Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)
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sys/kern/kern_rwlock.c
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1 /*- 2 * Copyright (c) 2006 John Baldwin <jhb@FreeBSD.org> 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * 1. Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * 2. Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in the 12 * documentation and/or other materials provided with the distribution. 13 * 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: releng/6.4/sys/kern/kern_rwlock.c 172355 2007-09-27 20:24:56Z 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/lock.h> 43 #include <sys/mutex.h> 44 #include <sys/proc.h> 45 #include <sys/rwlock.h> 46 #include <sys/systm.h> 47 #include <sys/turnstile.h> 48 49 #include <machine/cpu.h> 50 51 CTASSERT((RW_RECURSE & LO_CLASSFLAGS) == RW_RECURSE); 52 53 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS) 54 #define ADAPTIVE_RWLOCKS 55 #endif 56 57 #ifdef DDB 58 #include <ddb/ddb.h> 59 60 static void db_show_rwlock(struct lock_object *lock); 61 #endif 62 63 struct lock_class lock_class_rw = { 64 .lc_name = "rw", 65 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 66 #ifdef DDB 67 .lc_ddb_show = db_show_rwlock, 68 #endif 69 }; 70 71 /* 72 * Return a pointer to the owning thread if the lock is write-locked or 73 * NULL if the lock is unlocked or read-locked. 74 */ 75 #define rw_wowner(rw) \ 76 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \ 77 (struct thread *)RW_OWNER((rw)->rw_lock)) 78 79 /* 80 * Returns if a write owner is recursed. Write ownership is not assured 81 * here and should be previously checked. 82 */ 83 #define rw_recursed(rw) ((rw)->rw_recurse != 0) 84 85 /* 86 * Return true if curthread helds the lock. 87 */ 88 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 89 90 /* 91 * Return a pointer to the owning thread for this lock who should receive 92 * any priority lent by threads that block on this lock. Currently this 93 * is identical to rw_wowner(). 94 */ 95 #define rw_owner(rw) rw_wowner(rw) 96 97 #ifndef INVARIANTS 98 #define _rw_assert(rw, what, file, line) 99 #endif 100 101 void 102 rw_init_flags(struct rwlock *rw, const char *name, int opts) 103 { 104 int flags; 105 106 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 107 RW_RECURSE)) == 0); 108 109 flags = LO_UPGRADABLE | LO_RECURSABLE; 110 if (opts & RW_DUPOK) 111 flags |= LO_DUPOK; 112 if (!(opts & RW_NOWITNESS)) 113 flags |= LO_WITNESS; 114 if (opts & RW_QUIET) 115 flags |= LO_QUIET; 116 flags |= opts & RW_RECURSE; 117 118 rw->rw_lock = RW_UNLOCKED; 119 rw->rw_recurse = 0; 120 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 121 } 122 123 void 124 rw_destroy(struct rwlock *rw) 125 { 126 127 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked")); 128 KASSERT(rw->rw_recurse == 0, ("rw lock still recursed")); 129 rw->rw_lock = RW_DESTROYED; 130 lock_destroy(&rw->lock_object); 131 } 132 133 void 134 rw_sysinit(void *arg) 135 { 136 struct rw_args *args = arg; 137 138 rw_init(args->ra_rw, args->ra_desc); 139 } 140 141 int 142 rw_wowned(struct rwlock *rw) 143 { 144 145 return (rw_wowner(rw) == curthread); 146 } 147 148 void 149 _rw_wlock(struct rwlock *rw, const char *file, int line) 150 { 151 152 MPASS(curthread != NULL); 153 KASSERT(rw->rw_lock != RW_DESTROYED, 154 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 155 KASSERT(rw_wowner(rw) != curthread, 156 ("%s (%s): wlock already held @ %s:%d", __func__, 157 rw->lock_object.lo_name, file, line)); 158 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 159 line); 160 __rw_wlock(rw, curthread, file, line); 161 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 162 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 163 curthread->td_locks++; 164 } 165 166 void 167 _rw_wunlock(struct rwlock *rw, const char *file, int line) 168 { 169 170 MPASS(curthread != NULL); 171 KASSERT(rw->rw_lock != RW_DESTROYED, 172 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 173 _rw_assert(rw, RA_WLOCKED, file, line); 174 curthread->td_locks--; 175 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 176 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 177 line); 178 if (!rw_recursed(rw)) 179 lock_profile_release_lock(&rw->lock_object); 180 __rw_wunlock(rw, curthread, file, line); 181 } 182 183 void 184 _rw_rlock(struct rwlock *rw, const char *file, int line) 185 { 186 #ifdef ADAPTIVE_RWLOCKS 187 volatile struct thread *owner; 188 #endif 189 //uint64_t waittime = 0; /* XXX: notsup */ 190 //int contested = 0; /* XXX: notsup */ 191 uintptr_t x; 192 193 KASSERT(rw->rw_lock != RW_DESTROYED, 194 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 195 KASSERT(rw_wowner(rw) != curthread, 196 ("%s (%s): wlock already held @ %s:%d", __func__, 197 rw->lock_object.lo_name, file, line)); 198 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line); 199 200 /* 201 * Note that we don't make any attempt to try to block read 202 * locks once a writer has blocked on the lock. The reason is 203 * that we currently allow for read locks to recurse and we 204 * don't keep track of all the holders of read locks. Thus, if 205 * we were to block readers once a writer blocked and a reader 206 * tried to recurse on their reader lock after a writer had 207 * blocked we would end up in a deadlock since the reader would 208 * be blocked on the writer, and the writer would be blocked 209 * waiting for the reader to release its original read lock. 210 */ 211 for (;;) { 212 /* 213 * Handle the easy case. If no other thread has a write 214 * lock, then try to bump up the count of read locks. Note 215 * that we have to preserve the current state of the 216 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 217 * read lock, then rw_lock must have changed, so restart 218 * the loop. Note that this handles the case of a 219 * completely unlocked rwlock since such a lock is encoded 220 * as a read lock with no waiters. 221 */ 222 x = rw->rw_lock; 223 if (x & RW_LOCK_READ) { 224 225 /* 226 * The RW_LOCK_READ_WAITERS flag should only be set 227 * if another thread currently holds a write lock, 228 * and in that case RW_LOCK_READ should be clear. 229 */ 230 MPASS((x & RW_LOCK_READ_WAITERS) == 0); 231 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, 232 x + RW_ONE_READER)) { 233 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 234 CTR4(KTR_LOCK, 235 "%s: %p succeed %p -> %p", __func__, 236 rw, (void *)x, 237 (void *)(x + RW_ONE_READER)); 238 if (RW_READERS(x) == 0) 239 lock_profile_obtain_lock_success( 240 &rw->lock_object, contested, waittime, 241 file, line); 242 break; 243 } 244 cpu_spinwait(); 245 continue; 246 } 247 lock_profile_obtain_lock_failed(&rw->lock_object, &contested, 248 &waittime); 249 250 /* 251 * Okay, now it's the hard case. Some other thread already 252 * has a write lock, so acquire the turnstile lock so we can 253 * begin the process of blocking. 254 */ 255 turnstile_lock(&rw->lock_object); 256 257 /* 258 * The lock might have been released while we spun, so 259 * recheck its state and restart the loop if there is no 260 * longer a write lock. 261 */ 262 x = rw->rw_lock; 263 if (x & RW_LOCK_READ) { 264 turnstile_release(&rw->lock_object); 265 cpu_spinwait(); 266 continue; 267 } 268 269 /* 270 * Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS 271 * flag is already set, then we can go ahead and block. If 272 * it is not set then try to set it. If we fail to set it 273 * drop the turnstile lock and restart the loop. 274 */ 275 if (!(x & RW_LOCK_READ_WAITERS)) { 276 if (!atomic_cmpset_ptr(&rw->rw_lock, x, 277 x | RW_LOCK_READ_WAITERS)) { 278 turnstile_release(&rw->lock_object); 279 cpu_spinwait(); 280 continue; 281 } 282 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 283 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 284 __func__, rw); 285 } 286 287 #ifdef ADAPTIVE_RWLOCKS 288 /* 289 * If the owner is running on another CPU, spin until 290 * the owner stops running or the state of the lock 291 * changes. 292 */ 293 owner = (struct thread *)RW_OWNER(x); 294 if (TD_IS_RUNNING(owner)) { 295 turnstile_release(&rw->lock_object); 296 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 297 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 298 __func__, rw, owner); 299 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 300 TD_IS_RUNNING(owner)) 301 cpu_spinwait(); 302 continue; 303 } 304 #endif 305 306 /* 307 * We were unable to acquire the lock and the read waiters 308 * flag is set, so we must block on the turnstile. 309 */ 310 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 311 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 312 rw); 313 turnstile_wait(&rw->lock_object, rw_owner(rw), 314 TS_SHARED_QUEUE); 315 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 316 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 317 __func__, rw); 318 } 319 320 /* 321 * TODO: acquire "owner of record" here. Here be turnstile dragons 322 * however. turnstiles don't like owners changing between calls to 323 * turnstile_wait() currently. 324 */ 325 326 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 327 WITNESS_LOCK(&rw->lock_object, 0, file, line); 328 curthread->td_locks++; 329 } 330 331 void 332 _rw_runlock(struct rwlock *rw, const char *file, int line) 333 { 334 struct turnstile *ts; 335 uintptr_t x; 336 337 KASSERT(rw->rw_lock != RW_DESTROYED, 338 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 339 _rw_assert(rw, RA_RLOCKED, file, line); 340 curthread->td_locks--; 341 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 342 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 343 344 /* TODO: drop "owner of record" here. */ 345 346 for (;;) { 347 /* 348 * See if there is more than one read lock held. If so, 349 * just drop one and return. 350 */ 351 x = rw->rw_lock; 352 if (RW_READERS(x) > 1) { 353 if (atomic_cmpset_ptr(&rw->rw_lock, x, 354 x - RW_ONE_READER)) { 355 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 356 CTR4(KTR_LOCK, 357 "%s: %p succeeded %p -> %p", 358 __func__, rw, (void *)x, 359 (void *)(x - RW_ONE_READER)); 360 break; 361 } 362 continue; 363 } 364 365 366 /* 367 * We should never have read waiters while at least one 368 * thread holds a read lock. (See note above) 369 */ 370 KASSERT(!(x & RW_LOCK_READ_WAITERS), 371 ("%s: waiting readers", __func__)); 372 373 /* 374 * If there aren't any waiters for a write lock, then try 375 * to drop it quickly. 376 */ 377 if (!(x & RW_LOCK_WRITE_WAITERS)) { 378 379 /* 380 * There shouldn't be any flags set and we should 381 * be the only read lock. If we fail to release 382 * the single read lock, then another thread might 383 * have just acquired a read lock, so go back up 384 * to the multiple read locks case. 385 */ 386 MPASS(x == RW_READERS_LOCK(1)); 387 if (atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 388 RW_UNLOCKED)) { 389 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 390 CTR2(KTR_LOCK, "%s: %p last succeeded", 391 __func__, rw); 392 break; 393 } 394 continue; 395 } 396 397 /* 398 * There should just be one reader with one or more 399 * writers waiting. 400 */ 401 MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS)); 402 403 /* 404 * Ok, we know we have a waiting writer and we think we 405 * are the last reader, so grab the turnstile lock. 406 */ 407 turnstile_lock(&rw->lock_object); 408 409 /* 410 * Try to drop our lock leaving the lock in a unlocked 411 * state. 412 * 413 * If you wanted to do explicit lock handoff you'd have to 414 * do it here. You'd also want to use turnstile_signal() 415 * and you'd have to handle the race where a higher 416 * priority thread blocks on the write lock before the 417 * thread you wakeup actually runs and have the new thread 418 * "steal" the lock. For now it's a lot simpler to just 419 * wakeup all of the waiters. 420 * 421 * As above, if we fail, then another thread might have 422 * acquired a read lock, so drop the turnstile lock and 423 * restart. 424 */ 425 if (!atomic_cmpset_ptr(&rw->rw_lock, 426 RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) { 427 turnstile_release(&rw->lock_object); 428 continue; 429 } 430 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 431 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 432 __func__, rw); 433 434 /* 435 * Ok. The lock is released and all that's left is to 436 * wake up the waiters. Note that the lock might not be 437 * free anymore, but in that case the writers will just 438 * block again if they run before the new lock holder(s) 439 * release the lock. 440 */ 441 ts = turnstile_lookup(&rw->lock_object); 442 MPASS(ts != NULL); 443 turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE); 444 turnstile_unpend(ts, TS_SHARED_LOCK); 445 break; 446 } 447 lock_profile_release_lock(&rw->lock_object); 448 } 449 450 /* 451 * This function is called when we are unable to obtain a write lock on the 452 * first try. This means that at least one other thread holds either a 453 * read or write lock. 454 */ 455 void 456 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 457 { 458 #ifdef ADAPTIVE_RWLOCKS 459 volatile struct thread *owner; 460 #endif 461 uintptr_t v; 462 463 if (rw_wlocked(rw)) { 464 KASSERT(rw->lock_object.lo_flags & RW_RECURSE, 465 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 466 __func__, rw->lock_object.lo_name, file, line)); 467 rw->rw_recurse++; 468 atomic_set_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 469 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 470 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 471 return; 472 } 473 474 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 475 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 476 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 477 478 while (!_rw_write_lock(rw, tid)) { 479 turnstile_lock(&rw->lock_object); 480 v = rw->rw_lock; 481 482 /* 483 * If the lock was released while spinning on the 484 * turnstile chain lock, try again. 485 */ 486 if (v == RW_UNLOCKED) { 487 turnstile_release(&rw->lock_object); 488 cpu_spinwait(); 489 continue; 490 } 491 492 /* 493 * If the lock was released by a writer with both readers 494 * and writers waiting and a reader hasn't woken up and 495 * acquired the lock yet, rw_lock will be set to the 496 * value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see 497 * that value, try to acquire it once. Note that we have 498 * to preserve the RW_LOCK_WRITE_WAITERS flag as there are 499 * other writers waiting still. If we fail, restart the 500 * loop. 501 */ 502 if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) { 503 if (atomic_cmpset_acq_ptr(&rw->rw_lock, 504 RW_UNLOCKED | RW_LOCK_WRITE_WAITERS, 505 tid | RW_LOCK_WRITE_WAITERS)) { 506 turnstile_claim(&rw->lock_object); 507 CTR2(KTR_LOCK, "%s: %p claimed by new writer", 508 __func__, rw); 509 break; 510 } 511 turnstile_release(&rw->lock_object); 512 cpu_spinwait(); 513 continue; 514 } 515 516 /* 517 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 518 * set it. If we fail to set it, then loop back and try 519 * again. 520 */ 521 if (!(v & RW_LOCK_WRITE_WAITERS)) { 522 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 523 v | RW_LOCK_WRITE_WAITERS)) { 524 turnstile_release(&rw->lock_object); 525 cpu_spinwait(); 526 continue; 527 } 528 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 529 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 530 __func__, rw); 531 } 532 533 #ifdef ADAPTIVE_RWLOCKS 534 /* 535 * If the lock is write locked and the owner is 536 * running on another CPU, spin until the owner stops 537 * running or the state of the lock changes. 538 */ 539 owner = (struct thread *)RW_OWNER(v); 540 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 541 turnstile_release(&rw->lock_object); 542 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 543 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 544 __func__, rw, owner); 545 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 546 TD_IS_RUNNING(owner)) 547 cpu_spinwait(); 548 continue; 549 } 550 #endif 551 552 /* 553 * We were unable to acquire the lock and the write waiters 554 * flag is set, so we must block on the turnstile. 555 */ 556 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 557 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 558 rw); 559 turnstile_wait(&rw->lock_object, rw_owner(rw), 560 TS_EXCLUSIVE_QUEUE); 561 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 562 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 563 __func__, rw); 564 } 565 } 566 567 /* 568 * This function is called if the first try at releasing a write lock failed. 569 * This means that one of the 2 waiter bits must be set indicating that at 570 * least one thread is waiting on this lock. 571 */ 572 void 573 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 574 { 575 struct turnstile *ts; 576 uintptr_t v; 577 int queue; 578 579 if (rw_wlocked(rw) && rw_recursed(rw)) { 580 if ((--rw->rw_recurse) == 0) 581 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 582 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 583 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 584 return; 585 } 586 587 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 588 ("%s: neither of the waiter flags are set", __func__)); 589 590 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 591 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 592 593 turnstile_lock(&rw->lock_object); 594 ts = turnstile_lookup(&rw->lock_object); 595 596 #ifdef ADAPTIVE_RWLOCKS 597 /* 598 * There might not be a turnstile for this lock if all of 599 * the waiters are adaptively spinning. In that case, just 600 * reset the lock to the unlocked state and return. 601 */ 602 if (ts == NULL) { 603 atomic_store_rel_ptr(&rw->rw_lock, RW_UNLOCKED); 604 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 605 CTR2(KTR_LOCK, "%s: %p no sleepers", __func__, rw); 606 turnstile_release(&rw->lock_object); 607 return; 608 } 609 #else 610 MPASS(ts != NULL); 611 #endif 612 613 /* 614 * Use the same algo as sx locks for now. Prefer waking up shared 615 * waiters if we have any over writers. This is probably not ideal. 616 * 617 * 'v' is the value we are going to write back to rw_lock. If we 618 * have waiters on both queues, we need to preserve the state of 619 * the waiter flag for the queue we don't wake up. For now this is 620 * hardcoded for the algorithm mentioned above. 621 * 622 * In the case of both readers and writers waiting we wakeup the 623 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 624 * new writer comes in before a reader it will claim the lock up 625 * above. There is probably a potential priority inversion in 626 * there that could be worked around either by waking both queues 627 * of waiters or doing some complicated lock handoff gymnastics. 628 * 629 * Note that in the ADAPTIVE_RWLOCKS case, if both flags are 630 * set, there might not be any actual writers on the turnstile 631 * as they might all be spinning. In that case, we don't want 632 * to preserve the RW_LOCK_WRITE_WAITERS flag as the turnstile 633 * is going to go away once we wakeup all the readers. 634 */ 635 v = RW_UNLOCKED; 636 if (rw->rw_lock & RW_LOCK_READ_WAITERS) { 637 queue = TS_SHARED_QUEUE; 638 #ifdef ADAPTIVE_RWLOCKS 639 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS && 640 !turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 641 v |= RW_LOCK_WRITE_WAITERS; 642 #else 643 v |= (rw->rw_lock & RW_LOCK_WRITE_WAITERS); 644 #endif 645 } else 646 queue = TS_EXCLUSIVE_QUEUE; 647 648 #ifdef ADAPTIVE_RWLOCKS 649 /* 650 * We have to make sure that we actually have waiters to 651 * wakeup. If they are all spinning, then we just need to 652 * disown the turnstile and return. 653 */ 654 if (turnstile_empty(ts, queue)) { 655 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 656 CTR2(KTR_LOCK, "%s: %p no sleepers 2", __func__, rw); 657 atomic_store_rel_ptr(&rw->rw_lock, v); 658 turnstile_disown(ts); 659 turnstile_release(&rw->lock_object); 660 return; 661 } 662 #endif 663 664 /* Wake up all waiters for the specific queue. */ 665 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 666 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 667 queue == TS_SHARED_QUEUE ? "read" : "write"); 668 turnstile_broadcast(ts, queue); 669 atomic_store_rel_ptr(&rw->rw_lock, v); 670 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 671 } 672 673 /* 674 * Attempt to do a non-blocking upgrade from a read lock to a write 675 * lock. This will only succeed if this thread holds a single read 676 * lock. Returns true if the upgrade succeeded and false otherwise. 677 */ 678 int 679 _rw_try_upgrade(struct rwlock *rw, const char *file, int line) 680 { 681 uintptr_t v, tid; 682 int success; 683 684 KASSERT(rw->rw_lock != RW_DESTROYED, 685 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 686 _rw_assert(rw, RA_RLOCKED, file, line); 687 688 /* 689 * Attempt to switch from one reader to a writer. If there 690 * are any write waiters, then we will have to lock the 691 * turnstile first to prevent races with another writer 692 * calling turnstile_wait() before we have claimed this 693 * turnstile. So, do the simple case of no waiters first. 694 */ 695 tid = (uintptr_t)curthread; 696 if (!(rw->rw_lock & RW_LOCK_WRITE_WAITERS)) { 697 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 698 tid); 699 goto out; 700 } 701 702 /* 703 * Ok, we think we have write waiters, so lock the 704 * turnstile. 705 */ 706 turnstile_lock(&rw->lock_object); 707 708 /* 709 * Try to switch from one reader to a writer again. This time 710 * we honor the current state of the RW_LOCK_WRITE_WAITERS 711 * flag. If we obtain the lock with the flag set, then claim 712 * ownership of the turnstile. In the ADAPTIVE_RWLOCKS case 713 * it is possible for there to not be an associated turnstile 714 * even though there are waiters if all of the waiters are 715 * spinning. 716 */ 717 v = rw->rw_lock & RW_LOCK_WRITE_WAITERS; 718 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 719 tid | v); 720 #ifdef ADAPTIVE_RWLOCKS 721 if (success && v && turnstile_lookup(&rw->lock_object) != NULL) 722 #else 723 if (success && v) 724 #endif 725 turnstile_claim(&rw->lock_object); 726 else 727 turnstile_release(&rw->lock_object); 728 out: 729 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 730 if (success) 731 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 732 file, line); 733 return (success); 734 } 735 736 /* 737 * Downgrade a write lock into a single read lock. 738 */ 739 void 740 _rw_downgrade(struct rwlock *rw, const char *file, int line) 741 { 742 struct turnstile *ts; 743 uintptr_t tid, v; 744 745 KASSERT(rw->rw_lock != RW_DESTROYED, 746 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 747 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line); 748 #ifndef INVARIANTS 749 if (rw_recursed(rw)) 750 panic("downgrade of a recursed lock"); 751 #endif 752 753 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 754 755 /* 756 * Convert from a writer to a single reader. First we handle 757 * the easy case with no waiters. If there are any waiters, we 758 * lock the turnstile, "disown" the lock, and awaken any read 759 * waiters. 760 */ 761 tid = (uintptr_t)curthread; 762 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 763 goto out; 764 765 /* 766 * Ok, we think we have waiters, so lock the turnstile so we can 767 * read the waiter flags without any races. 768 */ 769 turnstile_lock(&rw->lock_object); 770 v = rw->rw_lock; 771 MPASS(v & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)); 772 773 /* 774 * Downgrade from a write lock while preserving 775 * RW_LOCK_WRITE_WAITERS and give up ownership of the 776 * turnstile. If there are any read waiters, wake them up. 777 * 778 * For ADAPTIVE_RWLOCKS, we have to allow for the fact that 779 * all of the read waiters might be spinning. In that case, 780 * act as if RW_LOCK_READ_WAITERS is not set. Also, only 781 * preserve the RW_LOCK_WRITE_WAITERS flag if at least one 782 * writer is blocked on the turnstile. 783 */ 784 ts = turnstile_lookup(&rw->lock_object); 785 #ifdef ADAPTIVE_RWLOCKS 786 if (ts == NULL) 787 v &= ~(RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS); 788 else if (v & RW_LOCK_READ_WAITERS && 789 turnstile_empty(ts, TS_SHARED_QUEUE)) 790 v &= ~RW_LOCK_READ_WAITERS; 791 else if (v & RW_LOCK_WRITE_WAITERS && 792 turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 793 v &= ~RW_LOCK_WRITE_WAITERS; 794 #else 795 MPASS(ts != NULL); 796 #endif 797 if (v & RW_LOCK_READ_WAITERS) 798 turnstile_broadcast(ts, TS_SHARED_QUEUE); 799 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | 800 (v & RW_LOCK_WRITE_WAITERS)); 801 if (v & RW_LOCK_READ_WAITERS) { 802 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 803 } else if (ts) { 804 turnstile_disown(ts); 805 turnstile_release(&rw->lock_object); 806 } 807 out: 808 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 809 } 810 811 #ifdef INVARIANT_SUPPORT 812 #ifndef INVARIANTS 813 #undef _rw_assert 814 #endif 815 816 /* 817 * In the non-WITNESS case, rw_assert() can only detect that at least 818 * *some* thread owns an rlock, but it cannot guarantee that *this* 819 * thread owns an rlock. 820 */ 821 void 822 _rw_assert(struct rwlock *rw, int what, const char *file, int line) 823 { 824 825 if (panicstr != NULL) 826 return; 827 switch (what) { 828 case RA_LOCKED: 829 case RA_LOCKED | RA_RECURSED: 830 case RA_LOCKED | RA_NOTRECURSED: 831 case RA_RLOCKED: 832 #ifdef WITNESS 833 witness_assert(&rw->lock_object, what, file, line); 834 #else 835 /* 836 * If some other thread has a write lock or we have one 837 * and are asserting a read lock, fail. Also, if no one 838 * has a lock at all, fail. 839 */ 840 if (rw->rw_lock == RW_UNLOCKED || 841 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED || 842 rw_wowner(rw) != curthread))) 843 panic("Lock %s not %slocked @ %s:%d\n", 844 rw->lock_object.lo_name, (what == RA_RLOCKED) ? 845 "read " : "", file, line); 846 847 if (!(rw->rw_lock & RW_LOCK_READ)) { 848 if (rw_recursed(rw)) { 849 if (what & RA_NOTRECURSED) 850 panic("Lock %s recursed @ %s:%d\n", 851 rw->lock_object.lo_name, file, 852 line); 853 } else if (what & RA_RECURSED) 854 panic("Lock %s not recursed @ %s:%d\n", 855 rw->lock_object.lo_name, file, line); 856 } 857 #endif 858 break; 859 case RA_WLOCKED: 860 case RA_WLOCKED | RA_RECURSED: 861 case RA_WLOCKED | RA_NOTRECURSED: 862 if (rw_wowner(rw) != curthread) 863 panic("Lock %s not exclusively locked @ %s:%d\n", 864 rw->lock_object.lo_name, file, line); 865 if (rw_recursed(rw)) { 866 if (what & RA_NOTRECURSED) 867 panic("Lock %s recursed @ %s:%d\n", 868 rw->lock_object.lo_name, file, line); 869 } else if (what & RA_RECURSED) 870 panic("Lock %s not recursed @ %s:%d\n", 871 rw->lock_object.lo_name, file, line); 872 break; 873 case RA_UNLOCKED: 874 #ifdef WITNESS 875 witness_assert(&rw->lock_object, what, file, line); 876 #else 877 /* 878 * If we hold a write lock fail. We can't reliably check 879 * to see if we hold a read lock or not. 880 */ 881 if (rw_wowner(rw) == curthread) 882 panic("Lock %s exclusively locked @ %s:%d\n", 883 rw->lock_object.lo_name, file, line); 884 #endif 885 break; 886 default: 887 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 888 line); 889 } 890 } 891 #endif /* INVARIANT_SUPPORT */ 892 893 #ifdef DDB 894 void 895 db_show_rwlock(struct lock_object *lock) 896 { 897 struct rwlock *rw; 898 struct thread *td; 899 900 rw = (struct rwlock *)lock; 901 902 db_printf(" state: "); 903 if (rw->rw_lock == RW_UNLOCKED) 904 db_printf("UNLOCKED\n"); 905 else if (rw->rw_lock == RW_DESTROYED) { 906 db_printf("DESTROYED\n"); 907 return; 908 } else if (rw->rw_lock & RW_LOCK_READ) 909 db_printf("RLOCK: %ju locks\n", 910 (uintmax_t)(RW_READERS(rw->rw_lock))); 911 else { 912 td = rw_wowner(rw); 913 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 914 td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm); 915 if (rw_recursed(rw)) 916 db_printf(" recursed: %u\n", rw->rw_recurse); 917 } 918 db_printf(" waiters: "); 919 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 920 case RW_LOCK_READ_WAITERS: 921 db_printf("readers\n"); 922 break; 923 case RW_LOCK_WRITE_WAITERS: 924 db_printf("writers\n"); 925 break; 926 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 927 db_printf("readers and writers\n"); 928 break; 929 default: 930 db_printf("none\n"); 931 break; 932 } 933 } 934 935 #endif
Cache object: 90014e14551f427b974d84e67ab8c684
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