1 /* $NetBSD: kern_rwlock.c,v 1.36 2010/02/08 09:54:27 skrll Exp $ */ 2 3 /*- 4 * Copyright (c) 2002, 2006, 2007, 2008, 2009 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by Jason R. Thorpe and Andrew Doran. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 19 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 21 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 22 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 23 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 24 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 25 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 26 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 27 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 28 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 29 * POSSIBILITY OF SUCH DAMAGE. 30 */ 31 32 /* 33 * Kernel reader/writer lock implementation, modeled after those 34 * found in Solaris, a description of which can be found in: 35 * 36 * Solaris Internals: Core Kernel Architecture, Jim Mauro and 37 * Richard McDougall. 38 */ 39 40 #include <sys/cdefs.h> 41 __KERNEL_RCSID(0, "$NetBSD: kern_rwlock.c,v 1.36 2010/02/08 09:54:27 skrll Exp $"); 42 43 #define __RWLOCK_PRIVATE 44 45 #include <sys/param.h> 46 #include <sys/proc.h> 47 #include <sys/rwlock.h> 48 #include <sys/sched.h> 49 #include <sys/sleepq.h> 50 #include <sys/systm.h> 51 #include <sys/lockdebug.h> 52 #include <sys/cpu.h> 53 #include <sys/atomic.h> 54 #include <sys/lock.h> 55 56 #include <dev/lockstat.h> 57 58 /* 59 * LOCKDEBUG 60 */ 61 62 #if defined(LOCKDEBUG) 63 64 #define RW_WANTLOCK(rw, op, t) \ 65 LOCKDEBUG_WANTLOCK(RW_DEBUG_P(rw), (rw), \ 66 (uintptr_t)__builtin_return_address(0), op == RW_READER, t); 67 #define RW_LOCKED(rw, op) \ 68 LOCKDEBUG_LOCKED(RW_DEBUG_P(rw), (rw), NULL, \ 69 (uintptr_t)__builtin_return_address(0), op == RW_READER); 70 #define RW_UNLOCKED(rw, op) \ 71 LOCKDEBUG_UNLOCKED(RW_DEBUG_P(rw), (rw), \ 72 (uintptr_t)__builtin_return_address(0), op == RW_READER); 73 #define RW_DASSERT(rw, cond) \ 74 do { \ 75 if (!(cond)) \ 76 rw_abort(rw, __func__, "assertion failed: " #cond); \ 77 } while (/* CONSTCOND */ 0); 78 79 #else /* LOCKDEBUG */ 80 81 #define RW_WANTLOCK(rw, op, t) /* nothing */ 82 #define RW_LOCKED(rw, op) /* nothing */ 83 #define RW_UNLOCKED(rw, op) /* nothing */ 84 #define RW_DASSERT(rw, cond) /* nothing */ 85 86 #endif /* LOCKDEBUG */ 87 88 /* 89 * DIAGNOSTIC 90 */ 91 92 #if defined(DIAGNOSTIC) 93 94 #define RW_ASSERT(rw, cond) \ 95 do { \ 96 if (!(cond)) \ 97 rw_abort(rw, __func__, "assertion failed: " #cond); \ 98 } while (/* CONSTCOND */ 0) 99 100 #else 101 102 #define RW_ASSERT(rw, cond) /* nothing */ 103 104 #endif /* DIAGNOSTIC */ 105 106 #define RW_SETDEBUG(rw, on) ((rw)->rw_owner |= (on) ? 0 : RW_NODEBUG) 107 #define RW_DEBUG_P(rw) (((rw)->rw_owner & RW_NODEBUG) == 0) 108 #if defined(LOCKDEBUG) 109 #define RW_INHERITDEBUG(new, old) (new) |= (old) & RW_NODEBUG 110 #else /* defined(LOCKDEBUG) */ 111 #define RW_INHERITDEBUG(new, old) /* nothing */ 112 #endif /* defined(LOCKDEBUG) */ 113 114 static void rw_abort(krwlock_t *, const char *, const char *); 115 static void rw_dump(volatile void *); 116 static lwp_t *rw_owner(wchan_t); 117 118 static inline uintptr_t 119 rw_cas(krwlock_t *rw, uintptr_t o, uintptr_t n) 120 { 121 122 RW_INHERITDEBUG(n, o); 123 return (uintptr_t)atomic_cas_ptr((volatile void *)&rw->rw_owner, 124 (void *)o, (void *)n); 125 } 126 127 static inline void 128 rw_swap(krwlock_t *rw, uintptr_t o, uintptr_t n) 129 { 130 131 RW_INHERITDEBUG(n, o); 132 n = (uintptr_t)atomic_swap_ptr((volatile void *)&rw->rw_owner, 133 (void *)n); 134 RW_DASSERT(rw, n == o); 135 } 136 137 /* 138 * For platforms that do not provide stubs, or for the LOCKDEBUG case. 139 */ 140 #ifdef LOCKDEBUG 141 #undef __HAVE_RW_STUBS 142 #endif 143 144 #ifndef __HAVE_RW_STUBS 145 __strong_alias(rw_enter,rw_vector_enter); 146 __strong_alias(rw_exit,rw_vector_exit); 147 __strong_alias(rw_tryenter,rw_vector_tryenter); 148 #endif 149 150 lockops_t rwlock_lockops = { 151 "Reader / writer lock", 152 LOCKOPS_SLEEP, 153 rw_dump 154 }; 155 156 syncobj_t rw_syncobj = { 157 SOBJ_SLEEPQ_SORTED, 158 turnstile_unsleep, 159 turnstile_changepri, 160 sleepq_lendpri, 161 rw_owner, 162 }; 163 164 /* 165 * rw_dump: 166 * 167 * Dump the contents of a rwlock structure. 168 */ 169 static void 170 rw_dump(volatile void *cookie) 171 { 172 volatile krwlock_t *rw = cookie; 173 174 printf_nolog("owner/count : %#018lx flags : %#018x\n", 175 (long)RW_OWNER(rw), (int)RW_FLAGS(rw)); 176 } 177 178 /* 179 * rw_abort: 180 * 181 * Dump information about an error and panic the system. This 182 * generates a lot of machine code in the DIAGNOSTIC case, so 183 * we ask the compiler to not inline it. 184 */ 185 static void __noinline 186 rw_abort(krwlock_t *rw, const char *func, const char *msg) 187 { 188 189 if (panicstr != NULL) 190 return; 191 192 LOCKDEBUG_ABORT(rw, &rwlock_lockops, func, msg); 193 } 194 195 /* 196 * rw_init: 197 * 198 * Initialize a rwlock for use. 199 */ 200 void 201 rw_init(krwlock_t *rw) 202 { 203 bool dodebug; 204 205 memset(rw, 0, sizeof(*rw)); 206 207 dodebug = LOCKDEBUG_ALLOC(rw, &rwlock_lockops, 208 (uintptr_t)__builtin_return_address(0)); 209 RW_SETDEBUG(rw, dodebug); 210 } 211 212 /* 213 * rw_destroy: 214 * 215 * Tear down a rwlock. 216 */ 217 void 218 rw_destroy(krwlock_t *rw) 219 { 220 221 RW_ASSERT(rw, (rw->rw_owner & ~RW_NODEBUG) == 0); 222 LOCKDEBUG_FREE(RW_DEBUG_P(rw), rw); 223 } 224 225 /* 226 * rw_onproc: 227 * 228 * Return true if an rwlock owner is running on a CPU in the system. 229 * If the target is waiting on the kernel big lock, then we must 230 * release it. This is necessary to avoid deadlock. 231 * 232 * Note that we can't use the rwlock owner field as an LWP pointer. We 233 * don't have full control over the timing of our execution, and so the 234 * pointer could be completely invalid by the time we dereference it. 235 */ 236 static int 237 rw_onproc(uintptr_t owner, struct cpu_info **cip) 238 { 239 #ifdef MULTIPROCESSOR 240 CPU_INFO_ITERATOR cii; 241 struct cpu_info *ci; 242 lwp_t *l; 243 244 if ((owner & (RW_WRITE_LOCKED|RW_HAS_WAITERS)) != RW_WRITE_LOCKED) 245 return 0; 246 l = (lwp_t *)(owner & RW_THREAD); 247 248 /* See if the target is running on a CPU somewhere. */ 249 if ((ci = *cip) != NULL && ci->ci_curlwp == l) 250 goto run; 251 for (CPU_INFO_FOREACH(cii, ci)) 252 if (ci->ci_curlwp == l) 253 goto run; 254 255 /* No: it may be safe to block now. */ 256 *cip = NULL; 257 return 0; 258 259 run: 260 /* Target is running; do we need to block? */ 261 *cip = ci; 262 return ci->ci_biglock_wanted != l; 263 #else 264 return 0; 265 #endif /* MULTIPROCESSOR */ 266 } 267 268 /* 269 * rw_vector_enter: 270 * 271 * Acquire a rwlock. 272 */ 273 void 274 rw_vector_enter(krwlock_t *rw, const krw_t op) 275 { 276 uintptr_t owner, incr, need_wait, set_wait, curthread, next; 277 struct cpu_info *ci; 278 turnstile_t *ts; 279 int queue; 280 lwp_t *l; 281 LOCKSTAT_TIMER(slptime); 282 LOCKSTAT_TIMER(slpcnt); 283 LOCKSTAT_TIMER(spintime); 284 LOCKSTAT_COUNTER(spincnt); 285 LOCKSTAT_FLAG(lsflag); 286 287 l = curlwp; 288 curthread = (uintptr_t)l; 289 290 RW_ASSERT(rw, !cpu_intr_p()); 291 RW_ASSERT(rw, curthread != 0); 292 RW_WANTLOCK(rw, op, false); 293 294 if (panicstr == NULL) { 295 LOCKDEBUG_BARRIER(&kernel_lock, 1); 296 } 297 298 /* 299 * We play a slight trick here. If we're a reader, we want 300 * increment the read count. If we're a writer, we want to 301 * set the owner field and whe WRITE_LOCKED bit. 302 * 303 * In the latter case, we expect those bits to be zero, 304 * therefore we can use an add operation to set them, which 305 * means an add operation for both cases. 306 */ 307 if (__predict_true(op == RW_READER)) { 308 incr = RW_READ_INCR; 309 set_wait = RW_HAS_WAITERS; 310 need_wait = RW_WRITE_LOCKED | RW_WRITE_WANTED; 311 queue = TS_READER_Q; 312 } else { 313 RW_DASSERT(rw, op == RW_WRITER); 314 incr = curthread | RW_WRITE_LOCKED; 315 set_wait = RW_HAS_WAITERS | RW_WRITE_WANTED; 316 need_wait = RW_WRITE_LOCKED | RW_THREAD; 317 queue = TS_WRITER_Q; 318 } 319 320 LOCKSTAT_ENTER(lsflag); 321 322 for (ci = NULL, owner = rw->rw_owner;;) { 323 /* 324 * Read the lock owner field. If the need-to-wait 325 * indicator is clear, then try to acquire the lock. 326 */ 327 if ((owner & need_wait) == 0) { 328 next = rw_cas(rw, owner, (owner + incr) & 329 ~RW_WRITE_WANTED); 330 if (__predict_true(next == owner)) { 331 /* Got it! */ 332 membar_enter(); 333 break; 334 } 335 336 /* 337 * Didn't get it -- spin around again (we'll 338 * probably sleep on the next iteration). 339 */ 340 owner = next; 341 continue; 342 } 343 344 if (__predict_false(panicstr != NULL)) 345 return; 346 if (__predict_false(RW_OWNER(rw) == curthread)) 347 rw_abort(rw, __func__, "locking against myself"); 348 349 /* 350 * If the lock owner is running on another CPU, and 351 * there are no existing waiters, then spin. 352 */ 353 if (rw_onproc(owner, &ci)) { 354 LOCKSTAT_START_TIMER(lsflag, spintime); 355 u_int count = SPINLOCK_BACKOFF_MIN; 356 do { 357 SPINLOCK_BACKOFF(count); 358 owner = rw->rw_owner; 359 } while (rw_onproc(owner, &ci)); 360 LOCKSTAT_STOP_TIMER(lsflag, spintime); 361 LOCKSTAT_COUNT(spincnt, 1); 362 if ((owner & need_wait) == 0) 363 continue; 364 } 365 366 /* 367 * Grab the turnstile chain lock. Once we have that, we 368 * can adjust the waiter bits and sleep queue. 369 */ 370 ts = turnstile_lookup(rw); 371 372 /* 373 * Mark the rwlock as having waiters. If the set fails, 374 * then we may not need to sleep and should spin again. 375 * Reload rw_owner because turnstile_lookup() may have 376 * spun on the turnstile chain lock. 377 */ 378 owner = rw->rw_owner; 379 if ((owner & need_wait) == 0 || rw_onproc(owner, &ci)) { 380 turnstile_exit(rw); 381 continue; 382 } 383 next = rw_cas(rw, owner, owner | set_wait); 384 if (__predict_false(next != owner)) { 385 turnstile_exit(rw); 386 owner = next; 387 continue; 388 } 389 390 LOCKSTAT_START_TIMER(lsflag, slptime); 391 turnstile_block(ts, queue, rw, &rw_syncobj); 392 LOCKSTAT_STOP_TIMER(lsflag, slptime); 393 LOCKSTAT_COUNT(slpcnt, 1); 394 395 /* 396 * No need for a memory barrier because of context switch. 397 * If not handed the lock, then spin again. 398 */ 399 if (op == RW_READER || (rw->rw_owner & RW_THREAD) == curthread) 400 break; 401 } 402 403 LOCKSTAT_EVENT(lsflag, rw, LB_RWLOCK | 404 (op == RW_WRITER ? LB_SLEEP1 : LB_SLEEP2), slpcnt, slptime); 405 LOCKSTAT_EVENT(lsflag, rw, LB_RWLOCK | LB_SPIN, spincnt, spintime); 406 LOCKSTAT_EXIT(lsflag); 407 408 RW_DASSERT(rw, (op != RW_READER && RW_OWNER(rw) == curthread) || 409 (op == RW_READER && RW_COUNT(rw) != 0)); 410 RW_LOCKED(rw, op); 411 } 412 413 /* 414 * rw_vector_exit: 415 * 416 * Release a rwlock. 417 */ 418 void 419 rw_vector_exit(krwlock_t *rw) 420 { 421 uintptr_t curthread, owner, decr, new, next; 422 turnstile_t *ts; 423 int rcnt, wcnt; 424 lwp_t *l; 425 426 curthread = (uintptr_t)curlwp; 427 RW_ASSERT(rw, curthread != 0); 428 429 if (__predict_false(panicstr != NULL)) 430 return; 431 432 /* 433 * Again, we use a trick. Since we used an add operation to 434 * set the required lock bits, we can use a subtract to clear 435 * them, which makes the read-release and write-release path 436 * the same. 437 */ 438 owner = rw->rw_owner; 439 if (__predict_false((owner & RW_WRITE_LOCKED) != 0)) { 440 RW_UNLOCKED(rw, RW_WRITER); 441 RW_ASSERT(rw, RW_OWNER(rw) == curthread); 442 decr = curthread | RW_WRITE_LOCKED; 443 } else { 444 RW_UNLOCKED(rw, RW_READER); 445 RW_ASSERT(rw, RW_COUNT(rw) != 0); 446 decr = RW_READ_INCR; 447 } 448 449 /* 450 * Compute what we expect the new value of the lock to be. Only 451 * proceed to do direct handoff if there are waiters, and if the 452 * lock would become unowned. 453 */ 454 membar_exit(); 455 for (;;) { 456 new = (owner - decr); 457 if ((new & (RW_THREAD | RW_HAS_WAITERS)) == RW_HAS_WAITERS) 458 break; 459 next = rw_cas(rw, owner, new); 460 if (__predict_true(next == owner)) 461 return; 462 owner = next; 463 } 464 465 /* 466 * Grab the turnstile chain lock. This gets the interlock 467 * on the sleep queue. Once we have that, we can adjust the 468 * waiter bits. 469 */ 470 ts = turnstile_lookup(rw); 471 owner = rw->rw_owner; 472 RW_DASSERT(rw, ts != NULL); 473 RW_DASSERT(rw, (owner & RW_HAS_WAITERS) != 0); 474 475 wcnt = TS_WAITERS(ts, TS_WRITER_Q); 476 rcnt = TS_WAITERS(ts, TS_READER_Q); 477 478 /* 479 * Give the lock away. 480 * 481 * If we are releasing a write lock, then prefer to wake all 482 * outstanding readers. Otherwise, wake one writer if there 483 * are outstanding readers, or all writers if there are no 484 * pending readers. If waking one specific writer, the writer 485 * is handed the lock here. If waking multiple writers, we 486 * set WRITE_WANTED to block out new readers, and let them 487 * do the work of acquring the lock in rw_vector_enter(). 488 */ 489 if (rcnt == 0 || decr == RW_READ_INCR) { 490 RW_DASSERT(rw, wcnt != 0); 491 RW_DASSERT(rw, (owner & RW_WRITE_WANTED) != 0); 492 493 if (rcnt != 0) { 494 /* Give the lock to the longest waiting writer. */ 495 l = TS_FIRST(ts, TS_WRITER_Q); 496 new = (uintptr_t)l | RW_WRITE_LOCKED | RW_HAS_WAITERS; 497 if (wcnt > 1) 498 new |= RW_WRITE_WANTED; 499 rw_swap(rw, owner, new); 500 turnstile_wakeup(ts, TS_WRITER_Q, 1, l); 501 } else { 502 /* Wake all writers and let them fight it out. */ 503 rw_swap(rw, owner, RW_WRITE_WANTED); 504 turnstile_wakeup(ts, TS_WRITER_Q, wcnt, NULL); 505 } 506 } else { 507 RW_DASSERT(rw, rcnt != 0); 508 509 /* 510 * Give the lock to all blocked readers. If there 511 * is a writer waiting, new readers that arrive 512 * after the release will be blocked out. 513 */ 514 new = rcnt << RW_READ_COUNT_SHIFT; 515 if (wcnt != 0) 516 new |= RW_HAS_WAITERS | RW_WRITE_WANTED; 517 518 /* Wake up all sleeping readers. */ 519 rw_swap(rw, owner, new); 520 turnstile_wakeup(ts, TS_READER_Q, rcnt, NULL); 521 } 522 } 523 524 /* 525 * rw_vector_tryenter: 526 * 527 * Try to acquire a rwlock. 528 */ 529 int 530 rw_vector_tryenter(krwlock_t *rw, const krw_t op) 531 { 532 uintptr_t curthread, owner, incr, need_wait, next; 533 534 curthread = (uintptr_t)curlwp; 535 536 RW_ASSERT(rw, curthread != 0); 537 538 if (op == RW_READER) { 539 incr = RW_READ_INCR; 540 need_wait = RW_WRITE_LOCKED | RW_WRITE_WANTED; 541 } else { 542 RW_DASSERT(rw, op == RW_WRITER); 543 incr = curthread | RW_WRITE_LOCKED; 544 need_wait = RW_WRITE_LOCKED | RW_THREAD; 545 } 546 547 for (owner = rw->rw_owner;; owner = next) { 548 owner = rw->rw_owner; 549 if (__predict_false((owner & need_wait) != 0)) 550 return 0; 551 next = rw_cas(rw, owner, owner + incr); 552 if (__predict_true(next == owner)) { 553 /* Got it! */ 554 membar_enter(); 555 break; 556 } 557 } 558 559 RW_WANTLOCK(rw, op, true); 560 RW_LOCKED(rw, op); 561 RW_DASSERT(rw, (op != RW_READER && RW_OWNER(rw) == curthread) || 562 (op == RW_READER && RW_COUNT(rw) != 0)); 563 564 return 1; 565 } 566 567 /* 568 * rw_downgrade: 569 * 570 * Downgrade a write lock to a read lock. 571 */ 572 void 573 rw_downgrade(krwlock_t *rw) 574 { 575 uintptr_t owner, curthread, new, next; 576 turnstile_t *ts; 577 int rcnt, wcnt; 578 579 curthread = (uintptr_t)curlwp; 580 RW_ASSERT(rw, curthread != 0); 581 RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) != 0); 582 RW_ASSERT(rw, RW_OWNER(rw) == curthread); 583 RW_UNLOCKED(rw, RW_WRITER); 584 585 membar_producer(); 586 owner = rw->rw_owner; 587 if ((owner & RW_HAS_WAITERS) == 0) { 588 /* 589 * There are no waiters, so we can do this the easy way. 590 * Try swapping us down to one read hold. If it fails, the 591 * lock condition has changed and we most likely now have 592 * waiters. 593 */ 594 next = rw_cas(rw, owner, RW_READ_INCR); 595 if (__predict_true(next == owner)) { 596 RW_LOCKED(rw, RW_READER); 597 RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0); 598 RW_DASSERT(rw, RW_COUNT(rw) != 0); 599 return; 600 } 601 owner = next; 602 } 603 604 /* 605 * Grab the turnstile chain lock. This gets the interlock 606 * on the sleep queue. Once we have that, we can adjust the 607 * waiter bits. 608 */ 609 for (;; owner = next) { 610 ts = turnstile_lookup(rw); 611 RW_DASSERT(rw, ts != NULL); 612 613 rcnt = TS_WAITERS(ts, TS_READER_Q); 614 wcnt = TS_WAITERS(ts, TS_WRITER_Q); 615 616 /* 617 * If there are no readers, just preserve the waiters 618 * bits, swap us down to one read hold and return. 619 */ 620 if (rcnt == 0) { 621 RW_DASSERT(rw, wcnt != 0); 622 RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_WANTED) != 0); 623 RW_DASSERT(rw, (rw->rw_owner & RW_HAS_WAITERS) != 0); 624 625 new = RW_READ_INCR | RW_HAS_WAITERS | RW_WRITE_WANTED; 626 next = rw_cas(rw, owner, new); 627 turnstile_exit(rw); 628 if (__predict_true(next == owner)) 629 break; 630 } else { 631 /* 632 * Give the lock to all blocked readers. We may 633 * retain one read hold if downgrading. If there 634 * is a writer waiting, new readers will be blocked 635 * out. 636 */ 637 new = (rcnt << RW_READ_COUNT_SHIFT) + RW_READ_INCR; 638 if (wcnt != 0) 639 new |= RW_HAS_WAITERS | RW_WRITE_WANTED; 640 641 next = rw_cas(rw, owner, new); 642 if (__predict_true(next == owner)) { 643 /* Wake up all sleeping readers. */ 644 turnstile_wakeup(ts, TS_READER_Q, rcnt, NULL); 645 break; 646 } 647 turnstile_exit(rw); 648 } 649 } 650 651 RW_WANTLOCK(rw, RW_READER, false); 652 RW_LOCKED(rw, RW_READER); 653 RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0); 654 RW_DASSERT(rw, RW_COUNT(rw) != 0); 655 } 656 657 /* 658 * rw_tryupgrade: 659 * 660 * Try to upgrade a read lock to a write lock. We must be the 661 * only reader. 662 */ 663 int 664 rw_tryupgrade(krwlock_t *rw) 665 { 666 uintptr_t owner, curthread, new, next; 667 668 curthread = (uintptr_t)curlwp; 669 RW_ASSERT(rw, curthread != 0); 670 RW_ASSERT(rw, rw_read_held(rw)); 671 672 for (owner = rw->rw_owner;; owner = next) { 673 RW_ASSERT(rw, (owner & RW_WRITE_LOCKED) == 0); 674 if (__predict_false((owner & RW_THREAD) != RW_READ_INCR)) { 675 RW_ASSERT(rw, (owner & RW_THREAD) != 0); 676 return 0; 677 } 678 new = curthread | RW_WRITE_LOCKED | (owner & ~RW_THREAD); 679 next = rw_cas(rw, owner, new); 680 if (__predict_true(next == owner)) { 681 membar_producer(); 682 break; 683 } 684 } 685 686 RW_UNLOCKED(rw, RW_READER); 687 RW_WANTLOCK(rw, RW_WRITER, true); 688 RW_LOCKED(rw, RW_WRITER); 689 RW_DASSERT(rw, rw->rw_owner & RW_WRITE_LOCKED); 690 RW_DASSERT(rw, RW_OWNER(rw) == curthread); 691 692 return 1; 693 } 694 695 /* 696 * rw_read_held: 697 * 698 * Returns true if the rwlock is held for reading. Must only be 699 * used for diagnostic assertions, and never be used to make 700 * decisions about how to use a rwlock. 701 */ 702 int 703 rw_read_held(krwlock_t *rw) 704 { 705 uintptr_t owner; 706 707 if (panicstr != NULL) 708 return 1; 709 if (rw == NULL) 710 return 0; 711 owner = rw->rw_owner; 712 return (owner & RW_WRITE_LOCKED) == 0 && (owner & RW_THREAD) != 0; 713 } 714 715 /* 716 * rw_write_held: 717 * 718 * Returns true if the rwlock is held for writing. Must only be 719 * used for diagnostic assertions, and never be used to make 720 * decisions about how to use a rwlock. 721 */ 722 int 723 rw_write_held(krwlock_t *rw) 724 { 725 726 if (panicstr != NULL) 727 return 1; 728 if (rw == NULL) 729 return 0; 730 return (rw->rw_owner & (RW_WRITE_LOCKED | RW_THREAD)) == 731 (RW_WRITE_LOCKED | (uintptr_t)curlwp); 732 } 733 734 /* 735 * rw_lock_held: 736 * 737 * Returns true if the rwlock is held for reading or writing. Must 738 * only be used for diagnostic assertions, and never be used to make 739 * decisions about how to use a rwlock. 740 */ 741 int 742 rw_lock_held(krwlock_t *rw) 743 { 744 745 if (panicstr != NULL) 746 return 1; 747 if (rw == NULL) 748 return 0; 749 return (rw->rw_owner & RW_THREAD) != 0; 750 } 751 752 /* 753 * rw_owner: 754 * 755 * Return the current owner of an RW lock, but only if it is write 756 * held. Used for priority inheritance. 757 */ 758 static lwp_t * 759 rw_owner(wchan_t obj) 760 { 761 krwlock_t *rw = (void *)(uintptr_t)obj; /* discard qualifiers */ 762 uintptr_t owner = rw->rw_owner; 763 764 if ((owner & RW_WRITE_LOCKED) == 0) 765 return NULL; 766 767 return (void *)(owner & RW_THREAD); 768 } 769