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 * 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("$FreeBSD$"); 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 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 156 line); 157 __rw_wlock(rw, curthread, file, line); 158 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 159 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 160 curthread->td_locks++; 161 } 162 163 void 164 _rw_wunlock(struct rwlock *rw, const char *file, int line) 165 { 166 167 MPASS(curthread != NULL); 168 KASSERT(rw->rw_lock != RW_DESTROYED, 169 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 170 _rw_assert(rw, RA_WLOCKED, file, line); 171 curthread->td_locks--; 172 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 173 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 174 line); 175 if (!rw_recursed(rw)) 176 lock_profile_release_lock(&rw->lock_object); 177 __rw_wunlock(rw, curthread, file, line); 178 } 179 180 void 181 _rw_rlock(struct rwlock *rw, const char *file, int line) 182 { 183 #ifdef ADAPTIVE_RWLOCKS 184 volatile struct thread *owner; 185 #endif 186 //uint64_t waittime = 0; /* XXX: notsup */ 187 //int contested = 0; /* XXX: notsup */ 188 uintptr_t x; 189 190 KASSERT(rw->rw_lock != RW_DESTROYED, 191 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 192 KASSERT(rw_wowner(rw) != curthread, 193 ("%s (%s): wlock already held @ %s:%d", __func__, 194 rw->lock_object.lo_name, file, line)); 195 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line); 196 197 /* 198 * Note that we don't make any attempt to try to block read 199 * locks once a writer has blocked on the lock. The reason is 200 * that we currently allow for read locks to recurse and we 201 * don't keep track of all the holders of read locks. Thus, if 202 * we were to block readers once a writer blocked and a reader 203 * tried to recurse on their reader lock after a writer had 204 * blocked we would end up in a deadlock since the reader would 205 * be blocked on the writer, and the writer would be blocked 206 * waiting for the reader to release its original read lock. 207 */ 208 for (;;) { 209 /* 210 * Handle the easy case. If no other thread has a write 211 * lock, then try to bump up the count of read locks. Note 212 * that we have to preserve the current state of the 213 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 214 * read lock, then rw_lock must have changed, so restart 215 * the loop. Note that this handles the case of a 216 * completely unlocked rwlock since such a lock is encoded 217 * as a read lock with no waiters. 218 */ 219 x = rw->rw_lock; 220 if (x & RW_LOCK_READ) { 221 222 /* 223 * The RW_LOCK_READ_WAITERS flag should only be set 224 * if another thread currently holds a write lock, 225 * and in that case RW_LOCK_READ should be clear. 226 */ 227 MPASS((x & RW_LOCK_READ_WAITERS) == 0); 228 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, 229 x + RW_ONE_READER)) { 230 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 231 CTR4(KTR_LOCK, 232 "%s: %p succeed %p -> %p", __func__, 233 rw, (void *)x, 234 (void *)(x + RW_ONE_READER)); 235 if (RW_READERS(x) == 0) 236 lock_profile_obtain_lock_success( 237 &rw->lock_object, contested, waittime, 238 file, line); 239 break; 240 } 241 cpu_spinwait(); 242 continue; 243 } 244 lock_profile_obtain_lock_failed(&rw->lock_object, &contested, 245 &waittime); 246 247 /* 248 * Okay, now it's the hard case. Some other thread already 249 * has a write lock, so acquire the turnstile lock so we can 250 * begin the process of blocking. 251 */ 252 turnstile_lock(&rw->lock_object); 253 254 /* 255 * The lock might have been released while we spun, so 256 * recheck its state and restart the loop if there is no 257 * longer a write lock. 258 */ 259 x = rw->rw_lock; 260 if (x & RW_LOCK_READ) { 261 turnstile_release(&rw->lock_object); 262 cpu_spinwait(); 263 continue; 264 } 265 266 /* 267 * Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS 268 * flag is already set, then we can go ahead and block. If 269 * it is not set then try to set it. If we fail to set it 270 * drop the turnstile lock and restart the loop. 271 */ 272 if (!(x & RW_LOCK_READ_WAITERS)) { 273 if (!atomic_cmpset_ptr(&rw->rw_lock, x, 274 x | RW_LOCK_READ_WAITERS)) { 275 turnstile_release(&rw->lock_object); 276 cpu_spinwait(); 277 continue; 278 } 279 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 280 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 281 __func__, rw); 282 } 283 284 #ifdef ADAPTIVE_RWLOCKS 285 /* 286 * If the owner is running on another CPU, spin until 287 * the owner stops running or the state of the lock 288 * changes. 289 */ 290 owner = (struct thread *)RW_OWNER(x); 291 if (TD_IS_RUNNING(owner)) { 292 turnstile_release(&rw->lock_object); 293 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 294 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 295 __func__, rw, owner); 296 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 297 TD_IS_RUNNING(owner)) 298 cpu_spinwait(); 299 continue; 300 } 301 #endif 302 303 /* 304 * We were unable to acquire the lock and the read waiters 305 * flag is set, so we must block on the turnstile. 306 */ 307 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 308 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 309 rw); 310 turnstile_wait(&rw->lock_object, rw_owner(rw), 311 TS_SHARED_QUEUE); 312 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 313 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 314 __func__, rw); 315 } 316 317 /* 318 * TODO: acquire "owner of record" here. Here be turnstile dragons 319 * however. turnstiles don't like owners changing between calls to 320 * turnstile_wait() currently. 321 */ 322 323 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 324 WITNESS_LOCK(&rw->lock_object, 0, file, line); 325 curthread->td_locks++; 326 } 327 328 void 329 _rw_runlock(struct rwlock *rw, const char *file, int line) 330 { 331 struct turnstile *ts; 332 uintptr_t x; 333 334 KASSERT(rw->rw_lock != RW_DESTROYED, 335 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 336 _rw_assert(rw, RA_RLOCKED, file, line); 337 curthread->td_locks--; 338 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 339 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 340 341 /* TODO: drop "owner of record" here. */ 342 343 for (;;) { 344 /* 345 * See if there is more than one read lock held. If so, 346 * just drop one and return. 347 */ 348 x = rw->rw_lock; 349 if (RW_READERS(x) > 1) { 350 if (atomic_cmpset_ptr(&rw->rw_lock, x, 351 x - RW_ONE_READER)) { 352 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 353 CTR4(KTR_LOCK, 354 "%s: %p succeeded %p -> %p", 355 __func__, rw, (void *)x, 356 (void *)(x - RW_ONE_READER)); 357 break; 358 } 359 continue; 360 } 361 362 363 /* 364 * We should never have read waiters while at least one 365 * thread holds a read lock. (See note above) 366 */ 367 KASSERT(!(x & RW_LOCK_READ_WAITERS), 368 ("%s: waiting readers", __func__)); 369 370 /* 371 * If there aren't any waiters for a write lock, then try 372 * to drop it quickly. 373 */ 374 if (!(x & RW_LOCK_WRITE_WAITERS)) { 375 376 /* 377 * There shouldn't be any flags set and we should 378 * be the only read lock. If we fail to release 379 * the single read lock, then another thread might 380 * have just acquired a read lock, so go back up 381 * to the multiple read locks case. 382 */ 383 MPASS(x == RW_READERS_LOCK(1)); 384 if (atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 385 RW_UNLOCKED)) { 386 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 387 CTR2(KTR_LOCK, "%s: %p last succeeded", 388 __func__, rw); 389 break; 390 } 391 continue; 392 } 393 394 /* 395 * There should just be one reader with one or more 396 * writers waiting. 397 */ 398 MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS)); 399 400 /* 401 * Ok, we know we have a waiting writer and we think we 402 * are the last reader, so grab the turnstile lock. 403 */ 404 turnstile_lock(&rw->lock_object); 405 406 /* 407 * Try to drop our lock leaving the lock in a unlocked 408 * state. 409 * 410 * If you wanted to do explicit lock handoff you'd have to 411 * do it here. You'd also want to use turnstile_signal() 412 * and you'd have to handle the race where a higher 413 * priority thread blocks on the write lock before the 414 * thread you wakeup actually runs and have the new thread 415 * "steal" the lock. For now it's a lot simpler to just 416 * wakeup all of the waiters. 417 * 418 * As above, if we fail, then another thread might have 419 * acquired a read lock, so drop the turnstile lock and 420 * restart. 421 */ 422 if (!atomic_cmpset_ptr(&rw->rw_lock, 423 RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) { 424 turnstile_release(&rw->lock_object); 425 continue; 426 } 427 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 428 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 429 __func__, rw); 430 431 /* 432 * Ok. The lock is released and all that's left is to 433 * wake up the waiters. Note that the lock might not be 434 * free anymore, but in that case the writers will just 435 * block again if they run before the new lock holder(s) 436 * release the lock. 437 */ 438 ts = turnstile_lookup(&rw->lock_object); 439 MPASS(ts != NULL); 440 turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE); 441 turnstile_unpend(ts, TS_SHARED_LOCK); 442 break; 443 } 444 lock_profile_release_lock(&rw->lock_object); 445 } 446 447 /* 448 * This function is called when we are unable to obtain a write lock on the 449 * first try. This means that at least one other thread holds either a 450 * read or write lock. 451 */ 452 void 453 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 454 { 455 #ifdef ADAPTIVE_RWLOCKS 456 volatile struct thread *owner; 457 #endif 458 uintptr_t v; 459 460 if (rw_wlocked(rw)) { 461 KASSERT(rw->lock_object.lo_flags & RW_RECURSE, 462 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 463 __func__, rw->lock_object.lo_name, file, line)); 464 rw->rw_recurse++; 465 atomic_set_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 466 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 467 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 468 return; 469 } 470 471 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 472 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 473 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 474 475 while (!_rw_write_lock(rw, tid)) { 476 turnstile_lock(&rw->lock_object); 477 v = rw->rw_lock; 478 479 /* 480 * If the lock was released while spinning on the 481 * turnstile chain lock, try again. 482 */ 483 if (v == RW_UNLOCKED) { 484 turnstile_release(&rw->lock_object); 485 cpu_spinwait(); 486 continue; 487 } 488 489 /* 490 * If the lock was released by a writer with both readers 491 * and writers waiting and a reader hasn't woken up and 492 * acquired the lock yet, rw_lock will be set to the 493 * value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see 494 * that value, try to acquire it once. Note that we have 495 * to preserve the RW_LOCK_WRITE_WAITERS flag as there are 496 * other writers waiting still. If we fail, restart the 497 * loop. 498 */ 499 if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) { 500 if (atomic_cmpset_acq_ptr(&rw->rw_lock, 501 RW_UNLOCKED | RW_LOCK_WRITE_WAITERS, 502 tid | RW_LOCK_WRITE_WAITERS)) { 503 turnstile_claim(&rw->lock_object); 504 CTR2(KTR_LOCK, "%s: %p claimed by new writer", 505 __func__, rw); 506 break; 507 } 508 turnstile_release(&rw->lock_object); 509 cpu_spinwait(); 510 continue; 511 } 512 513 /* 514 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 515 * set it. If we fail to set it, then loop back and try 516 * again. 517 */ 518 if (!(v & RW_LOCK_WRITE_WAITERS)) { 519 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 520 v | RW_LOCK_WRITE_WAITERS)) { 521 turnstile_release(&rw->lock_object); 522 cpu_spinwait(); 523 continue; 524 } 525 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 526 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 527 __func__, rw); 528 } 529 530 #ifdef ADAPTIVE_RWLOCKS 531 /* 532 * If the lock is write locked and the owner is 533 * running on another CPU, spin until the owner stops 534 * running or the state of the lock changes. 535 */ 536 owner = (struct thread *)RW_OWNER(v); 537 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 538 turnstile_release(&rw->lock_object); 539 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 540 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 541 __func__, rw, owner); 542 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 543 TD_IS_RUNNING(owner)) 544 cpu_spinwait(); 545 continue; 546 } 547 #endif 548 549 /* 550 * We were unable to acquire the lock and the write waiters 551 * flag is set, so we must block on the turnstile. 552 */ 553 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 554 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 555 rw); 556 turnstile_wait(&rw->lock_object, rw_owner(rw), 557 TS_EXCLUSIVE_QUEUE); 558 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 559 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 560 __func__, rw); 561 } 562 } 563 564 /* 565 * This function is called if the first try at releasing a write lock failed. 566 * This means that one of the 2 waiter bits must be set indicating that at 567 * least one thread is waiting on this lock. 568 */ 569 void 570 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 571 { 572 struct turnstile *ts; 573 uintptr_t v; 574 int queue; 575 576 if (rw_wlocked(rw) && rw_recursed(rw)) { 577 if ((--rw->rw_recurse) == 0) 578 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 579 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 580 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 581 return; 582 } 583 584 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 585 ("%s: neither of the waiter flags are set", __func__)); 586 587 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 588 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 589 590 turnstile_lock(&rw->lock_object); 591 ts = turnstile_lookup(&rw->lock_object); 592 593 #ifdef ADAPTIVE_RWLOCKS 594 /* 595 * There might not be a turnstile for this lock if all of 596 * the waiters are adaptively spinning. In that case, just 597 * reset the lock to the unlocked state and return. 598 */ 599 if (ts == NULL) { 600 atomic_store_rel_ptr(&rw->rw_lock, RW_UNLOCKED); 601 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 602 CTR2(KTR_LOCK, "%s: %p no sleepers", __func__, rw); 603 turnstile_release(&rw->lock_object); 604 return; 605 } 606 #else 607 MPASS(ts != NULL); 608 #endif 609 610 /* 611 * Use the same algo as sx locks for now. Prefer waking up shared 612 * waiters if we have any over writers. This is probably not ideal. 613 * 614 * 'v' is the value we are going to write back to rw_lock. If we 615 * have waiters on both queues, we need to preserve the state of 616 * the waiter flag for the queue we don't wake up. For now this is 617 * hardcoded for the algorithm mentioned above. 618 * 619 * In the case of both readers and writers waiting we wakeup the 620 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 621 * new writer comes in before a reader it will claim the lock up 622 * above. There is probably a potential priority inversion in 623 * there that could be worked around either by waking both queues 624 * of waiters or doing some complicated lock handoff gymnastics. 625 * 626 * Note that in the ADAPTIVE_RWLOCKS case, if both flags are 627 * set, there might not be any actual writers on the turnstile 628 * as they might all be spinning. In that case, we don't want 629 * to preserve the RW_LOCK_WRITE_WAITERS flag as the turnstile 630 * is going to go away once we wakeup all the readers. 631 */ 632 v = RW_UNLOCKED; 633 if (rw->rw_lock & RW_LOCK_READ_WAITERS) { 634 queue = TS_SHARED_QUEUE; 635 #ifdef ADAPTIVE_RWLOCKS 636 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS && 637 !turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 638 v |= RW_LOCK_WRITE_WAITERS; 639 #else 640 v |= (rw->rw_lock & RW_LOCK_WRITE_WAITERS); 641 #endif 642 } else 643 queue = TS_EXCLUSIVE_QUEUE; 644 645 #ifdef ADAPTIVE_RWLOCKS 646 /* 647 * We have to make sure that we actually have waiters to 648 * wakeup. If they are all spinning, then we just need to 649 * disown the turnstile and return. 650 */ 651 if (turnstile_empty(ts, queue)) { 652 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 653 CTR2(KTR_LOCK, "%s: %p no sleepers 2", __func__, rw); 654 atomic_store_rel_ptr(&rw->rw_lock, v); 655 turnstile_disown(ts); 656 turnstile_release(&rw->lock_object); 657 return; 658 } 659 #endif 660 661 /* Wake up all waiters for the specific queue. */ 662 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 663 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 664 queue == TS_SHARED_QUEUE ? "read" : "write"); 665 turnstile_broadcast(ts, queue); 666 atomic_store_rel_ptr(&rw->rw_lock, v); 667 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 668 } 669 670 /* 671 * Attempt to do a non-blocking upgrade from a read lock to a write 672 * lock. This will only succeed if this thread holds a single read 673 * lock. Returns true if the upgrade succeeded and false otherwise. 674 */ 675 int 676 _rw_try_upgrade(struct rwlock *rw, const char *file, int line) 677 { 678 uintptr_t v, tid; 679 int success; 680 681 KASSERT(rw->rw_lock != RW_DESTROYED, 682 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 683 _rw_assert(rw, RA_RLOCKED, file, line); 684 685 /* 686 * Attempt to switch from one reader to a writer. If there 687 * are any write waiters, then we will have to lock the 688 * turnstile first to prevent races with another writer 689 * calling turnstile_wait() before we have claimed this 690 * turnstile. So, do the simple case of no waiters first. 691 */ 692 tid = (uintptr_t)curthread; 693 if (!(rw->rw_lock & RW_LOCK_WRITE_WAITERS)) { 694 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 695 tid); 696 goto out; 697 } 698 699 /* 700 * Ok, we think we have write waiters, so lock the 701 * turnstile. 702 */ 703 turnstile_lock(&rw->lock_object); 704 705 /* 706 * Try to switch from one reader to a writer again. This time 707 * we honor the current state of the RW_LOCK_WRITE_WAITERS 708 * flag. If we obtain the lock with the flag set, then claim 709 * ownership of the turnstile. In the ADAPTIVE_RWLOCKS case 710 * it is possible for there to not be an associated turnstile 711 * even though there are waiters if all of the waiters are 712 * spinning. 713 */ 714 v = rw->rw_lock & RW_LOCK_WRITE_WAITERS; 715 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 716 tid | v); 717 #ifdef ADAPTIVE_RWLOCKS 718 if (success && v && turnstile_lookup(&rw->lock_object) != NULL) 719 #else 720 if (success && v) 721 #endif 722 turnstile_claim(&rw->lock_object); 723 else 724 turnstile_release(&rw->lock_object); 725 out: 726 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 727 if (success) 728 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 729 file, line); 730 return (success); 731 } 732 733 /* 734 * Downgrade a write lock into a single read lock. 735 */ 736 void 737 _rw_downgrade(struct rwlock *rw, const char *file, int line) 738 { 739 struct turnstile *ts; 740 uintptr_t tid, v; 741 742 KASSERT(rw->rw_lock != RW_DESTROYED, 743 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 744 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line); 745 #ifndef INVARIANTS 746 if (rw_recursed(rw)) 747 panic("downgrade of a recursed lock"); 748 #endif 749 750 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 751 752 /* 753 * Convert from a writer to a single reader. First we handle 754 * the easy case with no waiters. If there are any waiters, we 755 * lock the turnstile, "disown" the lock, and awaken any read 756 * waiters. 757 */ 758 tid = (uintptr_t)curthread; 759 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 760 goto out; 761 762 /* 763 * Ok, we think we have waiters, so lock the turnstile so we can 764 * read the waiter flags without any races. 765 */ 766 turnstile_lock(&rw->lock_object); 767 v = rw->rw_lock; 768 MPASS(v & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)); 769 770 /* 771 * Downgrade from a write lock while preserving 772 * RW_LOCK_WRITE_WAITERS and give up ownership of the 773 * turnstile. If there are any read waiters, wake them up. 774 * 775 * For ADAPTIVE_RWLOCKS, we have to allow for the fact that 776 * all of the read waiters might be spinning. In that case, 777 * act as if RW_LOCK_READ_WAITERS is not set. Also, only 778 * preserve the RW_LOCK_WRITE_WAITERS flag if at least one 779 * writer is blocked on the turnstile. 780 */ 781 ts = turnstile_lookup(&rw->lock_object); 782 #ifdef ADAPTIVE_RWLOCKS 783 if (ts == NULL) 784 v &= ~(RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS); 785 else if (v & RW_LOCK_READ_WAITERS && 786 turnstile_empty(ts, TS_SHARED_QUEUE)) 787 v &= ~RW_LOCK_READ_WAITERS; 788 else if (v & RW_LOCK_WRITE_WAITERS && 789 turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 790 v &= ~RW_LOCK_WRITE_WAITERS; 791 #else 792 MPASS(ts != NULL); 793 #endif 794 if (v & RW_LOCK_READ_WAITERS) 795 turnstile_broadcast(ts, TS_SHARED_QUEUE); 796 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | 797 (v & RW_LOCK_WRITE_WAITERS)); 798 if (v & RW_LOCK_READ_WAITERS) { 799 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 800 } else if (ts) { 801 turnstile_disown(ts); 802 turnstile_release(&rw->lock_object); 803 } 804 out: 805 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 806 } 807 808 #ifdef INVARIANT_SUPPORT 809 #ifndef INVARIANTS 810 #undef _rw_assert 811 #endif 812 813 /* 814 * In the non-WITNESS case, rw_assert() can only detect that at least 815 * *some* thread owns an rlock, but it cannot guarantee that *this* 816 * thread owns an rlock. 817 */ 818 void 819 _rw_assert(struct rwlock *rw, int what, const char *file, int line) 820 { 821 822 if (panicstr != NULL) 823 return; 824 switch (what) { 825 case RA_LOCKED: 826 case RA_LOCKED | RA_RECURSED: 827 case RA_LOCKED | RA_NOTRECURSED: 828 case RA_RLOCKED: 829 #ifdef WITNESS 830 witness_assert(&rw->lock_object, what, file, line); 831 #else 832 /* 833 * If some other thread has a write lock or we have one 834 * and are asserting a read lock, fail. Also, if no one 835 * has a lock at all, fail. 836 */ 837 if (rw->rw_lock == RW_UNLOCKED || 838 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED || 839 rw_wowner(rw) != curthread))) 840 panic("Lock %s not %slocked @ %s:%d\n", 841 rw->lock_object.lo_name, (what == RA_RLOCKED) ? 842 "read " : "", file, line); 843 844 if (!(rw->rw_lock & RW_LOCK_READ)) { 845 if (rw_recursed(rw)) { 846 if (what & RA_NOTRECURSED) 847 panic("Lock %s recursed @ %s:%d\n", 848 rw->lock_object.lo_name, file, 849 line); 850 } else if (what & RA_RECURSED) 851 panic("Lock %s not recursed @ %s:%d\n", 852 rw->lock_object.lo_name, file, line); 853 } 854 #endif 855 break; 856 case RA_WLOCKED: 857 case RA_WLOCKED | RA_RECURSED: 858 case RA_WLOCKED | RA_NOTRECURSED: 859 if (rw_wowner(rw) != curthread) 860 panic("Lock %s not exclusively locked @ %s:%d\n", 861 rw->lock_object.lo_name, file, line); 862 if (rw_recursed(rw)) { 863 if (what & RA_NOTRECURSED) 864 panic("Lock %s recursed @ %s:%d\n", 865 rw->lock_object.lo_name, file, line); 866 } else if (what & RA_RECURSED) 867 panic("Lock %s not recursed @ %s:%d\n", 868 rw->lock_object.lo_name, file, line); 869 break; 870 case RA_UNLOCKED: 871 #ifdef WITNESS 872 witness_assert(&rw->lock_object, what, file, line); 873 #else 874 /* 875 * If we hold a write lock fail. We can't reliably check 876 * to see if we hold a read lock or not. 877 */ 878 if (rw_wowner(rw) == curthread) 879 panic("Lock %s exclusively locked @ %s:%d\n", 880 rw->lock_object.lo_name, file, line); 881 #endif 882 break; 883 default: 884 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 885 line); 886 } 887 } 888 #endif /* INVARIANT_SUPPORT */ 889 890 #ifdef DDB 891 void 892 db_show_rwlock(struct lock_object *lock) 893 { 894 struct rwlock *rw; 895 struct thread *td; 896 897 rw = (struct rwlock *)lock; 898 899 db_printf(" state: "); 900 if (rw->rw_lock == RW_UNLOCKED) 901 db_printf("UNLOCKED\n"); 902 else if (rw->rw_lock == RW_DESTROYED) { 903 db_printf("DESTROYED\n"); 904 return; 905 } else if (rw->rw_lock & RW_LOCK_READ) 906 db_printf("RLOCK: %ju locks\n", 907 (uintmax_t)(RW_READERS(rw->rw_lock))); 908 else { 909 td = rw_wowner(rw); 910 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 911 td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm); 912 if (rw_recursed(rw)) 913 db_printf(" recursed: %u\n", rw->rw_recurse); 914 } 915 db_printf(" waiters: "); 916 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 917 case RW_LOCK_READ_WAITERS: 918 db_printf("readers\n"); 919 break; 920 case RW_LOCK_WRITE_WAITERS: 921 db_printf("writers\n"); 922 break; 923 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 924 db_printf("readers and writers\n"); 925 break; 926 default: 927 db_printf("none\n"); 928 break; 929 } 930 } 931 932 #endif
Cache object: e6370c0e7914236e31b901239fc03502
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