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_kdtrace.h" 39 #include "opt_no_adaptive_rwlocks.h" 40 41 #include <sys/param.h> 42 #include <sys/ktr.h> 43 #include <sys/kernel.h> 44 #include <sys/lock.h> 45 #include <sys/mutex.h> 46 #include <sys/proc.h> 47 #include <sys/rwlock.h> 48 #include <sys/sysctl.h> 49 #include <sys/systm.h> 50 #include <sys/turnstile.h> 51 52 #include <machine/cpu.h> 53 54 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS) 55 #define ADAPTIVE_RWLOCKS 56 #endif 57 58 #ifdef ADAPTIVE_RWLOCKS 59 #define ROWNER_RETRIES 10 60 #define ROWNER_LOOPS 10000 61 #endif 62 63 #ifdef DDB 64 #include <ddb/ddb.h> 65 66 static void db_show_rwlock(struct lock_object *lock); 67 #endif 68 static void assert_rw(struct lock_object *lock, int what); 69 static void lock_rw(struct lock_object *lock, int how); 70 #ifdef KDTRACE_HOOKS 71 static int owner_rw(struct lock_object *lock, struct thread **owner); 72 #endif 73 static int unlock_rw(struct lock_object *lock); 74 75 struct lock_class lock_class_rw = { 76 .lc_name = "rw", 77 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 78 .lc_assert = assert_rw, 79 #ifdef DDB 80 .lc_ddb_show = db_show_rwlock, 81 #endif 82 .lc_lock = lock_rw, 83 .lc_unlock = unlock_rw, 84 #ifdef KDTRACE_HOOKS 85 .lc_owner = owner_rw, 86 #endif 87 }; 88 89 /* 90 * Return a pointer to the owning thread if the lock is write-locked or 91 * NULL if the lock is unlocked or read-locked. 92 */ 93 #define rw_wowner(rw) \ 94 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \ 95 (struct thread *)RW_OWNER((rw)->rw_lock)) 96 97 /* 98 * Returns if a write owner is recursed. Write ownership is not assured 99 * here and should be previously checked. 100 */ 101 #define rw_recursed(rw) ((rw)->rw_recurse != 0) 102 103 /* 104 * Return true if curthread helds the lock. 105 */ 106 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 107 108 /* 109 * Return a pointer to the owning thread for this lock who should receive 110 * any priority lent by threads that block on this lock. Currently this 111 * is identical to rw_wowner(). 112 */ 113 #define rw_owner(rw) rw_wowner(rw) 114 115 #ifndef INVARIANTS 116 #define _rw_assert(rw, what, file, line) 117 #endif 118 119 void 120 assert_rw(struct lock_object *lock, int what) 121 { 122 123 rw_assert((struct rwlock *)lock, what); 124 } 125 126 void 127 lock_rw(struct lock_object *lock, int how) 128 { 129 struct rwlock *rw; 130 131 rw = (struct rwlock *)lock; 132 if (how) 133 rw_wlock(rw); 134 else 135 rw_rlock(rw); 136 } 137 138 int 139 unlock_rw(struct lock_object *lock) 140 { 141 struct rwlock *rw; 142 143 rw = (struct rwlock *)lock; 144 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED); 145 if (rw->rw_lock & RW_LOCK_READ) { 146 rw_runlock(rw); 147 return (0); 148 } else { 149 rw_wunlock(rw); 150 return (1); 151 } 152 } 153 154 #ifdef KDTRACE_HOOKS 155 int 156 owner_rw(struct lock_object *lock, struct thread **owner) 157 { 158 struct rwlock *rw = (struct rwlock *)lock; 159 uintptr_t x = rw->rw_lock; 160 161 *owner = rw_wowner(rw); 162 return ((x & RW_LOCK_READ) != 0 ? (RW_READERS(x) != 0) : 163 (*owner != NULL)); 164 } 165 #endif 166 167 void 168 rw_init_flags(struct rwlock *rw, const char *name, int opts) 169 { 170 int flags; 171 172 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 173 RW_RECURSE)) == 0); 174 ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock, 175 ("%s: rw_lock not aligned for %s: %p", __func__, name, 176 &rw->rw_lock)); 177 178 flags = LO_UPGRADABLE; 179 if (opts & RW_DUPOK) 180 flags |= LO_DUPOK; 181 if (opts & RW_NOPROFILE) 182 flags |= LO_NOPROFILE; 183 if (!(opts & RW_NOWITNESS)) 184 flags |= LO_WITNESS; 185 if (opts & RW_RECURSE) 186 flags |= LO_RECURSABLE; 187 if (opts & RW_QUIET) 188 flags |= LO_QUIET; 189 190 rw->rw_lock = RW_UNLOCKED; 191 rw->rw_recurse = 0; 192 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 193 } 194 195 void 196 rw_destroy(struct rwlock *rw) 197 { 198 199 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw)); 200 KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw)); 201 rw->rw_lock = RW_DESTROYED; 202 lock_destroy(&rw->lock_object); 203 } 204 205 void 206 rw_sysinit(void *arg) 207 { 208 struct rw_args *args = arg; 209 210 rw_init(args->ra_rw, args->ra_desc); 211 } 212 213 void 214 rw_sysinit_flags(void *arg) 215 { 216 struct rw_args_flags *args = arg; 217 218 rw_init_flags(args->ra_rw, args->ra_desc, args->ra_flags); 219 } 220 221 int 222 rw_wowned(struct rwlock *rw) 223 { 224 225 return (rw_wowner(rw) == curthread); 226 } 227 228 void 229 _rw_wlock(struct rwlock *rw, const char *file, int line) 230 { 231 232 if (SCHEDULER_STOPPED()) 233 return; 234 MPASS(curthread != NULL); 235 KASSERT(rw->rw_lock != RW_DESTROYED, 236 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 237 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 238 line, NULL); 239 __rw_wlock(rw, curthread, file, line); 240 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 241 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 242 curthread->td_locks++; 243 } 244 245 int 246 _rw_try_wlock(struct rwlock *rw, const char *file, int line) 247 { 248 int rval; 249 250 if (SCHEDULER_STOPPED()) 251 return (1); 252 253 KASSERT(rw->rw_lock != RW_DESTROYED, 254 ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line)); 255 256 if (rw_wlocked(rw) && 257 (rw->lock_object.lo_flags & LO_RECURSABLE) != 0) { 258 rw->rw_recurse++; 259 rval = 1; 260 } else 261 rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED, 262 (uintptr_t)curthread); 263 264 LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line); 265 if (rval) { 266 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 267 file, line); 268 curthread->td_locks++; 269 } 270 return (rval); 271 } 272 273 void 274 _rw_wunlock(struct rwlock *rw, const char *file, int line) 275 { 276 277 if (SCHEDULER_STOPPED()) 278 return; 279 MPASS(curthread != NULL); 280 KASSERT(rw->rw_lock != RW_DESTROYED, 281 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 282 _rw_assert(rw, RA_WLOCKED, file, line); 283 curthread->td_locks--; 284 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 285 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 286 line); 287 if (!rw_recursed(rw)) 288 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_WUNLOCK_RELEASE, rw); 289 __rw_wunlock(rw, curthread, file, line); 290 } 291 /* 292 * Determines whether a new reader can acquire a lock. Succeeds if the 293 * reader already owns a read lock and the lock is locked for read to 294 * prevent deadlock from reader recursion. Also succeeds if the lock 295 * is unlocked and has no writer waiters or spinners. Failing otherwise 296 * prioritizes writers before readers. 297 */ 298 #define RW_CAN_READ(_rw) \ 299 ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) & \ 300 (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) == \ 301 RW_LOCK_READ) 302 303 void 304 _rw_rlock(struct rwlock *rw, const char *file, int line) 305 { 306 struct turnstile *ts; 307 #ifdef ADAPTIVE_RWLOCKS 308 volatile struct thread *owner; 309 int spintries = 0; 310 int i; 311 #endif 312 #ifdef LOCK_PROFILING 313 uint64_t waittime = 0; 314 int contested = 0; 315 #endif 316 uintptr_t v; 317 #ifdef KDTRACE_HOOKS 318 uint64_t spin_cnt = 0; 319 uint64_t sleep_cnt = 0; 320 int64_t sleep_time = 0; 321 #endif 322 323 if (SCHEDULER_STOPPED()) 324 return; 325 326 KASSERT(rw->rw_lock != RW_DESTROYED, 327 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 328 KASSERT(rw_wowner(rw) != curthread, 329 ("rw_rlock: wlock already held for %s @ %s:%d", 330 rw->lock_object.lo_name, file, line)); 331 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL); 332 333 for (;;) { 334 #ifdef KDTRACE_HOOKS 335 spin_cnt++; 336 #endif 337 /* 338 * Handle the easy case. If no other thread has a write 339 * lock, then try to bump up the count of read locks. Note 340 * that we have to preserve the current state of the 341 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 342 * read lock, then rw_lock must have changed, so restart 343 * the loop. Note that this handles the case of a 344 * completely unlocked rwlock since such a lock is encoded 345 * as a read lock with no waiters. 346 */ 347 v = rw->rw_lock; 348 if (RW_CAN_READ(v)) { 349 /* 350 * The RW_LOCK_READ_WAITERS flag should only be set 351 * if the lock has been unlocked and write waiters 352 * were present. 353 */ 354 if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, 355 v + RW_ONE_READER)) { 356 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 357 CTR4(KTR_LOCK, 358 "%s: %p succeed %p -> %p", __func__, 359 rw, (void *)v, 360 (void *)(v + RW_ONE_READER)); 361 break; 362 } 363 continue; 364 } 365 lock_profile_obtain_lock_failed(&rw->lock_object, 366 &contested, &waittime); 367 368 #ifdef ADAPTIVE_RWLOCKS 369 /* 370 * If the owner is running on another CPU, spin until 371 * the owner stops running or the state of the lock 372 * changes. 373 */ 374 if ((v & RW_LOCK_READ) == 0) { 375 owner = (struct thread *)RW_OWNER(v); 376 if (TD_IS_RUNNING(owner)) { 377 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 378 CTR3(KTR_LOCK, 379 "%s: spinning on %p held by %p", 380 __func__, rw, owner); 381 while ((struct thread*)RW_OWNER(rw->rw_lock) == 382 owner && TD_IS_RUNNING(owner)) { 383 cpu_spinwait(); 384 #ifdef KDTRACE_HOOKS 385 spin_cnt++; 386 #endif 387 } 388 continue; 389 } 390 } else if (spintries < ROWNER_RETRIES) { 391 spintries++; 392 for (i = 0; i < ROWNER_LOOPS; i++) { 393 v = rw->rw_lock; 394 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v)) 395 break; 396 cpu_spinwait(); 397 } 398 if (i != ROWNER_LOOPS) 399 continue; 400 } 401 #endif 402 403 /* 404 * Okay, now it's the hard case. Some other thread already 405 * has a write lock or there are write waiters present, 406 * acquire the turnstile lock so we can begin the process 407 * of blocking. 408 */ 409 ts = turnstile_trywait(&rw->lock_object); 410 411 /* 412 * The lock might have been released while we spun, so 413 * recheck its state and restart the loop if needed. 414 */ 415 v = rw->rw_lock; 416 if (RW_CAN_READ(v)) { 417 turnstile_cancel(ts); 418 continue; 419 } 420 421 #ifdef ADAPTIVE_RWLOCKS 422 /* 423 * The current lock owner might have started executing 424 * on another CPU (or the lock could have changed 425 * owners) while we were waiting on the turnstile 426 * chain lock. If so, drop the turnstile lock and try 427 * again. 428 */ 429 if ((v & RW_LOCK_READ) == 0) { 430 owner = (struct thread *)RW_OWNER(v); 431 if (TD_IS_RUNNING(owner)) { 432 turnstile_cancel(ts); 433 continue; 434 } 435 } 436 #endif 437 438 /* 439 * The lock is held in write mode or it already has waiters. 440 */ 441 MPASS(!RW_CAN_READ(v)); 442 443 /* 444 * If the RW_LOCK_READ_WAITERS flag is already set, then 445 * we can go ahead and block. If it is not set then try 446 * to set it. If we fail to set it drop the turnstile 447 * lock and restart the loop. 448 */ 449 if (!(v & RW_LOCK_READ_WAITERS)) { 450 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 451 v | RW_LOCK_READ_WAITERS)) { 452 turnstile_cancel(ts); 453 continue; 454 } 455 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 456 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 457 __func__, rw); 458 } 459 460 /* 461 * We were unable to acquire the lock and the read waiters 462 * flag is set, so we must block on the turnstile. 463 */ 464 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 465 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 466 rw); 467 #ifdef KDTRACE_HOOKS 468 sleep_time -= lockstat_nsecs(); 469 #endif 470 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE); 471 #ifdef KDTRACE_HOOKS 472 sleep_time += lockstat_nsecs(); 473 sleep_cnt++; 474 #endif 475 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 476 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 477 __func__, rw); 478 } 479 480 /* 481 * TODO: acquire "owner of record" here. Here be turnstile dragons 482 * however. turnstiles don't like owners changing between calls to 483 * turnstile_wait() currently. 484 */ 485 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_RLOCK_ACQUIRE, rw, contested, 486 waittime, file, line); 487 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 488 WITNESS_LOCK(&rw->lock_object, 0, file, line); 489 curthread->td_locks++; 490 curthread->td_rw_rlocks++; 491 #ifdef KDTRACE_HOOKS 492 if (sleep_time) 493 LOCKSTAT_RECORD1(LS_RW_RLOCK_BLOCK, rw, sleep_time); 494 495 /* 496 * Record only the loops spinning and not sleeping. 497 */ 498 if (spin_cnt > sleep_cnt) 499 LOCKSTAT_RECORD1(LS_RW_RLOCK_SPIN, rw, (spin_cnt - sleep_cnt)); 500 #endif 501 } 502 503 int 504 _rw_try_rlock(struct rwlock *rw, const char *file, int line) 505 { 506 uintptr_t x; 507 508 if (SCHEDULER_STOPPED()) 509 return (1); 510 511 for (;;) { 512 x = rw->rw_lock; 513 KASSERT(rw->rw_lock != RW_DESTROYED, 514 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line)); 515 if (!(x & RW_LOCK_READ)) 516 break; 517 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) { 518 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file, 519 line); 520 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line); 521 curthread->td_locks++; 522 curthread->td_rw_rlocks++; 523 return (1); 524 } 525 } 526 527 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line); 528 return (0); 529 } 530 531 void 532 _rw_runlock(struct rwlock *rw, const char *file, int line) 533 { 534 struct turnstile *ts; 535 uintptr_t x, v, queue; 536 537 if (SCHEDULER_STOPPED()) 538 return; 539 540 KASSERT(rw->rw_lock != RW_DESTROYED, 541 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 542 _rw_assert(rw, RA_RLOCKED, file, line); 543 curthread->td_locks--; 544 curthread->td_rw_rlocks--; 545 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 546 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 547 548 /* TODO: drop "owner of record" here. */ 549 550 for (;;) { 551 /* 552 * See if there is more than one read lock held. If so, 553 * just drop one and return. 554 */ 555 x = rw->rw_lock; 556 if (RW_READERS(x) > 1) { 557 if (atomic_cmpset_rel_ptr(&rw->rw_lock, x, 558 x - RW_ONE_READER)) { 559 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 560 CTR4(KTR_LOCK, 561 "%s: %p succeeded %p -> %p", 562 __func__, rw, (void *)x, 563 (void *)(x - RW_ONE_READER)); 564 break; 565 } 566 continue; 567 } 568 /* 569 * If there aren't any waiters for a write lock, then try 570 * to drop it quickly. 571 */ 572 if (!(x & RW_LOCK_WAITERS)) { 573 MPASS((x & ~RW_LOCK_WRITE_SPINNER) == 574 RW_READERS_LOCK(1)); 575 if (atomic_cmpset_rel_ptr(&rw->rw_lock, x, 576 RW_UNLOCKED)) { 577 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 578 CTR2(KTR_LOCK, "%s: %p last succeeded", 579 __func__, rw); 580 break; 581 } 582 continue; 583 } 584 /* 585 * Ok, we know we have waiters and we think we are the 586 * last reader, so grab the turnstile lock. 587 */ 588 turnstile_chain_lock(&rw->lock_object); 589 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 590 MPASS(v & RW_LOCK_WAITERS); 591 592 /* 593 * Try to drop our lock leaving the lock in a unlocked 594 * state. 595 * 596 * If you wanted to do explicit lock handoff you'd have to 597 * do it here. You'd also want to use turnstile_signal() 598 * and you'd have to handle the race where a higher 599 * priority thread blocks on the write lock before the 600 * thread you wakeup actually runs and have the new thread 601 * "steal" the lock. For now it's a lot simpler to just 602 * wakeup all of the waiters. 603 * 604 * As above, if we fail, then another thread might have 605 * acquired a read lock, so drop the turnstile lock and 606 * restart. 607 */ 608 x = RW_UNLOCKED; 609 if (v & RW_LOCK_WRITE_WAITERS) { 610 queue = TS_EXCLUSIVE_QUEUE; 611 x |= (v & RW_LOCK_READ_WAITERS); 612 } else 613 queue = TS_SHARED_QUEUE; 614 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 615 x)) { 616 turnstile_chain_unlock(&rw->lock_object); 617 continue; 618 } 619 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 620 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 621 __func__, rw); 622 623 /* 624 * Ok. The lock is released and all that's left is to 625 * wake up the waiters. Note that the lock might not be 626 * free anymore, but in that case the writers will just 627 * block again if they run before the new lock holder(s) 628 * release the lock. 629 */ 630 ts = turnstile_lookup(&rw->lock_object); 631 MPASS(ts != NULL); 632 turnstile_broadcast(ts, queue); 633 turnstile_unpend(ts, TS_SHARED_LOCK); 634 turnstile_chain_unlock(&rw->lock_object); 635 break; 636 } 637 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_RUNLOCK_RELEASE, rw); 638 } 639 640 /* 641 * This function is called when we are unable to obtain a write lock on the 642 * first try. This means that at least one other thread holds either a 643 * read or write lock. 644 */ 645 void 646 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 647 { 648 struct turnstile *ts; 649 #ifdef ADAPTIVE_RWLOCKS 650 volatile struct thread *owner; 651 int spintries = 0; 652 int i; 653 #endif 654 uintptr_t v, x; 655 #ifdef LOCK_PROFILING 656 uint64_t waittime = 0; 657 int contested = 0; 658 #endif 659 #ifdef KDTRACE_HOOKS 660 uint64_t spin_cnt = 0; 661 uint64_t sleep_cnt = 0; 662 int64_t sleep_time = 0; 663 #endif 664 665 if (SCHEDULER_STOPPED()) 666 return; 667 668 if (rw_wlocked(rw)) { 669 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE, 670 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 671 __func__, rw->lock_object.lo_name, file, line)); 672 rw->rw_recurse++; 673 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 674 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 675 return; 676 } 677 678 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 679 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 680 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 681 682 while (!_rw_write_lock(rw, tid)) { 683 #ifdef KDTRACE_HOOKS 684 spin_cnt++; 685 #endif 686 lock_profile_obtain_lock_failed(&rw->lock_object, 687 &contested, &waittime); 688 #ifdef ADAPTIVE_RWLOCKS 689 /* 690 * If the lock is write locked and the owner is 691 * running on another CPU, spin until the owner stops 692 * running or the state of the lock changes. 693 */ 694 v = rw->rw_lock; 695 owner = (struct thread *)RW_OWNER(v); 696 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 697 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 698 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 699 __func__, rw, owner); 700 while ((struct thread*)RW_OWNER(rw->rw_lock) == owner && 701 TD_IS_RUNNING(owner)) { 702 cpu_spinwait(); 703 #ifdef KDTRACE_HOOKS 704 spin_cnt++; 705 #endif 706 } 707 continue; 708 } 709 if ((v & RW_LOCK_READ) && RW_READERS(v) && 710 spintries < ROWNER_RETRIES) { 711 if (!(v & RW_LOCK_WRITE_SPINNER)) { 712 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 713 v | RW_LOCK_WRITE_SPINNER)) { 714 continue; 715 } 716 } 717 spintries++; 718 for (i = 0; i < ROWNER_LOOPS; i++) { 719 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0) 720 break; 721 cpu_spinwait(); 722 } 723 #ifdef KDTRACE_HOOKS 724 spin_cnt += ROWNER_LOOPS - i; 725 #endif 726 if (i != ROWNER_LOOPS) 727 continue; 728 } 729 #endif 730 ts = turnstile_trywait(&rw->lock_object); 731 v = rw->rw_lock; 732 733 #ifdef ADAPTIVE_RWLOCKS 734 /* 735 * The current lock owner might have started executing 736 * on another CPU (or the lock could have changed 737 * owners) while we were waiting on the turnstile 738 * chain lock. If so, drop the turnstile lock and try 739 * again. 740 */ 741 if (!(v & RW_LOCK_READ)) { 742 owner = (struct thread *)RW_OWNER(v); 743 if (TD_IS_RUNNING(owner)) { 744 turnstile_cancel(ts); 745 continue; 746 } 747 } 748 #endif 749 /* 750 * Check for the waiters flags about this rwlock. 751 * If the lock was released, without maintain any pending 752 * waiters queue, simply try to acquire it. 753 * If a pending waiters queue is present, claim the lock 754 * ownership and maintain the pending queue. 755 */ 756 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 757 if ((v & ~x) == RW_UNLOCKED) { 758 x &= ~RW_LOCK_WRITE_SPINNER; 759 if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) { 760 if (x) 761 turnstile_claim(ts); 762 else 763 turnstile_cancel(ts); 764 break; 765 } 766 turnstile_cancel(ts); 767 continue; 768 } 769 /* 770 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 771 * set it. If we fail to set it, then loop back and try 772 * again. 773 */ 774 if (!(v & RW_LOCK_WRITE_WAITERS)) { 775 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 776 v | RW_LOCK_WRITE_WAITERS)) { 777 turnstile_cancel(ts); 778 continue; 779 } 780 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 781 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 782 __func__, rw); 783 } 784 /* 785 * We were unable to acquire the lock and the write waiters 786 * flag is set, so we must block on the turnstile. 787 */ 788 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 789 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 790 rw); 791 #ifdef KDTRACE_HOOKS 792 sleep_time -= lockstat_nsecs(); 793 #endif 794 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE); 795 #ifdef KDTRACE_HOOKS 796 sleep_time += lockstat_nsecs(); 797 sleep_cnt++; 798 #endif 799 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 800 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 801 __func__, rw); 802 #ifdef ADAPTIVE_RWLOCKS 803 spintries = 0; 804 #endif 805 } 806 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_WLOCK_ACQUIRE, rw, contested, 807 waittime, file, line); 808 #ifdef KDTRACE_HOOKS 809 if (sleep_time) 810 LOCKSTAT_RECORD1(LS_RW_WLOCK_BLOCK, rw, sleep_time); 811 812 /* 813 * Record only the loops spinning and not sleeping. 814 */ 815 if (spin_cnt > sleep_cnt) 816 LOCKSTAT_RECORD1(LS_RW_WLOCK_SPIN, rw, (spin_cnt - sleep_cnt)); 817 #endif 818 } 819 820 /* 821 * This function is called if the first try at releasing a write lock failed. 822 * This means that one of the 2 waiter bits must be set indicating that at 823 * least one thread is waiting on this lock. 824 */ 825 void 826 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line) 827 { 828 struct turnstile *ts; 829 uintptr_t v; 830 int queue; 831 832 if (SCHEDULER_STOPPED()) 833 return; 834 835 if (rw_wlocked(rw) && rw_recursed(rw)) { 836 rw->rw_recurse--; 837 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 838 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 839 return; 840 } 841 842 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 843 ("%s: neither of the waiter flags are set", __func__)); 844 845 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 846 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 847 848 turnstile_chain_lock(&rw->lock_object); 849 ts = turnstile_lookup(&rw->lock_object); 850 MPASS(ts != NULL); 851 852 /* 853 * Use the same algo as sx locks for now. Prefer waking up shared 854 * waiters if we have any over writers. This is probably not ideal. 855 * 856 * 'v' is the value we are going to write back to rw_lock. If we 857 * have waiters on both queues, we need to preserve the state of 858 * the waiter flag for the queue we don't wake up. For now this is 859 * hardcoded for the algorithm mentioned above. 860 * 861 * In the case of both readers and writers waiting we wakeup the 862 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 863 * new writer comes in before a reader it will claim the lock up 864 * above. There is probably a potential priority inversion in 865 * there that could be worked around either by waking both queues 866 * of waiters or doing some complicated lock handoff gymnastics. 867 */ 868 v = RW_UNLOCKED; 869 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) { 870 queue = TS_EXCLUSIVE_QUEUE; 871 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS); 872 } else 873 queue = TS_SHARED_QUEUE; 874 875 /* Wake up all waiters for the specific queue. */ 876 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 877 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 878 queue == TS_SHARED_QUEUE ? "read" : "write"); 879 turnstile_broadcast(ts, queue); 880 atomic_store_rel_ptr(&rw->rw_lock, v); 881 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 882 turnstile_chain_unlock(&rw->lock_object); 883 } 884 885 /* 886 * Attempt to do a non-blocking upgrade from a read lock to a write 887 * lock. This will only succeed if this thread holds a single read 888 * lock. Returns true if the upgrade succeeded and false otherwise. 889 */ 890 int 891 _rw_try_upgrade(struct rwlock *rw, const char *file, int line) 892 { 893 uintptr_t v, x, tid; 894 struct turnstile *ts; 895 int success; 896 897 if (SCHEDULER_STOPPED()) 898 return (1); 899 900 KASSERT(rw->rw_lock != RW_DESTROYED, 901 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 902 _rw_assert(rw, RA_RLOCKED, file, line); 903 904 /* 905 * Attempt to switch from one reader to a writer. If there 906 * are any write waiters, then we will have to lock the 907 * turnstile first to prevent races with another writer 908 * calling turnstile_wait() before we have claimed this 909 * turnstile. So, do the simple case of no waiters first. 910 */ 911 tid = (uintptr_t)curthread; 912 success = 0; 913 for (;;) { 914 v = rw->rw_lock; 915 if (RW_READERS(v) > 1) 916 break; 917 if (!(v & RW_LOCK_WAITERS)) { 918 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid); 919 if (!success) 920 continue; 921 break; 922 } 923 924 /* 925 * Ok, we think we have waiters, so lock the turnstile. 926 */ 927 ts = turnstile_trywait(&rw->lock_object); 928 v = rw->rw_lock; 929 if (RW_READERS(v) > 1) { 930 turnstile_cancel(ts); 931 break; 932 } 933 /* 934 * Try to switch from one reader to a writer again. This time 935 * we honor the current state of the waiters flags. 936 * If we obtain the lock with the flags set, then claim 937 * ownership of the turnstile. 938 */ 939 x = rw->rw_lock & RW_LOCK_WAITERS; 940 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x); 941 if (success) { 942 if (x) 943 turnstile_claim(ts); 944 else 945 turnstile_cancel(ts); 946 break; 947 } 948 turnstile_cancel(ts); 949 } 950 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 951 if (success) { 952 curthread->td_rw_rlocks--; 953 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 954 file, line); 955 LOCKSTAT_RECORD0(LS_RW_TRYUPGRADE_UPGRADE, rw); 956 } 957 return (success); 958 } 959 960 /* 961 * Downgrade a write lock into a single read lock. 962 */ 963 void 964 _rw_downgrade(struct rwlock *rw, const char *file, int line) 965 { 966 struct turnstile *ts; 967 uintptr_t tid, v; 968 int rwait, wwait; 969 970 if (SCHEDULER_STOPPED()) 971 return; 972 973 KASSERT(rw->rw_lock != RW_DESTROYED, 974 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 975 _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line); 976 #ifndef INVARIANTS 977 if (rw_recursed(rw)) 978 panic("downgrade of a recursed lock"); 979 #endif 980 981 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 982 983 /* 984 * Convert from a writer to a single reader. First we handle 985 * the easy case with no waiters. If there are any waiters, we 986 * lock the turnstile and "disown" the lock. 987 */ 988 tid = (uintptr_t)curthread; 989 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 990 goto out; 991 992 /* 993 * Ok, we think we have waiters, so lock the turnstile so we can 994 * read the waiter flags without any races. 995 */ 996 turnstile_chain_lock(&rw->lock_object); 997 v = rw->rw_lock & RW_LOCK_WAITERS; 998 rwait = v & RW_LOCK_READ_WAITERS; 999 wwait = v & RW_LOCK_WRITE_WAITERS; 1000 MPASS(rwait | wwait); 1001 1002 /* 1003 * Downgrade from a write lock while preserving waiters flag 1004 * and give up ownership of the turnstile. 1005 */ 1006 ts = turnstile_lookup(&rw->lock_object); 1007 MPASS(ts != NULL); 1008 if (!wwait) 1009 v &= ~RW_LOCK_READ_WAITERS; 1010 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v); 1011 /* 1012 * Wake other readers if there are no writers pending. Otherwise they 1013 * won't be able to acquire the lock anyway. 1014 */ 1015 if (rwait && !wwait) { 1016 turnstile_broadcast(ts, TS_SHARED_QUEUE); 1017 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1018 } else 1019 turnstile_disown(ts); 1020 turnstile_chain_unlock(&rw->lock_object); 1021 out: 1022 curthread->td_rw_rlocks++; 1023 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 1024 LOCKSTAT_RECORD0(LS_RW_DOWNGRADE_DOWNGRADE, rw); 1025 } 1026 1027 #ifdef INVARIANT_SUPPORT 1028 #ifndef INVARIANTS 1029 #undef _rw_assert 1030 #endif 1031 1032 /* 1033 * In the non-WITNESS case, rw_assert() can only detect that at least 1034 * *some* thread owns an rlock, but it cannot guarantee that *this* 1035 * thread owns an rlock. 1036 */ 1037 void 1038 _rw_assert(struct rwlock *rw, int what, const char *file, int line) 1039 { 1040 1041 if (panicstr != NULL) 1042 return; 1043 switch (what) { 1044 case RA_LOCKED: 1045 case RA_LOCKED | RA_RECURSED: 1046 case RA_LOCKED | RA_NOTRECURSED: 1047 case RA_RLOCKED: 1048 case RA_RLOCKED | RA_RECURSED: 1049 case RA_RLOCKED | RA_NOTRECURSED: 1050 #ifdef WITNESS 1051 witness_assert(&rw->lock_object, what, file, line); 1052 #else 1053 /* 1054 * If some other thread has a write lock or we have one 1055 * and are asserting a read lock, fail. Also, if no one 1056 * has a lock at all, fail. 1057 */ 1058 if (rw->rw_lock == RW_UNLOCKED || 1059 (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED || 1060 rw_wowner(rw) != curthread))) 1061 panic("Lock %s not %slocked @ %s:%d\n", 1062 rw->lock_object.lo_name, (what & RA_RLOCKED) ? 1063 "read " : "", file, line); 1064 1065 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) { 1066 if (rw_recursed(rw)) { 1067 if (what & RA_NOTRECURSED) 1068 panic("Lock %s recursed @ %s:%d\n", 1069 rw->lock_object.lo_name, file, 1070 line); 1071 } else if (what & RA_RECURSED) 1072 panic("Lock %s not recursed @ %s:%d\n", 1073 rw->lock_object.lo_name, file, line); 1074 } 1075 #endif 1076 break; 1077 case RA_WLOCKED: 1078 case RA_WLOCKED | RA_RECURSED: 1079 case RA_WLOCKED | RA_NOTRECURSED: 1080 if (rw_wowner(rw) != curthread) 1081 panic("Lock %s not exclusively locked @ %s:%d\n", 1082 rw->lock_object.lo_name, file, line); 1083 if (rw_recursed(rw)) { 1084 if (what & RA_NOTRECURSED) 1085 panic("Lock %s recursed @ %s:%d\n", 1086 rw->lock_object.lo_name, file, line); 1087 } else if (what & RA_RECURSED) 1088 panic("Lock %s not recursed @ %s:%d\n", 1089 rw->lock_object.lo_name, file, line); 1090 break; 1091 case RA_UNLOCKED: 1092 #ifdef WITNESS 1093 witness_assert(&rw->lock_object, what, file, line); 1094 #else 1095 /* 1096 * If we hold a write lock fail. We can't reliably check 1097 * to see if we hold a read lock or not. 1098 */ 1099 if (rw_wowner(rw) == curthread) 1100 panic("Lock %s exclusively locked @ %s:%d\n", 1101 rw->lock_object.lo_name, file, line); 1102 #endif 1103 break; 1104 default: 1105 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 1106 line); 1107 } 1108 } 1109 #endif /* INVARIANT_SUPPORT */ 1110 1111 #ifdef DDB 1112 void 1113 db_show_rwlock(struct lock_object *lock) 1114 { 1115 struct rwlock *rw; 1116 struct thread *td; 1117 1118 rw = (struct rwlock *)lock; 1119 1120 db_printf(" state: "); 1121 if (rw->rw_lock == RW_UNLOCKED) 1122 db_printf("UNLOCKED\n"); 1123 else if (rw->rw_lock == RW_DESTROYED) { 1124 db_printf("DESTROYED\n"); 1125 return; 1126 } else if (rw->rw_lock & RW_LOCK_READ) 1127 db_printf("RLOCK: %ju locks\n", 1128 (uintmax_t)(RW_READERS(rw->rw_lock))); 1129 else { 1130 td = rw_wowner(rw); 1131 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 1132 td->td_tid, td->td_proc->p_pid, td->td_name); 1133 if (rw_recursed(rw)) 1134 db_printf(" recursed: %u\n", rw->rw_recurse); 1135 } 1136 db_printf(" waiters: "); 1137 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 1138 case RW_LOCK_READ_WAITERS: 1139 db_printf("readers\n"); 1140 break; 1141 case RW_LOCK_WRITE_WAITERS: 1142 db_printf("writers\n"); 1143 break; 1144 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 1145 db_printf("readers and writers\n"); 1146 break; 1147 default: 1148 db_printf("none\n"); 1149 break; 1150 } 1151 } 1152 1153 #endif
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