1 /* $NetBSD: kern_mutex.c,v 1.90 2020/03/08 00:26:06 chs Exp $ */ 2 3 /*- 4 * Copyright (c) 2002, 2006, 2007, 2008, 2019 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 mutex implementation, modeled after those found in Solaris, 34 * a description of which can be found in: 35 * 36 * Solaris Internals: Core Kernel Architecture, Jim Mauro and 37 * Richard McDougall. 38 */ 39 40 #define __MUTEX_PRIVATE 41 42 #include <sys/cdefs.h> 43 __KERNEL_RCSID(0, "$NetBSD: kern_mutex.c,v 1.90 2020/03/08 00:26:06 chs Exp $"); 44 45 #include <sys/param.h> 46 #include <sys/atomic.h> 47 #include <sys/proc.h> 48 #include <sys/mutex.h> 49 #include <sys/sched.h> 50 #include <sys/sleepq.h> 51 #include <sys/systm.h> 52 #include <sys/lockdebug.h> 53 #include <sys/kernel.h> 54 #include <sys/intr.h> 55 #include <sys/lock.h> 56 #include <sys/types.h> 57 #include <sys/cpu.h> 58 #include <sys/pserialize.h> 59 60 #include <dev/lockstat.h> 61 62 #include <machine/lock.h> 63 64 /* 65 * When not running a debug kernel, spin mutexes are not much 66 * more than an splraiseipl() and splx() pair. 67 */ 68 69 #if defined(DIAGNOSTIC) || defined(MULTIPROCESSOR) || defined(LOCKDEBUG) 70 #define FULL 71 #endif 72 73 /* 74 * Debugging support. 75 */ 76 77 #define MUTEX_WANTLOCK(mtx) \ 78 LOCKDEBUG_WANTLOCK(MUTEX_DEBUG_P(mtx), (mtx), \ 79 (uintptr_t)__builtin_return_address(0), 0) 80 #define MUTEX_TESTLOCK(mtx) \ 81 LOCKDEBUG_WANTLOCK(MUTEX_DEBUG_P(mtx), (mtx), \ 82 (uintptr_t)__builtin_return_address(0), -1) 83 #define MUTEX_LOCKED(mtx) \ 84 LOCKDEBUG_LOCKED(MUTEX_DEBUG_P(mtx), (mtx), NULL, \ 85 (uintptr_t)__builtin_return_address(0), 0) 86 #define MUTEX_UNLOCKED(mtx) \ 87 LOCKDEBUG_UNLOCKED(MUTEX_DEBUG_P(mtx), (mtx), \ 88 (uintptr_t)__builtin_return_address(0), 0) 89 #define MUTEX_ABORT(mtx, msg) \ 90 mutex_abort(__func__, __LINE__, mtx, msg) 91 92 #if defined(LOCKDEBUG) 93 94 #define MUTEX_DASSERT(mtx, cond) \ 95 do { \ 96 if (__predict_false(!(cond))) \ 97 MUTEX_ABORT(mtx, "assertion failed: " #cond); \ 98 } while (/* CONSTCOND */ 0) 99 100 #else /* LOCKDEBUG */ 101 102 #define MUTEX_DASSERT(mtx, cond) /* nothing */ 103 104 #endif /* LOCKDEBUG */ 105 106 #if defined(DIAGNOSTIC) 107 108 #define MUTEX_ASSERT(mtx, cond) \ 109 do { \ 110 if (__predict_false(!(cond))) \ 111 MUTEX_ABORT(mtx, "assertion failed: " #cond); \ 112 } while (/* CONSTCOND */ 0) 113 114 #else /* DIAGNOSTIC */ 115 116 #define MUTEX_ASSERT(mtx, cond) /* nothing */ 117 118 #endif /* DIAGNOSTIC */ 119 120 /* 121 * Some architectures can't use __cpu_simple_lock as is so allow a way 122 * for them to use an alternate definition. 123 */ 124 #ifndef MUTEX_SPINBIT_LOCK_INIT 125 #define MUTEX_SPINBIT_LOCK_INIT(mtx) __cpu_simple_lock_init(&(mtx)->mtx_lock) 126 #endif 127 #ifndef MUTEX_SPINBIT_LOCKED_P 128 #define MUTEX_SPINBIT_LOCKED_P(mtx) __SIMPLELOCK_LOCKED_P(&(mtx)->mtx_lock) 129 #endif 130 #ifndef MUTEX_SPINBIT_LOCK_TRY 131 #define MUTEX_SPINBIT_LOCK_TRY(mtx) __cpu_simple_lock_try(&(mtx)->mtx_lock) 132 #endif 133 #ifndef MUTEX_SPINBIT_LOCK_UNLOCK 134 #define MUTEX_SPINBIT_LOCK_UNLOCK(mtx) __cpu_simple_unlock(&(mtx)->mtx_lock) 135 #endif 136 137 #ifndef MUTEX_INITIALIZE_SPIN_IPL 138 #define MUTEX_INITIALIZE_SPIN_IPL(mtx, ipl) \ 139 ((mtx)->mtx_ipl = makeiplcookie((ipl))) 140 #endif 141 142 /* 143 * Spin mutex SPL save / restore. 144 */ 145 146 #define MUTEX_SPIN_SPLRAISE(mtx) \ 147 do { \ 148 struct cpu_info *x__ci; \ 149 int x__cnt, s; \ 150 s = splraiseipl(MUTEX_SPIN_IPL(mtx)); \ 151 x__ci = curcpu(); \ 152 x__cnt = x__ci->ci_mtx_count--; \ 153 __insn_barrier(); \ 154 if (x__cnt == 0) \ 155 x__ci->ci_mtx_oldspl = (s); \ 156 } while (/* CONSTCOND */ 0) 157 158 #define MUTEX_SPIN_SPLRESTORE(mtx) \ 159 do { \ 160 struct cpu_info *x__ci = curcpu(); \ 161 int s = x__ci->ci_mtx_oldspl; \ 162 __insn_barrier(); \ 163 if (++(x__ci->ci_mtx_count) == 0) \ 164 splx(s); \ 165 } while (/* CONSTCOND */ 0) 166 167 /* 168 * Memory barriers. 169 */ 170 #ifdef __HAVE_ATOMIC_AS_MEMBAR 171 #define MUTEX_MEMBAR_ENTER() 172 #define MUTEX_MEMBAR_EXIT() 173 #else 174 #define MUTEX_MEMBAR_ENTER() membar_enter() 175 #define MUTEX_MEMBAR_EXIT() membar_exit() 176 #endif 177 178 /* 179 * For architectures that provide 'simple' mutexes: they provide a 180 * CAS function that is either MP-safe, or does not need to be MP 181 * safe. Adaptive mutexes on these architectures do not require an 182 * additional interlock. 183 */ 184 185 #ifdef __HAVE_SIMPLE_MUTEXES 186 187 #define MUTEX_OWNER(owner) \ 188 (owner & MUTEX_THREAD) 189 #define MUTEX_HAS_WAITERS(mtx) \ 190 (((int)(mtx)->mtx_owner & MUTEX_BIT_WAITERS) != 0) 191 192 #define MUTEX_INITIALIZE_ADAPTIVE(mtx, dodebug) \ 193 do { \ 194 if (!dodebug) \ 195 (mtx)->mtx_owner |= MUTEX_BIT_NODEBUG; \ 196 } while (/* CONSTCOND */ 0) 197 198 #define MUTEX_INITIALIZE_SPIN(mtx, dodebug, ipl) \ 199 do { \ 200 (mtx)->mtx_owner = MUTEX_BIT_SPIN; \ 201 if (!dodebug) \ 202 (mtx)->mtx_owner |= MUTEX_BIT_NODEBUG; \ 203 MUTEX_INITIALIZE_SPIN_IPL((mtx), (ipl)); \ 204 MUTEX_SPINBIT_LOCK_INIT((mtx)); \ 205 } while (/* CONSTCOND */ 0) 206 207 #define MUTEX_DESTROY(mtx) \ 208 do { \ 209 (mtx)->mtx_owner = MUTEX_THREAD; \ 210 } while (/* CONSTCOND */ 0) 211 212 #define MUTEX_SPIN_P(owner) \ 213 (((owner) & MUTEX_BIT_SPIN) != 0) 214 #define MUTEX_ADAPTIVE_P(owner) \ 215 (((owner) & MUTEX_BIT_SPIN) == 0) 216 217 #define MUTEX_DEBUG_P(mtx) (((mtx)->mtx_owner & MUTEX_BIT_NODEBUG) == 0) 218 #if defined(LOCKDEBUG) 219 #define MUTEX_OWNED(owner) (((owner) & ~MUTEX_BIT_NODEBUG) != 0) 220 #define MUTEX_INHERITDEBUG(n, o) (n) |= (o) & MUTEX_BIT_NODEBUG 221 #else /* defined(LOCKDEBUG) */ 222 #define MUTEX_OWNED(owner) ((owner) != 0) 223 #define MUTEX_INHERITDEBUG(n, o) /* nothing */ 224 #endif /* defined(LOCKDEBUG) */ 225 226 static inline int 227 MUTEX_ACQUIRE(kmutex_t *mtx, uintptr_t curthread) 228 { 229 int rv; 230 uintptr_t oldown = 0; 231 uintptr_t newown = curthread; 232 233 MUTEX_INHERITDEBUG(oldown, mtx->mtx_owner); 234 MUTEX_INHERITDEBUG(newown, oldown); 235 rv = MUTEX_CAS(&mtx->mtx_owner, oldown, newown); 236 MUTEX_MEMBAR_ENTER(); 237 return rv; 238 } 239 240 static inline int 241 MUTEX_SET_WAITERS(kmutex_t *mtx, uintptr_t owner) 242 { 243 int rv; 244 rv = MUTEX_CAS(&mtx->mtx_owner, owner, owner | MUTEX_BIT_WAITERS); 245 MUTEX_MEMBAR_ENTER(); 246 return rv; 247 } 248 249 static inline void 250 MUTEX_RELEASE(kmutex_t *mtx) 251 { 252 uintptr_t newown; 253 254 MUTEX_MEMBAR_EXIT(); 255 newown = 0; 256 MUTEX_INHERITDEBUG(newown, mtx->mtx_owner); 257 mtx->mtx_owner = newown; 258 } 259 #endif /* __HAVE_SIMPLE_MUTEXES */ 260 261 /* 262 * Patch in stubs via strong alias where they are not available. 263 */ 264 265 #if defined(LOCKDEBUG) 266 #undef __HAVE_MUTEX_STUBS 267 #undef __HAVE_SPIN_MUTEX_STUBS 268 #endif 269 270 #ifndef __HAVE_MUTEX_STUBS 271 __strong_alias(mutex_enter,mutex_vector_enter); 272 __strong_alias(mutex_exit,mutex_vector_exit); 273 #endif 274 275 #ifndef __HAVE_SPIN_MUTEX_STUBS 276 __strong_alias(mutex_spin_enter,mutex_vector_enter); 277 __strong_alias(mutex_spin_exit,mutex_vector_exit); 278 #endif 279 280 static void mutex_abort(const char *, size_t, const kmutex_t *, 281 const char *); 282 static void mutex_dump(const volatile void *, lockop_printer_t); 283 284 lockops_t mutex_spin_lockops = { 285 .lo_name = "Mutex", 286 .lo_type = LOCKOPS_SPIN, 287 .lo_dump = mutex_dump, 288 }; 289 290 lockops_t mutex_adaptive_lockops = { 291 .lo_name = "Mutex", 292 .lo_type = LOCKOPS_SLEEP, 293 .lo_dump = mutex_dump, 294 }; 295 296 syncobj_t mutex_syncobj = { 297 .sobj_flag = SOBJ_SLEEPQ_SORTED, 298 .sobj_unsleep = turnstile_unsleep, 299 .sobj_changepri = turnstile_changepri, 300 .sobj_lendpri = sleepq_lendpri, 301 .sobj_owner = (void *)mutex_owner, 302 }; 303 304 /* 305 * mutex_dump: 306 * 307 * Dump the contents of a mutex structure. 308 */ 309 static void 310 mutex_dump(const volatile void *cookie, lockop_printer_t pr) 311 { 312 const volatile kmutex_t *mtx = cookie; 313 uintptr_t owner = mtx->mtx_owner; 314 315 pr("owner field : %#018lx wait/spin: %16d/%d\n", 316 (long)MUTEX_OWNER(owner), MUTEX_HAS_WAITERS(mtx), 317 MUTEX_SPIN_P(owner)); 318 } 319 320 /* 321 * mutex_abort: 322 * 323 * Dump information about an error and panic the system. This 324 * generates a lot of machine code in the DIAGNOSTIC case, so 325 * we ask the compiler to not inline it. 326 */ 327 static void __noinline 328 mutex_abort(const char *func, size_t line, const kmutex_t *mtx, const char *msg) 329 { 330 331 LOCKDEBUG_ABORT(func, line, mtx, (MUTEX_SPIN_P(mtx->mtx_owner) ? 332 &mutex_spin_lockops : &mutex_adaptive_lockops), msg); 333 } 334 335 /* 336 * mutex_init: 337 * 338 * Initialize a mutex for use. Note that adaptive mutexes are in 339 * essence spin mutexes that can sleep to avoid deadlock and wasting 340 * CPU time. We can't easily provide a type of mutex that always 341 * sleeps - see comments in mutex_vector_enter() about releasing 342 * mutexes unlocked. 343 */ 344 void _mutex_init(kmutex_t *, kmutex_type_t, int, uintptr_t); 345 void 346 _mutex_init(kmutex_t *mtx, kmutex_type_t type, int ipl, 347 uintptr_t return_address) 348 { 349 lockops_t *lockops __unused; 350 bool dodebug; 351 352 memset(mtx, 0, sizeof(*mtx)); 353 354 if (ipl == IPL_NONE || ipl == IPL_SOFTCLOCK || 355 ipl == IPL_SOFTBIO || ipl == IPL_SOFTNET || 356 ipl == IPL_SOFTSERIAL) { 357 lockops = (type == MUTEX_NODEBUG ? 358 NULL : &mutex_adaptive_lockops); 359 dodebug = LOCKDEBUG_ALLOC(mtx, lockops, return_address); 360 MUTEX_INITIALIZE_ADAPTIVE(mtx, dodebug); 361 } else { 362 lockops = (type == MUTEX_NODEBUG ? 363 NULL : &mutex_spin_lockops); 364 dodebug = LOCKDEBUG_ALLOC(mtx, lockops, return_address); 365 MUTEX_INITIALIZE_SPIN(mtx, dodebug, ipl); 366 } 367 } 368 369 void 370 mutex_init(kmutex_t *mtx, kmutex_type_t type, int ipl) 371 { 372 373 _mutex_init(mtx, type, ipl, (uintptr_t)__builtin_return_address(0)); 374 } 375 376 /* 377 * mutex_destroy: 378 * 379 * Tear down a mutex. 380 */ 381 void 382 mutex_destroy(kmutex_t *mtx) 383 { 384 uintptr_t owner = mtx->mtx_owner; 385 386 if (MUTEX_ADAPTIVE_P(owner)) { 387 MUTEX_ASSERT(mtx, !MUTEX_OWNED(owner)); 388 MUTEX_ASSERT(mtx, !MUTEX_HAS_WAITERS(mtx)); 389 } else { 390 MUTEX_ASSERT(mtx, !MUTEX_SPINBIT_LOCKED_P(mtx)); 391 } 392 393 LOCKDEBUG_FREE(MUTEX_DEBUG_P(mtx), mtx); 394 MUTEX_DESTROY(mtx); 395 } 396 397 #ifdef MULTIPROCESSOR 398 /* 399 * mutex_oncpu: 400 * 401 * Return true if an adaptive mutex owner is running on a CPU in the 402 * system. If the target is waiting on the kernel big lock, then we 403 * must release it. This is necessary to avoid deadlock. 404 */ 405 static bool 406 mutex_oncpu(uintptr_t owner) 407 { 408 struct cpu_info *ci; 409 lwp_t *l; 410 411 KASSERT(kpreempt_disabled()); 412 413 if (!MUTEX_OWNED(owner)) { 414 return false; 415 } 416 417 /* 418 * See lwp_dtor() why dereference of the LWP pointer is safe. 419 * We must have kernel preemption disabled for that. 420 */ 421 l = (lwp_t *)MUTEX_OWNER(owner); 422 ci = l->l_cpu; 423 424 if (ci && ci->ci_curlwp == l) { 425 /* Target is running; do we need to block? */ 426 return (ci->ci_biglock_wanted != l); 427 } 428 429 /* Not running. It may be safe to block now. */ 430 return false; 431 } 432 #endif /* MULTIPROCESSOR */ 433 434 /* 435 * mutex_vector_enter: 436 * 437 * Support routine for mutex_enter() that must handle all cases. In 438 * the LOCKDEBUG case, mutex_enter() is always aliased here, even if 439 * fast-path stubs are available. If a mutex_spin_enter() stub is 440 * not available, then it is also aliased directly here. 441 */ 442 void 443 mutex_vector_enter(kmutex_t *mtx) 444 { 445 uintptr_t owner, curthread; 446 turnstile_t *ts; 447 #ifdef MULTIPROCESSOR 448 u_int count; 449 #endif 450 LOCKSTAT_COUNTER(spincnt); 451 LOCKSTAT_COUNTER(slpcnt); 452 LOCKSTAT_TIMER(spintime); 453 LOCKSTAT_TIMER(slptime); 454 LOCKSTAT_FLAG(lsflag); 455 456 /* 457 * Handle spin mutexes. 458 */ 459 owner = mtx->mtx_owner; 460 if (MUTEX_SPIN_P(owner)) { 461 #if defined(LOCKDEBUG) && defined(MULTIPROCESSOR) 462 u_int spins = 0; 463 #endif 464 MUTEX_SPIN_SPLRAISE(mtx); 465 MUTEX_WANTLOCK(mtx); 466 #ifdef FULL 467 if (MUTEX_SPINBIT_LOCK_TRY(mtx)) { 468 MUTEX_LOCKED(mtx); 469 return; 470 } 471 #if !defined(MULTIPROCESSOR) 472 MUTEX_ABORT(mtx, "locking against myself"); 473 #else /* !MULTIPROCESSOR */ 474 475 LOCKSTAT_ENTER(lsflag); 476 LOCKSTAT_START_TIMER(lsflag, spintime); 477 count = SPINLOCK_BACKOFF_MIN; 478 479 /* 480 * Spin testing the lock word and do exponential backoff 481 * to reduce cache line ping-ponging between CPUs. 482 */ 483 do { 484 while (MUTEX_SPINBIT_LOCKED_P(mtx)) { 485 SPINLOCK_BACKOFF(count); 486 #ifdef LOCKDEBUG 487 if (SPINLOCK_SPINOUT(spins)) 488 MUTEX_ABORT(mtx, "spinout"); 489 #endif /* LOCKDEBUG */ 490 } 491 } while (!MUTEX_SPINBIT_LOCK_TRY(mtx)); 492 493 if (count != SPINLOCK_BACKOFF_MIN) { 494 LOCKSTAT_STOP_TIMER(lsflag, spintime); 495 LOCKSTAT_EVENT(lsflag, mtx, 496 LB_SPIN_MUTEX | LB_SPIN, 1, spintime); 497 } 498 LOCKSTAT_EXIT(lsflag); 499 #endif /* !MULTIPROCESSOR */ 500 #endif /* FULL */ 501 MUTEX_LOCKED(mtx); 502 return; 503 } 504 505 curthread = (uintptr_t)curlwp; 506 507 MUTEX_DASSERT(mtx, MUTEX_ADAPTIVE_P(owner)); 508 MUTEX_ASSERT(mtx, curthread != 0); 509 MUTEX_ASSERT(mtx, !cpu_intr_p()); 510 MUTEX_WANTLOCK(mtx); 511 512 if (panicstr == NULL) { 513 KDASSERT(pserialize_not_in_read_section()); 514 LOCKDEBUG_BARRIER(&kernel_lock, 1); 515 } 516 517 LOCKSTAT_ENTER(lsflag); 518 519 /* 520 * Adaptive mutex; spin trying to acquire the mutex. If we 521 * determine that the owner is not running on a processor, 522 * then we stop spinning, and sleep instead. 523 */ 524 KPREEMPT_DISABLE(curlwp); 525 for (;;) { 526 if (!MUTEX_OWNED(owner)) { 527 /* 528 * Mutex owner clear could mean two things: 529 * 530 * * The mutex has been released. 531 * * The owner field hasn't been set yet. 532 * 533 * Try to acquire it again. If that fails, 534 * we'll just loop again. 535 */ 536 if (MUTEX_ACQUIRE(mtx, curthread)) 537 break; 538 owner = mtx->mtx_owner; 539 continue; 540 } 541 if (__predict_false(MUTEX_OWNER(owner) == curthread)) { 542 MUTEX_ABORT(mtx, "locking against myself"); 543 } 544 #ifdef MULTIPROCESSOR 545 /* 546 * Check to see if the owner is running on a processor. 547 * If so, then we should just spin, as the owner will 548 * likely release the lock very soon. 549 */ 550 if (mutex_oncpu(owner)) { 551 LOCKSTAT_START_TIMER(lsflag, spintime); 552 count = SPINLOCK_BACKOFF_MIN; 553 do { 554 KPREEMPT_ENABLE(curlwp); 555 SPINLOCK_BACKOFF(count); 556 KPREEMPT_DISABLE(curlwp); 557 owner = mtx->mtx_owner; 558 } while (mutex_oncpu(owner)); 559 LOCKSTAT_STOP_TIMER(lsflag, spintime); 560 LOCKSTAT_COUNT(spincnt, 1); 561 if (!MUTEX_OWNED(owner)) 562 continue; 563 } 564 #endif 565 566 ts = turnstile_lookup(mtx); 567 568 /* 569 * Once we have the turnstile chain interlock, mark the 570 * mutex as having waiters. If that fails, spin again: 571 * chances are that the mutex has been released. 572 */ 573 if (!MUTEX_SET_WAITERS(mtx, owner)) { 574 turnstile_exit(mtx); 575 owner = mtx->mtx_owner; 576 continue; 577 } 578 579 #ifdef MULTIPROCESSOR 580 /* 581 * mutex_exit() is permitted to release the mutex without 582 * any interlocking instructions, and the following can 583 * occur as a result: 584 * 585 * CPU 1: MUTEX_SET_WAITERS() CPU2: mutex_exit() 586 * ---------------------------- ---------------------------- 587 * .. acquire cache line 588 * .. test for waiters 589 * acquire cache line <- lose cache line 590 * lock cache line .. 591 * verify mutex is held .. 592 * set waiters .. 593 * unlock cache line .. 594 * lose cache line -> acquire cache line 595 * .. clear lock word, waiters 596 * return success 597 * 598 * There is another race that can occur: a third CPU could 599 * acquire the mutex as soon as it is released. Since 600 * adaptive mutexes are primarily spin mutexes, this is not 601 * something that we need to worry about too much. What we 602 * do need to ensure is that the waiters bit gets set. 603 * 604 * To allow the unlocked release, we need to make some 605 * assumptions here: 606 * 607 * o Release is the only non-atomic/unlocked operation 608 * that can be performed on the mutex. (It must still 609 * be atomic on the local CPU, e.g. in case interrupted 610 * or preempted). 611 * 612 * o At any given time, MUTEX_SET_WAITERS() can only ever 613 * be in progress on one CPU in the system - guaranteed 614 * by the turnstile chain lock. 615 * 616 * o No other operations other than MUTEX_SET_WAITERS() 617 * and release can modify a mutex with a non-zero 618 * owner field. 619 * 620 * o The result of a successful MUTEX_SET_WAITERS() call 621 * is an unbuffered write that is immediately visible 622 * to all other processors in the system. 623 * 624 * o If the holding LWP switches away, it posts a store 625 * fence before changing curlwp, ensuring that any 626 * overwrite of the mutex waiters flag by mutex_exit() 627 * completes before the modification of curlwp becomes 628 * visible to this CPU. 629 * 630 * o mi_switch() posts a store fence before setting curlwp 631 * and before resuming execution of an LWP. 632 * 633 * o _kernel_lock() posts a store fence before setting 634 * curcpu()->ci_biglock_wanted, and after clearing it. 635 * This ensures that any overwrite of the mutex waiters 636 * flag by mutex_exit() completes before the modification 637 * of ci_biglock_wanted becomes visible. 638 * 639 * We now post a read memory barrier (after setting the 640 * waiters field) and check the lock holder's status again. 641 * Some of the possible outcomes (not an exhaustive list): 642 * 643 * 1. The on-CPU check returns true: the holding LWP is 644 * running again. The lock may be released soon and 645 * we should spin. Importantly, we can't trust the 646 * value of the waiters flag. 647 * 648 * 2. The on-CPU check returns false: the holding LWP is 649 * not running. We now have the opportunity to check 650 * if mutex_exit() has blatted the modifications made 651 * by MUTEX_SET_WAITERS(). 652 * 653 * 3. The on-CPU check returns false: the holding LWP may 654 * or may not be running. It has context switched at 655 * some point during our check. Again, we have the 656 * chance to see if the waiters bit is still set or 657 * has been overwritten. 658 * 659 * 4. The on-CPU check returns false: the holding LWP is 660 * running on a CPU, but wants the big lock. It's OK 661 * to check the waiters field in this case. 662 * 663 * 5. The has-waiters check fails: the mutex has been 664 * released, the waiters flag cleared and another LWP 665 * now owns the mutex. 666 * 667 * 6. The has-waiters check fails: the mutex has been 668 * released. 669 * 670 * If the waiters bit is not set it's unsafe to go asleep, 671 * as we might never be awoken. 672 */ 673 membar_consumer(); 674 if (mutex_oncpu(owner)) { 675 turnstile_exit(mtx); 676 owner = mtx->mtx_owner; 677 continue; 678 } 679 membar_consumer(); 680 if (!MUTEX_HAS_WAITERS(mtx)) { 681 turnstile_exit(mtx); 682 owner = mtx->mtx_owner; 683 continue; 684 } 685 #endif /* MULTIPROCESSOR */ 686 687 LOCKSTAT_START_TIMER(lsflag, slptime); 688 689 turnstile_block(ts, TS_WRITER_Q, mtx, &mutex_syncobj); 690 691 LOCKSTAT_STOP_TIMER(lsflag, slptime); 692 LOCKSTAT_COUNT(slpcnt, 1); 693 694 owner = mtx->mtx_owner; 695 } 696 KPREEMPT_ENABLE(curlwp); 697 698 LOCKSTAT_EVENT(lsflag, mtx, LB_ADAPTIVE_MUTEX | LB_SLEEP1, 699 slpcnt, slptime); 700 LOCKSTAT_EVENT(lsflag, mtx, LB_ADAPTIVE_MUTEX | LB_SPIN, 701 spincnt, spintime); 702 LOCKSTAT_EXIT(lsflag); 703 704 MUTEX_DASSERT(mtx, MUTEX_OWNER(mtx->mtx_owner) == curthread); 705 MUTEX_LOCKED(mtx); 706 } 707 708 /* 709 * mutex_vector_exit: 710 * 711 * Support routine for mutex_exit() that handles all cases. 712 */ 713 void 714 mutex_vector_exit(kmutex_t *mtx) 715 { 716 turnstile_t *ts; 717 uintptr_t curthread; 718 719 if (MUTEX_SPIN_P(mtx->mtx_owner)) { 720 #ifdef FULL 721 if (__predict_false(!MUTEX_SPINBIT_LOCKED_P(mtx))) { 722 MUTEX_ABORT(mtx, "exiting unheld spin mutex"); 723 } 724 MUTEX_UNLOCKED(mtx); 725 MUTEX_SPINBIT_LOCK_UNLOCK(mtx); 726 #endif 727 MUTEX_SPIN_SPLRESTORE(mtx); 728 return; 729 } 730 731 #ifndef __HAVE_MUTEX_STUBS 732 /* 733 * On some architectures without mutex stubs, we can enter here to 734 * release mutexes before interrupts and whatnot are up and running. 735 * We need this hack to keep them sweet. 736 */ 737 if (__predict_false(cold)) { 738 MUTEX_UNLOCKED(mtx); 739 MUTEX_RELEASE(mtx); 740 return; 741 } 742 #endif 743 744 curthread = (uintptr_t)curlwp; 745 MUTEX_DASSERT(mtx, curthread != 0); 746 MUTEX_ASSERT(mtx, MUTEX_OWNER(mtx->mtx_owner) == curthread); 747 MUTEX_UNLOCKED(mtx); 748 #if !defined(LOCKDEBUG) 749 __USE(curthread); 750 #endif 751 752 #ifdef LOCKDEBUG 753 /* 754 * Avoid having to take the turnstile chain lock every time 755 * around. Raise the priority level to splhigh() in order 756 * to disable preemption and so make the following atomic. 757 */ 758 { 759 int s = splhigh(); 760 if (!MUTEX_HAS_WAITERS(mtx)) { 761 MUTEX_RELEASE(mtx); 762 splx(s); 763 return; 764 } 765 splx(s); 766 } 767 #endif 768 769 /* 770 * Get this lock's turnstile. This gets the interlock on 771 * the sleep queue. Once we have that, we can clear the 772 * lock. If there was no turnstile for the lock, there 773 * were no waiters remaining. 774 */ 775 ts = turnstile_lookup(mtx); 776 777 if (ts == NULL) { 778 MUTEX_RELEASE(mtx); 779 turnstile_exit(mtx); 780 } else { 781 MUTEX_RELEASE(mtx); 782 turnstile_wakeup(ts, TS_WRITER_Q, 783 TS_WAITERS(ts, TS_WRITER_Q), NULL); 784 } 785 } 786 787 #ifndef __HAVE_SIMPLE_MUTEXES 788 /* 789 * mutex_wakeup: 790 * 791 * Support routine for mutex_exit() that wakes up all waiters. 792 * We assume that the mutex has been released, but it need not 793 * be. 794 */ 795 void 796 mutex_wakeup(kmutex_t *mtx) 797 { 798 turnstile_t *ts; 799 800 ts = turnstile_lookup(mtx); 801 if (ts == NULL) { 802 turnstile_exit(mtx); 803 return; 804 } 805 MUTEX_CLEAR_WAITERS(mtx); 806 turnstile_wakeup(ts, TS_WRITER_Q, TS_WAITERS(ts, TS_WRITER_Q), NULL); 807 } 808 #endif /* !__HAVE_SIMPLE_MUTEXES */ 809 810 /* 811 * mutex_owned: 812 * 813 * Return true if the current LWP (adaptive) or CPU (spin) 814 * holds the mutex. 815 */ 816 int 817 mutex_owned(const kmutex_t *mtx) 818 { 819 820 if (mtx == NULL) 821 return 0; 822 if (MUTEX_ADAPTIVE_P(mtx->mtx_owner)) 823 return MUTEX_OWNER(mtx->mtx_owner) == (uintptr_t)curlwp; 824 #ifdef FULL 825 return MUTEX_SPINBIT_LOCKED_P(mtx); 826 #else 827 return 1; 828 #endif 829 } 830 831 /* 832 * mutex_owner: 833 * 834 * Return the current owner of an adaptive mutex. Used for 835 * priority inheritance. 836 */ 837 lwp_t * 838 mutex_owner(const kmutex_t *mtx) 839 { 840 841 MUTEX_ASSERT(mtx, MUTEX_ADAPTIVE_P(mtx->mtx_owner)); 842 return (struct lwp *)MUTEX_OWNER(mtx->mtx_owner); 843 } 844 845 /* 846 * mutex_owner_running: 847 * 848 * Return true if an adaptive mutex is unheld, or held and the owner is 849 * running on a CPU. For the pagedaemon only - do not document or use 850 * in other code. 851 */ 852 bool 853 mutex_owner_running(const kmutex_t *mtx) 854 { 855 #ifdef MULTIPROCESSOR 856 uintptr_t owner; 857 bool rv; 858 859 MUTEX_ASSERT(mtx, MUTEX_ADAPTIVE_P(mtx->mtx_owner)); 860 kpreempt_disable(); 861 owner = mtx->mtx_owner; 862 rv = !MUTEX_OWNED(owner) || mutex_oncpu(MUTEX_OWNER(owner)); 863 kpreempt_enable(); 864 return rv; 865 #else 866 return mutex_owner(mtx) == curlwp; 867 #endif 868 } 869 870 /* 871 * mutex_ownable: 872 * 873 * When compiled with DEBUG and LOCKDEBUG defined, ensure that 874 * the mutex is available. We cannot use !mutex_owned() since 875 * that won't work correctly for spin mutexes. 876 */ 877 int 878 mutex_ownable(const kmutex_t *mtx) 879 { 880 881 #ifdef LOCKDEBUG 882 MUTEX_TESTLOCK(mtx); 883 #endif 884 return 1; 885 } 886 887 /* 888 * mutex_tryenter: 889 * 890 * Try to acquire the mutex; return non-zero if we did. 891 */ 892 int 893 mutex_tryenter(kmutex_t *mtx) 894 { 895 uintptr_t curthread; 896 897 /* 898 * Handle spin mutexes. 899 */ 900 if (MUTEX_SPIN_P(mtx->mtx_owner)) { 901 MUTEX_SPIN_SPLRAISE(mtx); 902 #ifdef FULL 903 if (MUTEX_SPINBIT_LOCK_TRY(mtx)) { 904 MUTEX_WANTLOCK(mtx); 905 MUTEX_LOCKED(mtx); 906 return 1; 907 } 908 MUTEX_SPIN_SPLRESTORE(mtx); 909 #else 910 MUTEX_WANTLOCK(mtx); 911 MUTEX_LOCKED(mtx); 912 return 1; 913 #endif 914 } else { 915 curthread = (uintptr_t)curlwp; 916 MUTEX_ASSERT(mtx, curthread != 0); 917 if (MUTEX_ACQUIRE(mtx, curthread)) { 918 MUTEX_WANTLOCK(mtx); 919 MUTEX_LOCKED(mtx); 920 MUTEX_DASSERT(mtx, 921 MUTEX_OWNER(mtx->mtx_owner) == curthread); 922 return 1; 923 } 924 } 925 926 return 0; 927 } 928 929 #if defined(__HAVE_SPIN_MUTEX_STUBS) || defined(FULL) 930 /* 931 * mutex_spin_retry: 932 * 933 * Support routine for mutex_spin_enter(). Assumes that the caller 934 * has already raised the SPL, and adjusted counters. 935 */ 936 void 937 mutex_spin_retry(kmutex_t *mtx) 938 { 939 #ifdef MULTIPROCESSOR 940 u_int count; 941 LOCKSTAT_TIMER(spintime); 942 LOCKSTAT_FLAG(lsflag); 943 #ifdef LOCKDEBUG 944 u_int spins = 0; 945 #endif /* LOCKDEBUG */ 946 947 MUTEX_WANTLOCK(mtx); 948 949 LOCKSTAT_ENTER(lsflag); 950 LOCKSTAT_START_TIMER(lsflag, spintime); 951 count = SPINLOCK_BACKOFF_MIN; 952 953 /* 954 * Spin testing the lock word and do exponential backoff 955 * to reduce cache line ping-ponging between CPUs. 956 */ 957 do { 958 while (MUTEX_SPINBIT_LOCKED_P(mtx)) { 959 SPINLOCK_BACKOFF(count); 960 #ifdef LOCKDEBUG 961 if (SPINLOCK_SPINOUT(spins)) 962 MUTEX_ABORT(mtx, "spinout"); 963 #endif /* LOCKDEBUG */ 964 } 965 } while (!MUTEX_SPINBIT_LOCK_TRY(mtx)); 966 967 LOCKSTAT_STOP_TIMER(lsflag, spintime); 968 LOCKSTAT_EVENT(lsflag, mtx, LB_SPIN_MUTEX | LB_SPIN, 1, spintime); 969 LOCKSTAT_EXIT(lsflag); 970 971 MUTEX_LOCKED(mtx); 972 #else /* MULTIPROCESSOR */ 973 MUTEX_ABORT(mtx, "locking against myself"); 974 #endif /* MULTIPROCESSOR */ 975 } 976 #endif /* defined(__HAVE_SPIN_MUTEX_STUBS) || defined(FULL) */ 977