kern_rwlock.c revision 182914
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 * 3. Neither the name of the author nor the names of any co-contributors 14 * may be used to endorse or promote products derived from this software 15 * without specific prior written permission. 16 * 17 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 18 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 21 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27 * SUCH DAMAGE. 28 */ 29 30/* 31 * Machine independent bits of reader/writer lock implementation. 32 */ 33 34#include <sys/cdefs.h> 35__FBSDID("$FreeBSD: head/sys/kern/kern_rwlock.c 182914 2008-09-10 19:13:30Z jhb $"); 36 37#include "opt_ddb.h" 38#include "opt_no_adaptive_rwlocks.h" 39 40#include <sys/param.h> 41#include <sys/ktr.h> 42#include <sys/kernel.h> 43#include <sys/lock.h> 44#include <sys/mutex.h> 45#include <sys/proc.h> 46#include <sys/rwlock.h> 47#include <sys/sysctl.h> 48#include <sys/systm.h> 49#include <sys/turnstile.h> 50 51#include <machine/cpu.h> 52 53CTASSERT((RW_RECURSE & LO_CLASSFLAGS) == RW_RECURSE); 54 55#if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS) 56#define ADAPTIVE_RWLOCKS 57#endif 58 59#ifdef ADAPTIVE_RWLOCKS 60static int rowner_retries = 10; 61static int rowner_loops = 10000; 62SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL, "rwlock debugging"); 63SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, ""); 64SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, ""); 65#endif 66 67#ifdef DDB 68#include <ddb/ddb.h> 69 70static void db_show_rwlock(struct lock_object *lock); 71#endif 72static void assert_rw(struct lock_object *lock, int what); 73static void lock_rw(struct lock_object *lock, int how); 74static int unlock_rw(struct lock_object *lock); 75 76struct lock_class lock_class_rw = { 77 .lc_name = "rw", 78 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 79 .lc_assert = assert_rw, 80#ifdef DDB 81 .lc_ddb_show = db_show_rwlock, 82#endif 83 .lc_lock = lock_rw, 84 .lc_unlock = unlock_rw, 85}; 86 87/* 88 * Return a pointer to the owning thread if the lock is write-locked or 89 * NULL if the lock is unlocked or read-locked. 90 */ 91#define rw_wowner(rw) \ 92 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \ 93 (struct thread *)RW_OWNER((rw)->rw_lock)) 94 95/* 96 * Returns if a write owner is recursed. Write ownership is not assured 97 * here and should be previously checked. 98 */ 99#define rw_recursed(rw) ((rw)->rw_recurse != 0) 100 101/* 102 * Return true if curthread helds the lock. 103 */ 104#define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 105 106/* 107 * Return a pointer to the owning thread for this lock who should receive 108 * any priority lent by threads that block on this lock. Currently this 109 * is identical to rw_wowner(). 110 */ 111#define rw_owner(rw) rw_wowner(rw) 112 113#ifndef INVARIANTS 114#define _rw_assert(rw, what, file, line) 115#endif 116 117void 118assert_rw(struct lock_object *lock, int what) 119{ 120 121 rw_assert((struct rwlock *)lock, what); 122} 123 124void 125lock_rw(struct lock_object *lock, int how) 126{ 127 struct rwlock *rw; 128 129 rw = (struct rwlock *)lock; 130 if (how) 131 rw_wlock(rw); 132 else 133 rw_rlock(rw); 134} 135 136int 137unlock_rw(struct lock_object *lock) 138{ 139 struct rwlock *rw; 140 141 rw = (struct rwlock *)lock; 142 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED); 143 if (rw->rw_lock & RW_LOCK_READ) { 144 rw_runlock(rw); 145 return (0); 146 } else { 147 rw_wunlock(rw); 148 return (1); 149 } 150} 151 152void 153rw_init_flags(struct rwlock *rw, const char *name, int opts) 154{ 155 int flags; 156 157 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 158 RW_RECURSE)) == 0); 159 160 flags = LO_UPGRADABLE | LO_RECURSABLE; 161 if (opts & RW_DUPOK) 162 flags |= LO_DUPOK; 163 if (opts & RW_NOPROFILE) 164 flags |= LO_NOPROFILE; 165 if (!(opts & RW_NOWITNESS)) 166 flags |= LO_WITNESS; 167 if (opts & RW_QUIET) 168 flags |= LO_QUIET; 169 flags |= opts & RW_RECURSE; 170 171 rw->rw_lock = RW_UNLOCKED; 172 rw->rw_recurse = 0; 173 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 174} 175 176void 177rw_destroy(struct rwlock *rw) 178{ 179 180 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked")); 181 KASSERT(rw->rw_recurse == 0, ("rw lock still recursed")); 182 rw->rw_lock = RW_DESTROYED; 183 lock_destroy(&rw->lock_object); 184} 185 186void 187rw_sysinit(void *arg) 188{ 189 struct rw_args *args = arg; 190 191 rw_init(args->ra_rw, args->ra_desc); 192} 193 194int 195rw_wowned(struct rwlock *rw) 196{ 197 198 return (rw_wowner(rw) == curthread); 199} 200 201void 202_rw_wlock(struct rwlock *rw, const char *file, int line) 203{ 204 205 MPASS(curthread != NULL); 206 KASSERT(rw->rw_lock != RW_DESTROYED, 207 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 208 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 209 line, NULL); 210 __rw_wlock(rw, curthread, file, line); 211 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 212 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 213 curthread->td_locks++; 214} 215 216int 217_rw_try_wlock(struct rwlock *rw, const char *file, int line) 218{ 219 int rval; 220 221 KASSERT(rw->rw_lock != RW_DESTROYED, 222 ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line)); 223 224 if (rw_wlocked(rw) && (rw->lock_object.lo_flags & RW_RECURSE) != 0) { 225 rw->rw_recurse++; 226 rval = 1; 227 } else 228 rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED, 229 (uintptr_t)curthread); 230 231 LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line); 232 if (rval) { 233 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 234 file, line); 235 curthread->td_locks++; 236 } 237 return (rval); 238} 239 240void 241_rw_wunlock(struct rwlock *rw, const char *file, int line) 242{ 243 244 MPASS(curthread != NULL); 245 KASSERT(rw->rw_lock != RW_DESTROYED, 246 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 247 _rw_assert(rw, RA_WLOCKED, file, line); 248 curthread->td_locks--; 249 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 250 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 251 line); 252 if (!rw_recursed(rw)) 253 lock_profile_release_lock(&rw->lock_object); 254 __rw_wunlock(rw, curthread, file, line); 255} 256/* 257 * Determines whether a new reader can acquire a lock. Succeeds if the 258 * reader already owns a read lock and the lock is locked for read to 259 * prevent deadlock from reader recursion. Also succeeds if the lock 260 * is unlocked and has no writer waiters or spinners. Failing otherwise 261 * prioritizes writers before readers. 262 */ 263#define RW_CAN_READ(_rw) \ 264 ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) & \ 265 (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) == \ 266 RW_LOCK_READ) 267 268void 269_rw_rlock(struct rwlock *rw, const char *file, int line) 270{ 271 struct turnstile *ts; 272#ifdef ADAPTIVE_RWLOCKS 273 volatile struct thread *owner; 274 int spintries = 0; 275 int i; 276#endif 277 uint64_t waittime = 0; 278 int contested = 0; 279 uintptr_t v; 280 281 KASSERT(rw->rw_lock != RW_DESTROYED, 282 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 283 KASSERT(rw_wowner(rw) != curthread, 284 ("%s (%s): wlock already held @ %s:%d", __func__, 285 rw->lock_object.lo_name, file, line)); 286 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL); 287 288 for (;;) { 289 /* 290 * Handle the easy case. If no other thread has a write 291 * lock, then try to bump up the count of read locks. Note 292 * that we have to preserve the current state of the 293 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 294 * read lock, then rw_lock must have changed, so restart 295 * the loop. Note that this handles the case of a 296 * completely unlocked rwlock since such a lock is encoded 297 * as a read lock with no waiters. 298 */ 299 v = rw->rw_lock; 300 if (RW_CAN_READ(v)) { 301 /* 302 * The RW_LOCK_READ_WAITERS flag should only be set 303 * if the lock has been unlocked and write waiters 304 * were present. 305 */ 306 if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, 307 v + RW_ONE_READER)) { 308 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 309 CTR4(KTR_LOCK, 310 "%s: %p succeed %p -> %p", __func__, 311 rw, (void *)v, 312 (void *)(v + RW_ONE_READER)); 313 break; 314 } 315 cpu_spinwait(); 316 continue; 317 } 318 lock_profile_obtain_lock_failed(&rw->lock_object, 319 &contested, &waittime); 320 321#ifdef ADAPTIVE_RWLOCKS 322 /* 323 * If the owner is running on another CPU, spin until 324 * the owner stops running or the state of the lock 325 * changes. 326 */ 327 if ((v & RW_LOCK_READ) == 0) { 328 owner = (struct thread *)RW_OWNER(v); 329 if (TD_IS_RUNNING(owner)) { 330 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 331 CTR3(KTR_LOCK, 332 "%s: spinning on %p held by %p", 333 __func__, rw, owner); 334 while ((struct thread*)RW_OWNER(rw->rw_lock) == 335 owner && TD_IS_RUNNING(owner)) 336 cpu_spinwait(); 337 continue; 338 } 339 } else if (spintries < rowner_retries) { 340 spintries++; 341 for (i = 0; i < rowner_loops; i++) { 342 v = rw->rw_lock; 343 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v)) 344 break; 345 cpu_spinwait(); 346 } 347 if (i != rowner_loops) 348 continue; 349 } 350#endif 351 352 /* 353 * Okay, now it's the hard case. Some other thread already 354 * has a write lock or there are write waiters present, 355 * acquire the turnstile lock so we can begin the process 356 * of blocking. 357 */ 358 ts = turnstile_trywait(&rw->lock_object); 359 360 /* 361 * The lock might have been released while we spun, so 362 * recheck its state and restart the loop if needed. 363 */ 364 v = rw->rw_lock; 365 if (RW_CAN_READ(v)) { 366 turnstile_cancel(ts); 367 cpu_spinwait(); 368 continue; 369 } 370 371#ifdef ADAPTIVE_RWLOCKS 372 /* 373 * If the current owner of the lock is executing on another 374 * CPU quit the hard path and try to spin. 375 */ 376 if ((v & RW_LOCK_READ) == 0) { 377 owner = (struct thread *)RW_OWNER(v); 378 if (TD_IS_RUNNING(owner)) { 379 turnstile_cancel(ts); 380 cpu_spinwait(); 381 continue; 382 } 383 } 384#endif 385 386 /* 387 * The lock is held in write mode or it already has waiters. 388 */ 389 MPASS(!RW_CAN_READ(v)); 390 391 /* 392 * If the RW_LOCK_READ_WAITERS flag is already set, then 393 * we can go ahead and block. If it is not set then try 394 * to set it. If we fail to set it drop the turnstile 395 * lock and restart the loop. 396 */ 397 if (!(v & RW_LOCK_READ_WAITERS)) { 398 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 399 v | RW_LOCK_READ_WAITERS)) { 400 turnstile_cancel(ts); 401 cpu_spinwait(); 402 continue; 403 } 404 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 405 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 406 __func__, rw); 407 } 408 409 /* 410 * We were unable to acquire the lock and the read waiters 411 * flag is set, so we must block on the turnstile. 412 */ 413 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 414 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 415 rw); 416 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE); 417 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 418 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 419 __func__, rw); 420 } 421 422 /* 423 * TODO: acquire "owner of record" here. Here be turnstile dragons 424 * however. turnstiles don't like owners changing between calls to 425 * turnstile_wait() currently. 426 */ 427 lock_profile_obtain_lock_success( &rw->lock_object, contested, 428 waittime, file, line); 429 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 430 WITNESS_LOCK(&rw->lock_object, 0, file, line); 431 curthread->td_locks++; 432 curthread->td_rw_rlocks++; 433} 434 435int 436_rw_try_rlock(struct rwlock *rw, const char *file, int line) 437{ 438 uintptr_t x; 439 440 for (;;) { 441 x = rw->rw_lock; 442 KASSERT(rw->rw_lock != RW_DESTROYED, 443 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line)); 444 if (!(x & RW_LOCK_READ)) 445 break; 446 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) { 447 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file, 448 line); 449 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line); 450 curthread->td_locks++; 451 curthread->td_rw_rlocks++; 452 return (1); 453 } 454 } 455 456 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line); 457 return (0); 458} 459 460void 461_rw_runlock(struct rwlock *rw, const char *file, int line) 462{ 463 struct turnstile *ts; 464 uintptr_t x, v, queue; 465 466 KASSERT(rw->rw_lock != RW_DESTROYED, 467 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 468 _rw_assert(rw, RA_RLOCKED, file, line); 469 curthread->td_locks--; 470 curthread->td_rw_rlocks--; 471 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 472 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 473 474 /* TODO: drop "owner of record" here. */ 475 476 for (;;) { 477 /* 478 * See if there is more than one read lock held. If so, 479 * just drop one and return. 480 */ 481 x = rw->rw_lock; 482 if (RW_READERS(x) > 1) { 483 if (atomic_cmpset_ptr(&rw->rw_lock, x, 484 x - RW_ONE_READER)) { 485 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 486 CTR4(KTR_LOCK, 487 "%s: %p succeeded %p -> %p", 488 __func__, rw, (void *)x, 489 (void *)(x - RW_ONE_READER)); 490 break; 491 } 492 continue; 493 } 494 /* 495 * If there aren't any waiters for a write lock, then try 496 * to drop it quickly. 497 */ 498 if (!(x & RW_LOCK_WAITERS)) { 499 MPASS((x & ~RW_LOCK_WRITE_SPINNER) == 500 RW_READERS_LOCK(1)); 501 if (atomic_cmpset_ptr(&rw->rw_lock, x, RW_UNLOCKED)) { 502 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 503 CTR2(KTR_LOCK, "%s: %p last succeeded", 504 __func__, rw); 505 break; 506 } 507 continue; 508 } 509 /* 510 * Ok, we know we have waiters and we think we are the 511 * last reader, so grab the turnstile lock. 512 */ 513 turnstile_chain_lock(&rw->lock_object); 514 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 515 MPASS(v & RW_LOCK_WAITERS); 516 517 /* 518 * Try to drop our lock leaving the lock in a unlocked 519 * state. 520 * 521 * If you wanted to do explicit lock handoff you'd have to 522 * do it here. You'd also want to use turnstile_signal() 523 * and you'd have to handle the race where a higher 524 * priority thread blocks on the write lock before the 525 * thread you wakeup actually runs and have the new thread 526 * "steal" the lock. For now it's a lot simpler to just 527 * wakeup all of the waiters. 528 * 529 * As above, if we fail, then another thread might have 530 * acquired a read lock, so drop the turnstile lock and 531 * restart. 532 */ 533 x = RW_UNLOCKED; 534 if (v & RW_LOCK_WRITE_WAITERS) { 535 queue = TS_EXCLUSIVE_QUEUE; 536 x |= (v & RW_LOCK_READ_WAITERS); 537 } else 538 queue = TS_SHARED_QUEUE; 539 if (!atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 540 x)) { 541 turnstile_chain_unlock(&rw->lock_object); 542 continue; 543 } 544 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 545 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 546 __func__, rw); 547 548 /* 549 * Ok. The lock is released and all that's left is to 550 * wake up the waiters. Note that the lock might not be 551 * free anymore, but in that case the writers will just 552 * block again if they run before the new lock holder(s) 553 * release the lock. 554 */ 555 ts = turnstile_lookup(&rw->lock_object); 556 MPASS(ts != NULL); 557 turnstile_broadcast(ts, queue); 558 turnstile_unpend(ts, TS_SHARED_LOCK); 559 turnstile_chain_unlock(&rw->lock_object); 560 break; 561 } 562 lock_profile_release_lock(&rw->lock_object); 563} 564 565/* 566 * This function is called when we are unable to obtain a write lock on the 567 * first try. This means that at least one other thread holds either a 568 * read or write lock. 569 */ 570void 571_rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 572{ 573 struct turnstile *ts; 574#ifdef ADAPTIVE_RWLOCKS 575 volatile struct thread *owner; 576 int spintries = 0; 577 int i; 578#endif 579 uint64_t waittime = 0; 580 uintptr_t v, x; 581 int contested = 0; 582 583 if (rw_wlocked(rw)) { 584 KASSERT(rw->lock_object.lo_flags & RW_RECURSE, 585 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 586 __func__, rw->lock_object.lo_name, file, line)); 587 rw->rw_recurse++; 588 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 589 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 590 return; 591 } 592 593 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 594 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 595 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 596 597 while (!_rw_write_lock(rw, tid)) { 598 lock_profile_obtain_lock_failed(&rw->lock_object, 599 &contested, &waittime); 600#ifdef ADAPTIVE_RWLOCKS 601 /* 602 * If the lock is write locked and the owner is 603 * running on another CPU, spin until the owner stops 604 * running or the state of the lock changes. 605 */ 606 v = rw->rw_lock; 607 owner = (struct thread *)RW_OWNER(v); 608 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 609 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 610 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 611 __func__, rw, owner); 612 while ((struct thread*)RW_OWNER(rw->rw_lock) == owner && 613 TD_IS_RUNNING(owner)) 614 cpu_spinwait(); 615 continue; 616 } 617 if ((v & RW_LOCK_READ) && RW_READERS(v) && 618 spintries < rowner_retries) { 619 if (!(v & RW_LOCK_WRITE_SPINNER)) { 620 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 621 v | RW_LOCK_WRITE_SPINNER)) { 622 cpu_spinwait(); 623 continue; 624 } 625 } 626 spintries++; 627 for (i = 0; i < rowner_loops; i++) { 628 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0) 629 break; 630 cpu_spinwait(); 631 } 632 if (i != rowner_loops) 633 continue; 634 } 635#endif 636 ts = turnstile_trywait(&rw->lock_object); 637 v = rw->rw_lock; 638 639#ifdef ADAPTIVE_RWLOCKS 640 /* 641 * If the current owner of the lock is executing on another 642 * CPU quit the hard path and try to spin. 643 */ 644 if (!(v & RW_LOCK_READ)) { 645 owner = (struct thread *)RW_OWNER(v); 646 if (TD_IS_RUNNING(owner)) { 647 turnstile_cancel(ts); 648 cpu_spinwait(); 649 continue; 650 } 651 } 652#endif 653 /* 654 * Check for the waiters flags about this rwlock. 655 * If the lock was released, without maintain any pending 656 * waiters queue, simply try to acquire it. 657 * If a pending waiters queue is present, claim the lock 658 * ownership and maintain the pending queue. 659 */ 660 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 661 if ((v & ~x) == RW_UNLOCKED) { 662 x &= ~RW_LOCK_WRITE_SPINNER; 663 if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) { 664 if (x) 665 turnstile_claim(ts); 666 else 667 turnstile_cancel(ts); 668 break; 669 } 670 turnstile_cancel(ts); 671 cpu_spinwait(); 672 continue; 673 } 674 /* 675 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 676 * set it. If we fail to set it, then loop back and try 677 * again. 678 */ 679 if (!(v & RW_LOCK_WRITE_WAITERS)) { 680 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 681 v | RW_LOCK_WRITE_WAITERS)) { 682 turnstile_cancel(ts); 683 cpu_spinwait(); 684 continue; 685 } 686 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 687 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 688 __func__, rw); 689 } 690 /* 691 * We were unable to acquire the lock and the write waiters 692 * flag is set, so we must block on the turnstile. 693 */ 694 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 695 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 696 rw); 697 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE); 698 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 699 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 700 __func__, rw); 701#ifdef ADAPTIVE_RWLOCKS 702 spintries = 0; 703#endif 704 } 705 lock_profile_obtain_lock_success(&rw->lock_object, contested, waittime, 706 file, line); 707} 708 709/* 710 * This function is called if the first try at releasing a write lock failed. 711 * This means that one of the 2 waiter bits must be set indicating that at 712 * least one thread is waiting on this lock. 713 */ 714void 715_rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 716{ 717 struct turnstile *ts; 718 uintptr_t v; 719 int queue; 720 721 if (rw_wlocked(rw) && rw_recursed(rw)) { 722 rw->rw_recurse--; 723 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 724 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 725 return; 726 } 727 v = rw->rw_lock; 728 729 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 730 ("%s: neither of the waiter flags are set", __func__)); 731 732 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 733 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 734 735 turnstile_chain_lock(&rw->lock_object); 736 ts = turnstile_lookup(&rw->lock_object); 737 MPASS(ts != NULL); 738 739 /* 740 * Use the same algo as sx locks for now. Prefer waking up shared 741 * waiters if we have any over writers. This is probably not ideal. 742 * 743 * 'v' is the value we are going to write back to rw_lock. If we 744 * have waiters on both queues, we need to preserve the state of 745 * the waiter flag for the queue we don't wake up. For now this is 746 * hardcoded for the algorithm mentioned above. 747 * 748 * In the case of both readers and writers waiting we wakeup the 749 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 750 * new writer comes in before a reader it will claim the lock up 751 * above. There is probably a potential priority inversion in 752 * there that could be worked around either by waking both queues 753 * of waiters or doing some complicated lock handoff gymnastics. 754 */ 755 v = RW_UNLOCKED; 756 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) { 757 queue = TS_EXCLUSIVE_QUEUE; 758 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS); 759 } else 760 queue = TS_SHARED_QUEUE; 761 762 /* Wake up all waiters for the specific queue. */ 763 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 764 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 765 queue == TS_SHARED_QUEUE ? "read" : "write"); 766 turnstile_broadcast(ts, queue); 767 atomic_store_rel_ptr(&rw->rw_lock, v); 768 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 769 turnstile_chain_unlock(&rw->lock_object); 770} 771 772/* 773 * Attempt to do a non-blocking upgrade from a read lock to a write 774 * lock. This will only succeed if this thread holds a single read 775 * lock. Returns true if the upgrade succeeded and false otherwise. 776 */ 777int 778_rw_try_upgrade(struct rwlock *rw, const char *file, int line) 779{ 780 uintptr_t v, x, tid; 781 struct turnstile *ts; 782 int success; 783 784 KASSERT(rw->rw_lock != RW_DESTROYED, 785 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 786 _rw_assert(rw, RA_RLOCKED, file, line); 787 788 /* 789 * Attempt to switch from one reader to a writer. If there 790 * are any write waiters, then we will have to lock the 791 * turnstile first to prevent races with another writer 792 * calling turnstile_wait() before we have claimed this 793 * turnstile. So, do the simple case of no waiters first. 794 */ 795 tid = (uintptr_t)curthread; 796 success = 0; 797 for (;;) { 798 v = rw->rw_lock; 799 if (RW_READERS(v) > 1) 800 break; 801 if (!(v & RW_LOCK_WAITERS)) { 802 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid); 803 if (!success) 804 continue; 805 break; 806 } 807 808 /* 809 * Ok, we think we have waiters, so lock the turnstile. 810 */ 811 ts = turnstile_trywait(&rw->lock_object); 812 v = rw->rw_lock; 813 if (RW_READERS(v) > 1) { 814 turnstile_cancel(ts); 815 break; 816 } 817 /* 818 * Try to switch from one reader to a writer again. This time 819 * we honor the current state of the waiters flags. 820 * If we obtain the lock with the flags set, then claim 821 * ownership of the turnstile. 822 */ 823 x = rw->rw_lock & RW_LOCK_WAITERS; 824 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x); 825 if (success) { 826 if (x) 827 turnstile_claim(ts); 828 else 829 turnstile_cancel(ts); 830 break; 831 } 832 turnstile_cancel(ts); 833 } 834 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 835 if (success) { 836 curthread->td_rw_rlocks--; 837 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 838 file, line); 839 } 840 return (success); 841} 842 843/* 844 * Downgrade a write lock into a single read lock. 845 */ 846void 847_rw_downgrade(struct rwlock *rw, const char *file, int line) 848{ 849 struct turnstile *ts; 850 uintptr_t tid, v; 851 int rwait, wwait; 852 853 KASSERT(rw->rw_lock != RW_DESTROYED, 854 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 855 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line); 856#ifndef INVARIANTS 857 if (rw_recursed(rw)) 858 panic("downgrade of a recursed lock"); 859#endif 860 861 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 862 863 /* 864 * Convert from a writer to a single reader. First we handle 865 * the easy case with no waiters. If there are any waiters, we 866 * lock the turnstile and "disown" the lock. 867 */ 868 tid = (uintptr_t)curthread; 869 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 870 goto out; 871 872 /* 873 * Ok, we think we have waiters, so lock the turnstile so we can 874 * read the waiter flags without any races. 875 */ 876 turnstile_chain_lock(&rw->lock_object); 877 v = rw->rw_lock & RW_LOCK_WAITERS; 878 rwait = v & RW_LOCK_READ_WAITERS; 879 wwait = v & RW_LOCK_WRITE_WAITERS; 880 MPASS(rwait | wwait); 881 882 /* 883 * Downgrade from a write lock while preserving waiters flag 884 * and give up ownership of the turnstile. 885 */ 886 ts = turnstile_lookup(&rw->lock_object); 887 MPASS(ts != NULL); 888 if (!wwait) 889 v &= ~RW_LOCK_READ_WAITERS; 890 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v); 891 /* 892 * Wake other readers if there are no writers pending. Otherwise they 893 * won't be able to acquire the lock anyway. 894 */ 895 if (rwait && !wwait) { 896 turnstile_broadcast(ts, TS_SHARED_QUEUE); 897 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 898 } else 899 turnstile_disown(ts); 900 turnstile_chain_unlock(&rw->lock_object); 901out: 902 curthread->td_rw_rlocks++; 903 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 904} 905 906#ifdef INVARIANT_SUPPORT 907#ifndef INVARIANTS 908#undef _rw_assert 909#endif 910 911/* 912 * In the non-WITNESS case, rw_assert() can only detect that at least 913 * *some* thread owns an rlock, but it cannot guarantee that *this* 914 * thread owns an rlock. 915 */ 916void 917_rw_assert(struct rwlock *rw, int what, const char *file, int line) 918{ 919 920 if (panicstr != NULL) 921 return; 922 switch (what) { 923 case RA_LOCKED: 924 case RA_LOCKED | RA_RECURSED: 925 case RA_LOCKED | RA_NOTRECURSED: 926 case RA_RLOCKED: 927#ifdef WITNESS 928 witness_assert(&rw->lock_object, what, file, line); 929#else 930 /* 931 * If some other thread has a write lock or we have one 932 * and are asserting a read lock, fail. Also, if no one 933 * has a lock at all, fail. 934 */ 935 if (rw->rw_lock == RW_UNLOCKED || 936 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED || 937 rw_wowner(rw) != curthread))) 938 panic("Lock %s not %slocked @ %s:%d\n", 939 rw->lock_object.lo_name, (what == RA_RLOCKED) ? 940 "read " : "", file, line); 941 942 if (!(rw->rw_lock & RW_LOCK_READ)) { 943 if (rw_recursed(rw)) { 944 if (what & RA_NOTRECURSED) 945 panic("Lock %s recursed @ %s:%d\n", 946 rw->lock_object.lo_name, file, 947 line); 948 } else if (what & RA_RECURSED) 949 panic("Lock %s not recursed @ %s:%d\n", 950 rw->lock_object.lo_name, file, line); 951 } 952#endif 953 break; 954 case RA_WLOCKED: 955 case RA_WLOCKED | RA_RECURSED: 956 case RA_WLOCKED | RA_NOTRECURSED: 957 if (rw_wowner(rw) != curthread) 958 panic("Lock %s not exclusively locked @ %s:%d\n", 959 rw->lock_object.lo_name, file, line); 960 if (rw_recursed(rw)) { 961 if (what & RA_NOTRECURSED) 962 panic("Lock %s recursed @ %s:%d\n", 963 rw->lock_object.lo_name, file, line); 964 } else if (what & RA_RECURSED) 965 panic("Lock %s not recursed @ %s:%d\n", 966 rw->lock_object.lo_name, file, line); 967 break; 968 case RA_UNLOCKED: 969#ifdef WITNESS 970 witness_assert(&rw->lock_object, what, file, line); 971#else 972 /* 973 * If we hold a write lock fail. We can't reliably check 974 * to see if we hold a read lock or not. 975 */ 976 if (rw_wowner(rw) == curthread) 977 panic("Lock %s exclusively locked @ %s:%d\n", 978 rw->lock_object.lo_name, file, line); 979#endif 980 break; 981 default: 982 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 983 line); 984 } 985} 986#endif /* INVARIANT_SUPPORT */ 987 988#ifdef DDB 989void 990db_show_rwlock(struct lock_object *lock) 991{ 992 struct rwlock *rw; 993 struct thread *td; 994 995 rw = (struct rwlock *)lock; 996 997 db_printf(" state: "); 998 if (rw->rw_lock == RW_UNLOCKED) 999 db_printf("UNLOCKED\n"); 1000 else if (rw->rw_lock == RW_DESTROYED) { 1001 db_printf("DESTROYED\n"); 1002 return; 1003 } else if (rw->rw_lock & RW_LOCK_READ) 1004 db_printf("RLOCK: %ju locks\n", 1005 (uintmax_t)(RW_READERS(rw->rw_lock))); 1006 else { 1007 td = rw_wowner(rw); 1008 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 1009 td->td_tid, td->td_proc->p_pid, td->td_name); 1010 if (rw_recursed(rw)) 1011 db_printf(" recursed: %u\n", rw->rw_recurse); 1012 } 1013 db_printf(" waiters: "); 1014 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 1015 case RW_LOCK_READ_WAITERS: 1016 db_printf("readers\n"); 1017 break; 1018 case RW_LOCK_WRITE_WAITERS: 1019 db_printf("writers\n"); 1020 break; 1021 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 1022 db_printf("readers and writers\n"); 1023 break; 1024 default: 1025 db_printf("none\n"); 1026 break; 1027 } 1028} 1029 1030#endif 1031