1 /* $NetBSD: kern_mutex.c,v 1.107 2023/05/01 12:18:08 riastradh 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 1.107 2023/05/01 12:18:08 riastradh 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 const int s = splraiseipl(MUTEX_SPIN_IPL(mtx)); \ 149 struct cpu_info * const x__ci = curcpu(); \ 150 const int x__cnt = x__ci->ci_mtx_count--; \ 151 __insn_barrier(); \ 152 if (x__cnt == 0) \ 153 x__ci->ci_mtx_oldspl = s; \ 154 } while (/* CONSTCOND */ 0) 155 156 #define MUTEX_SPIN_SPLRESTORE(mtx) \ 157 do { \ 158 struct cpu_info * const x__ci = curcpu(); \ 159 const int s = x__ci->ci_mtx_oldspl; \ 160 __insn_barrier(); \ 161 if (++(x__ci->ci_mtx_count) == 0) \ 162 splx(s); \ 163 } while (/* CONSTCOND */ 0) 164 165 /* 166 * Memory barriers. 167 */ 168 #ifdef __HAVE_ATOMIC_AS_MEMBAR 169 #define MUTEX_MEMBAR_ENTER() 170 #else 171 #define MUTEX_MEMBAR_ENTER() membar_enter() 172 #endif 173 174 /* 175 * For architectures that provide 'simple' mutexes: they provide a 176 * CAS function that is either MP-safe, or does not need to be MP 177 * safe. Adaptive mutexes on these architectures do not require an 178 * additional interlock. 179 */ 180 181 #ifdef __HAVE_SIMPLE_MUTEXES 182 183 #define MUTEX_OWNER(owner) \ 184 (owner & MUTEX_THREAD) 185 #define MUTEX_HAS_WAITERS(mtx) \ 186 (((int)(mtx)->mtx_owner & MUTEX_BIT_WAITERS) != 0) 187 188 #define MUTEX_INITIALIZE_ADAPTIVE(mtx, dodebug) \ 189 do { \ 190 if (!dodebug) \ 191 (mtx)->mtx_owner |= MUTEX_BIT_NODEBUG; \ 192 } while (/* CONSTCOND */ 0) 193 194 #define MUTEX_INITIALIZE_SPIN(mtx, dodebug, ipl) \ 195 do { \ 196 (mtx)->mtx_owner = MUTEX_BIT_SPIN; \ 197 if (!dodebug) \ 198 (mtx)->mtx_owner |= MUTEX_BIT_NODEBUG; \ 199 MUTEX_INITIALIZE_SPIN_IPL((mtx), (ipl)); \ 200 MUTEX_SPINBIT_LOCK_INIT((mtx)); \ 201 } while (/* CONSTCOND */ 0) 202 203 #define MUTEX_DESTROY(mtx) \ 204 do { \ 205 (mtx)->mtx_owner = MUTEX_THREAD; \ 206 } while (/* CONSTCOND */ 0) 207 208 #define MUTEX_SPIN_P(owner) \ 209 (((owner) & MUTEX_BIT_SPIN) != 0) 210 #define MUTEX_ADAPTIVE_P(owner) \ 211 (((owner) & MUTEX_BIT_SPIN) == 0) 212 213 #ifndef MUTEX_CAS 214 #define MUTEX_CAS(p, o, n) \ 215 (atomic_cas_ulong((volatile unsigned long *)(p), (o), (n)) == (o)) 216 #endif /* MUTEX_CAS */ 217 218 #define MUTEX_DEBUG_P(mtx) (((mtx)->mtx_owner & MUTEX_BIT_NODEBUG) == 0) 219 #if defined(LOCKDEBUG) 220 #define MUTEX_OWNED(owner) (((owner) & ~MUTEX_BIT_NODEBUG) != 0) 221 #define MUTEX_INHERITDEBUG(n, o) (n) |= (o) & MUTEX_BIT_NODEBUG 222 #else /* defined(LOCKDEBUG) */ 223 #define MUTEX_OWNED(owner) ((owner) != 0) 224 #define MUTEX_INHERITDEBUG(n, o) /* nothing */ 225 #endif /* defined(LOCKDEBUG) */ 226 227 static inline int 228 MUTEX_ACQUIRE(kmutex_t *mtx, uintptr_t curthread) 229 { 230 int rv; 231 uintptr_t oldown = 0; 232 uintptr_t newown = curthread; 233 234 MUTEX_INHERITDEBUG(oldown, mtx->mtx_owner); 235 MUTEX_INHERITDEBUG(newown, oldown); 236 rv = MUTEX_CAS(&mtx->mtx_owner, oldown, newown); 237 membar_acquire(); 238 return rv; 239 } 240 241 static inline int 242 MUTEX_SET_WAITERS(kmutex_t *mtx, uintptr_t owner) 243 { 244 int rv; 245 246 rv = MUTEX_CAS(&mtx->mtx_owner, owner, owner | MUTEX_BIT_WAITERS); 247 MUTEX_MEMBAR_ENTER(); 248 return rv; 249 } 250 251 static inline void 252 MUTEX_RELEASE(kmutex_t *mtx) 253 { 254 uintptr_t newown; 255 256 newown = 0; 257 MUTEX_INHERITDEBUG(newown, mtx->mtx_owner); 258 atomic_store_release(&mtx->mtx_owner, newown); 259 } 260 #endif /* __HAVE_SIMPLE_MUTEXES */ 261 262 /* 263 * Patch in stubs via strong alias where they are not available. 264 */ 265 266 #if defined(LOCKDEBUG) 267 #undef __HAVE_MUTEX_STUBS 268 #undef __HAVE_SPIN_MUTEX_STUBS 269 #endif 270 271 #ifndef __HAVE_MUTEX_STUBS 272 __strong_alias(mutex_enter,mutex_vector_enter); 273 __strong_alias(mutex_exit,mutex_vector_exit); 274 #endif 275 276 #ifndef __HAVE_SPIN_MUTEX_STUBS 277 __strong_alias(mutex_spin_enter,mutex_vector_enter); 278 __strong_alias(mutex_spin_exit,mutex_vector_exit); 279 #endif 280 281 static void mutex_abort(const char *, size_t, volatile const kmutex_t *, 282 const char *); 283 static void mutex_dump(const volatile void *, lockop_printer_t); 284 static lwp_t *mutex_owner(wchan_t); 285 286 lockops_t mutex_spin_lockops = { 287 .lo_name = "Mutex", 288 .lo_type = LOCKOPS_SPIN, 289 .lo_dump = mutex_dump, 290 }; 291 292 lockops_t mutex_adaptive_lockops = { 293 .lo_name = "Mutex", 294 .lo_type = LOCKOPS_SLEEP, 295 .lo_dump = mutex_dump, 296 }; 297 298 syncobj_t mutex_syncobj = { 299 .sobj_flag = SOBJ_SLEEPQ_SORTED, 300 .sobj_unsleep = turnstile_unsleep, 301 .sobj_changepri = turnstile_changepri, 302 .sobj_lendpri = sleepq_lendpri, 303 .sobj_owner = mutex_owner, 304 }; 305 306 /* 307 * mutex_dump: 308 * 309 * Dump the contents of a mutex structure. 310 */ 311 static void 312 mutex_dump(const volatile void *cookie, lockop_printer_t pr) 313 { 314 const volatile kmutex_t *mtx = cookie; 315 uintptr_t owner = mtx->mtx_owner; 316 317 pr("owner field : %#018lx wait/spin: %16d/%d\n", 318 (long)MUTEX_OWNER(owner), MUTEX_HAS_WAITERS(mtx), 319 MUTEX_SPIN_P(owner)); 320 } 321 322 /* 323 * mutex_abort: 324 * 325 * Dump information about an error and panic the system. This 326 * generates a lot of machine code in the DIAGNOSTIC case, so 327 * we ask the compiler to not inline it. 328 */ 329 static void __noinline 330 mutex_abort(const char *func, size_t line, volatile const kmutex_t *mtx, 331 const char *msg) 332 { 333 334 LOCKDEBUG_ABORT(func, line, mtx, (MUTEX_SPIN_P(mtx->mtx_owner) ? 335 &mutex_spin_lockops : &mutex_adaptive_lockops), msg); 336 } 337 338 /* 339 * mutex_init: 340 * 341 * Initialize a mutex for use. Note that adaptive mutexes are in 342 * essence spin mutexes that can sleep to avoid deadlock and wasting 343 * CPU time. We can't easily provide a type of mutex that always 344 * sleeps - see comments in mutex_vector_enter() about releasing 345 * mutexes unlocked. 346 */ 347 void 348 _mutex_init(kmutex_t *mtx, kmutex_type_t type, int ipl, 349 uintptr_t return_address) 350 { 351 lockops_t *lockops __unused; 352 bool dodebug; 353 354 memset(mtx, 0, sizeof(*mtx)); 355 356 if (ipl == IPL_NONE || ipl == IPL_SOFTCLOCK || 357 ipl == IPL_SOFTBIO || ipl == IPL_SOFTNET || 358 ipl == IPL_SOFTSERIAL) { 359 lockops = (type == MUTEX_NODEBUG ? 360 NULL : &mutex_adaptive_lockops); 361 dodebug = LOCKDEBUG_ALLOC(mtx, lockops, return_address); 362 MUTEX_INITIALIZE_ADAPTIVE(mtx, dodebug); 363 } else { 364 lockops = (type == MUTEX_NODEBUG ? 365 NULL : &mutex_spin_lockops); 366 dodebug = LOCKDEBUG_ALLOC(mtx, lockops, return_address); 367 MUTEX_INITIALIZE_SPIN(mtx, dodebug, ipl); 368 } 369 } 370 371 void 372 mutex_init(kmutex_t *mtx, kmutex_type_t type, int ipl) 373 { 374 375 _mutex_init(mtx, type, ipl, (uintptr_t)__builtin_return_address(0)); 376 } 377 378 /* 379 * mutex_destroy: 380 * 381 * Tear down a mutex. 382 */ 383 void 384 mutex_destroy(kmutex_t *mtx) 385 { 386 uintptr_t owner = mtx->mtx_owner; 387 388 if (MUTEX_ADAPTIVE_P(owner)) { 389 MUTEX_ASSERT(mtx, !MUTEX_OWNED(owner)); 390 MUTEX_ASSERT(mtx, !MUTEX_HAS_WAITERS(mtx)); 391 } else { 392 MUTEX_ASSERT(mtx, !MUTEX_SPINBIT_LOCKED_P(mtx)); 393 } 394 395 LOCKDEBUG_FREE(MUTEX_DEBUG_P(mtx), mtx); 396 MUTEX_DESTROY(mtx); 397 } 398 399 #ifdef MULTIPROCESSOR 400 /* 401 * mutex_oncpu: 402 * 403 * Return true if an adaptive mutex owner is running on a CPU in the 404 * system. If the target is waiting on the kernel big lock, then we 405 * must release it. This is necessary to avoid deadlock. 406 */ 407 static bool 408 mutex_oncpu(uintptr_t owner) 409 { 410 struct cpu_info *ci; 411 lwp_t *l; 412 413 KASSERT(kpreempt_disabled()); 414 415 if (!MUTEX_OWNED(owner)) { 416 return false; 417 } 418 419 /* 420 * See lwp_dtor() why dereference of the LWP pointer is safe. 421 * We must have kernel preemption disabled for that. 422 */ 423 l = (lwp_t *)MUTEX_OWNER(owner); 424 ci = l->l_cpu; 425 426 if (ci && ci->ci_curlwp == l) { 427 /* Target is running; do we need to block? */ 428 return (atomic_load_relaxed(&ci->ci_biglock_wanted) != l); 429 } 430 431 /* Not running. It may be safe to block now. */ 432 return false; 433 } 434 #endif /* MULTIPROCESSOR */ 435 436 /* 437 * mutex_vector_enter: 438 * 439 * Support routine for mutex_enter() that must handle all cases. In 440 * the LOCKDEBUG case, mutex_enter() is always aliased here, even if 441 * fast-path stubs are available. If a mutex_spin_enter() stub is 442 * not available, then it is also aliased directly here. 443 */ 444 void 445 mutex_vector_enter(kmutex_t *mtx) 446 { 447 uintptr_t owner, curthread; 448 turnstile_t *ts; 449 #ifdef MULTIPROCESSOR 450 u_int count; 451 #endif 452 LOCKSTAT_COUNTER(spincnt); 453 LOCKSTAT_COUNTER(slpcnt); 454 LOCKSTAT_TIMER(spintime); 455 LOCKSTAT_TIMER(slptime); 456 LOCKSTAT_FLAG(lsflag); 457 458 /* 459 * Handle spin mutexes. 460 */ 461 KPREEMPT_DISABLE(curlwp); 462 owner = mtx->mtx_owner; 463 if (MUTEX_SPIN_P(owner)) { 464 #if defined(LOCKDEBUG) && defined(MULTIPROCESSOR) 465 u_int spins = 0; 466 #endif 467 KPREEMPT_ENABLE(curlwp); 468 MUTEX_SPIN_SPLRAISE(mtx); 469 MUTEX_WANTLOCK(mtx); 470 #ifdef FULL 471 if (MUTEX_SPINBIT_LOCK_TRY(mtx)) { 472 MUTEX_LOCKED(mtx); 473 return; 474 } 475 #if !defined(MULTIPROCESSOR) 476 MUTEX_ABORT(mtx, "locking against myself"); 477 #else /* !MULTIPROCESSOR */ 478 479 LOCKSTAT_ENTER(lsflag); 480 LOCKSTAT_START_TIMER(lsflag, spintime); 481 count = SPINLOCK_BACKOFF_MIN; 482 483 /* 484 * Spin testing the lock word and do exponential backoff 485 * to reduce cache line ping-ponging between CPUs. 486 */ 487 do { 488 while (MUTEX_SPINBIT_LOCKED_P(mtx)) { 489 SPINLOCK_SPIN_HOOK; 490 SPINLOCK_BACKOFF(count); 491 #ifdef LOCKDEBUG 492 if (SPINLOCK_SPINOUT(spins)) 493 MUTEX_ABORT(mtx, "spinout"); 494 #endif /* LOCKDEBUG */ 495 } 496 } while (!MUTEX_SPINBIT_LOCK_TRY(mtx)); 497 498 if (count != SPINLOCK_BACKOFF_MIN) { 499 LOCKSTAT_STOP_TIMER(lsflag, spintime); 500 LOCKSTAT_EVENT(lsflag, mtx, 501 LB_SPIN_MUTEX | LB_SPIN, 1, spintime); 502 } 503 LOCKSTAT_EXIT(lsflag); 504 #endif /* !MULTIPROCESSOR */ 505 #endif /* FULL */ 506 MUTEX_LOCKED(mtx); 507 return; 508 } 509 510 curthread = (uintptr_t)curlwp; 511 512 MUTEX_DASSERT(mtx, MUTEX_ADAPTIVE_P(owner)); 513 MUTEX_ASSERT(mtx, curthread != 0); 514 MUTEX_ASSERT(mtx, !cpu_intr_p()); 515 MUTEX_WANTLOCK(mtx); 516 517 if (__predict_true(panicstr == NULL)) { 518 KDASSERT(pserialize_not_in_read_section()); 519 LOCKDEBUG_BARRIER(&kernel_lock, 1); 520 } 521 522 LOCKSTAT_ENTER(lsflag); 523 524 /* 525 * Adaptive mutex; spin trying to acquire the mutex. If we 526 * determine that the owner is not running on a processor, 527 * then we stop spinning, and sleep instead. 528 */ 529 for (;;) { 530 if (!MUTEX_OWNED(owner)) { 531 /* 532 * Mutex owner clear could mean two things: 533 * 534 * * The mutex has been released. 535 * * The owner field hasn't been set yet. 536 * 537 * Try to acquire it again. If that fails, 538 * we'll just loop again. 539 */ 540 if (MUTEX_ACQUIRE(mtx, curthread)) 541 break; 542 owner = mtx->mtx_owner; 543 continue; 544 } 545 if (__predict_false(MUTEX_OWNER(owner) == curthread)) { 546 MUTEX_ABORT(mtx, "locking against myself"); 547 } 548 #ifdef MULTIPROCESSOR 549 /* 550 * Check to see if the owner is running on a processor. 551 * If so, then we should just spin, as the owner will 552 * likely release the lock very soon. 553 */ 554 if (mutex_oncpu(owner)) { 555 LOCKSTAT_START_TIMER(lsflag, spintime); 556 count = SPINLOCK_BACKOFF_MIN; 557 do { 558 KPREEMPT_ENABLE(curlwp); 559 SPINLOCK_BACKOFF(count); 560 KPREEMPT_DISABLE(curlwp); 561 owner = mtx->mtx_owner; 562 } while (mutex_oncpu(owner)); 563 LOCKSTAT_STOP_TIMER(lsflag, spintime); 564 LOCKSTAT_COUNT(spincnt, 1); 565 if (!MUTEX_OWNED(owner)) 566 continue; 567 } 568 #endif 569 570 ts = turnstile_lookup(mtx); 571 572 /* 573 * Once we have the turnstile chain interlock, mark the 574 * mutex as having waiters. If that fails, spin again: 575 * chances are that the mutex has been released. 576 */ 577 if (!MUTEX_SET_WAITERS(mtx, owner)) { 578 turnstile_exit(mtx); 579 owner = mtx->mtx_owner; 580 continue; 581 } 582 583 #ifdef MULTIPROCESSOR 584 /* 585 * mutex_exit() is permitted to release the mutex without 586 * any interlocking instructions, and the following can 587 * occur as a result: 588 * 589 * CPU 1: MUTEX_SET_WAITERS() CPU2: mutex_exit() 590 * ---------------------------- ---------------------------- 591 * .. load mtx->mtx_owner 592 * .. see has-waiters bit clear 593 * set has-waiters bit .. 594 * .. store mtx->mtx_owner := 0 595 * return success 596 * 597 * There is another race that can occur: a third CPU could 598 * acquire the mutex as soon as it is released. Since 599 * adaptive mutexes are primarily spin mutexes, this is not 600 * something that we need to worry about too much. What we 601 * do need to ensure is that the waiters bit gets set. 602 * 603 * To allow the unlocked release, we need to make some 604 * assumptions here: 605 * 606 * o Release is the only non-atomic/unlocked operation 607 * that can be performed on the mutex. (It must still 608 * be atomic on the local CPU, e.g. in case interrupted 609 * or preempted). 610 * 611 * o At any given time on each mutex, MUTEX_SET_WAITERS() 612 * can only ever be in progress on one CPU in the 613 * system - guaranteed by the turnstile chain lock. 614 * 615 * o No other operations other than MUTEX_SET_WAITERS() 616 * and release can modify a mutex with a non-zero 617 * owner field. 618 * 619 * o If the holding LWP switches away, it posts a store 620 * fence before changing curlwp, ensuring that any 621 * overwrite of the mutex waiters flag by mutex_exit() 622 * completes before the modification of curlwp becomes 623 * visible to this CPU. 624 * 625 * o cpu_switchto() posts a store fence after setting curlwp 626 * and before resuming execution of an LWP. 627 * 628 * o _kernel_lock() posts a store fence before setting 629 * curcpu()->ci_biglock_wanted, and after clearing it. 630 * This ensures that any overwrite of the mutex waiters 631 * flag by mutex_exit() completes before the modification 632 * of ci_biglock_wanted becomes visible. 633 * 634 * After MUTEX_SET_WAITERS() succeeds, simultaneously 635 * confirming that the same LWP still holds the mutex 636 * since we took the turnstile lock and notifying it that 637 * we're waiting, we check the lock holder's status again. 638 * Some of the possible outcomes (not an exhaustive list; 639 * XXX this should be made exhaustive): 640 * 641 * 1. The on-CPU check returns true: the holding LWP is 642 * running again. The lock may be released soon and 643 * we should spin. Importantly, we can't trust the 644 * value of the waiters flag. 645 * 646 * 2. The on-CPU check returns false: the holding LWP is 647 * not running. We now have the opportunity to check 648 * if mutex_exit() has blatted the modifications made 649 * by MUTEX_SET_WAITERS(). 650 * 651 * 3. The on-CPU check returns false: the holding LWP may 652 * or may not be running. It has context switched at 653 * some point during our check. Again, we have the 654 * chance to see if the waiters bit is still set or 655 * has been overwritten. 656 * 657 * 4. The on-CPU check returns false: the holding LWP is 658 * running on a CPU, but wants the big lock. It's OK 659 * to check the waiters field in this case. 660 * 661 * 5. The has-waiters check fails: the mutex has been 662 * released, the waiters flag cleared and another LWP 663 * now owns the mutex. 664 * 665 * 6. The has-waiters check fails: the mutex has been 666 * released. 667 * 668 * If the waiters bit is not set it's unsafe to go asleep, 669 * as we might never be awoken. 670 */ 671 if (mutex_oncpu(owner)) { 672 turnstile_exit(mtx); 673 owner = mtx->mtx_owner; 674 continue; 675 } 676 membar_consumer(); 677 if (!MUTEX_HAS_WAITERS(mtx)) { 678 turnstile_exit(mtx); 679 owner = mtx->mtx_owner; 680 continue; 681 } 682 #endif /* MULTIPROCESSOR */ 683 684 LOCKSTAT_START_TIMER(lsflag, slptime); 685 686 turnstile_block(ts, TS_WRITER_Q, mtx, &mutex_syncobj); 687 688 LOCKSTAT_STOP_TIMER(lsflag, slptime); 689 LOCKSTAT_COUNT(slpcnt, 1); 690 691 owner = mtx->mtx_owner; 692 } 693 KPREEMPT_ENABLE(curlwp); 694 695 LOCKSTAT_EVENT(lsflag, mtx, LB_ADAPTIVE_MUTEX | LB_SLEEP1, 696 slpcnt, slptime); 697 LOCKSTAT_EVENT(lsflag, mtx, LB_ADAPTIVE_MUTEX | LB_SPIN, 698 spincnt, spintime); 699 LOCKSTAT_EXIT(lsflag); 700 701 MUTEX_DASSERT(mtx, MUTEX_OWNER(mtx->mtx_owner) == curthread); 702 MUTEX_LOCKED(mtx); 703 } 704 705 /* 706 * mutex_vector_exit: 707 * 708 * Support routine for mutex_exit() that handles all cases. 709 */ 710 void 711 mutex_vector_exit(kmutex_t *mtx) 712 { 713 turnstile_t *ts; 714 uintptr_t curthread; 715 716 if (MUTEX_SPIN_P(mtx->mtx_owner)) { 717 #ifdef FULL 718 if (__predict_false(!MUTEX_SPINBIT_LOCKED_P(mtx))) { 719 MUTEX_ABORT(mtx, "exiting unheld spin mutex"); 720 } 721 MUTEX_UNLOCKED(mtx); 722 MUTEX_SPINBIT_LOCK_UNLOCK(mtx); 723 #endif 724 MUTEX_SPIN_SPLRESTORE(mtx); 725 return; 726 } 727 728 #ifndef __HAVE_MUTEX_STUBS 729 /* 730 * On some architectures without mutex stubs, we can enter here to 731 * release mutexes before interrupts and whatnot are up and running. 732 * We need this hack to keep them sweet. 733 */ 734 if (__predict_false(cold)) { 735 MUTEX_UNLOCKED(mtx); 736 MUTEX_RELEASE(mtx); 737 return; 738 } 739 #endif 740 741 curthread = (uintptr_t)curlwp; 742 MUTEX_DASSERT(mtx, curthread != 0); 743 MUTEX_ASSERT(mtx, MUTEX_OWNER(mtx->mtx_owner) == curthread); 744 MUTEX_UNLOCKED(mtx); 745 #if !defined(LOCKDEBUG) 746 __USE(curthread); 747 #endif 748 749 #ifdef LOCKDEBUG 750 /* 751 * Avoid having to take the turnstile chain lock every time 752 * around. Raise the priority level to splhigh() in order 753 * to disable preemption and so make the following atomic. 754 */ 755 { 756 int s = splhigh(); 757 if (!MUTEX_HAS_WAITERS(mtx)) { 758 MUTEX_RELEASE(mtx); 759 splx(s); 760 return; 761 } 762 splx(s); 763 } 764 #endif 765 766 /* 767 * Get this lock's turnstile. This gets the interlock on 768 * the sleep queue. Once we have that, we can clear the 769 * lock. If there was no turnstile for the lock, there 770 * were no waiters remaining. 771 */ 772 ts = turnstile_lookup(mtx); 773 774 if (ts == NULL) { 775 MUTEX_RELEASE(mtx); 776 turnstile_exit(mtx); 777 } else { 778 MUTEX_RELEASE(mtx); 779 turnstile_wakeup(ts, TS_WRITER_Q, 780 TS_WAITERS(ts, TS_WRITER_Q), NULL); 781 } 782 } 783 784 #ifndef __HAVE_SIMPLE_MUTEXES 785 /* 786 * mutex_wakeup: 787 * 788 * Support routine for mutex_exit() that wakes up all waiters. 789 * We assume that the mutex has been released, but it need not 790 * be. 791 */ 792 void 793 mutex_wakeup(kmutex_t *mtx) 794 { 795 turnstile_t *ts; 796 797 ts = turnstile_lookup(mtx); 798 if (ts == NULL) { 799 turnstile_exit(mtx); 800 return; 801 } 802 MUTEX_CLEAR_WAITERS(mtx); 803 turnstile_wakeup(ts, TS_WRITER_Q, TS_WAITERS(ts, TS_WRITER_Q), NULL); 804 } 805 #endif /* !__HAVE_SIMPLE_MUTEXES */ 806 807 /* 808 * mutex_owned: 809 * 810 * Return true if the current LWP (adaptive) or CPU (spin) 811 * holds the mutex. 812 */ 813 int 814 mutex_owned(const kmutex_t *mtx) 815 { 816 817 if (mtx == NULL) 818 return 0; 819 if (MUTEX_ADAPTIVE_P(mtx->mtx_owner)) 820 return MUTEX_OWNER(mtx->mtx_owner) == (uintptr_t)curlwp; 821 #ifdef FULL 822 return MUTEX_SPINBIT_LOCKED_P(mtx); 823 #else 824 return 1; 825 #endif 826 } 827 828 /* 829 * mutex_owner: 830 * 831 * Return the current owner of an adaptive mutex. Used for 832 * priority inheritance. 833 */ 834 static lwp_t * 835 mutex_owner(wchan_t wchan) 836 { 837 volatile const kmutex_t *mtx = wchan; 838 839 MUTEX_ASSERT(mtx, MUTEX_ADAPTIVE_P(mtx->mtx_owner)); 840 return (struct lwp *)MUTEX_OWNER(mtx->mtx_owner); 841 } 842 843 /* 844 * mutex_owner_running: 845 * 846 * Return true if an adaptive mutex is unheld, or held and the owner is 847 * running on a CPU. For the pagedaemon only - do not document or use 848 * in other code. 849 */ 850 bool 851 mutex_owner_running(const kmutex_t *mtx) 852 { 853 #ifdef MULTIPROCESSOR 854 uintptr_t owner; 855 bool rv; 856 857 MUTEX_ASSERT(mtx, MUTEX_ADAPTIVE_P(mtx->mtx_owner)); 858 kpreempt_disable(); 859 owner = mtx->mtx_owner; 860 rv = !MUTEX_OWNED(owner) || mutex_oncpu(MUTEX_OWNER(owner)); 861 kpreempt_enable(); 862 return rv; 863 #else 864 return mutex_owner(mtx) == curlwp; 865 #endif 866 } 867 868 /* 869 * mutex_ownable: 870 * 871 * When compiled with DEBUG and LOCKDEBUG defined, ensure that 872 * the mutex is available. We cannot use !mutex_owned() since 873 * that won't work correctly for spin mutexes. 874 */ 875 int 876 mutex_ownable(const kmutex_t *mtx) 877 { 878 879 #ifdef LOCKDEBUG 880 MUTEX_TESTLOCK(mtx); 881 #endif 882 return 1; 883 } 884 885 /* 886 * mutex_tryenter: 887 * 888 * Try to acquire the mutex; return non-zero if we did. 889 */ 890 int 891 mutex_tryenter(kmutex_t *mtx) 892 { 893 uintptr_t curthread; 894 895 /* 896 * Handle spin mutexes. 897 */ 898 if (MUTEX_SPIN_P(mtx->mtx_owner)) { 899 MUTEX_SPIN_SPLRAISE(mtx); 900 #ifdef FULL 901 if (MUTEX_SPINBIT_LOCK_TRY(mtx)) { 902 MUTEX_WANTLOCK(mtx); 903 MUTEX_LOCKED(mtx); 904 return 1; 905 } 906 MUTEX_SPIN_SPLRESTORE(mtx); 907 #else 908 MUTEX_WANTLOCK(mtx); 909 MUTEX_LOCKED(mtx); 910 return 1; 911 #endif 912 } else { 913 curthread = (uintptr_t)curlwp; 914 MUTEX_ASSERT(mtx, curthread != 0); 915 if (MUTEX_ACQUIRE(mtx, curthread)) { 916 MUTEX_WANTLOCK(mtx); 917 MUTEX_LOCKED(mtx); 918 MUTEX_DASSERT(mtx, 919 MUTEX_OWNER(mtx->mtx_owner) == curthread); 920 return 1; 921 } 922 } 923 924 return 0; 925 } 926 927 #if defined(__HAVE_SPIN_MUTEX_STUBS) || defined(FULL) 928 /* 929 * mutex_spin_retry: 930 * 931 * Support routine for mutex_spin_enter(). Assumes that the caller 932 * has already raised the SPL, and adjusted counters. 933 */ 934 void 935 mutex_spin_retry(kmutex_t *mtx) 936 { 937 #ifdef MULTIPROCESSOR 938 u_int count; 939 LOCKSTAT_TIMER(spintime); 940 LOCKSTAT_FLAG(lsflag); 941 #ifdef LOCKDEBUG 942 u_int spins = 0; 943 #endif /* LOCKDEBUG */ 944 945 MUTEX_WANTLOCK(mtx); 946 947 LOCKSTAT_ENTER(lsflag); 948 LOCKSTAT_START_TIMER(lsflag, spintime); 949 count = SPINLOCK_BACKOFF_MIN; 950 951 /* 952 * Spin testing the lock word and do exponential backoff 953 * to reduce cache line ping-ponging between CPUs. 954 */ 955 do { 956 while (MUTEX_SPINBIT_LOCKED_P(mtx)) { 957 SPINLOCK_BACKOFF(count); 958 #ifdef LOCKDEBUG 959 if (SPINLOCK_SPINOUT(spins)) 960 MUTEX_ABORT(mtx, "spinout"); 961 #endif /* LOCKDEBUG */ 962 } 963 } while (!MUTEX_SPINBIT_LOCK_TRY(mtx)); 964 965 LOCKSTAT_STOP_TIMER(lsflag, spintime); 966 LOCKSTAT_EVENT(lsflag, mtx, LB_SPIN_MUTEX | LB_SPIN, 1, spintime); 967 LOCKSTAT_EXIT(lsflag); 968 969 MUTEX_LOCKED(mtx); 970 #else /* MULTIPROCESSOR */ 971 MUTEX_ABORT(mtx, "locking against myself"); 972 #endif /* MULTIPROCESSOR */ 973 } 974 #endif /* defined(__HAVE_SPIN_MUTEX_STUBS) || defined(FULL) */ 975