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 static void lock_rw(struct lock_object *lock, int how); 63 static int unlock_rw(struct lock_object *lock); 64 65 struct lock_class lock_class_rw = { 66 .lc_name = "rw", 67 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 68 #ifdef DDB 69 .lc_ddb_show = db_show_rwlock, 70 #endif 71 .lc_lock = lock_rw, 72 .lc_unlock = unlock_rw, 73 }; 74 75 /* 76 * Return a pointer to the owning thread if the lock is write-locked or 77 * NULL if the lock is unlocked or read-locked. 78 */ 79 #define rw_wowner(rw) \ 80 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \ 81 (struct thread *)RW_OWNER((rw)->rw_lock)) 82 83 /* 84 * Returns if a write owner is recursed. Write ownership is not assured 85 * here and should be previously checked. 86 */ 87 #define rw_recursed(rw) ((rw)->rw_recurse != 0) 88 89 /* 90 * Return true if curthread helds the lock. 91 */ 92 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 93 94 /* 95 * Return a pointer to the owning thread for this lock who should receive 96 * any priority lent by threads that block on this lock. Currently this 97 * is identical to rw_wowner(). 98 */ 99 #define rw_owner(rw) rw_wowner(rw) 100 101 #ifndef INVARIANTS 102 #define _rw_assert(rw, what, file, line) 103 #endif 104 105 void 106 lock_rw(struct lock_object *lock, int how) 107 { 108 struct rwlock *rw; 109 110 rw = (struct rwlock *)lock; 111 if (how) 112 rw_wlock(rw); 113 else 114 rw_rlock(rw); 115 } 116 117 int 118 unlock_rw(struct lock_object *lock) 119 { 120 struct rwlock *rw; 121 122 rw = (struct rwlock *)lock; 123 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED); 124 if (rw->rw_lock & RW_LOCK_READ) { 125 rw_runlock(rw); 126 return (0); 127 } else { 128 rw_wunlock(rw); 129 return (1); 130 } 131 } 132 133 void 134 rw_init_flags(struct rwlock *rw, const char *name, int opts) 135 { 136 int flags; 137 138 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 139 RW_RECURSE)) == 0); 140 141 flags = LO_UPGRADABLE | LO_RECURSABLE; 142 if (opts & RW_DUPOK) 143 flags |= LO_DUPOK; 144 if (opts & RW_NOPROFILE) 145 flags |= LO_NOPROFILE; 146 if (!(opts & RW_NOWITNESS)) 147 flags |= LO_WITNESS; 148 if (opts & RW_QUIET) 149 flags |= LO_QUIET; 150 flags |= opts & RW_RECURSE; 151 152 rw->rw_lock = RW_UNLOCKED; 153 rw->rw_recurse = 0; 154 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 155 } 156 157 void 158 rw_destroy(struct rwlock *rw) 159 { 160 161 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked")); 162 KASSERT(rw->rw_recurse == 0, ("rw lock still recursed")); 163 rw->rw_lock = RW_DESTROYED; 164 lock_destroy(&rw->lock_object); 165 } 166 167 void 168 rw_sysinit(void *arg) 169 { 170 struct rw_args *args = arg; 171 172 rw_init(args->ra_rw, args->ra_desc); 173 } 174 175 int 176 rw_wowned(struct rwlock *rw) 177 { 178 179 return (rw_wowner(rw) == curthread); 180 } 181 182 void 183 _rw_wlock(struct rwlock *rw, const char *file, int line) 184 { 185 186 MPASS(curthread != NULL); 187 KASSERT(rw->rw_lock != RW_DESTROYED, 188 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 189 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 190 line); 191 __rw_wlock(rw, curthread, file, line); 192 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 193 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 194 curthread->td_locks++; 195 } 196 197 void 198 _rw_wunlock(struct rwlock *rw, const char *file, int line) 199 { 200 201 MPASS(curthread != NULL); 202 KASSERT(rw->rw_lock != RW_DESTROYED, 203 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 204 _rw_assert(rw, RA_WLOCKED, file, line); 205 curthread->td_locks--; 206 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 207 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 208 line); 209 if (!rw_recursed(rw)) 210 lock_profile_release_lock(&rw->lock_object); 211 __rw_wunlock(rw, curthread, file, line); 212 } 213 214 void 215 _rw_rlock(struct rwlock *rw, const char *file, int line) 216 { 217 struct turnstile *ts; 218 #ifdef ADAPTIVE_RWLOCKS 219 volatile struct thread *owner; 220 #endif 221 #ifdef LOCK_PROFILING_SHARED 222 uint64_t waittime = 0; 223 int contested = 0; 224 #endif 225 uintptr_t x; 226 227 KASSERT(rw->rw_lock != RW_DESTROYED, 228 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 229 KASSERT(rw_wowner(rw) != curthread, 230 ("%s (%s): wlock already held @ %s:%d", __func__, 231 rw->lock_object.lo_name, file, line)); 232 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line); 233 234 /* 235 * Note that we don't make any attempt to try to block read 236 * locks once a writer has blocked on the lock. The reason is 237 * that we currently allow for read locks to recurse and we 238 * don't keep track of all the holders of read locks. Thus, if 239 * we were to block readers once a writer blocked and a reader 240 * tried to recurse on their reader lock after a writer had 241 * blocked we would end up in a deadlock since the reader would 242 * be blocked on the writer, and the writer would be blocked 243 * waiting for the reader to release its original read lock. 244 */ 245 for (;;) { 246 /* 247 * Handle the easy case. If no other thread has a write 248 * lock, then try to bump up the count of read locks. Note 249 * that we have to preserve the current state of the 250 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 251 * read lock, then rw_lock must have changed, so restart 252 * the loop. Note that this handles the case of a 253 * completely unlocked rwlock since such a lock is encoded 254 * as a read lock with no waiters. 255 */ 256 x = rw->rw_lock; 257 if (x & RW_LOCK_READ) { 258 259 /* 260 * The RW_LOCK_READ_WAITERS flag should only be set 261 * if another thread currently holds a write lock, 262 * and in that case RW_LOCK_READ should be clear. 263 */ 264 MPASS((x & RW_LOCK_READ_WAITERS) == 0); 265 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, 266 x + RW_ONE_READER)) { 267 #ifdef LOCK_PROFILING_SHARED 268 if (RW_READERS(x) == 0) 269 lock_profile_obtain_lock_success( 270 &rw->lock_object, contested, 271 waittime, file, line); 272 #endif 273 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 274 CTR4(KTR_LOCK, 275 "%s: %p succeed %p -> %p", __func__, 276 rw, (void *)x, 277 (void *)(x + RW_ONE_READER)); 278 break; 279 } 280 cpu_spinwait(); 281 continue; 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 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 293 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 294 __func__, rw, owner); 295 #ifdef LOCK_PROFILING_SHARED 296 lock_profile_obtain_lock_failed(&rw->lock_object, 297 &contested, &waittime); 298 #endif 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 * Okay, now it's the hard case. Some other thread already 308 * has a write lock, so acquire the turnstile lock so we can 309 * begin the process of blocking. 310 */ 311 ts = turnstile_trywait(&rw->lock_object); 312 313 /* 314 * The lock might have been released while we spun, so 315 * recheck its state and restart the loop if there is no 316 * longer a write lock. 317 */ 318 x = rw->rw_lock; 319 if (x & RW_LOCK_READ) { 320 turnstile_cancel(ts); 321 cpu_spinwait(); 322 continue; 323 } 324 325 #ifdef ADAPTIVE_RWLOCKS 326 /* 327 * If the current owner of the lock is executing on another 328 * CPU quit the hard path and try to spin. 329 */ 330 owner = (struct thread *)RW_OWNER(x); 331 if (TD_IS_RUNNING(owner)) { 332 turnstile_cancel(ts); 333 cpu_spinwait(); 334 continue; 335 } 336 #endif 337 338 /* 339 * Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS 340 * flag is already set, then we can go ahead and block. If 341 * it is not set then try to set it. If we fail to set it 342 * drop the turnstile lock and restart the loop. 343 */ 344 if (!(x & RW_LOCK_READ_WAITERS)) { 345 if (!atomic_cmpset_ptr(&rw->rw_lock, x, 346 x | RW_LOCK_READ_WAITERS)) { 347 turnstile_cancel(ts); 348 cpu_spinwait(); 349 continue; 350 } 351 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 352 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 353 __func__, rw); 354 } 355 356 /* 357 * We were unable to acquire the lock and the read waiters 358 * flag is set, so we must block on the turnstile. 359 */ 360 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 361 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 362 rw); 363 #ifdef LOCK_PROFILING_SHARED 364 lock_profile_obtain_lock_failed(&rw->lock_object, &contested, 365 &waittime); 366 #endif 367 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE); 368 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 369 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 370 __func__, rw); 371 } 372 373 /* 374 * TODO: acquire "owner of record" here. Here be turnstile dragons 375 * however. turnstiles don't like owners changing between calls to 376 * turnstile_wait() currently. 377 */ 378 379 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 380 WITNESS_LOCK(&rw->lock_object, 0, file, line); 381 curthread->td_locks++; 382 } 383 384 void 385 _rw_runlock(struct rwlock *rw, const char *file, int line) 386 { 387 struct turnstile *ts; 388 uintptr_t x; 389 390 KASSERT(rw->rw_lock != RW_DESTROYED, 391 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 392 _rw_assert(rw, RA_RLOCKED, file, line); 393 curthread->td_locks--; 394 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 395 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 396 397 /* TODO: drop "owner of record" here. */ 398 399 for (;;) { 400 /* 401 * See if there is more than one read lock held. If so, 402 * just drop one and return. 403 */ 404 x = rw->rw_lock; 405 if (RW_READERS(x) > 1) { 406 if (atomic_cmpset_ptr(&rw->rw_lock, x, 407 x - RW_ONE_READER)) { 408 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 409 CTR4(KTR_LOCK, 410 "%s: %p succeeded %p -> %p", 411 __func__, rw, (void *)x, 412 (void *)(x - RW_ONE_READER)); 413 break; 414 } 415 continue; 416 } 417 418 419 /* 420 * We should never have read waiters while at least one 421 * thread holds a read lock. (See note above) 422 */ 423 KASSERT(!(x & RW_LOCK_READ_WAITERS), 424 ("%s: waiting readers", __func__)); 425 #ifdef LOCK_PROFILING_SHARED 426 lock_profile_release_lock(&rw->lock_object); 427 #endif 428 429 /* 430 * If there aren't any waiters for a write lock, then try 431 * to drop it quickly. 432 */ 433 if (!(x & RW_LOCK_WRITE_WAITERS)) { 434 435 /* 436 * There shouldn't be any flags set and we should 437 * be the only read lock. If we fail to release 438 * the single read lock, then another thread might 439 * have just acquired a read lock, so go back up 440 * to the multiple read locks case. 441 */ 442 MPASS(x == RW_READERS_LOCK(1)); 443 if (atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 444 RW_UNLOCKED)) { 445 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 446 CTR2(KTR_LOCK, "%s: %p last succeeded", 447 __func__, rw); 448 break; 449 } 450 continue; 451 } 452 453 /* 454 * There should just be one reader with one or more 455 * writers waiting. 456 */ 457 MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS)); 458 459 /* 460 * Ok, we know we have a waiting writer and we think we 461 * are the last reader, so grab the turnstile lock. 462 */ 463 turnstile_chain_lock(&rw->lock_object); 464 465 /* 466 * Try to drop our lock leaving the lock in a unlocked 467 * state. 468 * 469 * If you wanted to do explicit lock handoff you'd have to 470 * do it here. You'd also want to use turnstile_signal() 471 * and you'd have to handle the race where a higher 472 * priority thread blocks on the write lock before the 473 * thread you wakeup actually runs and have the new thread 474 * "steal" the lock. For now it's a lot simpler to just 475 * wakeup all of the waiters. 476 * 477 * As above, if we fail, then another thread might have 478 * acquired a read lock, so drop the turnstile lock and 479 * restart. 480 */ 481 if (!atomic_cmpset_ptr(&rw->rw_lock, 482 RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) { 483 turnstile_chain_unlock(&rw->lock_object); 484 continue; 485 } 486 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 487 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 488 __func__, rw); 489 490 /* 491 * Ok. The lock is released and all that's left is to 492 * wake up the waiters. Note that the lock might not be 493 * free anymore, but in that case the writers will just 494 * block again if they run before the new lock holder(s) 495 * release the lock. 496 */ 497 ts = turnstile_lookup(&rw->lock_object); 498 MPASS(ts != NULL); 499 turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE); 500 turnstile_unpend(ts, TS_SHARED_LOCK); 501 turnstile_chain_unlock(&rw->lock_object); 502 break; 503 } 504 } 505 506 /* 507 * This function is called when we are unable to obtain a write lock on the 508 * first try. This means that at least one other thread holds either a 509 * read or write lock. 510 */ 511 void 512 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 513 { 514 struct turnstile *ts; 515 #ifdef ADAPTIVE_RWLOCKS 516 volatile struct thread *owner; 517 #endif 518 uint64_t waittime = 0; 519 uintptr_t v; 520 int contested = 0; 521 522 if (rw_wlocked(rw)) { 523 KASSERT(rw->lock_object.lo_flags & RW_RECURSE, 524 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 525 __func__, rw->lock_object.lo_name, file, line)); 526 rw->rw_recurse++; 527 atomic_set_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 528 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 529 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 530 return; 531 } 532 533 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 534 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 535 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 536 537 while (!_rw_write_lock(rw, tid)) { 538 #ifdef ADAPTIVE_RWLOCKS 539 /* 540 * If the lock is write locked and the owner is 541 * running on another CPU, spin until the owner stops 542 * running or the state of the lock changes. 543 */ 544 v = rw->rw_lock; 545 owner = (struct thread *)RW_OWNER(v); 546 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 547 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 548 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 549 __func__, rw, owner); 550 lock_profile_obtain_lock_failed(&rw->lock_object, 551 &contested, &waittime); 552 while ((struct thread*)RW_OWNER(rw->rw_lock) == owner && 553 TD_IS_RUNNING(owner)) 554 cpu_spinwait(); 555 continue; 556 } 557 #endif 558 559 ts = turnstile_trywait(&rw->lock_object); 560 v = rw->rw_lock; 561 562 /* 563 * If the lock was released while spinning on the 564 * turnstile chain lock, try again. 565 */ 566 if (v == RW_UNLOCKED) { 567 turnstile_cancel(ts); 568 cpu_spinwait(); 569 continue; 570 } 571 572 #ifdef ADAPTIVE_RWLOCKS 573 /* 574 * If the current owner of the lock is executing on another 575 * CPU quit the hard path and try to spin. 576 */ 577 if (!(v & RW_LOCK_READ)) { 578 owner = (struct thread *)RW_OWNER(v); 579 if (TD_IS_RUNNING(owner)) { 580 turnstile_cancel(ts); 581 cpu_spinwait(); 582 continue; 583 } 584 } 585 #endif 586 587 /* 588 * If the lock was released by a writer with both readers 589 * and writers waiting and a reader hasn't woken up and 590 * acquired the lock yet, rw_lock will be set to the 591 * value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see 592 * that value, try to acquire it once. Note that we have 593 * to preserve the RW_LOCK_WRITE_WAITERS flag as there are 594 * other writers waiting still. If we fail, restart the 595 * loop. 596 */ 597 if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) { 598 if (atomic_cmpset_acq_ptr(&rw->rw_lock, 599 RW_UNLOCKED | RW_LOCK_WRITE_WAITERS, 600 tid | RW_LOCK_WRITE_WAITERS)) { 601 turnstile_claim(ts); 602 CTR2(KTR_LOCK, "%s: %p claimed by new writer", 603 __func__, rw); 604 break; 605 } 606 turnstile_cancel(ts); 607 cpu_spinwait(); 608 continue; 609 } 610 611 /* 612 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 613 * set it. If we fail to set it, then loop back and try 614 * again. 615 */ 616 if (!(v & RW_LOCK_WRITE_WAITERS)) { 617 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 618 v | RW_LOCK_WRITE_WAITERS)) { 619 turnstile_cancel(ts); 620 cpu_spinwait(); 621 continue; 622 } 623 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 624 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 625 __func__, rw); 626 } 627 628 /* 629 * We were unable to acquire the lock and the write waiters 630 * flag is set, so we must block on the turnstile. 631 */ 632 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 633 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 634 rw); 635 lock_profile_obtain_lock_failed(&rw->lock_object, &contested, 636 &waittime); 637 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE); 638 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 639 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 640 __func__, rw); 641 } 642 lock_profile_obtain_lock_success(&rw->lock_object, contested, waittime, 643 file, line); 644 } 645 646 /* 647 * This function is called if the first try at releasing a write lock failed. 648 * This means that one of the 2 waiter bits must be set indicating that at 649 * least one thread is waiting on this lock. 650 */ 651 void 652 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 653 { 654 struct turnstile *ts; 655 uintptr_t v; 656 int queue; 657 658 if (rw_wlocked(rw) && rw_recursed(rw)) { 659 if ((--rw->rw_recurse) == 0) 660 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 661 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 662 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 663 return; 664 } 665 666 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 667 ("%s: neither of the waiter flags are set", __func__)); 668 669 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 670 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 671 672 turnstile_chain_lock(&rw->lock_object); 673 ts = turnstile_lookup(&rw->lock_object); 674 675 MPASS(ts != NULL); 676 677 /* 678 * Use the same algo as sx locks for now. Prefer waking up shared 679 * waiters if we have any over writers. This is probably not ideal. 680 * 681 * 'v' is the value we are going to write back to rw_lock. If we 682 * have waiters on both queues, we need to preserve the state of 683 * the waiter flag for the queue we don't wake up. For now this is 684 * hardcoded for the algorithm mentioned above. 685 * 686 * In the case of both readers and writers waiting we wakeup the 687 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 688 * new writer comes in before a reader it will claim the lock up 689 * above. There is probably a potential priority inversion in 690 * there that could be worked around either by waking both queues 691 * of waiters or doing some complicated lock handoff gymnastics. 692 */ 693 v = RW_UNLOCKED; 694 if (rw->rw_lock & RW_LOCK_READ_WAITERS) { 695 queue = TS_SHARED_QUEUE; 696 v |= (rw->rw_lock & RW_LOCK_WRITE_WAITERS); 697 } else 698 queue = TS_EXCLUSIVE_QUEUE; 699 700 /* Wake up all waiters for the specific queue. */ 701 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 702 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 703 queue == TS_SHARED_QUEUE ? "read" : "write"); 704 turnstile_broadcast(ts, queue); 705 atomic_store_rel_ptr(&rw->rw_lock, v); 706 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 707 turnstile_chain_unlock(&rw->lock_object); 708 } 709 710 /* 711 * Attempt to do a non-blocking upgrade from a read lock to a write 712 * lock. This will only succeed if this thread holds a single read 713 * lock. Returns true if the upgrade succeeded and false otherwise. 714 */ 715 int 716 _rw_try_upgrade(struct rwlock *rw, const char *file, int line) 717 { 718 uintptr_t v, tid; 719 struct turnstile *ts; 720 int success; 721 722 KASSERT(rw->rw_lock != RW_DESTROYED, 723 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 724 _rw_assert(rw, RA_RLOCKED, file, line); 725 726 /* 727 * Attempt to switch from one reader to a writer. If there 728 * are any write waiters, then we will have to lock the 729 * turnstile first to prevent races with another writer 730 * calling turnstile_wait() before we have claimed this 731 * turnstile. So, do the simple case of no waiters first. 732 */ 733 tid = (uintptr_t)curthread; 734 if (!(rw->rw_lock & RW_LOCK_WRITE_WAITERS)) { 735 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 736 tid); 737 goto out; 738 } 739 740 /* 741 * Ok, we think we have write waiters, so lock the 742 * turnstile. 743 */ 744 ts = turnstile_trywait(&rw->lock_object); 745 746 /* 747 * Try to switch from one reader to a writer again. This time 748 * we honor the current state of the RW_LOCK_WRITE_WAITERS 749 * flag. If we obtain the lock with the flag set, then claim 750 * ownership of the turnstile. 751 */ 752 v = rw->rw_lock & RW_LOCK_WRITE_WAITERS; 753 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 754 tid | v); 755 if (success && v) 756 turnstile_claim(ts); 757 else 758 turnstile_cancel(ts); 759 out: 760 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 761 if (success) 762 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 763 file, line); 764 return (success); 765 } 766 767 /* 768 * Downgrade a write lock into a single read lock. 769 */ 770 void 771 _rw_downgrade(struct rwlock *rw, const char *file, int line) 772 { 773 struct turnstile *ts; 774 uintptr_t tid, v; 775 776 KASSERT(rw->rw_lock != RW_DESTROYED, 777 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 778 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line); 779 #ifndef INVARIANTS 780 if (rw_recursed(rw)) 781 panic("downgrade of a recursed lock"); 782 #endif 783 784 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 785 786 /* 787 * Convert from a writer to a single reader. First we handle 788 * the easy case with no waiters. If there are any waiters, we 789 * lock the turnstile, "disown" the lock, and awaken any read 790 * waiters. 791 */ 792 tid = (uintptr_t)curthread; 793 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 794 goto out; 795 796 /* 797 * Ok, we think we have waiters, so lock the turnstile so we can 798 * read the waiter flags without any races. 799 */ 800 turnstile_chain_lock(&rw->lock_object); 801 v = rw->rw_lock; 802 MPASS(v & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)); 803 804 /* 805 * Downgrade from a write lock while preserving 806 * RW_LOCK_WRITE_WAITERS and give up ownership of the 807 * turnstile. If there are any read waiters, wake them up. 808 */ 809 ts = turnstile_lookup(&rw->lock_object); 810 MPASS(ts != NULL); 811 if (v & RW_LOCK_READ_WAITERS) 812 turnstile_broadcast(ts, TS_SHARED_QUEUE); 813 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | 814 (v & RW_LOCK_WRITE_WAITERS)); 815 if (v & RW_LOCK_READ_WAITERS) 816 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 817 else if (ts) 818 turnstile_disown(ts); 819 turnstile_chain_unlock(&rw->lock_object); 820 out: 821 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 822 } 823 824 #ifdef INVARIANT_SUPPORT 825 #ifndef INVARIANTS 826 #undef _rw_assert 827 #endif 828 829 /* 830 * In the non-WITNESS case, rw_assert() can only detect that at least 831 * *some* thread owns an rlock, but it cannot guarantee that *this* 832 * thread owns an rlock. 833 */ 834 void 835 _rw_assert(struct rwlock *rw, int what, const char *file, int line) 836 { 837 838 if (panicstr != NULL) 839 return; 840 switch (what) { 841 case RA_LOCKED: 842 case RA_LOCKED | RA_RECURSED: 843 case RA_LOCKED | RA_NOTRECURSED: 844 case RA_RLOCKED: 845 #ifdef WITNESS 846 witness_assert(&rw->lock_object, what, file, line); 847 #else 848 /* 849 * If some other thread has a write lock or we have one 850 * and are asserting a read lock, fail. Also, if no one 851 * has a lock at all, fail. 852 */ 853 if (rw->rw_lock == RW_UNLOCKED || 854 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED || 855 rw_wowner(rw) != curthread))) 856 panic("Lock %s not %slocked @ %s:%d\n", 857 rw->lock_object.lo_name, (what == RA_RLOCKED) ? 858 "read " : "", file, line); 859 860 if (!(rw->rw_lock & RW_LOCK_READ)) { 861 if (rw_recursed(rw)) { 862 if (what & RA_NOTRECURSED) 863 panic("Lock %s recursed @ %s:%d\n", 864 rw->lock_object.lo_name, file, 865 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 } 870 #endif 871 break; 872 case RA_WLOCKED: 873 case RA_WLOCKED | RA_RECURSED: 874 case RA_WLOCKED | RA_NOTRECURSED: 875 if (rw_wowner(rw) != curthread) 876 panic("Lock %s not exclusively locked @ %s:%d\n", 877 rw->lock_object.lo_name, file, line); 878 if (rw_recursed(rw)) { 879 if (what & RA_NOTRECURSED) 880 panic("Lock %s recursed @ %s:%d\n", 881 rw->lock_object.lo_name, file, line); 882 } else if (what & RA_RECURSED) 883 panic("Lock %s not recursed @ %s:%d\n", 884 rw->lock_object.lo_name, file, line); 885 break; 886 case RA_UNLOCKED: 887 #ifdef WITNESS 888 witness_assert(&rw->lock_object, what, file, line); 889 #else 890 /* 891 * If we hold a write lock fail. We can't reliably check 892 * to see if we hold a read lock or not. 893 */ 894 if (rw_wowner(rw) == curthread) 895 panic("Lock %s exclusively locked @ %s:%d\n", 896 rw->lock_object.lo_name, file, line); 897 #endif 898 break; 899 default: 900 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 901 line); 902 } 903 } 904 #endif /* INVARIANT_SUPPORT */ 905 906 #ifdef DDB 907 void 908 db_show_rwlock(struct lock_object *lock) 909 { 910 struct rwlock *rw; 911 struct thread *td; 912 913 rw = (struct rwlock *)lock; 914 915 db_printf(" state: "); 916 if (rw->rw_lock == RW_UNLOCKED) 917 db_printf("UNLOCKED\n"); 918 else if (rw->rw_lock == RW_DESTROYED) { 919 db_printf("DESTROYED\n"); 920 return; 921 } else if (rw->rw_lock & RW_LOCK_READ) 922 db_printf("RLOCK: %ju locks\n", 923 (uintmax_t)(RW_READERS(rw->rw_lock))); 924 else { 925 td = rw_wowner(rw); 926 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 927 td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm); 928 if (rw_recursed(rw)) 929 db_printf(" recursed: %u\n", rw->rw_recurse); 930 } 931 db_printf(" waiters: "); 932 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 933 case RW_LOCK_READ_WAITERS: 934 db_printf("readers\n"); 935 break; 936 case RW_LOCK_WRITE_WAITERS: 937 db_printf("writers\n"); 938 break; 939 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 940 db_printf("readers and writers\n"); 941 break; 942 default: 943 db_printf("none\n"); 944 break; 945 } 946 } 947 948 #endif
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