw_wowner(rw) != td, 626 ("rw_rlock: wlock already held for %s @ %s:%d", 627 rw->lock_object.lo_name, file, line)); 628 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL); 629 630 v = RW_READ_VALUE(rw); 631 if (__predict_false(LOCKSTAT_OOL_PROFILE_ENABLED(rw__acquire) || 632 !__rw_rlock_try(rw, td, &v, file, line))) 633 __rw_rlock_hard(c, td, v, file, line); 634 635 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 636 WITNESS_LOCK(&rw->lock_object, 0, file, line); 637 TD_LOCKS_INC(curthread); 638 } 639 640 int 641 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line) 642 { 643 struct rwlock *rw; 644 uintptr_t x; 645 646 if (SCHEDULER_STOPPED()) 647 return (1); 648 649 rw = rwlock2rw(c); 650 651 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 652 ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d", 653 curthread, rw->lock_object.lo_name, file, line)); 654 655 x = rw->rw_lock; 656 for (;;) { 657 KASSERT(rw->rw_lock != RW_DESTROYED, 658 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line)); 659 if (!(x & RW_LOCK_READ)) 660 break; 661 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &x, x + RW_ONE_READER)) { 662 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file, 663 line); 664 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line); 665 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, 666 rw, 0, 0, file, line, LOCKSTAT_READER); 667 TD_LOCKS_INC(curthread); 668 curthread->td_rw_rlocks++; 669 return (1); 670 } 671 } 672 673 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line); 674 return (0); 675 } 676 677 static bool __always_inline 678 __rw_runlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp) 679 { 680 681 for (;;) { 682 /* 683 * See if there is more than one read lock held. If so, 684 * just drop one and return. 685 */ 686 if (RW_READERS(*vp) > 1) { 687 if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp, 688 *vp - RW_ONE_READER)) { 689 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 690 CTR4(KTR_LOCK, 691 "%s: %p succeeded %p -> %p", 692 __func__, rw, (void *)*vp, 693 (void *)(*vp - RW_ONE_READER)); 694 td->td_rw_rlocks--; 695 return (true); 696 } 697 continue; 698 } 699 /* 700 * If there aren't any waiters for a write lock, then try 701 * to drop it quickly. 702 */ 703 if (!(*vp & RW_LOCK_WAITERS)) { 704 MPASS((*vp & ~RW_LOCK_WRITE_SPINNER) == 705 RW_READERS_LOCK(1)); 706 if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp, 707 RW_UNLOCKED)) { 708 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 709 CTR2(KTR_LOCK, "%s: %p last succeeded", 710 __func__, rw); 711 td->td_rw_rlocks--; 712 return (true); 713 } 714 continue; 715 } 716 break; 717 } 718 return (false); 719 } 720 721 static void __noinline 722 __rw_runlock_hard(volatile uintptr_t *c, struct thread *td, uintptr_t v, 723 const char *file, int line) 724 { 725 struct rwlock *rw; 726 struct turnstile *ts; 727 uintptr_t x, queue; 728 729 if (SCHEDULER_STOPPED()) 730 return; 731 732 rw = rwlock2rw(c); 733 734 for (;;) { 735 if (__rw_runlock_try(rw, td, &v)) 736 break; 737 738 /* 739 * Ok, we know we have waiters and we think we are the 740 * last reader, so grab the turnstile lock. 741 */ 742 turnstile_chain_lock(&rw->lock_object); 743 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 744 MPASS(v & RW_LOCK_WAITERS); 745 746 /* 747 * Try to drop our lock leaving the lock in a unlocked 748 * state. 749 * 750 * If you wanted to do explicit lock handoff you'd have to 751 * do it here. You'd also want to use turnstile_signal() 752 * and you'd have to handle the race where a higher 753 * priority thread blocks on the write lock before the 754 * thread you wakeup actually runs and have the new thread 755 * "steal" the lock. For now it's a lot simpler to just 756 * wakeup all of the waiters. 757 * 758 * As above, if we fail, then another thread might have 759 * acquired a read lock, so drop the turnstile lock and 760 * restart. 761 */ 762 x = RW_UNLOCKED; 763 if (v & RW_LOCK_WRITE_WAITERS) { 764 queue = TS_EXCLUSIVE_QUEUE; 765 x |= (v & RW_LOCK_READ_WAITERS); 766 } else 767 queue = TS_SHARED_QUEUE; 768 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 769 x)) { 770 turnstile_chain_unlock(&rw->lock_object); 771 v = RW_READ_VALUE(rw); 772 continue; 773 } 774 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 775 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 776 __func__, rw); 777 778 /* 779 * Ok. The lock is released and all that's left is to 780 * wake up the waiters. Note that the lock might not be 781 * free anymore, but in that case the writers will just 782 * block again if they run before the new lock holder(s) 783 * release the lock. 784 */ 785 ts = turnstile_lookup(&rw->lock_object); 786 MPASS(ts != NULL); 787 turnstile_broadcast(ts, queue); 788 turnstile_unpend(ts, TS_SHARED_LOCK); 789 turnstile_chain_unlock(&rw->lock_object); 790 td->td_rw_rlocks--; 791 break; 792 } 793 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_READER); 794 } 795 796 void 797 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line) 798 { 799 struct rwlock *rw; 800 struct thread *td; 801 uintptr_t v; 802 803 rw = rwlock2rw(c); 804 805 KASSERT(rw->rw_lock != RW_DESTROYED, 806 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 807 __rw_assert(c, RA_RLOCKED, file, line); 808 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 809 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 810 811 td = curthread; 812 v = RW_READ_VALUE(rw); 813 814 if (__predict_false(LOCKSTAT_OOL_PROFILE_ENABLED(rw__release) || 815 !__rw_runlock_try(rw, td, &v))) 816 __rw_runlock_hard(c, td, v, file, line); 817 818 TD_LOCKS_DEC(curthread); 819 } 820 821 /* 822 * This function is called when we are unable to obtain a write lock on the 823 * first try. This means that at least one other thread holds either a 824 * read or write lock. 825 */ 826 void 827 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v, uintptr_t tid, 828 const char *file, int line) 829 { 830 struct rwlock *rw; 831 struct turnstile *ts; 832 #ifdef ADAPTIVE_RWLOCKS 833 volatile struct thread *owner; 834 int spintries = 0; 835 int i; 836 #endif 837 uintptr_t x; 838 #ifdef LOCK_PROFILING 839 uint64_t waittime = 0; 840 int contested = 0; 841 #endif 842 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS) 843 struct lock_delay_arg lda; 844 #endif 845 #ifdef KDTRACE_HOOKS 846 uintptr_t state; 847 u_int sleep_cnt = 0; 848 int64_t sleep_time = 0; 849 int64_t all_time = 0; 850 #endif 851 852 if (SCHEDULER_STOPPED()) 853 return; 854 855 #if defined(ADAPTIVE_RWLOCKS) 856 lock_delay_arg_init(&lda, &rw_delay); 857 #elif defined(KDTRACE_HOOKS) 858 lock_delay_arg_init(&lda, NULL); 859 #endif 860 rw = rwlock2rw(c); 861 if (__predict_false(v == RW_UNLOCKED)) 862 v = RW_READ_VALUE(rw); 863 864 if (__predict_false(lv_rw_wowner(v) == (struct thread *)tid)) { 865 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE, 866 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 867 __func__, rw->lock_object.lo_name, file, line)); 868 rw->rw_recurse++; 869 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 870 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 871 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 872 return; 873 } 874 875 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 876 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 877 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 878 879 #ifdef KDTRACE_HOOKS 880 all_time -= lockstat_nsecs(&rw->lock_object); 881 state = v; 882 #endif 883 for (;;) { 884 if (v == RW_UNLOCKED) { 885 if (_rw_write_lock_fetch(rw, &v, tid)) 886 break; 887 continue; 888 } 889 #ifdef KDTRACE_HOOKS 890 lda.spin_cnt++; 891 #endif 892 #ifdef HWPMC_HOOKS 893 PMC_SOFT_CALL( , , lock, failed); 894 #endif 895 lock_profile_obtain_lock_failed(&rw->lock_object, 896 &contested, &waittime); 897 #ifdef ADAPTIVE_RWLOCKS 898 /* 899 * If the lock is write locked and the owner is 900 * running on another CPU, spin until the owner stops 901 * running or the state of the lock changes. 902 */ 903 owner = lv_rw_wowner(v); 904 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 905 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 906 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 907 __func__, rw, owner); 908 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 909 "spinning", "lockname:\"%s\"", 910 rw->lock_object.lo_name); 911 do { 912 lock_delay(&lda); 913 v = RW_READ_VALUE(rw); 914 owner = lv_rw_wowner(v); 915 } while (owner != NULL && TD_IS_RUNNING(owner)); 916 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 917 "running"); 918 continue; 919 } 920 if ((v & RW_LOCK_READ) && RW_READERS(v) && 921 spintries < rowner_retries) { 922 if (!(v & RW_LOCK_WRITE_SPINNER)) { 923 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 924 v | RW_LOCK_WRITE_SPINNER)) { 925 v = RW_READ_VALUE(rw); 926 continue; 927 } 928 } 929 spintries++; 930 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread), 931 "spinning", "lockname:\"%s\"", 932 rw->lock_object.lo_name); 933 for (i = 0; i < rowner_loops; i++) { 934 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0) 935 break; 936 cpu_spinwait(); 937 } 938 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread), 939 "running"); 940 v = RW_READ_VALUE(rw); 941 #ifdef KDTRACE_HOOKS 942 lda.spin_cnt += rowner_loops - i; 943 #endif 944 if (i != rowner_loops) 945 continue; 946 } 947 #endif 948 ts = turnstile_trywait(&rw->lock_object); 949 v = RW_READ_VALUE(rw); 950 951 #ifdef ADAPTIVE_RWLOCKS 952 /* 953 * The current lock owner might have started executing 954 * on another CPU (or the lock could have changed 955 * owners) while we were waiting on the turnstile 956 * chain lock. If so, drop the turnstile lock and try 957 * again. 958 */ 959 if (!(v & RW_LOCK_READ)) { 960 owner = (struct thread *)RW_OWNER(v); 961 if (TD_IS_RUNNING(owner)) { 962 turnstile_cancel(ts); 963 continue; 964 } 965 } 966 #endif 967 /* 968 * Check for the waiters flags about this rwlock. 969 * If the lock was released, without maintain any pending 970 * waiters queue, simply try to acquire it. 971 * If a pending waiters queue is present, claim the lock 972 * ownership and maintain the pending queue. 973 */ 974 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 975 if ((v & ~x) == RW_UNLOCKED) { 976 x &= ~RW_LOCK_WRITE_SPINNER; 977 if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) { 978 if (x) 979 turnstile_claim(ts); 980 else 981 turnstile_cancel(ts); 982 break; 983 } 984 turnstile_cancel(ts); 985 v = RW_READ_VALUE(rw); 986 continue; 987 } 988 /* 989 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 990 * set it. If we fail to set it, then loop back and try 991 * again. 992 */ 993 if (!(v & RW_LOCK_WRITE_WAITERS)) { 994 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 995 v | RW_LOCK_WRITE_WAITERS)) { 996 turnstile_cancel(ts); 997 v = RW_READ_VALUE(rw); 998 continue; 999 } 1000 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1001 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 1002 __func__, rw); 1003 } 1004 /* 1005 * We were unable to acquire the lock and the write waiters 1006 * flag is set, so we must block on the turnstile. 1007 */ 1008 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1009 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 1010 rw); 1011 #ifdef KDTRACE_HOOKS 1012 sleep_time -= lockstat_nsecs(&rw->lock_object); 1013 #endif 1014 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE); 1015 #ifdef KDTRACE_HOOKS 1016 sleep_time += lockstat_nsecs(&rw->lock_object); 1017 sleep_cnt++; 1018 #endif 1019 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1020 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 1021 __func__, rw); 1022 #ifdef ADAPTIVE_RWLOCKS 1023 spintries = 0; 1024 #endif 1025 v = RW_READ_VALUE(rw); 1026 } 1027 #ifdef KDTRACE_HOOKS 1028 all_time += lockstat_nsecs(&rw->lock_object); 1029 if (sleep_time) 1030 LOCKSTAT_RECORD4(rw__block, rw, sleep_time, 1031 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1032 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1033 1034 /* Record only the loops spinning and not sleeping. */ 1035 if (lda.spin_cnt > sleep_cnt) 1036 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time, 1037 LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0, 1038 (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state)); 1039 #endif 1040 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested, 1041 waittime, file, line, LOCKSTAT_WRITER); 1042 } 1043 1044 /* 1045 * This function is called if lockstat is active or the first try at releasing 1046 * a write lock failed. The latter means that the lock is recursed or one of 1047 * the 2 waiter bits must be set indicating that at least one thread is waiting 1048 * on this lock. 1049 */ 1050 void 1051 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t tid, const char *file, 1052 int line) 1053 { 1054 struct rwlock *rw; 1055 struct turnstile *ts; 1056 uintptr_t v; 1057 int queue; 1058 1059 if (SCHEDULER_STOPPED()) 1060 return; 1061 1062 rw = rwlock2rw(c); 1063 v = RW_READ_VALUE(rw); 1064 if (v & RW_LOCK_WRITER_RECURSED) { 1065 if (--(rw->rw_recurse) == 0) 1066 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED); 1067 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1068 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 1069 return; 1070 } 1071 1072 LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_WRITER); 1073 if (v == tid && _rw_write_unlock(rw, tid)) 1074 return; 1075 1076 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 1077 ("%s: neither of the waiter flags are set", __func__)); 1078 1079 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1080 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 1081 1082 turnstile_chain_lock(&rw->lock_object); 1083 ts = turnstile_lookup(&rw->lock_object); 1084 MPASS(ts != NULL); 1085 1086 /* 1087 * Use the same algo as sx locks for now. Prefer waking up shared 1088 * waiters if we have any over writers. This is probably not ideal. 1089 * 1090 * 'v' is the value we are going to write back to rw_lock. If we 1091 * have waiters on both queues, we need to preserve the state of 1092 * the waiter flag for the queue we don't wake up. For now this is 1093 * hardcoded for the algorithm mentioned above. 1094 * 1095 * In the case of both readers and writers waiting we wakeup the 1096 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 1097 * new writer comes in before a reader it will claim the lock up 1098 * above. There is probably a potential priority inversion in 1099 * there that could be worked around either by waking both queues 1100 * of waiters or doing some complicated lock handoff gymnastics. 1101 */ 1102 v = RW_UNLOCKED; 1103 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) { 1104 queue = TS_EXCLUSIVE_QUEUE; 1105 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS); 1106 } else 1107 queue = TS_SHARED_QUEUE; 1108 1109 /* Wake up all waiters for the specific queue. */ 1110 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 1111 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 1112 queue == TS_SHARED_QUEUE ? "read" : "write"); 1113 turnstile_broadcast(ts, queue); 1114 atomic_store_rel_ptr(&rw->rw_lock, v); 1115 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1116 turnstile_chain_unlock(&rw->lock_object); 1117 } 1118 1119 /* 1120 * Attempt to do a non-blocking upgrade from a read lock to a write 1121 * lock. This will only succeed if this thread holds a single read 1122 * lock. Returns true if the upgrade succeeded and false otherwise. 1123 */ 1124 int 1125 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line) 1126 { 1127 struct rwlock *rw; 1128 uintptr_t v, x, tid; 1129 struct turnstile *ts; 1130 int success; 1131 1132 if (SCHEDULER_STOPPED()) 1133 return (1); 1134 1135 rw = rwlock2rw(c); 1136 1137 KASSERT(rw->rw_lock != RW_DESTROYED, 1138 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 1139 __rw_assert(c, RA_RLOCKED, file, line); 1140 1141 /* 1142 * Attempt to switch from one reader to a writer. If there 1143 * are any write waiters, then we will have to lock the 1144 * turnstile first to prevent races with another writer 1145 * calling turnstile_wait() before we have claimed this 1146 * turnstile. So, do the simple case of no waiters first. 1147 */ 1148 tid = (uintptr_t)curthread; 1149 success = 0; 1150 for (;;) { 1151 v = rw->rw_lock; 1152 if (RW_READERS(v) > 1) 1153 break; 1154 if (!(v & RW_LOCK_WAITERS)) { 1155 success = atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid); 1156 if (!success) 1157 continue; 1158 break; 1159 } 1160 1161 /* 1162 * Ok, we think we have waiters, so lock the turnstile. 1163 */ 1164 ts = turnstile_trywait(&rw->lock_object); 1165 v = rw->rw_lock; 1166 if (RW_READERS(v) > 1) { 1167 turnstile_cancel(ts); 1168 break; 1169 } 1170 /* 1171 * Try to switch from one reader to a writer again. This time 1172 * we honor the current state of the waiters flags. 1173 * If we obtain the lock with the flags set, then claim 1174 * ownership of the turnstile. 1175 */ 1176 x = rw->rw_lock & RW_LOCK_WAITERS; 1177 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x); 1178 if (success) { 1179 if (x) 1180 turnstile_claim(ts); 1181 else 1182 turnstile_cancel(ts); 1183 break; 1184 } 1185 turnstile_cancel(ts); 1186 } 1187 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 1188 if (success) { 1189 curthread->td_rw_rlocks--; 1190 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 1191 file, line); 1192 LOCKSTAT_RECORD0(rw__upgrade, rw); 1193 } 1194 return (success); 1195 } 1196 1197 /* 1198 * Downgrade a write lock into a single read lock. 1199 */ 1200 void 1201 __rw_downgrade(volatile uintptr_t *c, const char *file, int line) 1202 { 1203 struct rwlock *rw; 1204 struct turnstile *ts; 1205 uintptr_t tid, v; 1206 int rwait, wwait; 1207 1208 if (SCHEDULER_STOPPED()) 1209 return; 1210 1211 rw = rwlock2rw(c); 1212 1213 KASSERT(rw->rw_lock != RW_DESTROYED, 1214 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 1215 __rw_assert(c, RA_WLOCKED | RA_NOTRECURSED, file, line); 1216 #ifndef INVARIANTS 1217 if (rw_recursed(rw)) 1218 panic("downgrade of a recursed lock"); 1219 #endif 1220 1221 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 1222 1223 /* 1224 * Convert from a writer to a single reader. First we handle 1225 * the easy case with no waiters. If there are any waiters, we 1226 * lock the turnstile and "disown" the lock. 1227 */ 1228 tid = (uintptr_t)curthread; 1229 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 1230 goto out; 1231 1232 /* 1233 * Ok, we think we have waiters, so lock the turnstile so we can 1234 * read the waiter flags without any races. 1235 */ 1236 turnstile_chain_lock(&rw->lock_object); 1237 v = rw->rw_lock & RW_LOCK_WAITERS; 1238 rwait = v & RW_LOCK_READ_WAITERS; 1239 wwait = v & RW_LOCK_WRITE_WAITERS; 1240 MPASS(rwait | wwait); 1241 1242 /* 1243 * Downgrade from a write lock while preserving waiters flag 1244 * and give up ownership of the turnstile. 1245 */ 1246 ts = turnstile_lookup(&rw->lock_object); 1247 MPASS(ts != NULL); 1248 if (!wwait) 1249 v &= ~RW_LOCK_READ_WAITERS; 1250 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v); 1251 /* 1252 * Wake other readers if there are no writers pending. Otherwise they 1253 * won't be able to acquire the lock anyway. 1254 */ 1255 if (rwait && !wwait) { 1256 turnstile_broadcast(ts, TS_SHARED_QUEUE); 1257 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1258 } else 1259 turnstile_disown(ts); 1260 turnstile_chain_unlock(&rw->lock_object); 1261 out: 1262 curthread->td_rw_rlocks++; 1263 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 1264 LOCKSTAT_RECORD0(rw__downgrade, rw); 1265 } 1266 1267 #ifdef INVARIANT_SUPPORT 1268 #ifndef INVARIANTS 1269 #undef __rw_assert 1270 #endif 1271 1272 /* 1273 * In the non-WITNESS case, rw_assert() can only detect that at least 1274 * *some* thread owns an rlock, but it cannot guarantee that *this* 1275 * thread owns an rlock. 1276 */ 1277 void 1278 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line) 1279 { 1280 const struct rwlock *rw; 1281 1282 if (panicstr != NULL) 1283 return; 1284 1285 rw = rwlock2rw(c); 1286 1287 switch (what) { 1288 case RA_LOCKED: 1289 case RA_LOCKED | RA_RECURSED: 1290 case RA_LOCKED | RA_NOTRECURSED: 1291 case RA_RLOCKED: 1292 case RA_RLOCKED | RA_RECURSED: 1293 case RA_RLOCKED | RA_NOTRECURSED: 1294 #ifdef WITNESS 1295 witness_assert(&rw->lock_object, what, file, line); 1296 #else 1297 /* 1298 * If some other thread has a write lock or we have one 1299 * and are asserting a read lock, fail. Also, if no one 1300 * has a lock at all, fail. 1301 */ 1302 if (rw->rw_lock == RW_UNLOCKED || 1303 (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED || 1304 rw_wowner(rw) != curthread))) 1305 panic("Lock %s not %slocked @ %s:%d\n", 1306 rw->lock_object.lo_name, (what & RA_RLOCKED) ? 1307 "read " : "", file, line); 1308 1309 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) { 1310 if (rw_recursed(rw)) { 1311 if (what & RA_NOTRECURSED) 1312 panic("Lock %s recursed @ %s:%d\n", 1313 rw->lock_object.lo_name, file, 1314 line); 1315 } else if (what & RA_RECURSED) 1316 panic("Lock %s not recursed @ %s:%d\n", 1317 rw->lock_object.lo_name, file, line); 1318 } 1319 #endif 1320 break; 1321 case RA_WLOCKED: 1322 case RA_WLOCKED | RA_RECURSED: 1323 case RA_WLOCKED | RA_NOTRECURSED: 1324 if (rw_wowner(rw) != curthread) 1325 panic("Lock %s not exclusively locked @ %s:%d\n", 1326 rw->lock_object.lo_name, file, line); 1327 if (rw_recursed(rw)) { 1328 if (what & RA_NOTRECURSED) 1329 panic("Lock %s recursed @ %s:%d\n", 1330 rw->lock_object.lo_name, file, line); 1331 } else if (what & RA_RECURSED) 1332 panic("Lock %s not recursed @ %s:%d\n", 1333 rw->lock_object.lo_name, file, line); 1334 break; 1335 case RA_UNLOCKED: 1336 #ifdef WITNESS 1337 witness_assert(&rw->lock_object, what, file, line); 1338 #else 1339 /* 1340 * If we hold a write lock fail. We can't reliably check 1341 * to see if we hold a read lock or not. 1342 */ 1343 if (rw_wowner(rw) == curthread) 1344 panic("Lock %s exclusively locked @ %s:%d\n", 1345 rw->lock_object.lo_name, file, line); 1346 #endif 1347 break; 1348 default: 1349 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 1350 line); 1351 } 1352 } 1353 #endif /* INVARIANT_SUPPORT */ 1354 1355 #ifdef DDB 1356 void 1357 db_show_rwlock(const struct lock_object *lock) 1358 { 1359 const struct rwlock *rw; 1360 struct thread *td; 1361 1362 rw = (const struct rwlock *)lock; 1363 1364 db_printf(" state: "); 1365 if (rw->rw_lock == RW_UNLOCKED) 1366 db_printf("UNLOCKED\n"); 1367 else if (rw->rw_lock == RW_DESTROYED) { 1368 db_printf("DESTROYED\n"); 1369 return; 1370 } else if (rw->rw_lock & RW_LOCK_READ) 1371 db_printf("RLOCK: %ju locks\n", 1372 (uintmax_t)(RW_READERS(rw->rw_lock))); 1373 else { 1374 td = rw_wowner(rw); 1375 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 1376 td->td_tid, td->td_proc->p_pid, td->td_name); 1377 if (rw_recursed(rw)) 1378 db_printf(" recursed: %u\n", rw->rw_recurse); 1379 } 1380 db_printf(" waiters: "); 1381 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 1382 case RW_LOCK_READ_WAITERS: 1383 db_printf("readers\n"); 1384 break; 1385 case RW_LOCK_WRITE_WAITERS: 1386 db_printf("writers\n"); 1387 break; 1388 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 1389 db_printf("readers and writers\n"); 1390 break; 1391 default: 1392 db_printf("none\n"); 1393 break; 1394 } 1395 } 1396 1397 #endif
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