Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)
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sys/kern/kern_rwlock.c
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1 /*- 2 * Copyright (c) 2006 John Baldwin <jhb@FreeBSD.org> 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * 1. Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * 2. Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in the 12 * documentation and/or other materials provided with the distribution. 13 * 14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24 * SUCH DAMAGE. 25 */ 26 27 /* 28 * Machine independent bits of reader/writer lock implementation. 29 */ 30 31 #include <sys/cdefs.h> 32 __FBSDID("$FreeBSD: releng/10.1/sys/kern/kern_rwlock.c 262192 2014-02-18 20:27:17Z jhb $"); 33 34 #include "opt_ddb.h" 35 #include "opt_hwpmc_hooks.h" 36 #include "opt_kdtrace.h" 37 #include "opt_no_adaptive_rwlocks.h" 38 39 #include <sys/param.h> 40 #include <sys/kdb.h> 41 #include <sys/ktr.h> 42 #include <sys/kernel.h> 43 #include <sys/lock.h> 44 #include <sys/mutex.h> 45 #include <sys/proc.h> 46 #include <sys/rwlock.h> 47 #include <sys/sysctl.h> 48 #include <sys/systm.h> 49 #include <sys/turnstile.h> 50 51 #include <machine/cpu.h> 52 53 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS) 54 #define ADAPTIVE_RWLOCKS 55 #endif 56 57 #ifdef HWPMC_HOOKS 58 #include <sys/pmckern.h> 59 PMC_SOFT_DECLARE( , , lock, failed); 60 #endif 61 62 /* 63 * Return the rwlock address when the lock cookie address is provided. 64 * This functionality assumes that struct rwlock* have a member named rw_lock. 65 */ 66 #define rwlock2rw(c) (__containerof(c, struct rwlock, rw_lock)) 67 68 #ifdef ADAPTIVE_RWLOCKS 69 static int rowner_retries = 10; 70 static int rowner_loops = 10000; 71 static SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL, 72 "rwlock debugging"); 73 SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, ""); 74 SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, ""); 75 #endif 76 77 #ifdef DDB 78 #include <ddb/ddb.h> 79 80 static void db_show_rwlock(const struct lock_object *lock); 81 #endif 82 static void assert_rw(const struct lock_object *lock, int what); 83 static void lock_rw(struct lock_object *lock, uintptr_t how); 84 #ifdef KDTRACE_HOOKS 85 static int owner_rw(const struct lock_object *lock, struct thread **owner); 86 #endif 87 static uintptr_t unlock_rw(struct lock_object *lock); 88 89 struct lock_class lock_class_rw = { 90 .lc_name = "rw", 91 .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE, 92 .lc_assert = assert_rw, 93 #ifdef DDB 94 .lc_ddb_show = db_show_rwlock, 95 #endif 96 .lc_lock = lock_rw, 97 .lc_unlock = unlock_rw, 98 #ifdef KDTRACE_HOOKS 99 .lc_owner = owner_rw, 100 #endif 101 }; 102 103 /* 104 * Return a pointer to the owning thread if the lock is write-locked or 105 * NULL if the lock is unlocked or read-locked. 106 */ 107 #define rw_wowner(rw) \ 108 ((rw)->rw_lock & RW_LOCK_READ ? NULL : \ 109 (struct thread *)RW_OWNER((rw)->rw_lock)) 110 111 /* 112 * Returns if a write owner is recursed. Write ownership is not assured 113 * here and should be previously checked. 114 */ 115 #define rw_recursed(rw) ((rw)->rw_recurse != 0) 116 117 /* 118 * Return true if curthread helds the lock. 119 */ 120 #define rw_wlocked(rw) (rw_wowner((rw)) == curthread) 121 122 /* 123 * Return a pointer to the owning thread for this lock who should receive 124 * any priority lent by threads that block on this lock. Currently this 125 * is identical to rw_wowner(). 126 */ 127 #define rw_owner(rw) rw_wowner(rw) 128 129 #ifndef INVARIANTS 130 #define __rw_assert(c, what, file, line) 131 #endif 132 133 void 134 assert_rw(const struct lock_object *lock, int what) 135 { 136 137 rw_assert((const struct rwlock *)lock, what); 138 } 139 140 void 141 lock_rw(struct lock_object *lock, uintptr_t how) 142 { 143 struct rwlock *rw; 144 145 rw = (struct rwlock *)lock; 146 if (how) 147 rw_rlock(rw); 148 else 149 rw_wlock(rw); 150 } 151 152 uintptr_t 153 unlock_rw(struct lock_object *lock) 154 { 155 struct rwlock *rw; 156 157 rw = (struct rwlock *)lock; 158 rw_assert(rw, RA_LOCKED | LA_NOTRECURSED); 159 if (rw->rw_lock & RW_LOCK_READ) { 160 rw_runlock(rw); 161 return (1); 162 } else { 163 rw_wunlock(rw); 164 return (0); 165 } 166 } 167 168 #ifdef KDTRACE_HOOKS 169 int 170 owner_rw(const struct lock_object *lock, struct thread **owner) 171 { 172 const struct rwlock *rw = (const struct rwlock *)lock; 173 uintptr_t x = rw->rw_lock; 174 175 *owner = rw_wowner(rw); 176 return ((x & RW_LOCK_READ) != 0 ? (RW_READERS(x) != 0) : 177 (*owner != NULL)); 178 } 179 #endif 180 181 void 182 _rw_init_flags(volatile uintptr_t *c, const char *name, int opts) 183 { 184 struct rwlock *rw; 185 int flags; 186 187 rw = rwlock2rw(c); 188 189 MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET | 190 RW_RECURSE)) == 0); 191 ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock, 192 ("%s: rw_lock not aligned for %s: %p", __func__, name, 193 &rw->rw_lock)); 194 195 flags = LO_UPGRADABLE; 196 if (opts & RW_DUPOK) 197 flags |= LO_DUPOK; 198 if (opts & RW_NOPROFILE) 199 flags |= LO_NOPROFILE; 200 if (!(opts & RW_NOWITNESS)) 201 flags |= LO_WITNESS; 202 if (opts & RW_RECURSE) 203 flags |= LO_RECURSABLE; 204 if (opts & RW_QUIET) 205 flags |= LO_QUIET; 206 207 lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags); 208 rw->rw_lock = RW_UNLOCKED; 209 rw->rw_recurse = 0; 210 } 211 212 void 213 _rw_destroy(volatile uintptr_t *c) 214 { 215 struct rwlock *rw; 216 217 rw = rwlock2rw(c); 218 219 KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw)); 220 KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw)); 221 rw->rw_lock = RW_DESTROYED; 222 lock_destroy(&rw->lock_object); 223 } 224 225 void 226 rw_sysinit(void *arg) 227 { 228 struct rw_args *args = arg; 229 230 rw_init((struct rwlock *)args->ra_rw, args->ra_desc); 231 } 232 233 void 234 rw_sysinit_flags(void *arg) 235 { 236 struct rw_args_flags *args = arg; 237 238 rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc, 239 args->ra_flags); 240 } 241 242 int 243 _rw_wowned(const volatile uintptr_t *c) 244 { 245 246 return (rw_wowner(rwlock2rw(c)) == curthread); 247 } 248 249 void 250 _rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line) 251 { 252 struct rwlock *rw; 253 254 if (SCHEDULER_STOPPED()) 255 return; 256 257 rw = rwlock2rw(c); 258 259 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 260 ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d", 261 curthread, rw->lock_object.lo_name, file, line)); 262 KASSERT(rw->rw_lock != RW_DESTROYED, 263 ("rw_wlock() of destroyed rwlock @ %s:%d", file, line)); 264 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file, 265 line, NULL); 266 __rw_wlock(rw, curthread, file, line); 267 LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line); 268 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 269 curthread->td_locks++; 270 } 271 272 int 273 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line) 274 { 275 struct rwlock *rw; 276 int rval; 277 278 if (SCHEDULER_STOPPED()) 279 return (1); 280 281 rw = rwlock2rw(c); 282 283 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 284 ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d", 285 curthread, rw->lock_object.lo_name, file, line)); 286 KASSERT(rw->rw_lock != RW_DESTROYED, 287 ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line)); 288 289 if (rw_wlocked(rw) && 290 (rw->lock_object.lo_flags & LO_RECURSABLE) != 0) { 291 rw->rw_recurse++; 292 rval = 1; 293 } else 294 rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED, 295 (uintptr_t)curthread); 296 297 LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line); 298 if (rval) { 299 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 300 file, line); 301 curthread->td_locks++; 302 } 303 return (rval); 304 } 305 306 void 307 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line) 308 { 309 struct rwlock *rw; 310 311 if (SCHEDULER_STOPPED()) 312 return; 313 314 rw = rwlock2rw(c); 315 316 KASSERT(rw->rw_lock != RW_DESTROYED, 317 ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line)); 318 __rw_assert(c, RA_WLOCKED, file, line); 319 WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line); 320 LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file, 321 line); 322 if (!rw_recursed(rw)) 323 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_WUNLOCK_RELEASE, rw); 324 __rw_wunlock(rw, curthread, file, line); 325 curthread->td_locks--; 326 } 327 /* 328 * Determines whether a new reader can acquire a lock. Succeeds if the 329 * reader already owns a read lock and the lock is locked for read to 330 * prevent deadlock from reader recursion. Also succeeds if the lock 331 * is unlocked and has no writer waiters or spinners. Failing otherwise 332 * prioritizes writers before readers. 333 */ 334 #define RW_CAN_READ(_rw) \ 335 ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) & \ 336 (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) == \ 337 RW_LOCK_READ) 338 339 void 340 __rw_rlock(volatile uintptr_t *c, const char *file, int line) 341 { 342 struct rwlock *rw; 343 struct turnstile *ts; 344 #ifdef ADAPTIVE_RWLOCKS 345 volatile struct thread *owner; 346 int spintries = 0; 347 int i; 348 #endif 349 #ifdef LOCK_PROFILING 350 uint64_t waittime = 0; 351 int contested = 0; 352 #endif 353 uintptr_t v; 354 #ifdef KDTRACE_HOOKS 355 uint64_t spin_cnt = 0; 356 uint64_t sleep_cnt = 0; 357 int64_t sleep_time = 0; 358 #endif 359 360 if (SCHEDULER_STOPPED()) 361 return; 362 363 rw = rwlock2rw(c); 364 365 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 366 ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d", 367 curthread, rw->lock_object.lo_name, file, line)); 368 KASSERT(rw->rw_lock != RW_DESTROYED, 369 ("rw_rlock() of destroyed rwlock @ %s:%d", file, line)); 370 KASSERT(rw_wowner(rw) != curthread, 371 ("rw_rlock: wlock already held for %s @ %s:%d", 372 rw->lock_object.lo_name, file, line)); 373 WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL); 374 375 for (;;) { 376 #ifdef KDTRACE_HOOKS 377 spin_cnt++; 378 #endif 379 /* 380 * Handle the easy case. If no other thread has a write 381 * lock, then try to bump up the count of read locks. Note 382 * that we have to preserve the current state of the 383 * RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a 384 * read lock, then rw_lock must have changed, so restart 385 * the loop. Note that this handles the case of a 386 * completely unlocked rwlock since such a lock is encoded 387 * as a read lock with no waiters. 388 */ 389 v = rw->rw_lock; 390 if (RW_CAN_READ(v)) { 391 /* 392 * The RW_LOCK_READ_WAITERS flag should only be set 393 * if the lock has been unlocked and write waiters 394 * were present. 395 */ 396 if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, 397 v + RW_ONE_READER)) { 398 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 399 CTR4(KTR_LOCK, 400 "%s: %p succeed %p -> %p", __func__, 401 rw, (void *)v, 402 (void *)(v + RW_ONE_READER)); 403 break; 404 } 405 continue; 406 } 407 #ifdef HWPMC_HOOKS 408 PMC_SOFT_CALL( , , lock, failed); 409 #endif 410 lock_profile_obtain_lock_failed(&rw->lock_object, 411 &contested, &waittime); 412 413 #ifdef ADAPTIVE_RWLOCKS 414 /* 415 * If the owner is running on another CPU, spin until 416 * the owner stops running or the state of the lock 417 * changes. 418 */ 419 if ((v & RW_LOCK_READ) == 0) { 420 owner = (struct thread *)RW_OWNER(v); 421 if (TD_IS_RUNNING(owner)) { 422 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 423 CTR3(KTR_LOCK, 424 "%s: spinning on %p held by %p", 425 __func__, rw, owner); 426 while ((struct thread*)RW_OWNER(rw->rw_lock) == 427 owner && TD_IS_RUNNING(owner)) { 428 cpu_spinwait(); 429 #ifdef KDTRACE_HOOKS 430 spin_cnt++; 431 #endif 432 } 433 continue; 434 } 435 } else if (spintries < rowner_retries) { 436 spintries++; 437 for (i = 0; i < rowner_loops; i++) { 438 v = rw->rw_lock; 439 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v)) 440 break; 441 cpu_spinwait(); 442 } 443 if (i != rowner_loops) 444 continue; 445 } 446 #endif 447 448 /* 449 * Okay, now it's the hard case. Some other thread already 450 * has a write lock or there are write waiters present, 451 * acquire the turnstile lock so we can begin the process 452 * of blocking. 453 */ 454 ts = turnstile_trywait(&rw->lock_object); 455 456 /* 457 * The lock might have been released while we spun, so 458 * recheck its state and restart the loop if needed. 459 */ 460 v = rw->rw_lock; 461 if (RW_CAN_READ(v)) { 462 turnstile_cancel(ts); 463 continue; 464 } 465 466 #ifdef ADAPTIVE_RWLOCKS 467 /* 468 * The current lock owner might have started executing 469 * on another CPU (or the lock could have changed 470 * owners) while we were waiting on the turnstile 471 * chain lock. If so, drop the turnstile lock and try 472 * again. 473 */ 474 if ((v & RW_LOCK_READ) == 0) { 475 owner = (struct thread *)RW_OWNER(v); 476 if (TD_IS_RUNNING(owner)) { 477 turnstile_cancel(ts); 478 continue; 479 } 480 } 481 #endif 482 483 /* 484 * The lock is held in write mode or it already has waiters. 485 */ 486 MPASS(!RW_CAN_READ(v)); 487 488 /* 489 * If the RW_LOCK_READ_WAITERS flag is already set, then 490 * we can go ahead and block. If it is not set then try 491 * to set it. If we fail to set it drop the turnstile 492 * lock and restart the loop. 493 */ 494 if (!(v & RW_LOCK_READ_WAITERS)) { 495 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 496 v | RW_LOCK_READ_WAITERS)) { 497 turnstile_cancel(ts); 498 continue; 499 } 500 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 501 CTR2(KTR_LOCK, "%s: %p set read waiters flag", 502 __func__, rw); 503 } 504 505 /* 506 * We were unable to acquire the lock and the read waiters 507 * flag is set, so we must block on the turnstile. 508 */ 509 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 510 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 511 rw); 512 #ifdef KDTRACE_HOOKS 513 sleep_time -= lockstat_nsecs(); 514 #endif 515 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE); 516 #ifdef KDTRACE_HOOKS 517 sleep_time += lockstat_nsecs(); 518 sleep_cnt++; 519 #endif 520 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 521 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 522 __func__, rw); 523 } 524 525 /* 526 * TODO: acquire "owner of record" here. Here be turnstile dragons 527 * however. turnstiles don't like owners changing between calls to 528 * turnstile_wait() currently. 529 */ 530 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_RLOCK_ACQUIRE, rw, contested, 531 waittime, file, line); 532 LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line); 533 WITNESS_LOCK(&rw->lock_object, 0, file, line); 534 curthread->td_locks++; 535 curthread->td_rw_rlocks++; 536 #ifdef KDTRACE_HOOKS 537 if (sleep_time) 538 LOCKSTAT_RECORD1(LS_RW_RLOCK_BLOCK, rw, sleep_time); 539 540 /* 541 * Record only the loops spinning and not sleeping. 542 */ 543 if (spin_cnt > sleep_cnt) 544 LOCKSTAT_RECORD1(LS_RW_RLOCK_SPIN, rw, (spin_cnt - sleep_cnt)); 545 #endif 546 } 547 548 int 549 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line) 550 { 551 struct rwlock *rw; 552 uintptr_t x; 553 554 if (SCHEDULER_STOPPED()) 555 return (1); 556 557 rw = rwlock2rw(c); 558 559 KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread), 560 ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d", 561 curthread, rw->lock_object.lo_name, file, line)); 562 563 for (;;) { 564 x = rw->rw_lock; 565 KASSERT(rw->rw_lock != RW_DESTROYED, 566 ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line)); 567 if (!(x & RW_LOCK_READ)) 568 break; 569 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) { 570 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file, 571 line); 572 WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line); 573 curthread->td_locks++; 574 curthread->td_rw_rlocks++; 575 return (1); 576 } 577 } 578 579 LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line); 580 return (0); 581 } 582 583 void 584 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line) 585 { 586 struct rwlock *rw; 587 struct turnstile *ts; 588 uintptr_t x, v, queue; 589 590 if (SCHEDULER_STOPPED()) 591 return; 592 593 rw = rwlock2rw(c); 594 595 KASSERT(rw->rw_lock != RW_DESTROYED, 596 ("rw_runlock() of destroyed rwlock @ %s:%d", file, line)); 597 __rw_assert(c, RA_RLOCKED, file, line); 598 WITNESS_UNLOCK(&rw->lock_object, 0, file, line); 599 LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line); 600 601 /* TODO: drop "owner of record" here. */ 602 603 for (;;) { 604 /* 605 * See if there is more than one read lock held. If so, 606 * just drop one and return. 607 */ 608 x = rw->rw_lock; 609 if (RW_READERS(x) > 1) { 610 if (atomic_cmpset_rel_ptr(&rw->rw_lock, x, 611 x - RW_ONE_READER)) { 612 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 613 CTR4(KTR_LOCK, 614 "%s: %p succeeded %p -> %p", 615 __func__, rw, (void *)x, 616 (void *)(x - RW_ONE_READER)); 617 break; 618 } 619 continue; 620 } 621 /* 622 * If there aren't any waiters for a write lock, then try 623 * to drop it quickly. 624 */ 625 if (!(x & RW_LOCK_WAITERS)) { 626 MPASS((x & ~RW_LOCK_WRITE_SPINNER) == 627 RW_READERS_LOCK(1)); 628 if (atomic_cmpset_rel_ptr(&rw->rw_lock, x, 629 RW_UNLOCKED)) { 630 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 631 CTR2(KTR_LOCK, "%s: %p last succeeded", 632 __func__, rw); 633 break; 634 } 635 continue; 636 } 637 /* 638 * Ok, we know we have waiters and we think we are the 639 * last reader, so grab the turnstile lock. 640 */ 641 turnstile_chain_lock(&rw->lock_object); 642 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 643 MPASS(v & RW_LOCK_WAITERS); 644 645 /* 646 * Try to drop our lock leaving the lock in a unlocked 647 * state. 648 * 649 * If you wanted to do explicit lock handoff you'd have to 650 * do it here. You'd also want to use turnstile_signal() 651 * and you'd have to handle the race where a higher 652 * priority thread blocks on the write lock before the 653 * thread you wakeup actually runs and have the new thread 654 * "steal" the lock. For now it's a lot simpler to just 655 * wakeup all of the waiters. 656 * 657 * As above, if we fail, then another thread might have 658 * acquired a read lock, so drop the turnstile lock and 659 * restart. 660 */ 661 x = RW_UNLOCKED; 662 if (v & RW_LOCK_WRITE_WAITERS) { 663 queue = TS_EXCLUSIVE_QUEUE; 664 x |= (v & RW_LOCK_READ_WAITERS); 665 } else 666 queue = TS_SHARED_QUEUE; 667 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v, 668 x)) { 669 turnstile_chain_unlock(&rw->lock_object); 670 continue; 671 } 672 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 673 CTR2(KTR_LOCK, "%s: %p last succeeded with waiters", 674 __func__, rw); 675 676 /* 677 * Ok. The lock is released and all that's left is to 678 * wake up the waiters. Note that the lock might not be 679 * free anymore, but in that case the writers will just 680 * block again if they run before the new lock holder(s) 681 * release the lock. 682 */ 683 ts = turnstile_lookup(&rw->lock_object); 684 MPASS(ts != NULL); 685 turnstile_broadcast(ts, queue); 686 turnstile_unpend(ts, TS_SHARED_LOCK); 687 turnstile_chain_unlock(&rw->lock_object); 688 break; 689 } 690 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_RUNLOCK_RELEASE, rw); 691 curthread->td_locks--; 692 curthread->td_rw_rlocks--; 693 } 694 695 /* 696 * This function is called when we are unable to obtain a write lock on the 697 * first try. This means that at least one other thread holds either a 698 * read or write lock. 699 */ 700 void 701 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t tid, const char *file, 702 int line) 703 { 704 struct rwlock *rw; 705 struct turnstile *ts; 706 #ifdef ADAPTIVE_RWLOCKS 707 volatile struct thread *owner; 708 int spintries = 0; 709 int i; 710 #endif 711 uintptr_t v, x; 712 #ifdef LOCK_PROFILING 713 uint64_t waittime = 0; 714 int contested = 0; 715 #endif 716 #ifdef KDTRACE_HOOKS 717 uint64_t spin_cnt = 0; 718 uint64_t sleep_cnt = 0; 719 int64_t sleep_time = 0; 720 #endif 721 722 if (SCHEDULER_STOPPED()) 723 return; 724 725 rw = rwlock2rw(c); 726 727 if (rw_wlocked(rw)) { 728 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE, 729 ("%s: recursing but non-recursive rw %s @ %s:%d\n", 730 __func__, rw->lock_object.lo_name, file, line)); 731 rw->rw_recurse++; 732 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 733 CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw); 734 return; 735 } 736 737 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 738 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__, 739 rw->lock_object.lo_name, (void *)rw->rw_lock, file, line); 740 741 while (!_rw_write_lock(rw, tid)) { 742 #ifdef KDTRACE_HOOKS 743 spin_cnt++; 744 #endif 745 #ifdef HWPMC_HOOKS 746 PMC_SOFT_CALL( , , lock, failed); 747 #endif 748 lock_profile_obtain_lock_failed(&rw->lock_object, 749 &contested, &waittime); 750 #ifdef ADAPTIVE_RWLOCKS 751 /* 752 * If the lock is write locked and the owner is 753 * running on another CPU, spin until the owner stops 754 * running or the state of the lock changes. 755 */ 756 v = rw->rw_lock; 757 owner = (struct thread *)RW_OWNER(v); 758 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) { 759 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 760 CTR3(KTR_LOCK, "%s: spinning on %p held by %p", 761 __func__, rw, owner); 762 while ((struct thread*)RW_OWNER(rw->rw_lock) == owner && 763 TD_IS_RUNNING(owner)) { 764 cpu_spinwait(); 765 #ifdef KDTRACE_HOOKS 766 spin_cnt++; 767 #endif 768 } 769 continue; 770 } 771 if ((v & RW_LOCK_READ) && RW_READERS(v) && 772 spintries < rowner_retries) { 773 if (!(v & RW_LOCK_WRITE_SPINNER)) { 774 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 775 v | RW_LOCK_WRITE_SPINNER)) { 776 continue; 777 } 778 } 779 spintries++; 780 for (i = 0; i < rowner_loops; i++) { 781 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0) 782 break; 783 cpu_spinwait(); 784 } 785 #ifdef KDTRACE_HOOKS 786 spin_cnt += rowner_loops - i; 787 #endif 788 if (i != rowner_loops) 789 continue; 790 } 791 #endif 792 ts = turnstile_trywait(&rw->lock_object); 793 v = rw->rw_lock; 794 795 #ifdef ADAPTIVE_RWLOCKS 796 /* 797 * The current lock owner might have started executing 798 * on another CPU (or the lock could have changed 799 * owners) while we were waiting on the turnstile 800 * chain lock. If so, drop the turnstile lock and try 801 * again. 802 */ 803 if (!(v & RW_LOCK_READ)) { 804 owner = (struct thread *)RW_OWNER(v); 805 if (TD_IS_RUNNING(owner)) { 806 turnstile_cancel(ts); 807 continue; 808 } 809 } 810 #endif 811 /* 812 * Check for the waiters flags about this rwlock. 813 * If the lock was released, without maintain any pending 814 * waiters queue, simply try to acquire it. 815 * If a pending waiters queue is present, claim the lock 816 * ownership and maintain the pending queue. 817 */ 818 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER); 819 if ((v & ~x) == RW_UNLOCKED) { 820 x &= ~RW_LOCK_WRITE_SPINNER; 821 if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) { 822 if (x) 823 turnstile_claim(ts); 824 else 825 turnstile_cancel(ts); 826 break; 827 } 828 turnstile_cancel(ts); 829 continue; 830 } 831 /* 832 * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to 833 * set it. If we fail to set it, then loop back and try 834 * again. 835 */ 836 if (!(v & RW_LOCK_WRITE_WAITERS)) { 837 if (!atomic_cmpset_ptr(&rw->rw_lock, v, 838 v | RW_LOCK_WRITE_WAITERS)) { 839 turnstile_cancel(ts); 840 continue; 841 } 842 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 843 CTR2(KTR_LOCK, "%s: %p set write waiters flag", 844 __func__, rw); 845 } 846 /* 847 * We were unable to acquire the lock and the write waiters 848 * flag is set, so we must block on the turnstile. 849 */ 850 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 851 CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__, 852 rw); 853 #ifdef KDTRACE_HOOKS 854 sleep_time -= lockstat_nsecs(); 855 #endif 856 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE); 857 #ifdef KDTRACE_HOOKS 858 sleep_time += lockstat_nsecs(); 859 sleep_cnt++; 860 #endif 861 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 862 CTR2(KTR_LOCK, "%s: %p resuming from turnstile", 863 __func__, rw); 864 #ifdef ADAPTIVE_RWLOCKS 865 spintries = 0; 866 #endif 867 } 868 LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_WLOCK_ACQUIRE, rw, contested, 869 waittime, file, line); 870 #ifdef KDTRACE_HOOKS 871 if (sleep_time) 872 LOCKSTAT_RECORD1(LS_RW_WLOCK_BLOCK, rw, sleep_time); 873 874 /* 875 * Record only the loops spinning and not sleeping. 876 */ 877 if (spin_cnt > sleep_cnt) 878 LOCKSTAT_RECORD1(LS_RW_WLOCK_SPIN, rw, (spin_cnt - sleep_cnt)); 879 #endif 880 } 881 882 /* 883 * This function is called if the first try at releasing a write lock failed. 884 * This means that one of the 2 waiter bits must be set indicating that at 885 * least one thread is waiting on this lock. 886 */ 887 void 888 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t tid, const char *file, 889 int line) 890 { 891 struct rwlock *rw; 892 struct turnstile *ts; 893 uintptr_t v; 894 int queue; 895 896 if (SCHEDULER_STOPPED()) 897 return; 898 899 rw = rwlock2rw(c); 900 901 if (rw_wlocked(rw) && rw_recursed(rw)) { 902 rw->rw_recurse--; 903 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 904 CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw); 905 return; 906 } 907 908 KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS), 909 ("%s: neither of the waiter flags are set", __func__)); 910 911 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 912 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw); 913 914 turnstile_chain_lock(&rw->lock_object); 915 ts = turnstile_lookup(&rw->lock_object); 916 MPASS(ts != NULL); 917 918 /* 919 * Use the same algo as sx locks for now. Prefer waking up shared 920 * waiters if we have any over writers. This is probably not ideal. 921 * 922 * 'v' is the value we are going to write back to rw_lock. If we 923 * have waiters on both queues, we need to preserve the state of 924 * the waiter flag for the queue we don't wake up. For now this is 925 * hardcoded for the algorithm mentioned above. 926 * 927 * In the case of both readers and writers waiting we wakeup the 928 * readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a 929 * new writer comes in before a reader it will claim the lock up 930 * above. There is probably a potential priority inversion in 931 * there that could be worked around either by waking both queues 932 * of waiters or doing some complicated lock handoff gymnastics. 933 */ 934 v = RW_UNLOCKED; 935 if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) { 936 queue = TS_EXCLUSIVE_QUEUE; 937 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS); 938 } else 939 queue = TS_SHARED_QUEUE; 940 941 /* Wake up all waiters for the specific queue. */ 942 if (LOCK_LOG_TEST(&rw->lock_object, 0)) 943 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw, 944 queue == TS_SHARED_QUEUE ? "read" : "write"); 945 turnstile_broadcast(ts, queue); 946 atomic_store_rel_ptr(&rw->rw_lock, v); 947 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 948 turnstile_chain_unlock(&rw->lock_object); 949 } 950 951 /* 952 * Attempt to do a non-blocking upgrade from a read lock to a write 953 * lock. This will only succeed if this thread holds a single read 954 * lock. Returns true if the upgrade succeeded and false otherwise. 955 */ 956 int 957 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line) 958 { 959 struct rwlock *rw; 960 uintptr_t v, x, tid; 961 struct turnstile *ts; 962 int success; 963 964 if (SCHEDULER_STOPPED()) 965 return (1); 966 967 rw = rwlock2rw(c); 968 969 KASSERT(rw->rw_lock != RW_DESTROYED, 970 ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line)); 971 __rw_assert(c, RA_RLOCKED, file, line); 972 973 /* 974 * Attempt to switch from one reader to a writer. If there 975 * are any write waiters, then we will have to lock the 976 * turnstile first to prevent races with another writer 977 * calling turnstile_wait() before we have claimed this 978 * turnstile. So, do the simple case of no waiters first. 979 */ 980 tid = (uintptr_t)curthread; 981 success = 0; 982 for (;;) { 983 v = rw->rw_lock; 984 if (RW_READERS(v) > 1) 985 break; 986 if (!(v & RW_LOCK_WAITERS)) { 987 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid); 988 if (!success) 989 continue; 990 break; 991 } 992 993 /* 994 * Ok, we think we have waiters, so lock the turnstile. 995 */ 996 ts = turnstile_trywait(&rw->lock_object); 997 v = rw->rw_lock; 998 if (RW_READERS(v) > 1) { 999 turnstile_cancel(ts); 1000 break; 1001 } 1002 /* 1003 * Try to switch from one reader to a writer again. This time 1004 * we honor the current state of the waiters flags. 1005 * If we obtain the lock with the flags set, then claim 1006 * ownership of the turnstile. 1007 */ 1008 x = rw->rw_lock & RW_LOCK_WAITERS; 1009 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x); 1010 if (success) { 1011 if (x) 1012 turnstile_claim(ts); 1013 else 1014 turnstile_cancel(ts); 1015 break; 1016 } 1017 turnstile_cancel(ts); 1018 } 1019 LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line); 1020 if (success) { 1021 curthread->td_rw_rlocks--; 1022 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK, 1023 file, line); 1024 LOCKSTAT_RECORD0(LS_RW_TRYUPGRADE_UPGRADE, rw); 1025 } 1026 return (success); 1027 } 1028 1029 /* 1030 * Downgrade a write lock into a single read lock. 1031 */ 1032 void 1033 __rw_downgrade(volatile uintptr_t *c, const char *file, int line) 1034 { 1035 struct rwlock *rw; 1036 struct turnstile *ts; 1037 uintptr_t tid, v; 1038 int rwait, wwait; 1039 1040 if (SCHEDULER_STOPPED()) 1041 return; 1042 1043 rw = rwlock2rw(c); 1044 1045 KASSERT(rw->rw_lock != RW_DESTROYED, 1046 ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line)); 1047 __rw_assert(c, RA_WLOCKED | RA_NOTRECURSED, file, line); 1048 #ifndef INVARIANTS 1049 if (rw_recursed(rw)) 1050 panic("downgrade of a recursed lock"); 1051 #endif 1052 1053 WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line); 1054 1055 /* 1056 * Convert from a writer to a single reader. First we handle 1057 * the easy case with no waiters. If there are any waiters, we 1058 * lock the turnstile and "disown" the lock. 1059 */ 1060 tid = (uintptr_t)curthread; 1061 if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1))) 1062 goto out; 1063 1064 /* 1065 * Ok, we think we have waiters, so lock the turnstile so we can 1066 * read the waiter flags without any races. 1067 */ 1068 turnstile_chain_lock(&rw->lock_object); 1069 v = rw->rw_lock & RW_LOCK_WAITERS; 1070 rwait = v & RW_LOCK_READ_WAITERS; 1071 wwait = v & RW_LOCK_WRITE_WAITERS; 1072 MPASS(rwait | wwait); 1073 1074 /* 1075 * Downgrade from a write lock while preserving waiters flag 1076 * and give up ownership of the turnstile. 1077 */ 1078 ts = turnstile_lookup(&rw->lock_object); 1079 MPASS(ts != NULL); 1080 if (!wwait) 1081 v &= ~RW_LOCK_READ_WAITERS; 1082 atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v); 1083 /* 1084 * Wake other readers if there are no writers pending. Otherwise they 1085 * won't be able to acquire the lock anyway. 1086 */ 1087 if (rwait && !wwait) { 1088 turnstile_broadcast(ts, TS_SHARED_QUEUE); 1089 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK); 1090 } else 1091 turnstile_disown(ts); 1092 turnstile_chain_unlock(&rw->lock_object); 1093 out: 1094 curthread->td_rw_rlocks++; 1095 LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line); 1096 LOCKSTAT_RECORD0(LS_RW_DOWNGRADE_DOWNGRADE, rw); 1097 } 1098 1099 #ifdef INVARIANT_SUPPORT 1100 #ifndef INVARIANTS 1101 #undef __rw_assert 1102 #endif 1103 1104 /* 1105 * In the non-WITNESS case, rw_assert() can only detect that at least 1106 * *some* thread owns an rlock, but it cannot guarantee that *this* 1107 * thread owns an rlock. 1108 */ 1109 void 1110 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line) 1111 { 1112 const struct rwlock *rw; 1113 1114 if (panicstr != NULL) 1115 return; 1116 1117 rw = rwlock2rw(c); 1118 1119 switch (what) { 1120 case RA_LOCKED: 1121 case RA_LOCKED | RA_RECURSED: 1122 case RA_LOCKED | RA_NOTRECURSED: 1123 case RA_RLOCKED: 1124 case RA_RLOCKED | RA_RECURSED: 1125 case RA_RLOCKED | RA_NOTRECURSED: 1126 #ifdef WITNESS 1127 witness_assert(&rw->lock_object, what, file, line); 1128 #else 1129 /* 1130 * If some other thread has a write lock or we have one 1131 * and are asserting a read lock, fail. Also, if no one 1132 * has a lock at all, fail. 1133 */ 1134 if (rw->rw_lock == RW_UNLOCKED || 1135 (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED || 1136 rw_wowner(rw) != curthread))) 1137 panic("Lock %s not %slocked @ %s:%d\n", 1138 rw->lock_object.lo_name, (what & RA_RLOCKED) ? 1139 "read " : "", file, line); 1140 1141 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) { 1142 if (rw_recursed(rw)) { 1143 if (what & RA_NOTRECURSED) 1144 panic("Lock %s recursed @ %s:%d\n", 1145 rw->lock_object.lo_name, file, 1146 line); 1147 } else if (what & RA_RECURSED) 1148 panic("Lock %s not recursed @ %s:%d\n", 1149 rw->lock_object.lo_name, file, line); 1150 } 1151 #endif 1152 break; 1153 case RA_WLOCKED: 1154 case RA_WLOCKED | RA_RECURSED: 1155 case RA_WLOCKED | RA_NOTRECURSED: 1156 if (rw_wowner(rw) != curthread) 1157 panic("Lock %s not exclusively locked @ %s:%d\n", 1158 rw->lock_object.lo_name, file, line); 1159 if (rw_recursed(rw)) { 1160 if (what & RA_NOTRECURSED) 1161 panic("Lock %s recursed @ %s:%d\n", 1162 rw->lock_object.lo_name, file, line); 1163 } else if (what & RA_RECURSED) 1164 panic("Lock %s not recursed @ %s:%d\n", 1165 rw->lock_object.lo_name, file, line); 1166 break; 1167 case RA_UNLOCKED: 1168 #ifdef WITNESS 1169 witness_assert(&rw->lock_object, what, file, line); 1170 #else 1171 /* 1172 * If we hold a write lock fail. We can't reliably check 1173 * to see if we hold a read lock or not. 1174 */ 1175 if (rw_wowner(rw) == curthread) 1176 panic("Lock %s exclusively locked @ %s:%d\n", 1177 rw->lock_object.lo_name, file, line); 1178 #endif 1179 break; 1180 default: 1181 panic("Unknown rw lock assertion: %d @ %s:%d", what, file, 1182 line); 1183 } 1184 } 1185 #endif /* INVARIANT_SUPPORT */ 1186 1187 #ifdef DDB 1188 void 1189 db_show_rwlock(const struct lock_object *lock) 1190 { 1191 const struct rwlock *rw; 1192 struct thread *td; 1193 1194 rw = (const struct rwlock *)lock; 1195 1196 db_printf(" state: "); 1197 if (rw->rw_lock == RW_UNLOCKED) 1198 db_printf("UNLOCKED\n"); 1199 else if (rw->rw_lock == RW_DESTROYED) { 1200 db_printf("DESTROYED\n"); 1201 return; 1202 } else if (rw->rw_lock & RW_LOCK_READ) 1203 db_printf("RLOCK: %ju locks\n", 1204 (uintmax_t)(RW_READERS(rw->rw_lock))); 1205 else { 1206 td = rw_wowner(rw); 1207 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td, 1208 td->td_tid, td->td_proc->p_pid, td->td_name); 1209 if (rw_recursed(rw)) 1210 db_printf(" recursed: %u\n", rw->rw_recurse); 1211 } 1212 db_printf(" waiters: "); 1213 switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) { 1214 case RW_LOCK_READ_WAITERS: 1215 db_printf("readers\n"); 1216 break; 1217 case RW_LOCK_WRITE_WAITERS: 1218 db_printf("writers\n"); 1219 break; 1220 case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS: 1221 db_printf("readers and writers\n"); 1222 break; 1223 default: 1224 db_printf("none\n"); 1225 break; 1226 } 1227 } 1228 1229 #endif
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