kern_rwlock.c revision 167365
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 167365 2007-03-09 16:04:44Z jhb $"); 36 37#include "opt_ddb.h" 38 39#include <sys/param.h> 40#include <sys/ktr.h> 41#include <sys/lock.h> 42#include <sys/mutex.h> 43#include <sys/proc.h> 44#include <sys/rwlock.h> 45#include <sys/systm.h> 46#include <sys/turnstile.h> 47#include <sys/lock_profile.h> 48#include <machine/cpu.h> 49 50#ifdef DDB 51#include <ddb/ddb.h> 52 53static void db_show_rwlock(struct lock_object *lock); 54#endif 55 56struct lock_class lock_class_rw = { 57 .lc_name = "rw", 58 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 59#ifdef DDB 60 .lc_ddb_show = db_show_rwlock, 61#endif 62}; 63 64/* 65 * Return a pointer to the owning thread if the lock is write-locked or 66 * NULL if the lock is unlocked or read-locked. 67 */ 68#define rw_wowner(rw) \ 69 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \ 70 (struct thread *)RW_OWNER((rw)->rw_lock)) 71 72/* 73 * Return a pointer to the owning thread for this lock who should receive 74 * any priority lent by threads that block on this lock. Currently this 75 * is identical to rw_wowner(). 76 */ 77#define rw_owner(rw) rw_wowner(rw) 78 79#ifndef INVARIANTS 80#define _rw_assert(rw, what, file, line) 81#endif 82 83void 84rw_init(struct rwlock *rw, const char *name) 85{ 86 87 rw->rw_lock = RW_UNLOCKED; 88 89 lock_profile_object_init(&rw->rw_object, &lock_class_rw, name); 90 lock_init(&rw->rw_object, &lock_class_rw, name, NULL, LO_WITNESS | 91 LO_RECURSABLE | LO_UPGRADABLE); 92} 93 94void 95rw_destroy(struct rwlock *rw) 96{ 97 98 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked")); 99 lock_profile_object_destroy(&rw->rw_object); 100 lock_destroy(&rw->rw_object); 101} 102 103void 104rw_sysinit(void *arg) 105{ 106 struct rw_args *args = arg; 107 108 rw_init(args->ra_rw, args->ra_desc); 109} 110 111int 112rw_wowned(struct rwlock *rw) 113{ 114 115 return (rw_wowner(rw) == curthread); 116} 117 118void 119_rw_wlock(struct rwlock *rw, const char *file, int line) 120{ 121 122 MPASS(curthread != NULL); 123 KASSERT(rw_wowner(rw) != curthread, 124 ("%s (%s): wlock already held @ %s:%d", __func__, 125 rw->rw_object.lo_name, file, line)); 126 WITNESS_CHECKORDER(&rw->rw_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 127 line); 128 __rw_wlock(rw, curthread, file, line); 129 LOCK_LOG_LOCK("WLOCK", &rw->rw_object, 0, 0, file, line); 130 WITNESS_LOCK(&rw->rw_object, LOP_EXCLUSIVE, file, line); 131 curthread->td_locks++; 132} 133 134void 135_rw_wunlock(struct rwlock *rw, const char *file, int line) 136{ 137 138 MPASS(curthread != NULL); 139 _rw_assert(rw, RA_WLOCKED, file, line); 140 curthread->td_locks--; 141 WITNESS_UNLOCK(&rw->rw_object, LOP_EXCLUSIVE, file, line); 142 LOCK_LOG_LOCK("WUNLOCK", &rw->rw_object, 0, 0, file, line); 143 lock_profile_release_lock(&rw->rw_object); 144 __rw_wunlock(rw, curthread, file, line); 145} 146 147void 148_rw_rlock(struct rwlock *rw, const char *file, int line) 149{ 150#ifdef SMP 151 volatile struct thread *owner; 152#endif 153 uint64_t waittime = 0; 154 int contested = 0; 155 uintptr_t x; 156 157 KASSERT(rw_wowner(rw) != curthread, 158 ("%s (%s): wlock already held @ %s:%d", __func__, 159 rw->rw_object.lo_name, file, line)); 160 WITNESS_CHECKORDER(&rw->rw_object, LOP_NEWORDER, file, line); 161 162 /* 163 * Note that we don't make any attempt to try to block read 164 * locks once a writer has blocked on the lock. The reason is 165 * that we currently allow for read locks to recurse and we 166 * don't keep track of all the holders of read locks. Thus, if 167 * we were to block readers once a writer blocked and a reader 168 * tried to recurse on their reader lock after a writer had 169 * blocked we would end up in a deadlock since the reader would 170 * be blocked on the writer, and the writer would be blocked 171 * waiting for the reader to release its original read lock. 172 */ 173 for (;;) { 174 /* 175 * Handle the easy case. If no other thread has a write 176 * lock, then try to bump up the count of read locks. Note 177 * that we have to preserve the current state of the 178 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 179 * read lock, then rw_lock must have changed, so restart 180 * the loop. Note that this handles the case of a 181 * completely unlocked rwlock since such a lock is encoded 182 * as a read lock with no waiters. 183 */ 184 x = rw->rw_lock; 185 if (x & RW_LOCK_READ) { 186 187 /* 188 * The RW_LOCK_READ_WAITERS flag should only be set 189 * if another thread currently holds a write lock, 190 * and in that case RW_LOCK_READ should be clear. 191 */ 192 MPASS((x & RW_LOCK_READ_WAITERS) == 0); 193 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, 194 x + RW_ONE_READER)) { 195 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 196 CTR4(KTR_LOCK, 197 "%s: %p succeed %p -> %p", __func__, 198 rw, (void *)x, 199 (void *)(x + RW_ONE_READER)); 200 if (RW_READERS(x) == 0) 201 lock_profile_obtain_lock_success( 202 &rw->rw_object, contested, waittime, 203 file, line); 204 break; 205 } 206 cpu_spinwait(); 207 continue; 208 } 209 lock_profile_obtain_lock_failed(&rw->rw_object, &contested, 210 &waittime); 211 212 /* 213 * Okay, now it's the hard case. Some other thread already 214 * has a write lock, so acquire the turnstile lock so we can 215 * begin the process of blocking. 216 */ 217 turnstile_lock(&rw->rw_object); 218 219 /* 220 * The lock might have been released while we spun, so 221 * recheck its state and restart the loop if there is no 222 * longer a write lock. 223 */ 224 x = rw->rw_lock; 225 if (x & RW_LOCK_READ) { 226 turnstile_release(&rw->rw_object); 227 cpu_spinwait(); 228 continue; 229 } 230 231 /* 232 * Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS 233 * flag is already set, then we can go ahead and block. If 234 * it is not set then try to set it. If we fail to set it 235 * drop the turnstile lock and restart the loop. 236 */ 237 if (!(x & RW_LOCK_READ_WAITERS)) { 238 if (!atomic_cmpset_ptr(&rw->rw_lock, x, 239 x | RW_LOCK_READ_WAITERS)) { 240 turnstile_release(&rw->rw_object); 241 cpu_spinwait(); 242 continue; 243 } 244 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 245 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 246 __func__, rw); 247 } 248 249#ifdef SMP 250 /* 251 * If the owner is running on another CPU, spin until 252 * the owner stops running or the state of the lock 253 * changes. 254 */ 255 owner = (struct thread *)RW_OWNER(x); 256 if (TD_IS_RUNNING(owner)) { 257 turnstile_release(&rw->rw_object); 258 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 259 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 260 __func__, rw, owner); 261 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 262 TD_IS_RUNNING(owner)) 263 cpu_spinwait(); 264 continue; 265 } 266#endif 267 268 /* 269 * We were unable to acquire the lock and the read waiters 270 * flag is set, so we must block on the turnstile. 271 */ 272 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 273 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 274 rw); 275 turnstile_wait(&rw->rw_object, rw_owner(rw), TS_SHARED_QUEUE); 276 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 277 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 278 __func__, rw); 279 } 280 281 /* 282 * TODO: acquire "owner of record" here. Here be turnstile dragons 283 * however. turnstiles don't like owners changing between calls to 284 * turnstile_wait() currently. 285 */ 286 287 LOCK_LOG_LOCK("RLOCK", &rw->rw_object, 0, 0, file, line); 288 WITNESS_LOCK(&rw->rw_object, 0, file, line); 289 curthread->td_locks++; 290} 291 292void 293_rw_runlock(struct rwlock *rw, const char *file, int line) 294{ 295 struct turnstile *ts; 296 uintptr_t x; 297 298 _rw_assert(rw, RA_RLOCKED, file, line); 299 curthread->td_locks--; 300 WITNESS_UNLOCK(&rw->rw_object, 0, file, line); 301 LOCK_LOG_LOCK("RUNLOCK", &rw->rw_object, 0, 0, file, line); 302 303 /* TODO: drop "owner of record" here. */ 304 305 for (;;) { 306 /* 307 * See if there is more than one read lock held. If so, 308 * just drop one and return. 309 */ 310 x = rw->rw_lock; 311 if (RW_READERS(x) > 1) { 312 if (atomic_cmpset_ptr(&rw->rw_lock, x, 313 x - RW_ONE_READER)) { 314 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 315 CTR4(KTR_LOCK, 316 "%s: %p succeeded %p -> %p", 317 __func__, rw, (void *)x, 318 (void *)(x - RW_ONE_READER)); 319 break; 320 } 321 continue; 322 } 323 324 325 /* 326 * We should never have read waiters while at least one 327 * thread holds a read lock. (See note above) 328 */ 329 KASSERT(!(x & RW_LOCK_READ_WAITERS), 330 ("%s: waiting readers", __func__)); 331 332 /* 333 * If there aren't any waiters for a write lock, then try 334 * to drop it quickly. 335 */ 336 if (!(x & RW_LOCK_WRITE_WAITERS)) { 337 338 /* 339 * There shouldn't be any flags set and we should 340 * be the only read lock. If we fail to release 341 * the single read lock, then another thread might 342 * have just acquired a read lock, so go back up 343 * to the multiple read locks case. 344 */ 345 MPASS(x == RW_READERS_LOCK(1)); 346 if (atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 347 RW_UNLOCKED)) { 348 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 349 CTR2(KTR_LOCK, "%s: %p last succeeded", 350 __func__, rw); 351 break; 352 } 353 continue; 354 } 355 356 /* 357 * There should just be one reader with one or more 358 * writers waiting. 359 */ 360 MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS)); 361 362 /* 363 * Ok, we know we have a waiting writer and we think we 364 * are the last reader, so grab the turnstile lock. 365 */ 366 turnstile_lock(&rw->rw_object); 367 368 /* 369 * Try to drop our lock leaving the lock in a unlocked 370 * state. 371 * 372 * If you wanted to do explicit lock handoff you'd have to 373 * do it here. You'd also want to use turnstile_signal() 374 * and you'd have to handle the race where a higher 375 * priority thread blocks on the write lock before the 376 * thread you wakeup actually runs and have the new thread 377 * "steal" the lock. For now it's a lot simpler to just 378 * wakeup all of the waiters. 379 * 380 * As above, if we fail, then another thread might have 381 * acquired a read lock, so drop the turnstile lock and 382 * restart. 383 */ 384 if (!atomic_cmpset_ptr(&rw->rw_lock, 385 RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) { 386 turnstile_release(&rw->rw_object); 387 continue; 388 } 389 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 390 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 391 __func__, rw); 392 393 /* 394 * Ok. The lock is released and all that's left is to 395 * wake up the waiters. Note that the lock might not be 396 * free anymore, but in that case the writers will just 397 * block again if they run before the new lock holder(s) 398 * release the lock. 399 */ 400 ts = turnstile_lookup(&rw->rw_object); 401 MPASS(ts != NULL); 402 turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE); 403 turnstile_unpend(ts, TS_SHARED_LOCK); 404 break; 405 } 406 lock_profile_release_lock(&rw->rw_object); 407} 408 409/* 410 * This function is called when we are unable to obtain a write lock on the 411 * first try. This means that at least one other thread holds either a 412 * read or write lock. 413 */ 414void 415_rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 416{ 417#ifdef SMP 418 volatile struct thread *owner; 419#endif 420 uintptr_t v; 421 422 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 423 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 424 rw->rw_object.lo_name, (void *)rw->rw_lock, file, line); 425 426 while (!_rw_write_lock(rw, tid)) { 427 turnstile_lock(&rw->rw_object); 428 v = rw->rw_lock; 429 430 /* 431 * If the lock was released while spinning on the 432 * turnstile chain lock, try again. 433 */ 434 if (v == RW_UNLOCKED) { 435 turnstile_release(&rw->rw_object); 436 cpu_spinwait(); 437 continue; 438 } 439 440 /* 441 * If the lock was released by a writer with both readers 442 * and writers waiting and a reader hasn't woken up and 443 * acquired the lock yet, rw_lock will be set to the 444 * value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see 445 * that value, try to acquire it once. Note that we have 446 * to preserve the RW_LOCK_WRITE_WAITERS flag as there are 447 * other writers waiting still. If we fail, restart the 448 * loop. 449 */ 450 if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) { 451 if (atomic_cmpset_acq_ptr(&rw->rw_lock, 452 RW_UNLOCKED | RW_LOCK_WRITE_WAITERS, 453 tid | RW_LOCK_WRITE_WAITERS)) { 454 turnstile_claim(&rw->rw_object); 455 CTR2(KTR_LOCK, "%s: %p claimed by new writer", 456 __func__, rw); 457 break; 458 } 459 turnstile_release(&rw->rw_object); 460 cpu_spinwait(); 461 continue; 462 } 463 464 /* 465 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 466 * set it. If we fail to set it, then loop back and try 467 * again. 468 */ 469 if (!(v & RW_LOCK_WRITE_WAITERS)) { 470 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 471 v | RW_LOCK_WRITE_WAITERS)) { 472 turnstile_release(&rw->rw_object); 473 cpu_spinwait(); 474 continue; 475 } 476 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 477 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 478 __func__, rw); 479 } 480 481#ifdef SMP 482 /* 483 * If the lock is write locked and the owner is 484 * running on another CPU, spin until the owner stops 485 * running or the state of the lock changes. 486 */ 487 owner = (struct thread *)RW_OWNER(v); 488 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 489 turnstile_release(&rw->rw_object); 490 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 491 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 492 __func__, rw, owner); 493 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 494 TD_IS_RUNNING(owner)) 495 cpu_spinwait(); 496 continue; 497 } 498#endif 499 500 /* 501 * We were unable to acquire the lock and the write waiters 502 * flag is set, so we must block on the turnstile. 503 */ 504 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 505 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 506 rw); 507 turnstile_wait(&rw->rw_object, rw_owner(rw), 508 TS_EXCLUSIVE_QUEUE); 509 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 510 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 511 __func__, rw); 512 } 513} 514 515/* 516 * This function is called if the first try at releasing a write lock failed. 517 * This means that one of the 2 waiter bits must be set indicating that at 518 * least one thread is waiting on this lock. 519 */ 520void 521_rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 522{ 523 struct turnstile *ts; 524 uintptr_t v; 525 int queue; 526 527 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 528 ("%s: neither of the waiter flags are set", __func__)); 529 530 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 531 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 532 533 turnstile_lock(&rw->rw_object); 534 ts = turnstile_lookup(&rw->rw_object); 535 536#ifdef SMP 537 /* 538 * There might not be a turnstile for this lock if all of 539 * the waiters are adaptively spinning. In that case, just 540 * reset the lock to the unlocked state and return. 541 */ 542 if (ts == NULL) { 543 atomic_store_rel_ptr(&rw->rw_lock, RW_UNLOCKED); 544 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 545 CTR2(KTR_LOCK, "%s: %p no sleepers", __func__, rw); 546 turnstile_release(&rw->rw_object); 547 return; 548 } 549#else 550 MPASS(ts != NULL); 551#endif 552 553 /* 554 * Use the same algo as sx locks for now. Prefer waking up shared 555 * waiters if we have any over writers. This is probably not ideal. 556 * 557 * 'v' is the value we are going to write back to rw_lock. If we 558 * have waiters on both queues, we need to preserve the state of 559 * the waiter flag for the queue we don't wake up. For now this is 560 * hardcoded for the algorithm mentioned above. 561 * 562 * In the case of both readers and writers waiting we wakeup the 563 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 564 * new writer comes in before a reader it will claim the lock up 565 * above. There is probably a potential priority inversion in 566 * there that could be worked around either by waking both queues 567 * of waiters or doing some complicated lock handoff gymnastics. 568 * 569 * Note that in the SMP case, if both flags are set, there might 570 * not be any actual writers on the turnstile as they might all 571 * be spinning. In that case, we don't want to preserve the 572 * RW_LOCK_WRITE_WAITERS flag as the turnstile is going to go 573 * away once we wakeup all the readers. 574 */ 575 v = RW_UNLOCKED; 576 if (rw->rw_lock & RW_LOCK_READ_WAITERS) { 577 queue = TS_SHARED_QUEUE; 578#ifdef SMP 579 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS && 580 !turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 581 v |= RW_LOCK_WRITE_WAITERS; 582#else 583 v |= (rw->rw_lock & RW_LOCK_WRITE_WAITERS); 584#endif 585 } else 586 queue = TS_EXCLUSIVE_QUEUE; 587 588#ifdef SMP 589 /* 590 * We have to make sure that we actually have waiters to 591 * wakeup. If they are all spinning, then we just need to 592 * disown the turnstile and return. 593 */ 594 if (turnstile_empty(ts, queue)) { 595 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 596 CTR2(KTR_LOCK, "%s: %p no sleepers 2", __func__, rw); 597 atomic_store_rel_ptr(&rw->rw_lock, v); 598 turnstile_disown(ts); 599 return; 600 } 601#endif 602 603 /* Wake up all waiters for the specific queue. */ 604 if (LOCK_LOG_TEST(&rw->rw_object, 0)) 605 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 606 queue == TS_SHARED_QUEUE ? "read" : "write"); 607 turnstile_broadcast(ts, queue); 608 atomic_store_rel_ptr(&rw->rw_lock, v); 609 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 610} 611 612/* 613 * Attempt to do a non-blocking upgrade from a read lock to a write 614 * lock. This will only succeed if this thread holds a single read 615 * lock. Returns true if the upgrade succeeded and false otherwise. 616 */ 617int 618_rw_try_upgrade(struct rwlock *rw, const char *file, int line) 619{ 620 uintptr_t v, tid; 621 int success; 622 623 _rw_assert(rw, RA_RLOCKED, file, line); 624 625 /* 626 * Attempt to switch from one reader to a writer. If there 627 * are any write waiters, then we will have to lock the 628 * turnstile first to prevent races with another writer 629 * calling turnstile_wait() before we have claimed this 630 * turnstile. So, do the simple case of no waiters first. 631 */ 632 tid = (uintptr_t)curthread; 633 if (!(rw->rw_lock & RW_LOCK_WRITE_WAITERS)) { 634 success = atomic_cmpset_acq_ptr(&rw->rw_lock, 635 RW_READERS_LOCK(1), tid); 636 goto out; 637 } 638 639 /* 640 * Ok, we think we have write waiters, so lock the 641 * turnstile. 642 */ 643 turnstile_lock(&rw->rw_object); 644 645 /* 646 * Try to switch from one reader to a writer again. This time 647 * we honor the current state of the RW_LOCK_WRITE_WAITERS 648 * flag. If we obtain the lock with the flag set, then claim 649 * ownership of the turnstile. In the SMP case it is possible 650 * for there to not be an associated turnstile even though there 651 * are waiters if all of the waiters are spinning. 652 */ 653 v = rw->rw_lock & RW_LOCK_WRITE_WAITERS; 654 success = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 655 tid | v); 656#ifdef SMP 657 if (success && v && turnstile_lookup(&rw->rw_object) != NULL) 658#else 659 if (success && v) 660#endif 661 turnstile_claim(&rw->rw_object); 662 else 663 turnstile_release(&rw->rw_object); 664out: 665 LOCK_LOG_TRY("WUPGRADE", &rw->rw_object, 0, success, file, line); 666 if (success) 667 WITNESS_UPGRADE(&rw->rw_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 668 file, line); 669 return (success); 670} 671 672/* 673 * Downgrade a write lock into a single read lock. 674 */ 675void 676_rw_downgrade(struct rwlock *rw, const char *file, int line) 677{ 678 struct turnstile *ts; 679 uintptr_t tid, v; 680 681 _rw_assert(rw, RA_WLOCKED, file, line); 682 683 WITNESS_DOWNGRADE(&rw->rw_object, 0, file, line); 684 685 /* 686 * Convert from a writer to a single reader. First we handle 687 * the easy case with no waiters. If there are any waiters, we 688 * lock the turnstile, "disown" the lock, and awaken any read 689 * waiters. 690 */ 691 tid = (uintptr_t)curthread; 692 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 693 goto out; 694 695 /* 696 * Ok, we think we have waiters, so lock the turnstile so we can 697 * read the waiter flags without any races. 698 */ 699 turnstile_lock(&rw->rw_object); 700 v = rw->rw_lock; 701 MPASS(v & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)); 702 703 /* 704 * Downgrade from a write lock while preserving 705 * RW_LOCK_WRITE_WAITERS and give up ownership of the 706 * turnstile. If there are any read waiters, wake them up. 707 * 708 * For SMP, we have to allow for the fact that all of the 709 * read waiters might be spinning. In that case, act as if 710 * RW_LOCK_READ_WAITERS is not set. Also, only preserve 711 * the RW_LOCK_WRITE_WAITERS flag if at least one writer is 712 * blocked on the turnstile. 713 */ 714 ts = turnstile_lookup(&rw->rw_object); 715#ifdef SMP 716 if (ts == NULL) 717 v &= ~(RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS); 718 else if (v & RW_LOCK_READ_WAITERS && 719 turnstile_empty(ts, TS_SHARED_QUEUE)) 720 v &= ~RW_LOCK_READ_WAITERS; 721 else if (v & RW_LOCK_WRITE_WAITERS && 722 turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 723 v &= ~RW_LOCK_WRITE_WAITERS; 724#else 725 MPASS(ts != NULL); 726#endif 727 if (v & RW_LOCK_READ_WAITERS) 728 turnstile_broadcast(ts, TS_SHARED_QUEUE); 729 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | 730 (v & RW_LOCK_WRITE_WAITERS)); 731 if (v & RW_LOCK_READ_WAITERS) 732 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 733#ifdef SMP 734 else if (ts == NULL) 735 turnstile_release(&rw->rw_object); 736#endif 737 else 738 turnstile_disown(ts); 739out: 740 LOCK_LOG_LOCK("WDOWNGRADE", &rw->rw_object, 0, 0, file, line); 741} 742 743#ifdef INVARIANT_SUPPORT 744#ifndef INVARIANTS 745#undef _rw_assert 746#endif 747 748/* 749 * In the non-WITNESS case, rw_assert() can only detect that at least 750 * *some* thread owns an rlock, but it cannot guarantee that *this* 751 * thread owns an rlock. 752 */ 753void 754_rw_assert(struct rwlock *rw, int what, const char *file, int line) 755{ 756 757 if (panicstr != NULL) 758 return; 759 switch (what) { 760 case RA_LOCKED: 761 case RA_RLOCKED: 762#ifdef WITNESS 763 witness_assert(&rw->rw_object, what, file, line); 764#else 765 /* 766 * If some other thread has a write lock or we have one 767 * and are asserting a read lock, fail. Also, if no one 768 * has a lock at all, fail. 769 */ 770 if (rw->rw_lock == RW_UNLOCKED || 771 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED || 772 rw_wowner(rw) != curthread))) 773 panic("Lock %s not %slocked @ %s:%d\n", 774 rw->rw_object.lo_name, (what == RA_RLOCKED) ? 775 "read " : "", file, line); 776#endif 777 break; 778 case RA_WLOCKED: 779 if (rw_wowner(rw) != curthread) 780 panic("Lock %s not exclusively locked @ %s:%d\n", 781 rw->rw_object.lo_name, file, line); 782 break; 783 case RA_UNLOCKED: 784#ifdef WITNESS 785 witness_assert(&rw->rw_object, what, file, line); 786#else 787 /* 788 * If we hold a write lock fail. We can't reliably check 789 * to see if we hold a read lock or not. 790 */ 791 if (rw_wowner(rw) == curthread) 792 panic("Lock %s exclusively locked @ %s:%d\n", 793 rw->rw_object.lo_name, file, line); 794#endif 795 break; 796 default: 797 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 798 line); 799 } 800} 801#endif /* INVARIANT_SUPPORT */ 802 803#ifdef DDB 804void 805db_show_rwlock(struct lock_object *lock) 806{ 807 struct rwlock *rw; 808 struct thread *td; 809 810 rw = (struct rwlock *)lock; 811 812 db_printf(" state: "); 813 if (rw->rw_lock == RW_UNLOCKED) 814 db_printf("UNLOCKED\n"); 815 else if (rw->rw_lock & RW_LOCK_READ) 816 db_printf("RLOCK: %jd locks\n", 817 (intmax_t)(RW_READERS(rw->rw_lock))); 818 else { 819 td = rw_wowner(rw); 820 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 821 td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm); 822 } 823 db_printf(" waiters: "); 824 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 825 case RW_LOCK_READ_WAITERS: 826 db_printf("readers\n"); 827 break; 828 case RW_LOCK_WRITE_WAITERS: 829 db_printf("writers\n"); 830 break; 831 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 832 db_printf("readers and waiters\n"); 833 break; 834 default: 835 db_printf("none\n"); 836 break; 837 } 838} 839 840#endif 841