| 1 | /* $NetBSD: kern_rwlock.c,v 1.45 2014/11/28 08:28:17 uebayasi 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 1.45 2014/11/28 08:28:17 uebayasi 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) \ |
| 65 | LOCKDEBUG_WANTLOCK(RW_DEBUG_P(rw), (rw), \ |
| 66 | (uintptr_t)__builtin_return_address(0), op == RW_READER); |
| 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) /* 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(n, o) (n) |= (o) & RW_NODEBUG |
| 110 | #else /* defined(LOCKDEBUG) */ |
| 111 | #define RW_INHERITDEBUG(n, o) /* 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_oncpu: |
| 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 | static bool |
| 233 | rw_oncpu(uintptr_t owner) |
| 234 | { |
| 235 | #ifdef MULTIPROCESSOR |
| 236 | struct cpu_info *ci; |
| 237 | lwp_t *l; |
| 238 | |
| 239 | KASSERT(kpreempt_disabled()); |
| 240 | |
| 241 | if ((owner & (RW_WRITE_LOCKED|RW_HAS_WAITERS)) != RW_WRITE_LOCKED) { |
| 242 | return false; |
| 243 | } |
| 244 | |
| 245 | /* |
| 246 | * See lwp_dtor() why dereference of the LWP pointer is safe. |
| 247 | * We must have kernel preemption disabled for that. |
| 248 | */ |
| 249 | l = (lwp_t *)(owner & RW_THREAD); |
| 250 | ci = l->l_cpu; |
| 251 | |
| 252 | if (ci && ci->ci_curlwp == l) { |
| 253 | /* Target is running; do we need to block? */ |
| 254 | return (ci->ci_biglock_wanted != l); |
| 255 | } |
| 256 | #endif |
| 257 | /* Not running. It may be safe to block now. */ |
| 258 | return false; |
| 259 | } |
| 260 | |
| 261 | /* |
| 262 | * rw_vector_enter: |
| 263 | * |
| 264 | * Acquire a rwlock. |
| 265 | */ |
| 266 | void |
| 267 | rw_vector_enter(krwlock_t *rw, const krw_t op) |
| 268 | { |
| 269 | uintptr_t owner, incr, need_wait, set_wait, curthread, next; |
| 270 | turnstile_t *ts; |
| 271 | int queue; |
| 272 | lwp_t *l; |
| 273 | LOCKSTAT_TIMER(slptime); |
| 274 | LOCKSTAT_TIMER(slpcnt); |
| 275 | LOCKSTAT_TIMER(spintime); |
| 276 | LOCKSTAT_COUNTER(spincnt); |
| 277 | LOCKSTAT_FLAG(lsflag); |
| 278 | |
| 279 | l = curlwp; |
| 280 | curthread = (uintptr_t)l; |
| 281 | |
| 282 | RW_ASSERT(rw, !cpu_intr_p()); |
| 283 | RW_ASSERT(rw, curthread != 0); |
| 284 | RW_WANTLOCK(rw, op); |
| 285 | |
| 286 | if (panicstr == NULL) { |
| 287 | LOCKDEBUG_BARRIER(&kernel_lock, 1); |
| 288 | } |
| 289 | |
| 290 | /* |
| 291 | * We play a slight trick here. If we're a reader, we want |
| 292 | * increment the read count. If we're a writer, we want to |
| 293 | * set the owner field and the WRITE_LOCKED bit. |
| 294 | * |
| 295 | * In the latter case, we expect those bits to be zero, |
| 296 | * therefore we can use an add operation to set them, which |
| 297 | * means an add operation for both cases. |
| 298 | */ |
| 299 | if (__predict_true(op == RW_READER)) { |
| 300 | incr = RW_READ_INCR; |
| 301 | set_wait = RW_HAS_WAITERS; |
| 302 | need_wait = RW_WRITE_LOCKED | RW_WRITE_WANTED; |
| 303 | queue = TS_READER_Q; |
| 304 | } else { |
| 305 | RW_DASSERT(rw, op == RW_WRITER); |
| 306 | incr = curthread | RW_WRITE_LOCKED; |
| 307 | set_wait = RW_HAS_WAITERS | RW_WRITE_WANTED; |
| 308 | need_wait = RW_WRITE_LOCKED | RW_THREAD; |
| 309 | queue = TS_WRITER_Q; |
| 310 | } |
| 311 | |
| 312 | LOCKSTAT_ENTER(lsflag); |
| 313 | |
| 314 | KPREEMPT_DISABLE(curlwp); |
| 315 | for (owner = rw->rw_owner; ;) { |
| 316 | /* |
| 317 | * Read the lock owner field. If the need-to-wait |
| 318 | * indicator is clear, then try to acquire the lock. |
| 319 | */ |
| 320 | if ((owner & need_wait) == 0) { |
| 321 | next = rw_cas(rw, owner, (owner + incr) & |
| 322 | ~RW_WRITE_WANTED); |
| 323 | if (__predict_true(next == owner)) { |
| 324 | /* Got it! */ |
| 325 | membar_enter(); |
| 326 | break; |
| 327 | } |
| 328 | |
| 329 | /* |
| 330 | * Didn't get it -- spin around again (we'll |
| 331 | * probably sleep on the next iteration). |
| 332 | */ |
| 333 | owner = next; |
| 334 | continue; |
| 335 | } |
| 336 | if (__predict_false(panicstr != NULL)) { |
| 337 | KPREEMPT_ENABLE(curlwp); |
| 338 | return; |
| 339 | } |
| 340 | if (__predict_false(RW_OWNER(rw) == curthread)) { |
| 341 | rw_abort(rw, __func__, "locking against myself" ); |
| 342 | } |
| 343 | /* |
| 344 | * If the lock owner is running on another CPU, and |
| 345 | * there are no existing waiters, then spin. |
| 346 | */ |
| 347 | if (rw_oncpu(owner)) { |
| 348 | LOCKSTAT_START_TIMER(lsflag, spintime); |
| 349 | u_int count = SPINLOCK_BACKOFF_MIN; |
| 350 | do { |
| 351 | KPREEMPT_ENABLE(curlwp); |
| 352 | SPINLOCK_BACKOFF(count); |
| 353 | KPREEMPT_DISABLE(curlwp); |
| 354 | owner = rw->rw_owner; |
| 355 | } while (rw_oncpu(owner)); |
| 356 | LOCKSTAT_STOP_TIMER(lsflag, spintime); |
| 357 | LOCKSTAT_COUNT(spincnt, 1); |
| 358 | if ((owner & need_wait) == 0) |
| 359 | continue; |
| 360 | } |
| 361 | |
| 362 | /* |
| 363 | * Grab the turnstile chain lock. Once we have that, we |
| 364 | * can adjust the waiter bits and sleep queue. |
| 365 | */ |
| 366 | ts = turnstile_lookup(rw); |
| 367 | |
| 368 | /* |
| 369 | * Mark the rwlock as having waiters. If the set fails, |
| 370 | * then we may not need to sleep and should spin again. |
| 371 | * Reload rw_owner because turnstile_lookup() may have |
| 372 | * spun on the turnstile chain lock. |
| 373 | */ |
| 374 | owner = rw->rw_owner; |
| 375 | if ((owner & need_wait) == 0 || rw_oncpu(owner)) { |
| 376 | turnstile_exit(rw); |
| 377 | continue; |
| 378 | } |
| 379 | next = rw_cas(rw, owner, owner | set_wait); |
| 380 | if (__predict_false(next != owner)) { |
| 381 | turnstile_exit(rw); |
| 382 | owner = next; |
| 383 | continue; |
| 384 | } |
| 385 | |
| 386 | LOCKSTAT_START_TIMER(lsflag, slptime); |
| 387 | turnstile_block(ts, queue, rw, &rw_syncobj); |
| 388 | LOCKSTAT_STOP_TIMER(lsflag, slptime); |
| 389 | LOCKSTAT_COUNT(slpcnt, 1); |
| 390 | |
| 391 | /* |
| 392 | * No need for a memory barrier because of context switch. |
| 393 | * If not handed the lock, then spin again. |
| 394 | */ |
| 395 | if (op == RW_READER || (rw->rw_owner & RW_THREAD) == curthread) |
| 396 | break; |
| 397 | |
| 398 | owner = rw->rw_owner; |
| 399 | } |
| 400 | KPREEMPT_ENABLE(curlwp); |
| 401 | |
| 402 | LOCKSTAT_EVENT(lsflag, rw, LB_RWLOCK | |
| 403 | (op == RW_WRITER ? LB_SLEEP1 : LB_SLEEP2), slpcnt, slptime); |
| 404 | LOCKSTAT_EVENT(lsflag, rw, LB_RWLOCK | LB_SPIN, spincnt, spintime); |
| 405 | LOCKSTAT_EXIT(lsflag); |
| 406 | |
| 407 | RW_DASSERT(rw, (op != RW_READER && RW_OWNER(rw) == curthread) || |
| 408 | (op == RW_READER && RW_COUNT(rw) != 0)); |
| 409 | RW_LOCKED(rw, op); |
| 410 | } |
| 411 | |
| 412 | /* |
| 413 | * rw_vector_exit: |
| 414 | * |
| 415 | * Release a rwlock. |
| 416 | */ |
| 417 | void |
| 418 | rw_vector_exit(krwlock_t *rw) |
| 419 | { |
| 420 | uintptr_t curthread, owner, decr, newown, next; |
| 421 | turnstile_t *ts; |
| 422 | int rcnt, wcnt; |
| 423 | lwp_t *l; |
| 424 | |
| 425 | curthread = (uintptr_t)curlwp; |
| 426 | RW_ASSERT(rw, curthread != 0); |
| 427 | |
| 428 | if (__predict_false(panicstr != NULL)) |
| 429 | return; |
| 430 | |
| 431 | /* |
| 432 | * Again, we use a trick. Since we used an add operation to |
| 433 | * set the required lock bits, we can use a subtract to clear |
| 434 | * them, which makes the read-release and write-release path |
| 435 | * the same. |
| 436 | */ |
| 437 | owner = rw->rw_owner; |
| 438 | if (__predict_false((owner & RW_WRITE_LOCKED) != 0)) { |
| 439 | RW_UNLOCKED(rw, RW_WRITER); |
| 440 | RW_ASSERT(rw, RW_OWNER(rw) == curthread); |
| 441 | decr = curthread | RW_WRITE_LOCKED; |
| 442 | } else { |
| 443 | RW_UNLOCKED(rw, RW_READER); |
| 444 | RW_ASSERT(rw, RW_COUNT(rw) != 0); |
| 445 | decr = RW_READ_INCR; |
| 446 | } |
| 447 | |
| 448 | /* |
| 449 | * Compute what we expect the new value of the lock to be. Only |
| 450 | * proceed to do direct handoff if there are waiters, and if the |
| 451 | * lock would become unowned. |
| 452 | */ |
| 453 | membar_exit(); |
| 454 | for (;;) { |
| 455 | newown = (owner - decr); |
| 456 | if ((newown & (RW_THREAD | RW_HAS_WAITERS)) == RW_HAS_WAITERS) |
| 457 | break; |
| 458 | next = rw_cas(rw, owner, newown); |
| 459 | if (__predict_true(next == owner)) |
| 460 | return; |
| 461 | owner = next; |
| 462 | } |
| 463 | |
| 464 | /* |
| 465 | * Grab the turnstile chain lock. This gets the interlock |
| 466 | * on the sleep queue. Once we have that, we can adjust the |
| 467 | * waiter bits. |
| 468 | */ |
| 469 | ts = turnstile_lookup(rw); |
| 470 | owner = rw->rw_owner; |
| 471 | RW_DASSERT(rw, ts != NULL); |
| 472 | RW_DASSERT(rw, (owner & RW_HAS_WAITERS) != 0); |
| 473 | |
| 474 | wcnt = TS_WAITERS(ts, TS_WRITER_Q); |
| 475 | rcnt = TS_WAITERS(ts, TS_READER_Q); |
| 476 | |
| 477 | /* |
| 478 | * Give the lock away. |
| 479 | * |
| 480 | * If we are releasing a write lock, then prefer to wake all |
| 481 | * outstanding readers. Otherwise, wake one writer if there |
| 482 | * are outstanding readers, or all writers if there are no |
| 483 | * pending readers. If waking one specific writer, the writer |
| 484 | * is handed the lock here. If waking multiple writers, we |
| 485 | * set WRITE_WANTED to block out new readers, and let them |
| 486 | * do the work of acquiring the lock in rw_vector_enter(). |
| 487 | */ |
| 488 | if (rcnt == 0 || decr == RW_READ_INCR) { |
| 489 | RW_DASSERT(rw, wcnt != 0); |
| 490 | RW_DASSERT(rw, (owner & RW_WRITE_WANTED) != 0); |
| 491 | |
| 492 | if (rcnt != 0) { |
| 493 | /* Give the lock to the longest waiting writer. */ |
| 494 | l = TS_FIRST(ts, TS_WRITER_Q); |
| 495 | newown = (uintptr_t)l | RW_WRITE_LOCKED | RW_HAS_WAITERS; |
| 496 | if (wcnt > 1) |
| 497 | newown |= RW_WRITE_WANTED; |
| 498 | rw_swap(rw, owner, newown); |
| 499 | turnstile_wakeup(ts, TS_WRITER_Q, 1, l); |
| 500 | } else { |
| 501 | /* Wake all writers and let them fight it out. */ |
| 502 | rw_swap(rw, owner, RW_WRITE_WANTED); |
| 503 | turnstile_wakeup(ts, TS_WRITER_Q, wcnt, NULL); |
| 504 | } |
| 505 | } else { |
| 506 | RW_DASSERT(rw, rcnt != 0); |
| 507 | |
| 508 | /* |
| 509 | * Give the lock to all blocked readers. If there |
| 510 | * is a writer waiting, new readers that arrive |
| 511 | * after the release will be blocked out. |
| 512 | */ |
| 513 | newown = rcnt << RW_READ_COUNT_SHIFT; |
| 514 | if (wcnt != 0) |
| 515 | newown |= RW_HAS_WAITERS | RW_WRITE_WANTED; |
| 516 | |
| 517 | /* Wake up all sleeping readers. */ |
| 518 | rw_swap(rw, owner, newown); |
| 519 | turnstile_wakeup(ts, TS_READER_Q, rcnt, NULL); |
| 520 | } |
| 521 | } |
| 522 | |
| 523 | /* |
| 524 | * rw_vector_tryenter: |
| 525 | * |
| 526 | * Try to acquire a rwlock. |
| 527 | */ |
| 528 | int |
| 529 | rw_vector_tryenter(krwlock_t *rw, const krw_t op) |
| 530 | { |
| 531 | uintptr_t curthread, owner, incr, need_wait, next; |
| 532 | |
| 533 | curthread = (uintptr_t)curlwp; |
| 534 | |
| 535 | RW_ASSERT(rw, curthread != 0); |
| 536 | |
| 537 | if (op == RW_READER) { |
| 538 | incr = RW_READ_INCR; |
| 539 | need_wait = RW_WRITE_LOCKED | RW_WRITE_WANTED; |
| 540 | } else { |
| 541 | RW_DASSERT(rw, op == RW_WRITER); |
| 542 | incr = curthread | RW_WRITE_LOCKED; |
| 543 | need_wait = RW_WRITE_LOCKED | RW_THREAD; |
| 544 | } |
| 545 | |
| 546 | for (owner = rw->rw_owner;; owner = next) { |
| 547 | owner = rw->rw_owner; |
| 548 | if (__predict_false((owner & need_wait) != 0)) |
| 549 | return 0; |
| 550 | next = rw_cas(rw, owner, owner + incr); |
| 551 | if (__predict_true(next == owner)) { |
| 552 | /* Got it! */ |
| 553 | membar_enter(); |
| 554 | break; |
| 555 | } |
| 556 | } |
| 557 | |
| 558 | RW_WANTLOCK(rw, op); |
| 559 | RW_LOCKED(rw, op); |
| 560 | RW_DASSERT(rw, (op != RW_READER && RW_OWNER(rw) == curthread) || |
| 561 | (op == RW_READER && RW_COUNT(rw) != 0)); |
| 562 | |
| 563 | return 1; |
| 564 | } |
| 565 | |
| 566 | /* |
| 567 | * rw_downgrade: |
| 568 | * |
| 569 | * Downgrade a write lock to a read lock. |
| 570 | */ |
| 571 | void |
| 572 | rw_downgrade(krwlock_t *rw) |
| 573 | { |
| 574 | uintptr_t owner, curthread, newown, next; |
| 575 | turnstile_t *ts; |
| 576 | int rcnt, wcnt; |
| 577 | |
| 578 | curthread = (uintptr_t)curlwp; |
| 579 | RW_ASSERT(rw, curthread != 0); |
| 580 | RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) != 0); |
| 581 | RW_ASSERT(rw, RW_OWNER(rw) == curthread); |
| 582 | RW_UNLOCKED(rw, RW_WRITER); |
| 583 | #if !defined(DIAGNOSTIC) |
| 584 | __USE(curthread); |
| 585 | #endif |
| 586 | |
| 587 | |
| 588 | membar_producer(); |
| 589 | owner = rw->rw_owner; |
| 590 | if ((owner & RW_HAS_WAITERS) == 0) { |
| 591 | /* |
| 592 | * There are no waiters, so we can do this the easy way. |
| 593 | * Try swapping us down to one read hold. If it fails, the |
| 594 | * lock condition has changed and we most likely now have |
| 595 | * waiters. |
| 596 | */ |
| 597 | next = rw_cas(rw, owner, RW_READ_INCR); |
| 598 | if (__predict_true(next == owner)) { |
| 599 | RW_LOCKED(rw, RW_READER); |
| 600 | RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0); |
| 601 | RW_DASSERT(rw, RW_COUNT(rw) != 0); |
| 602 | return; |
| 603 | } |
| 604 | owner = next; |
| 605 | } |
| 606 | |
| 607 | /* |
| 608 | * Grab the turnstile chain lock. This gets the interlock |
| 609 | * on the sleep queue. Once we have that, we can adjust the |
| 610 | * waiter bits. |
| 611 | */ |
| 612 | for (;; owner = next) { |
| 613 | ts = turnstile_lookup(rw); |
| 614 | RW_DASSERT(rw, ts != NULL); |
| 615 | |
| 616 | rcnt = TS_WAITERS(ts, TS_READER_Q); |
| 617 | wcnt = TS_WAITERS(ts, TS_WRITER_Q); |
| 618 | |
| 619 | /* |
| 620 | * If there are no readers, just preserve the waiters |
| 621 | * bits, swap us down to one read hold and return. |
| 622 | */ |
| 623 | if (rcnt == 0) { |
| 624 | RW_DASSERT(rw, wcnt != 0); |
| 625 | RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_WANTED) != 0); |
| 626 | RW_DASSERT(rw, (rw->rw_owner & RW_HAS_WAITERS) != 0); |
| 627 | |
| 628 | newown = RW_READ_INCR | RW_HAS_WAITERS | RW_WRITE_WANTED; |
| 629 | next = rw_cas(rw, owner, newown); |
| 630 | turnstile_exit(rw); |
| 631 | if (__predict_true(next == owner)) |
| 632 | break; |
| 633 | } else { |
| 634 | /* |
| 635 | * Give the lock to all blocked readers. We may |
| 636 | * retain one read hold if downgrading. If there |
| 637 | * is a writer waiting, new readers will be blocked |
| 638 | * out. |
| 639 | */ |
| 640 | newown = (rcnt << RW_READ_COUNT_SHIFT) + RW_READ_INCR; |
| 641 | if (wcnt != 0) |
| 642 | newown |= RW_HAS_WAITERS | RW_WRITE_WANTED; |
| 643 | |
| 644 | next = rw_cas(rw, owner, newown); |
| 645 | if (__predict_true(next == owner)) { |
| 646 | /* Wake up all sleeping readers. */ |
| 647 | turnstile_wakeup(ts, TS_READER_Q, rcnt, NULL); |
| 648 | break; |
| 649 | } |
| 650 | turnstile_exit(rw); |
| 651 | } |
| 652 | } |
| 653 | |
| 654 | RW_WANTLOCK(rw, RW_READER); |
| 655 | RW_LOCKED(rw, RW_READER); |
| 656 | RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0); |
| 657 | RW_DASSERT(rw, RW_COUNT(rw) != 0); |
| 658 | } |
| 659 | |
| 660 | /* |
| 661 | * rw_tryupgrade: |
| 662 | * |
| 663 | * Try to upgrade a read lock to a write lock. We must be the |
| 664 | * only reader. |
| 665 | */ |
| 666 | int |
| 667 | rw_tryupgrade(krwlock_t *rw) |
| 668 | { |
| 669 | uintptr_t owner, curthread, newown, next; |
| 670 | |
| 671 | curthread = (uintptr_t)curlwp; |
| 672 | RW_ASSERT(rw, curthread != 0); |
| 673 | RW_ASSERT(rw, rw_read_held(rw)); |
| 674 | |
| 675 | for (owner = rw->rw_owner;; owner = next) { |
| 676 | RW_ASSERT(rw, (owner & RW_WRITE_LOCKED) == 0); |
| 677 | if (__predict_false((owner & RW_THREAD) != RW_READ_INCR)) { |
| 678 | RW_ASSERT(rw, (owner & RW_THREAD) != 0); |
| 679 | return 0; |
| 680 | } |
| 681 | newown = curthread | RW_WRITE_LOCKED | (owner & ~RW_THREAD); |
| 682 | next = rw_cas(rw, owner, newown); |
| 683 | if (__predict_true(next == owner)) { |
| 684 | membar_producer(); |
| 685 | break; |
| 686 | } |
| 687 | } |
| 688 | |
| 689 | RW_UNLOCKED(rw, RW_READER); |
| 690 | RW_WANTLOCK(rw, RW_WRITER); |
| 691 | RW_LOCKED(rw, RW_WRITER); |
| 692 | RW_DASSERT(rw, rw->rw_owner & RW_WRITE_LOCKED); |
| 693 | RW_DASSERT(rw, RW_OWNER(rw) == curthread); |
| 694 | |
| 695 | return 1; |
| 696 | } |
| 697 | |
| 698 | /* |
| 699 | * rw_read_held: |
| 700 | * |
| 701 | * Returns true if the rwlock is held for reading. Must only be |
| 702 | * used for diagnostic assertions, and never be used to make |
| 703 | * decisions about how to use a rwlock. |
| 704 | */ |
| 705 | int |
| 706 | rw_read_held(krwlock_t *rw) |
| 707 | { |
| 708 | uintptr_t owner; |
| 709 | |
| 710 | if (panicstr != NULL) |
| 711 | return 1; |
| 712 | if (rw == NULL) |
| 713 | return 0; |
| 714 | owner = rw->rw_owner; |
| 715 | return (owner & RW_WRITE_LOCKED) == 0 && (owner & RW_THREAD) != 0; |
| 716 | } |
| 717 | |
| 718 | /* |
| 719 | * rw_write_held: |
| 720 | * |
| 721 | * Returns true if the rwlock is held for writing. Must only be |
| 722 | * used for diagnostic assertions, and never be used to make |
| 723 | * decisions about how to use a rwlock. |
| 724 | */ |
| 725 | int |
| 726 | rw_write_held(krwlock_t *rw) |
| 727 | { |
| 728 | |
| 729 | if (panicstr != NULL) |
| 730 | return 1; |
| 731 | if (rw == NULL) |
| 732 | return 0; |
| 733 | return (rw->rw_owner & (RW_WRITE_LOCKED | RW_THREAD)) == |
| 734 | (RW_WRITE_LOCKED | (uintptr_t)curlwp); |
| 735 | } |
| 736 | |
| 737 | /* |
| 738 | * rw_lock_held: |
| 739 | * |
| 740 | * Returns true if the rwlock is held for reading or writing. Must |
| 741 | * only be used for diagnostic assertions, and never be used to make |
| 742 | * decisions about how to use a rwlock. |
| 743 | */ |
| 744 | int |
| 745 | rw_lock_held(krwlock_t *rw) |
| 746 | { |
| 747 | |
| 748 | if (panicstr != NULL) |
| 749 | return 1; |
| 750 | if (rw == NULL) |
| 751 | return 0; |
| 752 | return (rw->rw_owner & RW_THREAD) != 0; |
| 753 | } |
| 754 | |
| 755 | /* |
| 756 | * rw_owner: |
| 757 | * |
| 758 | * Return the current owner of an RW lock, but only if it is write |
| 759 | * held. Used for priority inheritance. |
| 760 | */ |
| 761 | static lwp_t * |
| 762 | rw_owner(wchan_t obj) |
| 763 | { |
| 764 | krwlock_t *rw = (void *)(uintptr_t)obj; /* discard qualifiers */ |
| 765 | uintptr_t owner = rw->rw_owner; |
| 766 | |
| 767 | if ((owner & RW_WRITE_LOCKED) == 0) |
| 768 | return NULL; |
| 769 | |
| 770 | return (void *)(owner & RW_THREAD); |
| 771 | } |
| 772 | |