kern_rwlock.c revision 171516
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 171516 2007-07-20 08:43:42Z attilio $"); 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 51CTASSERT((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 60static void db_show_rwlock(struct lock_object *lock); 61#endif 62static void lock_rw(struct lock_object *lock, int how); 63static int unlock_rw(struct lock_object *lock); 64 65struct 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 105void 106lock_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 117int 118unlock_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 133void 134rw_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 157void 158rw_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 167void 168rw_sysinit(void *arg) 169{ 170 struct rw_args *args = arg; 171 172 rw_init(args->ra_rw, args->ra_desc); 173} 174 175int 176rw_wowned(struct rwlock *rw) 177{ 178 179 return (rw_wowner(rw) == curthread); 180} 181 182void 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 KASSERT(rw_wowner(rw) != curthread, 190 ("%s (%s): wlock already held @ %s:%d", __func__, 191 rw->lock_object.lo_name, file, line)); 192 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 193 line); 194 __rw_wlock(rw, curthread, file, line); 195 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 196 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 197 curthread->td_locks++; 198} 199 200void 201_rw_wunlock(struct rwlock *rw, const char *file, int line) 202{ 203 204 MPASS(curthread != NULL); 205 KASSERT(rw->rw_lock != RW_DESTROYED, 206 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 207 _rw_assert(rw, RA_WLOCKED, file, line); 208 curthread->td_locks--; 209 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 210 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 211 line); 212 if (!rw_recursed(rw)) 213 lock_profile_release_lock(&rw->lock_object); 214 __rw_wunlock(rw, curthread, file, line); 215} 216 217void 218_rw_rlock(struct rwlock *rw, const char *file, int line) 219{ 220 struct turnstile *ts; 221#ifdef ADAPTIVE_RWLOCKS 222 volatile struct thread *owner; 223#endif 224#ifdef LOCK_PROFILING_SHARED 225 uint64_t waittime = 0; 226 int contested = 0; 227#endif 228 uintptr_t x; 229 230 KASSERT(rw->rw_lock != RW_DESTROYED, 231 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 232 KASSERT(rw_wowner(rw) != curthread, 233 ("%s (%s): wlock already held @ %s:%d", __func__, 234 rw->lock_object.lo_name, file, line)); 235 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line); 236 237 /* 238 * Note that we don't make any attempt to try to block read 239 * locks once a writer has blocked on the lock. The reason is 240 * that we currently allow for read locks to recurse and we 241 * don't keep track of all the holders of read locks. Thus, if 242 * we were to block readers once a writer blocked and a reader 243 * tried to recurse on their reader lock after a writer had 244 * blocked we would end up in a deadlock since the reader would 245 * be blocked on the writer, and the writer would be blocked 246 * waiting for the reader to release its original read lock. 247 */ 248 for (;;) { 249 /* 250 * Handle the easy case. If no other thread has a write 251 * lock, then try to bump up the count of read locks. Note 252 * that we have to preserve the current state of the 253 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 254 * read lock, then rw_lock must have changed, so restart 255 * the loop. Note that this handles the case of a 256 * completely unlocked rwlock since such a lock is encoded 257 * as a read lock with no waiters. 258 */ 259 x = rw->rw_lock; 260 if (x & RW_LOCK_READ) { 261 262 /* 263 * The RW_LOCK_READ_WAITERS flag should only be set 264 * if another thread currently holds a write lock, 265 * and in that case RW_LOCK_READ should be clear. 266 */ 267 MPASS((x & RW_LOCK_READ_WAITERS) == 0); 268 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, 269 x + RW_ONE_READER)) { 270#ifdef LOCK_PROFILING_SHARED 271 if (RW_READERS(x) == 0) 272 lock_profile_obtain_lock_success( 273 &rw->lock_object, contested, 274 waittime, file, line); 275#endif 276 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 277 CTR4(KTR_LOCK, 278 "%s: %p succeed %p -> %p", __func__, 279 rw, (void *)x, 280 (void *)(x + RW_ONE_READER)); 281 break; 282 } 283 cpu_spinwait(); 284 continue; 285 } 286 287 /* 288 * Okay, now it's the hard case. Some other thread already 289 * has a write lock, so acquire the turnstile lock so we can 290 * begin the process of blocking. 291 */ 292 ts = turnstile_trywait(&rw->lock_object); 293 294 /* 295 * The lock might have been released while we spun, so 296 * recheck its state and restart the loop if there is no 297 * longer a write lock. 298 */ 299 x = rw->rw_lock; 300 if (x & RW_LOCK_READ) { 301 turnstile_cancel(ts); 302 cpu_spinwait(); 303 continue; 304 } 305 306 /* 307 * Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS 308 * flag is already set, then we can go ahead and block. If 309 * it is not set then try to set it. If we fail to set it 310 * drop the turnstile lock and restart the loop. 311 */ 312 if (!(x & RW_LOCK_READ_WAITERS)) { 313 if (!atomic_cmpset_ptr(&rw->rw_lock, x, 314 x | RW_LOCK_READ_WAITERS)) { 315 turnstile_cancel(ts); 316 cpu_spinwait(); 317 continue; 318 } 319 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 320 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 321 __func__, rw); 322 } 323 324#ifdef ADAPTIVE_RWLOCKS 325 /* 326 * If the owner is running on another CPU, spin until 327 * the owner stops running or the state of the lock 328 * changes. 329 */ 330 owner = (struct thread *)RW_OWNER(x); 331 if (TD_IS_RUNNING(owner)) { 332 turnstile_cancel(ts); 333 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 334 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 335 __func__, rw, owner); 336#ifdef LOCK_PROFILING_SHARED 337 lock_profile_obtain_lock_failed(&rw->lock_object, 338 &contested, &waittime); 339#endif 340 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 341 TD_IS_RUNNING(owner)) 342 cpu_spinwait(); 343 continue; 344 } 345#endif 346 347 /* 348 * We were unable to acquire the lock and the read waiters 349 * flag is set, so we must block on the turnstile. 350 */ 351 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 352 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 353 rw); 354#ifdef LOCK_PROFILING_SHARED 355 lock_profile_obtain_lock_failed(&rw->lock_object, &contested, 356 &waittime); 357#endif 358 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE); 359 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 360 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 361 __func__, rw); 362 } 363 364 /* 365 * TODO: acquire "owner of record" here. Here be turnstile dragons 366 * however. turnstiles don't like owners changing between calls to 367 * turnstile_wait() currently. 368 */ 369 370 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 371 WITNESS_LOCK(&rw->lock_object, 0, file, line); 372 curthread->td_locks++; 373} 374 375void 376_rw_runlock(struct rwlock *rw, const char *file, int line) 377{ 378 struct turnstile *ts; 379 uintptr_t x; 380 381 KASSERT(rw->rw_lock != RW_DESTROYED, 382 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 383 _rw_assert(rw, RA_RLOCKED, file, line); 384 curthread->td_locks--; 385 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 386 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 387 388 /* TODO: drop "owner of record" here. */ 389 390 for (;;) { 391 /* 392 * See if there is more than one read lock held. If so, 393 * just drop one and return. 394 */ 395 x = rw->rw_lock; 396 if (RW_READERS(x) > 1) { 397 if (atomic_cmpset_ptr(&rw->rw_lock, x, 398 x - RW_ONE_READER)) { 399 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 400 CTR4(KTR_LOCK, 401 "%s: %p succeeded %p -> %p", 402 __func__, rw, (void *)x, 403 (void *)(x - RW_ONE_READER)); 404 break; 405 } 406 continue; 407 } 408 409 410 /* 411 * We should never have read waiters while at least one 412 * thread holds a read lock. (See note above) 413 */ 414 KASSERT(!(x & RW_LOCK_READ_WAITERS), 415 ("%s: waiting readers", __func__)); 416#ifdef LOCK_PROFILING_SHARED 417 lock_profile_release_lock(&rw->lock_object); 418#endif 419 420 /* 421 * If there aren't any waiters for a write lock, then try 422 * to drop it quickly. 423 */ 424 if (!(x & RW_LOCK_WRITE_WAITERS)) { 425 426 /* 427 * There shouldn't be any flags set and we should 428 * be the only read lock. If we fail to release 429 * the single read lock, then another thread might 430 * have just acquired a read lock, so go back up 431 * to the multiple read locks case. 432 */ 433 MPASS(x == RW_READERS_LOCK(1)); 434 if (atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 435 RW_UNLOCKED)) { 436 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 437 CTR2(KTR_LOCK, "%s: %p last succeeded", 438 __func__, rw); 439 break; 440 } 441 continue; 442 } 443 444 /* 445 * There should just be one reader with one or more 446 * writers waiting. 447 */ 448 MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS)); 449 450 /* 451 * Ok, we know we have a waiting writer and we think we 452 * are the last reader, so grab the turnstile lock. 453 */ 454 turnstile_chain_lock(&rw->lock_object); 455 456 /* 457 * Try to drop our lock leaving the lock in a unlocked 458 * state. 459 * 460 * If you wanted to do explicit lock handoff you'd have to 461 * do it here. You'd also want to use turnstile_signal() 462 * and you'd have to handle the race where a higher 463 * priority thread blocks on the write lock before the 464 * thread you wakeup actually runs and have the new thread 465 * "steal" the lock. For now it's a lot simpler to just 466 * wakeup all of the waiters. 467 * 468 * As above, if we fail, then another thread might have 469 * acquired a read lock, so drop the turnstile lock and 470 * restart. 471 */ 472 if (!atomic_cmpset_ptr(&rw->rw_lock, 473 RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) { 474 turnstile_chain_unlock(&rw->lock_object); 475 continue; 476 } 477 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 478 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 479 __func__, rw); 480 481 /* 482 * Ok. The lock is released and all that's left is to 483 * wake up the waiters. Note that the lock might not be 484 * free anymore, but in that case the writers will just 485 * block again if they run before the new lock holder(s) 486 * release the lock. 487 */ 488 ts = turnstile_lookup(&rw->lock_object); 489 MPASS(ts != NULL); 490 turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE); 491 turnstile_unpend(ts, TS_SHARED_LOCK); 492 turnstile_chain_unlock(&rw->lock_object); 493 break; 494 } 495} 496 497/* 498 * This function is called when we are unable to obtain a write lock on the 499 * first try. This means that at least one other thread holds either a 500 * read or write lock. 501 */ 502void 503_rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 504{ 505 struct turnstile *ts; 506#ifdef ADAPTIVE_RWLOCKS 507 volatile struct thread *owner; 508#endif 509 uint64_t waittime = 0; 510 uintptr_t v; 511 int contested = 0; 512 513 if (rw_wlocked(rw)) { 514 KASSERT(rw->lock_object.lo_flags & RW_RECURSE, 515 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 516 __func__, rw->lock_object.lo_name, file, line)); 517 rw->rw_recurse++; 518 atomic_set_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 519 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 520 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 521 return; 522 } 523 524 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 525 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 526 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 527 528 while (!_rw_write_lock(rw, tid)) { 529 ts = turnstile_trywait(&rw->lock_object); 530 v = rw->rw_lock; 531 532 /* 533 * If the lock was released while spinning on the 534 * turnstile chain lock, try again. 535 */ 536 if (v == RW_UNLOCKED) { 537 turnstile_cancel(ts); 538 cpu_spinwait(); 539 continue; 540 } 541 542 /* 543 * If the lock was released by a writer with both readers 544 * and writers waiting and a reader hasn't woken up and 545 * acquired the lock yet, rw_lock will be set to the 546 * value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see 547 * that value, try to acquire it once. Note that we have 548 * to preserve the RW_LOCK_WRITE_WAITERS flag as there are 549 * other writers waiting still. If we fail, restart the 550 * loop. 551 */ 552 if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) { 553 if (atomic_cmpset_acq_ptr(&rw->rw_lock, 554 RW_UNLOCKED | RW_LOCK_WRITE_WAITERS, 555 tid | RW_LOCK_WRITE_WAITERS)) { 556 turnstile_claim(ts); 557 CTR2(KTR_LOCK, "%s: %p claimed by new writer", 558 __func__, rw); 559 break; 560 } 561 turnstile_cancel(ts); 562 cpu_spinwait(); 563 continue; 564 } 565 566 /* 567 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 568 * set it. If we fail to set it, then loop back and try 569 * again. 570 */ 571 if (!(v & RW_LOCK_WRITE_WAITERS)) { 572 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 573 v | RW_LOCK_WRITE_WAITERS)) { 574 turnstile_cancel(ts); 575 cpu_spinwait(); 576 continue; 577 } 578 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 579 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 580 __func__, rw); 581 } 582 583#ifdef ADAPTIVE_RWLOCKS 584 /* 585 * If the lock is write locked and the owner is 586 * running on another CPU, spin until the owner stops 587 * running or the state of the lock changes. 588 */ 589 owner = (struct thread *)RW_OWNER(v); 590 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 591 turnstile_cancel(ts); 592 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 593 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 594 __func__, rw, owner); 595 lock_profile_obtain_lock_failed(&rw->lock_object, 596 &contested, &waittime); 597 while ((struct thread*)RW_OWNER(rw->rw_lock)== owner && 598 TD_IS_RUNNING(owner)) 599 cpu_spinwait(); 600 continue; 601 } 602#endif 603 604 /* 605 * We were unable to acquire the lock and the write waiters 606 * flag is set, so we must block on the turnstile. 607 */ 608 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 609 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 610 rw); 611 lock_profile_obtain_lock_failed(&rw->lock_object, &contested, 612 &waittime); 613 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE); 614 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 615 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 616 __func__, rw); 617 } 618 lock_profile_obtain_lock_success(&rw->lock_object, contested, waittime, 619 file, line); 620} 621 622/* 623 * This function is called if the first try at releasing a write lock failed. 624 * This means that one of the 2 waiter bits must be set indicating that at 625 * least one thread is waiting on this lock. 626 */ 627void 628_rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 629{ 630 struct turnstile *ts; 631 uintptr_t v; 632 int queue; 633 634 if (rw_wlocked(rw) && rw_recursed(rw)) { 635 if ((--rw->rw_recurse) == 0) 636 atomic_clear_ptr(&rw->rw_lock, RW_LOCK_RECURSED); 637 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 638 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 639 return; 640 } 641 642 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 643 ("%s: neither of the waiter flags are set", __func__)); 644 645 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 646 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 647 648 turnstile_chain_lock(&rw->lock_object); 649 ts = turnstile_lookup(&rw->lock_object); 650 651#ifdef ADAPTIVE_RWLOCKS 652 /* 653 * There might not be a turnstile for this lock if all of 654 * the waiters are adaptively spinning. In that case, just 655 * reset the lock to the unlocked state and return. 656 */ 657 if (ts == NULL) { 658 atomic_store_rel_ptr(&rw->rw_lock, RW_UNLOCKED); 659 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 660 CTR2(KTR_LOCK, "%s: %p no sleepers", __func__, rw); 661 turnstile_chain_unlock(&rw->lock_object); 662 return; 663 } 664#else 665 MPASS(ts != NULL); 666#endif 667 668 /* 669 * Use the same algo as sx locks for now. Prefer waking up shared 670 * waiters if we have any over writers. This is probably not ideal. 671 * 672 * 'v' is the value we are going to write back to rw_lock. If we 673 * have waiters on both queues, we need to preserve the state of 674 * the waiter flag for the queue we don't wake up. For now this is 675 * hardcoded for the algorithm mentioned above. 676 * 677 * In the case of both readers and writers waiting we wakeup the 678 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 679 * new writer comes in before a reader it will claim the lock up 680 * above. There is probably a potential priority inversion in 681 * there that could be worked around either by waking both queues 682 * of waiters or doing some complicated lock handoff gymnastics. 683 * 684 * Note that in the ADAPTIVE_RWLOCKS case, if both flags are 685 * set, there might not be any actual writers on the turnstile 686 * as they might all be spinning. In that case, we don't want 687 * to preserve the RW_LOCK_WRITE_WAITERS flag as the turnstile 688 * is going to go away once we wakeup all the readers. 689 */ 690 v = RW_UNLOCKED; 691 if (rw->rw_lock & RW_LOCK_READ_WAITERS) { 692 queue = TS_SHARED_QUEUE; 693#ifdef ADAPTIVE_RWLOCKS 694 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS && 695 !turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 696 v |= RW_LOCK_WRITE_WAITERS; 697#else 698 v |= (rw->rw_lock & RW_LOCK_WRITE_WAITERS); 699#endif 700 } else 701 queue = TS_EXCLUSIVE_QUEUE; 702 703#ifdef ADAPTIVE_RWLOCKS 704 /* 705 * We have to make sure that we actually have waiters to 706 * wakeup. If they are all spinning, then we just need to 707 * disown the turnstile and return. 708 */ 709 if (turnstile_empty(ts, queue)) { 710 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 711 CTR2(KTR_LOCK, "%s: %p no sleepers 2", __func__, rw); 712 atomic_store_rel_ptr(&rw->rw_lock, v); 713 turnstile_disown(ts); 714 turnstile_chain_unlock(&rw->lock_object); 715 return; 716 } 717#endif 718 719 /* Wake up all waiters for the specific queue. */ 720 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 721 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 722 queue == TS_SHARED_QUEUE ? "read" : "write"); 723 turnstile_broadcast(ts, queue); 724 atomic_store_rel_ptr(&rw->rw_lock, v); 725 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 726 turnstile_chain_unlock(&rw->lock_object); 727} 728 729/* 730 * Attempt to do a non-blocking upgrade from a read lock to a write 731 * lock. This will only succeed if this thread holds a single read 732 * lock. Returns true if the upgrade succeeded and false otherwise. 733 */ 734int 735_rw_try_upgrade(struct rwlock *rw, const char *file, int line) 736{ 737 uintptr_t v, tid; 738 struct turnstile *ts; 739 int success; 740 741 KASSERT(rw->rw_lock != RW_DESTROYED, 742 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 743 _rw_assert(rw, RA_RLOCKED, file, line); 744 745 /* 746 * Attempt to switch from one reader to a writer. If there 747 * are any write waiters, then we will have to lock the 748 * turnstile first to prevent races with another writer 749 * calling turnstile_wait() before we have claimed this 750 * turnstile. So, do the simple case of no waiters first. 751 */ 752 tid = (uintptr_t)curthread; 753 if (!(rw->rw_lock & RW_LOCK_WRITE_WAITERS)) { 754 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1), 755 tid); 756 goto out; 757 } 758 759 /* 760 * Ok, we think we have write waiters, so lock the 761 * turnstile. 762 */ 763 ts = turnstile_trywait(&rw->lock_object); 764 765 /* 766 * Try to switch from one reader to a writer again. This time 767 * we honor the current state of the RW_LOCK_WRITE_WAITERS 768 * flag. If we obtain the lock with the flag set, then claim 769 * ownership of the turnstile. In the ADAPTIVE_RWLOCKS case 770 * it is possible for there to not be an associated turnstile 771 * even though there are waiters if all of the waiters are 772 * spinning. 773 */ 774 v = rw->rw_lock & RW_LOCK_WRITE_WAITERS; 775 success = atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 776 tid | v); 777#ifdef ADAPTIVE_RWLOCKS 778 if (success && v && turnstile_lookup(&rw->lock_object) != NULL) 779#else 780 if (success && v) 781#endif 782 turnstile_claim(ts); 783 else 784 turnstile_cancel(ts); 785out: 786 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 787 if (success) 788 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 789 file, line); 790 return (success); 791} 792 793/* 794 * Downgrade a write lock into a single read lock. 795 */ 796void 797_rw_downgrade(struct rwlock *rw, const char *file, int line) 798{ 799 struct turnstile *ts; 800 uintptr_t tid, v; 801 802 KASSERT(rw->rw_lock != RW_DESTROYED, 803 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 804 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line); 805#ifndef INVARIANTS 806 if (rw_recursed(rw)) 807 panic("downgrade of a recursed lock"); 808#endif 809 810 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 811 812 /* 813 * Convert from a writer to a single reader. First we handle 814 * the easy case with no waiters. If there are any waiters, we 815 * lock the turnstile, "disown" the lock, and awaken any read 816 * waiters. 817 */ 818 tid = (uintptr_t)curthread; 819 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 820 goto out; 821 822 /* 823 * Ok, we think we have waiters, so lock the turnstile so we can 824 * read the waiter flags without any races. 825 */ 826 turnstile_chain_lock(&rw->lock_object); 827 v = rw->rw_lock; 828 MPASS(v & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)); 829 830 /* 831 * Downgrade from a write lock while preserving 832 * RW_LOCK_WRITE_WAITERS and give up ownership of the 833 * turnstile. If there are any read waiters, wake them up. 834 * 835 * For ADAPTIVE_RWLOCKS, we have to allow for the fact that 836 * all of the read waiters might be spinning. In that case, 837 * act as if RW_LOCK_READ_WAITERS is not set. Also, only 838 * preserve the RW_LOCK_WRITE_WAITERS flag if at least one 839 * writer is blocked on the turnstile. 840 */ 841 ts = turnstile_lookup(&rw->lock_object); 842#ifdef ADAPTIVE_RWLOCKS 843 if (ts == NULL) 844 v &= ~(RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS); 845 else if (v & RW_LOCK_READ_WAITERS && 846 turnstile_empty(ts, TS_SHARED_QUEUE)) 847 v &= ~RW_LOCK_READ_WAITERS; 848 else if (v & RW_LOCK_WRITE_WAITERS && 849 turnstile_empty(ts, TS_EXCLUSIVE_QUEUE)) 850 v &= ~RW_LOCK_WRITE_WAITERS; 851#else 852 MPASS(ts != NULL); 853#endif 854 if (v & RW_LOCK_READ_WAITERS) 855 turnstile_broadcast(ts, TS_SHARED_QUEUE); 856 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | 857 (v & RW_LOCK_WRITE_WAITERS)); 858 if (v & RW_LOCK_READ_WAITERS) 859 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 860 else if (ts) 861 turnstile_disown(ts); 862 turnstile_chain_unlock(&rw->lock_object); 863out: 864 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 865} 866 867#ifdef INVARIANT_SUPPORT 868#ifndef INVARIANTS 869#undef _rw_assert 870#endif 871 872/* 873 * In the non-WITNESS case, rw_assert() can only detect that at least 874 * *some* thread owns an rlock, but it cannot guarantee that *this* 875 * thread owns an rlock. 876 */ 877void 878_rw_assert(struct rwlock *rw, int what, const char *file, int line) 879{ 880 881 if (panicstr != NULL) 882 return; 883 switch (what) { 884 case RA_LOCKED: 885 case RA_LOCKED | RA_RECURSED: 886 case RA_LOCKED | RA_NOTRECURSED: 887 case RA_RLOCKED: 888#ifdef WITNESS 889 witness_assert(&rw->lock_object, what, file, line); 890#else 891 /* 892 * If some other thread has a write lock or we have one 893 * and are asserting a read lock, fail. Also, if no one 894 * has a lock at all, fail. 895 */ 896 if (rw->rw_lock == RW_UNLOCKED || 897 (!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED || 898 rw_wowner(rw) != curthread))) 899 panic("Lock %s not %slocked @ %s:%d\n", 900 rw->lock_object.lo_name, (what == RA_RLOCKED) ? 901 "read " : "", file, line); 902 903 if (!(rw->rw_lock & RW_LOCK_READ)) { 904 if (rw_recursed(rw)) { 905 if (what & RA_NOTRECURSED) 906 panic("Lock %s recursed @ %s:%d\n", 907 rw->lock_object.lo_name, file, 908 line); 909 } else if (what & RA_RECURSED) 910 panic("Lock %s not recursed @ %s:%d\n", 911 rw->lock_object.lo_name, file, line); 912 } 913#endif 914 break; 915 case RA_WLOCKED: 916 case RA_WLOCKED | RA_RECURSED: 917 case RA_WLOCKED | RA_NOTRECURSED: 918 if (rw_wowner(rw) != curthread) 919 panic("Lock %s not exclusively locked @ %s:%d\n", 920 rw->lock_object.lo_name, file, line); 921 if (rw_recursed(rw)) { 922 if (what & RA_NOTRECURSED) 923 panic("Lock %s recursed @ %s:%d\n", 924 rw->lock_object.lo_name, file, line); 925 } else if (what & RA_RECURSED) 926 panic("Lock %s not recursed @ %s:%d\n", 927 rw->lock_object.lo_name, file, line); 928 break; 929 case RA_UNLOCKED: 930#ifdef WITNESS 931 witness_assert(&rw->lock_object, what, file, line); 932#else 933 /* 934 * If we hold a write lock fail. We can't reliably check 935 * to see if we hold a read lock or not. 936 */ 937 if (rw_wowner(rw) == curthread) 938 panic("Lock %s exclusively locked @ %s:%d\n", 939 rw->lock_object.lo_name, file, line); 940#endif 941 break; 942 default: 943 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 944 line); 945 } 946} 947#endif /* INVARIANT_SUPPORT */ 948 949#ifdef DDB 950void 951db_show_rwlock(struct lock_object *lock) 952{ 953 struct rwlock *rw; 954 struct thread *td; 955 956 rw = (struct rwlock *)lock; 957 958 db_printf(" state: "); 959 if (rw->rw_lock == RW_UNLOCKED) 960 db_printf("UNLOCKED\n"); 961 else if (rw->rw_lock == RW_DESTROYED) { 962 db_printf("DESTROYED\n"); 963 return; 964 } else if (rw->rw_lock & RW_LOCK_READ) 965 db_printf("RLOCK: %ju locks\n", 966 (uintmax_t)(RW_READERS(rw->rw_lock))); 967 else { 968 td = rw_wowner(rw); 969 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 970 td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm); 971 if (rw_recursed(rw)) 972 db_printf(" recursed: %u\n", rw->rw_recurse); 973 } 974 db_printf(" waiters: "); 975 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 976 case RW_LOCK_READ_WAITERS: 977 db_printf("readers\n"); 978 break; 979 case RW_LOCK_WRITE_WAITERS: 980 db_printf("writers\n"); 981 break; 982 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 983 db_printf("readers and writers\n"); 984 break; 985 default: 986 db_printf("none\n"); 987 break; 988 } 989} 990 991#endif 992