pthread_mutex.cpp revision ff03a7aaade5826e3708f6e320d0612d4cdbdb72
1/* 2 * Copyright (C) 2008 The Android Open Source Project 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * * Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * * Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in 12 * the documentation and/or other materials provided with the 13 * distribution. 14 * 15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 16 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 17 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 18 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 19 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 21 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS 22 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED 23 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 24 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 25 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 * SUCH DAMAGE. 27 */ 28 29#include <pthread.h> 30 31#include <errno.h> 32#include <limits.h> 33#include <sys/mman.h> 34#include <unistd.h> 35 36#include "pthread_internal.h" 37 38#include "private/bionic_atomic_inline.h" 39#include "private/bionic_futex.h" 40#include "private/bionic_tls.h" 41 42extern void pthread_debug_mutex_lock_check(pthread_mutex_t *mutex); 43extern void pthread_debug_mutex_unlock_check(pthread_mutex_t *mutex); 44 45/* a mutex is implemented as a 32-bit integer holding the following fields 46 * 47 * bits: name description 48 * 31-16 tid owner thread's tid (recursive and errorcheck only) 49 * 15-14 type mutex type 50 * 13 shared process-shared flag 51 * 12-2 counter counter of recursive mutexes 52 * 1-0 state lock state (0, 1 or 2) 53 */ 54 55/* Convenience macro, creates a mask of 'bits' bits that starts from 56 * the 'shift'-th least significant bit in a 32-bit word. 57 * 58 * Examples: FIELD_MASK(0,4) -> 0xf 59 * FIELD_MASK(16,9) -> 0x1ff0000 60 */ 61#define FIELD_MASK(shift,bits) (((1 << (bits))-1) << (shift)) 62 63/* This one is used to create a bit pattern from a given field value */ 64#define FIELD_TO_BITS(val,shift,bits) (((val) & ((1 << (bits))-1)) << (shift)) 65 66/* And this one does the opposite, i.e. extract a field's value from a bit pattern */ 67#define FIELD_FROM_BITS(val,shift,bits) (((val) >> (shift)) & ((1 << (bits))-1)) 68 69/* Mutex state: 70 * 71 * 0 for unlocked 72 * 1 for locked, no waiters 73 * 2 for locked, maybe waiters 74 */ 75#define MUTEX_STATE_SHIFT 0 76#define MUTEX_STATE_LEN 2 77 78#define MUTEX_STATE_MASK FIELD_MASK(MUTEX_STATE_SHIFT, MUTEX_STATE_LEN) 79#define MUTEX_STATE_FROM_BITS(v) FIELD_FROM_BITS(v, MUTEX_STATE_SHIFT, MUTEX_STATE_LEN) 80#define MUTEX_STATE_TO_BITS(v) FIELD_TO_BITS(v, MUTEX_STATE_SHIFT, MUTEX_STATE_LEN) 81 82#define MUTEX_STATE_UNLOCKED 0 /* must be 0 to match __PTHREAD_MUTEX_INIT_VALUE */ 83#define MUTEX_STATE_LOCKED_UNCONTENDED 1 /* must be 1 due to atomic dec in unlock operation */ 84#define MUTEX_STATE_LOCKED_CONTENDED 2 /* must be 1 + LOCKED_UNCONTENDED due to atomic dec */ 85 86#define MUTEX_STATE_FROM_BITS(v) FIELD_FROM_BITS(v, MUTEX_STATE_SHIFT, MUTEX_STATE_LEN) 87#define MUTEX_STATE_TO_BITS(v) FIELD_TO_BITS(v, MUTEX_STATE_SHIFT, MUTEX_STATE_LEN) 88 89#define MUTEX_STATE_BITS_UNLOCKED MUTEX_STATE_TO_BITS(MUTEX_STATE_UNLOCKED) 90#define MUTEX_STATE_BITS_LOCKED_UNCONTENDED MUTEX_STATE_TO_BITS(MUTEX_STATE_LOCKED_UNCONTENDED) 91#define MUTEX_STATE_BITS_LOCKED_CONTENDED MUTEX_STATE_TO_BITS(MUTEX_STATE_LOCKED_CONTENDED) 92 93/* return true iff the mutex if locked with no waiters */ 94#define MUTEX_STATE_BITS_IS_LOCKED_UNCONTENDED(v) (((v) & MUTEX_STATE_MASK) == MUTEX_STATE_BITS_LOCKED_UNCONTENDED) 95 96/* return true iff the mutex if locked with maybe waiters */ 97#define MUTEX_STATE_BITS_IS_LOCKED_CONTENDED(v) (((v) & MUTEX_STATE_MASK) == MUTEX_STATE_BITS_LOCKED_CONTENDED) 98 99/* used to flip from LOCKED_UNCONTENDED to LOCKED_CONTENDED */ 100#define MUTEX_STATE_BITS_FLIP_CONTENTION(v) ((v) ^ (MUTEX_STATE_BITS_LOCKED_CONTENDED ^ MUTEX_STATE_BITS_LOCKED_UNCONTENDED)) 101 102/* Mutex counter: 103 * 104 * We need to check for overflow before incrementing, and we also need to 105 * detect when the counter is 0 106 */ 107#define MUTEX_COUNTER_SHIFT 2 108#define MUTEX_COUNTER_LEN 11 109#define MUTEX_COUNTER_MASK FIELD_MASK(MUTEX_COUNTER_SHIFT, MUTEX_COUNTER_LEN) 110 111#define MUTEX_COUNTER_BITS_WILL_OVERFLOW(v) (((v) & MUTEX_COUNTER_MASK) == MUTEX_COUNTER_MASK) 112#define MUTEX_COUNTER_BITS_IS_ZERO(v) (((v) & MUTEX_COUNTER_MASK) == 0) 113 114/* Used to increment the counter directly after overflow has been checked */ 115#define MUTEX_COUNTER_BITS_ONE FIELD_TO_BITS(1,MUTEX_COUNTER_SHIFT,MUTEX_COUNTER_LEN) 116 117/* Returns true iff the counter is 0 */ 118#define MUTEX_COUNTER_BITS_ARE_ZERO(v) (((v) & MUTEX_COUNTER_MASK) == 0) 119 120/* Mutex shared bit flag 121 * 122 * This flag is set to indicate that the mutex is shared among processes. 123 * This changes the futex opcode we use for futex wait/wake operations 124 * (non-shared operations are much faster). 125 */ 126#define MUTEX_SHARED_SHIFT 13 127#define MUTEX_SHARED_MASK FIELD_MASK(MUTEX_SHARED_SHIFT,1) 128 129/* Mutex type: 130 * 131 * We support normal, recursive and errorcheck mutexes. 132 * 133 * The constants defined here *cannot* be changed because they must match 134 * the C library ABI which defines the following initialization values in 135 * <pthread.h>: 136 * 137 * __PTHREAD_MUTEX_INIT_VALUE 138 * __PTHREAD_RECURSIVE_MUTEX_VALUE 139 * __PTHREAD_ERRORCHECK_MUTEX_INIT_VALUE 140 */ 141#define MUTEX_TYPE_SHIFT 14 142#define MUTEX_TYPE_LEN 2 143#define MUTEX_TYPE_MASK FIELD_MASK(MUTEX_TYPE_SHIFT,MUTEX_TYPE_LEN) 144 145#define MUTEX_TYPE_NORMAL 0 /* Must be 0 to match __PTHREAD_MUTEX_INIT_VALUE */ 146#define MUTEX_TYPE_RECURSIVE 1 147#define MUTEX_TYPE_ERRORCHECK 2 148 149#define MUTEX_TYPE_TO_BITS(t) FIELD_TO_BITS(t, MUTEX_TYPE_SHIFT, MUTEX_TYPE_LEN) 150 151#define MUTEX_TYPE_BITS_NORMAL MUTEX_TYPE_TO_BITS(MUTEX_TYPE_NORMAL) 152#define MUTEX_TYPE_BITS_RECURSIVE MUTEX_TYPE_TO_BITS(MUTEX_TYPE_RECURSIVE) 153#define MUTEX_TYPE_BITS_ERRORCHECK MUTEX_TYPE_TO_BITS(MUTEX_TYPE_ERRORCHECK) 154 155/* Mutex owner field: 156 * 157 * This is only used for recursive and errorcheck mutexes. It holds the 158 * tid of the owning thread. Note that this works because the Linux 159 * kernel _only_ uses 16-bit values for tids. 160 * 161 * More specifically, it will wrap to 10000 when it reaches over 32768 for 162 * application processes. You can check this by running the following inside 163 * an adb shell session: 164 * 165 OLDPID=$$; 166 while true; do 167 NEWPID=$(sh -c 'echo $$') 168 if [ "$NEWPID" -gt 32768 ]; then 169 echo "AARGH: new PID $NEWPID is too high!" 170 exit 1 171 fi 172 if [ "$NEWPID" -lt "$OLDPID" ]; then 173 echo "****** Wrapping from PID $OLDPID to $NEWPID. *******" 174 else 175 echo -n "$NEWPID!" 176 fi 177 OLDPID=$NEWPID 178 done 179 180 * Note that you can run the same example on a desktop Linux system, 181 * the wrapping will also happen at 32768, but will go back to 300 instead. 182 */ 183#define MUTEX_OWNER_SHIFT 16 184#define MUTEX_OWNER_LEN 16 185 186#define MUTEX_OWNER_FROM_BITS(v) FIELD_FROM_BITS(v,MUTEX_OWNER_SHIFT,MUTEX_OWNER_LEN) 187#define MUTEX_OWNER_TO_BITS(v) FIELD_TO_BITS(v,MUTEX_OWNER_SHIFT,MUTEX_OWNER_LEN) 188 189/* Convenience macros. 190 * 191 * These are used to form or modify the bit pattern of a given mutex value 192 */ 193 194 195 196/* a mutex attribute holds the following fields 197 * 198 * bits: name description 199 * 0-3 type type of mutex 200 * 4 shared process-shared flag 201 */ 202#define MUTEXATTR_TYPE_MASK 0x000f 203#define MUTEXATTR_SHARED_MASK 0x0010 204 205 206int pthread_mutexattr_init(pthread_mutexattr_t *attr) 207{ 208 *attr = PTHREAD_MUTEX_DEFAULT; 209 return 0; 210} 211 212int pthread_mutexattr_destroy(pthread_mutexattr_t *attr) 213{ 214 *attr = -1; 215 return 0; 216} 217 218int pthread_mutexattr_gettype(const pthread_mutexattr_t *attr, int *type_p) 219{ 220 int type = (*attr & MUTEXATTR_TYPE_MASK); 221 222 if (type < PTHREAD_MUTEX_NORMAL || type > PTHREAD_MUTEX_ERRORCHECK) { 223 return EINVAL; 224 } 225 226 *type_p = type; 227 return 0; 228} 229 230int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type) 231{ 232 if (type < PTHREAD_MUTEX_NORMAL || type > PTHREAD_MUTEX_ERRORCHECK ) { 233 return EINVAL; 234 } 235 236 *attr = (*attr & ~MUTEXATTR_TYPE_MASK) | type; 237 return 0; 238} 239 240/* process-shared mutexes are not supported at the moment */ 241 242int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr, int pshared) 243{ 244 switch (pshared) { 245 case PTHREAD_PROCESS_PRIVATE: 246 *attr &= ~MUTEXATTR_SHARED_MASK; 247 return 0; 248 249 case PTHREAD_PROCESS_SHARED: 250 /* our current implementation of pthread actually supports shared 251 * mutexes but won't cleanup if a process dies with the mutex held. 252 * Nevertheless, it's better than nothing. Shared mutexes are used 253 * by surfaceflinger and audioflinger. 254 */ 255 *attr |= MUTEXATTR_SHARED_MASK; 256 return 0; 257 } 258 return EINVAL; 259} 260 261int pthread_mutexattr_getpshared(const pthread_mutexattr_t* attr, int* pshared) { 262 *pshared = (*attr & MUTEXATTR_SHARED_MASK) ? PTHREAD_PROCESS_SHARED : PTHREAD_PROCESS_PRIVATE; 263 return 0; 264} 265 266int pthread_mutex_init(pthread_mutex_t* mutex, const pthread_mutexattr_t* attr) { 267 if (__predict_true(attr == NULL)) { 268 mutex->value = MUTEX_TYPE_BITS_NORMAL; 269 return 0; 270 } 271 272 int value = 0; 273 if ((*attr & MUTEXATTR_SHARED_MASK) != 0) { 274 value |= MUTEX_SHARED_MASK; 275 } 276 277 switch (*attr & MUTEXATTR_TYPE_MASK) { 278 case PTHREAD_MUTEX_NORMAL: 279 value |= MUTEX_TYPE_BITS_NORMAL; 280 break; 281 case PTHREAD_MUTEX_RECURSIVE: 282 value |= MUTEX_TYPE_BITS_RECURSIVE; 283 break; 284 case PTHREAD_MUTEX_ERRORCHECK: 285 value |= MUTEX_TYPE_BITS_ERRORCHECK; 286 break; 287 default: 288 return EINVAL; 289 } 290 291 mutex->value = value; 292 return 0; 293} 294 295 296/* 297 * Lock a non-recursive mutex. 298 * 299 * As noted above, there are three states: 300 * 0 (unlocked, no contention) 301 * 1 (locked, no contention) 302 * 2 (locked, contention) 303 * 304 * Non-recursive mutexes don't use the thread-id or counter fields, and the 305 * "type" value is zero, so the only bits that will be set are the ones in 306 * the lock state field. 307 */ 308static __inline__ void 309_normal_lock(pthread_mutex_t* mutex, int shared) 310{ 311 /* convenience shortcuts */ 312 const int unlocked = shared | MUTEX_STATE_BITS_UNLOCKED; 313 const int locked_uncontended = shared | MUTEX_STATE_BITS_LOCKED_UNCONTENDED; 314 /* 315 * The common case is an unlocked mutex, so we begin by trying to 316 * change the lock's state from 0 (UNLOCKED) to 1 (LOCKED). 317 * __bionic_cmpxchg() returns 0 if it made the swap successfully. 318 * If the result is nonzero, this lock is already held by another thread. 319 */ 320 if (__bionic_cmpxchg(unlocked, locked_uncontended, &mutex->value) != 0) { 321 const int locked_contended = shared | MUTEX_STATE_BITS_LOCKED_CONTENDED; 322 /* 323 * We want to go to sleep until the mutex is available, which 324 * requires promoting it to state 2 (CONTENDED). We need to 325 * swap in the new state value and then wait until somebody wakes us up. 326 * 327 * __bionic_swap() returns the previous value. We swap 2 in and 328 * see if we got zero back; if so, we have acquired the lock. If 329 * not, another thread still holds the lock and we wait again. 330 * 331 * The second argument to the __futex_wait() call is compared 332 * against the current value. If it doesn't match, __futex_wait() 333 * returns immediately (otherwise, it sleeps for a time specified 334 * by the third argument; 0 means sleep forever). This ensures 335 * that the mutex is in state 2 when we go to sleep on it, which 336 * guarantees a wake-up call. 337 */ 338 while (__bionic_swap(locked_contended, &mutex->value) != unlocked) 339 __futex_wait_ex(&mutex->value, shared, locked_contended, 0); 340 } 341 ANDROID_MEMBAR_FULL(); 342} 343 344/* 345 * Release a non-recursive mutex. The caller is responsible for determining 346 * that we are in fact the owner of this lock. 347 */ 348static __inline__ void 349_normal_unlock(pthread_mutex_t* mutex, int shared) 350{ 351 ANDROID_MEMBAR_FULL(); 352 353 /* 354 * The mutex state will be 1 or (rarely) 2. We use an atomic decrement 355 * to release the lock. __bionic_atomic_dec() returns the previous value; 356 * if it wasn't 1 we have to do some additional work. 357 */ 358 if (__bionic_atomic_dec(&mutex->value) != (shared|MUTEX_STATE_BITS_LOCKED_UNCONTENDED)) { 359 /* 360 * Start by releasing the lock. The decrement changed it from 361 * "contended lock" to "uncontended lock", which means we still 362 * hold it, and anybody who tries to sneak in will push it back 363 * to state 2. 364 * 365 * Once we set it to zero the lock is up for grabs. We follow 366 * this with a __futex_wake() to ensure that one of the waiting 367 * threads has a chance to grab it. 368 * 369 * This doesn't cause a race with the swap/wait pair in 370 * _normal_lock(), because the __futex_wait() call there will 371 * return immediately if the mutex value isn't 2. 372 */ 373 mutex->value = shared; 374 375 /* 376 * Wake up one waiting thread. We don't know which thread will be 377 * woken or when it'll start executing -- futexes make no guarantees 378 * here. There may not even be a thread waiting. 379 * 380 * The newly-woken thread will replace the 0 we just set above 381 * with 2, which means that when it eventually releases the mutex 382 * it will also call FUTEX_WAKE. This results in one extra wake 383 * call whenever a lock is contended, but lets us avoid forgetting 384 * anyone without requiring us to track the number of sleepers. 385 * 386 * It's possible for another thread to sneak in and grab the lock 387 * between the zero assignment above and the wake call below. If 388 * the new thread is "slow" and holds the lock for a while, we'll 389 * wake up a sleeper, which will swap in a 2 and then go back to 390 * sleep since the lock is still held. If the new thread is "fast", 391 * running to completion before we call wake, the thread we 392 * eventually wake will find an unlocked mutex and will execute. 393 * Either way we have correct behavior and nobody is orphaned on 394 * the wait queue. 395 */ 396 __futex_wake_ex(&mutex->value, shared, 1); 397 } 398} 399 400/* This common inlined function is used to increment the counter of an 401 * errorcheck or recursive mutex. 402 * 403 * For errorcheck mutexes, it will return EDEADLK 404 * If the counter overflows, it will return EAGAIN 405 * Otherwise, it atomically increments the counter and returns 0 406 * after providing an acquire barrier. 407 * 408 * mtype is the current mutex type 409 * mvalue is the current mutex value (already loaded) 410 * mutex pointers to the mutex. 411 */ 412static __inline__ __attribute__((always_inline)) int 413_recursive_increment(pthread_mutex_t* mutex, int mvalue, int mtype) 414{ 415 if (mtype == MUTEX_TYPE_BITS_ERRORCHECK) { 416 /* trying to re-lock a mutex we already acquired */ 417 return EDEADLK; 418 } 419 420 /* Detect recursive lock overflow and return EAGAIN. 421 * This is safe because only the owner thread can modify the 422 * counter bits in the mutex value. 423 */ 424 if (MUTEX_COUNTER_BITS_WILL_OVERFLOW(mvalue)) { 425 return EAGAIN; 426 } 427 428 /* We own the mutex, but other threads are able to change 429 * the lower bits (e.g. promoting it to "contended"), so we 430 * need to use an atomic cmpxchg loop to update the counter. 431 */ 432 for (;;) { 433 /* increment counter, overflow was already checked */ 434 int newval = mvalue + MUTEX_COUNTER_BITS_ONE; 435 if (__predict_true(__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0)) { 436 /* mutex is still locked, not need for a memory barrier */ 437 return 0; 438 } 439 /* the value was changed, this happens when another thread changes 440 * the lower state bits from 1 to 2 to indicate contention. This 441 * cannot change the counter, so simply reload and try again. 442 */ 443 mvalue = mutex->value; 444 } 445} 446 447int pthread_mutex_lock(pthread_mutex_t* mutex) { 448 int mvalue, mtype, tid, shared; 449 450 mvalue = mutex->value; 451 mtype = (mvalue & MUTEX_TYPE_MASK); 452 shared = (mvalue & MUTEX_SHARED_MASK); 453 454 /* Handle non-recursive case first */ 455 if ( __predict_true(mtype == MUTEX_TYPE_BITS_NORMAL) ) { 456 _normal_lock(mutex, shared); 457 return 0; 458 } 459 460 /* Do we already own this recursive or error-check mutex ? */ 461 tid = __get_thread()->tid; 462 if ( tid == MUTEX_OWNER_FROM_BITS(mvalue) ) 463 return _recursive_increment(mutex, mvalue, mtype); 464 465 /* Add in shared state to avoid extra 'or' operations below */ 466 mtype |= shared; 467 468 /* First, if the mutex is unlocked, try to quickly acquire it. 469 * In the optimistic case where this works, set the state to 1 to 470 * indicate locked with no contention */ 471 if (mvalue == mtype) { 472 int newval = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_UNCONTENDED; 473 if (__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0) { 474 ANDROID_MEMBAR_FULL(); 475 return 0; 476 } 477 /* argh, the value changed, reload before entering the loop */ 478 mvalue = mutex->value; 479 } 480 481 for (;;) { 482 int newval; 483 484 /* if the mutex is unlocked, its value should be 'mtype' and 485 * we try to acquire it by setting its owner and state atomically. 486 * NOTE: We put the state to 2 since we _know_ there is contention 487 * when we are in this loop. This ensures all waiters will be 488 * unlocked. 489 */ 490 if (mvalue == mtype) { 491 newval = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_CONTENDED; 492 /* TODO: Change this to __bionic_cmpxchg_acquire when we 493 * implement it to get rid of the explicit memory 494 * barrier below. 495 */ 496 if (__predict_false(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) { 497 mvalue = mutex->value; 498 continue; 499 } 500 ANDROID_MEMBAR_FULL(); 501 return 0; 502 } 503 504 /* the mutex is already locked by another thread, if its state is 1 505 * we will change it to 2 to indicate contention. */ 506 if (MUTEX_STATE_BITS_IS_LOCKED_UNCONTENDED(mvalue)) { 507 newval = MUTEX_STATE_BITS_FLIP_CONTENTION(mvalue); /* locked state 1 => state 2 */ 508 if (__predict_false(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) { 509 mvalue = mutex->value; 510 continue; 511 } 512 mvalue = newval; 513 } 514 515 /* wait until the mutex is unlocked */ 516 __futex_wait_ex(&mutex->value, shared, mvalue, NULL); 517 518 mvalue = mutex->value; 519 } 520 /* NOTREACHED */ 521} 522 523int pthread_mutex_unlock(pthread_mutex_t* mutex) { 524 int mvalue, mtype, tid, shared; 525 526 mvalue = mutex->value; 527 mtype = (mvalue & MUTEX_TYPE_MASK); 528 shared = (mvalue & MUTEX_SHARED_MASK); 529 530 /* Handle common case first */ 531 if (__predict_true(mtype == MUTEX_TYPE_BITS_NORMAL)) { 532 _normal_unlock(mutex, shared); 533 return 0; 534 } 535 536 /* Do we already own this recursive or error-check mutex ? */ 537 tid = __get_thread()->tid; 538 if ( tid != MUTEX_OWNER_FROM_BITS(mvalue) ) 539 return EPERM; 540 541 /* If the counter is > 0, we can simply decrement it atomically. 542 * Since other threads can mutate the lower state bits (and only the 543 * lower state bits), use a cmpxchg to do it. 544 */ 545 if (!MUTEX_COUNTER_BITS_IS_ZERO(mvalue)) { 546 for (;;) { 547 int newval = mvalue - MUTEX_COUNTER_BITS_ONE; 548 if (__predict_true(__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0)) { 549 /* success: we still own the mutex, so no memory barrier */ 550 return 0; 551 } 552 /* the value changed, so reload and loop */ 553 mvalue = mutex->value; 554 } 555 } 556 557 /* the counter is 0, so we're going to unlock the mutex by resetting 558 * its value to 'unlocked'. We need to perform a swap in order 559 * to read the current state, which will be 2 if there are waiters 560 * to awake. 561 * 562 * TODO: Change this to __bionic_swap_release when we implement it 563 * to get rid of the explicit memory barrier below. 564 */ 565 ANDROID_MEMBAR_FULL(); /* RELEASE BARRIER */ 566 mvalue = __bionic_swap(mtype | shared | MUTEX_STATE_BITS_UNLOCKED, &mutex->value); 567 568 /* Wake one waiting thread, if any */ 569 if (MUTEX_STATE_BITS_IS_LOCKED_CONTENDED(mvalue)) { 570 __futex_wake_ex(&mutex->value, shared, 1); 571 } 572 return 0; 573} 574 575int pthread_mutex_trylock(pthread_mutex_t* mutex) { 576 int mvalue, mtype, tid, shared; 577 578 mvalue = mutex->value; 579 mtype = (mvalue & MUTEX_TYPE_MASK); 580 shared = (mvalue & MUTEX_SHARED_MASK); 581 582 /* Handle common case first */ 583 if ( __predict_true(mtype == MUTEX_TYPE_BITS_NORMAL) ) 584 { 585 if (__bionic_cmpxchg(shared|MUTEX_STATE_BITS_UNLOCKED, 586 shared|MUTEX_STATE_BITS_LOCKED_UNCONTENDED, 587 &mutex->value) == 0) { 588 ANDROID_MEMBAR_FULL(); 589 return 0; 590 } 591 592 return EBUSY; 593 } 594 595 /* Do we already own this recursive or error-check mutex ? */ 596 tid = __get_thread()->tid; 597 if ( tid == MUTEX_OWNER_FROM_BITS(mvalue) ) 598 return _recursive_increment(mutex, mvalue, mtype); 599 600 /* Same as pthread_mutex_lock, except that we don't want to wait, and 601 * the only operation that can succeed is a single cmpxchg to acquire the 602 * lock if it is released / not owned by anyone. No need for a complex loop. 603 */ 604 mtype |= shared | MUTEX_STATE_BITS_UNLOCKED; 605 mvalue = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_UNCONTENDED; 606 607 if (__predict_true(__bionic_cmpxchg(mtype, mvalue, &mutex->value) == 0)) { 608 ANDROID_MEMBAR_FULL(); 609 return 0; 610 } 611 612 return EBUSY; 613} 614 615static int __pthread_mutex_timedlock(pthread_mutex_t* mutex, const timespec* abs_timeout, clockid_t clock) { 616 timespec ts; 617 618 int mvalue = mutex->value; 619 int mtype = (mvalue & MUTEX_TYPE_MASK); 620 int shared = (mvalue & MUTEX_SHARED_MASK); 621 622 // Handle common case first. 623 if (__predict_true(mtype == MUTEX_TYPE_BITS_NORMAL)) { 624 const int unlocked = shared | MUTEX_STATE_BITS_UNLOCKED; 625 const int locked_uncontended = shared | MUTEX_STATE_BITS_LOCKED_UNCONTENDED; 626 const int locked_contended = shared | MUTEX_STATE_BITS_LOCKED_CONTENDED; 627 628 // Fast path for uncontended lock. Note: MUTEX_TYPE_BITS_NORMAL is 0. 629 if (__bionic_cmpxchg(unlocked, locked_uncontended, &mutex->value) == 0) { 630 ANDROID_MEMBAR_FULL(); 631 return 0; 632 } 633 634 // Loop while needed. 635 while (__bionic_swap(locked_contended, &mutex->value) != unlocked) { 636 if (__timespec_from_absolute(&ts, abs_timeout, clock) < 0) { 637 return ETIMEDOUT; 638 } 639 __futex_wait_ex(&mutex->value, shared, locked_contended, &ts); 640 } 641 ANDROID_MEMBAR_FULL(); 642 return 0; 643 } 644 645 // Do we already own this recursive or error-check mutex? 646 pid_t tid = __get_thread()->tid; 647 if (tid == MUTEX_OWNER_FROM_BITS(mvalue)) { 648 return _recursive_increment(mutex, mvalue, mtype); 649 } 650 651 // The following implements the same loop as pthread_mutex_lock_impl 652 // but adds checks to ensure that the operation never exceeds the 653 // absolute expiration time. 654 mtype |= shared; 655 656 // First try a quick lock. 657 if (mvalue == mtype) { 658 mvalue = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_UNCONTENDED; 659 if (__predict_true(__bionic_cmpxchg(mtype, mvalue, &mutex->value) == 0)) { 660 ANDROID_MEMBAR_FULL(); 661 return 0; 662 } 663 mvalue = mutex->value; 664 } 665 666 while (true) { 667 // If the value is 'unlocked', try to acquire it directly. 668 // NOTE: put state to 2 since we know there is contention. 669 if (mvalue == mtype) { // Unlocked. 670 mvalue = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_CONTENDED; 671 if (__bionic_cmpxchg(mtype, mvalue, &mutex->value) == 0) { 672 ANDROID_MEMBAR_FULL(); 673 return 0; 674 } 675 // The value changed before we could lock it. We need to check 676 // the time to avoid livelocks, reload the value, then loop again. 677 if (__timespec_from_absolute(&ts, abs_timeout, clock) < 0) { 678 return ETIMEDOUT; 679 } 680 681 mvalue = mutex->value; 682 continue; 683 } 684 685 // The value is locked. If 'uncontended', try to switch its state 686 // to 'contented' to ensure we get woken up later. 687 if (MUTEX_STATE_BITS_IS_LOCKED_UNCONTENDED(mvalue)) { 688 int newval = MUTEX_STATE_BITS_FLIP_CONTENTION(mvalue); 689 if (__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0) { 690 // This failed because the value changed, reload it. 691 mvalue = mutex->value; 692 } else { 693 // This succeeded, update mvalue. 694 mvalue = newval; 695 } 696 } 697 698 // Check time and update 'ts'. 699 if (__timespec_from_absolute(&ts, abs_timeout, clock) < 0) { 700 return ETIMEDOUT; 701 } 702 703 // Only wait to be woken up if the state is '2', otherwise we'll 704 // simply loop right now. This can happen when the second cmpxchg 705 // in our loop failed because the mutex was unlocked by another thread. 706 if (MUTEX_STATE_BITS_IS_LOCKED_CONTENDED(mvalue)) { 707 if (__futex_wait_ex(&mutex->value, shared, mvalue, &ts) == -ETIMEDOUT) { 708 return ETIMEDOUT; 709 } 710 mvalue = mutex->value; 711 } 712 } 713 /* NOTREACHED */ 714} 715 716#if !defined(__LP64__) 717extern "C" int pthread_mutex_lock_timeout_np(pthread_mutex_t* mutex, unsigned ms) { 718 timespec abs_timeout; 719 clock_gettime(CLOCK_MONOTONIC, &abs_timeout); 720 abs_timeout.tv_sec += ms / 1000; 721 abs_timeout.tv_nsec += (ms % 1000) * 1000000; 722 if (abs_timeout.tv_nsec >= 1000000000) { 723 abs_timeout.tv_sec++; 724 abs_timeout.tv_nsec -= 1000000000; 725 } 726 727 int error = __pthread_mutex_timedlock(mutex, &abs_timeout, CLOCK_MONOTONIC); 728 if (error == ETIMEDOUT) { 729 error = EBUSY; 730 } 731 return error; 732} 733#endif 734 735int pthread_mutex_timedlock(pthread_mutex_t* mutex, const timespec* abs_timeout) { 736 return __pthread_mutex_timedlock(mutex, abs_timeout, CLOCK_REALTIME); 737} 738 739int pthread_mutex_destroy(pthread_mutex_t* mutex) { 740 // Use trylock to ensure that the mutex is valid and not already locked. 741 int error = pthread_mutex_trylock(mutex); 742 if (error != 0) { 743 return error; 744 } 745 mutex->value = 0xdead10cc; 746 return 0; 747} 748