linux/kernel/mutex.c
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   1/*
   2 * kernel/mutex.c
   3 *
   4 * Mutexes: blocking mutual exclusion locks
   5 *
   6 * Started by Ingo Molnar:
   7 *
   8 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
   9 *
  10 * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
  11 * David Howells for suggestions and improvements.
  12 *
  13 *  - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
  14 *    from the -rt tree, where it was originally implemented for rtmutexes
  15 *    by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
  16 *    and Sven Dietrich.
  17 *
  18 * Also see Documentation/mutex-design.txt.
  19 */
  20#include <linux/mutex.h>
  21#include <linux/sched.h>
  22#include <linux/module.h>
  23#include <linux/spinlock.h>
  24#include <linux/interrupt.h>
  25#include <linux/debug_locks.h>
  26
  27/*
  28 * In the DEBUG case we are using the "NULL fastpath" for mutexes,
  29 * which forces all calls into the slowpath:
  30 */
  31#ifdef CONFIG_DEBUG_MUTEXES
  32# include "mutex-debug.h"
  33# include <asm-generic/mutex-null.h>
  34#else
  35# include "mutex.h"
  36# include <asm/mutex.h>
  37#endif
  38
  39void
  40__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
  41{
  42        atomic_set(&lock->count, 1);
  43        spin_lock_init(&lock->wait_lock);
  44        INIT_LIST_HEAD(&lock->wait_list);
  45        mutex_clear_owner(lock);
  46
  47        debug_mutex_init(lock, name, key);
  48}
  49
  50EXPORT_SYMBOL(__mutex_init);
  51
  52#ifndef CONFIG_DEBUG_LOCK_ALLOC
  53/*
  54 * We split the mutex lock/unlock logic into separate fastpath and
  55 * slowpath functions, to reduce the register pressure on the fastpath.
  56 * We also put the fastpath first in the kernel image, to make sure the
  57 * branch is predicted by the CPU as default-untaken.
  58 */
  59static __used noinline void __sched
  60__mutex_lock_slowpath(atomic_t *lock_count);
  61
  62/**
  63 * mutex_lock - acquire the mutex
  64 * @lock: the mutex to be acquired
  65 *
  66 * Lock the mutex exclusively for this task. If the mutex is not
  67 * available right now, it will sleep until it can get it.
  68 *
  69 * The mutex must later on be released by the same task that
  70 * acquired it. Recursive locking is not allowed. The task
  71 * may not exit without first unlocking the mutex. Also, kernel
  72 * memory where the mutex resides mutex must not be freed with
  73 * the mutex still locked. The mutex must first be initialized
  74 * (or statically defined) before it can be locked. memset()-ing
  75 * the mutex to 0 is not allowed.
  76 *
  77 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
  78 *   checks that will enforce the restrictions and will also do
  79 *   deadlock debugging. )
  80 *
  81 * This function is similar to (but not equivalent to) down().
  82 */
  83void __sched mutex_lock(struct mutex *lock)
  84{
  85        might_sleep();
  86        /*
  87         * The locking fastpath is the 1->0 transition from
  88         * 'unlocked' into 'locked' state.
  89         */
  90        __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
  91        mutex_set_owner(lock);
  92}
  93
  94EXPORT_SYMBOL(mutex_lock);
  95#endif
  96
  97static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
  98
  99/**
 100 * mutex_unlock - release the mutex
 101 * @lock: the mutex to be released
 102 *
 103 * Unlock a mutex that has been locked by this task previously.
 104 *
 105 * This function must not be used in interrupt context. Unlocking
 106 * of a not locked mutex is not allowed.
 107 *
 108 * This function is similar to (but not equivalent to) up().
 109 */
 110void __sched mutex_unlock(struct mutex *lock)
 111{
 112        /*
 113         * The unlocking fastpath is the 0->1 transition from 'locked'
 114         * into 'unlocked' state:
 115         */
 116#ifndef CONFIG_DEBUG_MUTEXES
 117        /*
 118         * When debugging is enabled we must not clear the owner before time,
 119         * the slow path will always be taken, and that clears the owner field
 120         * after verifying that it was indeed current.
 121         */
 122        mutex_clear_owner(lock);
 123#endif
 124        __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
 125}
 126
 127EXPORT_SYMBOL(mutex_unlock);
 128
 129/*
 130 * Lock a mutex (possibly interruptible), slowpath:
 131 */
 132static inline int __sched
 133__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 134                    struct lockdep_map *nest_lock, unsigned long ip)
 135{
 136        struct task_struct *task = current;
 137        struct mutex_waiter waiter;
 138        unsigned long flags;
 139
 140        preempt_disable();
 141        mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
 142
 143#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
 144        /*
 145         * Optimistic spinning.
 146         *
 147         * We try to spin for acquisition when we find that there are no
 148         * pending waiters and the lock owner is currently running on a
 149         * (different) CPU.
 150         *
 151         * The rationale is that if the lock owner is running, it is likely to
 152         * release the lock soon.
 153         *
 154         * Since this needs the lock owner, and this mutex implementation
 155         * doesn't track the owner atomically in the lock field, we need to
 156         * track it non-atomically.
 157         *
 158         * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
 159         * to serialize everything.
 160         */
 161
 162        for (;;) {
 163                struct task_struct *owner;
 164
 165                /*
 166                 * If there's an owner, wait for it to either
 167                 * release the lock or go to sleep.
 168                 */
 169                owner = ACCESS_ONCE(lock->owner);
 170                if (owner && !mutex_spin_on_owner(lock, owner))
 171                        break;
 172
 173                if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
 174                        lock_acquired(&lock->dep_map, ip);
 175                        mutex_set_owner(lock);
 176                        preempt_enable();
 177                        return 0;
 178                }
 179
 180                /*
 181                 * When there's no owner, we might have preempted between the
 182                 * owner acquiring the lock and setting the owner field. If
 183                 * we're an RT task that will live-lock because we won't let
 184                 * the owner complete.
 185                 */
 186                if (!owner && (need_resched() || rt_task(task)))
 187                        break;
 188
 189                /*
 190                 * The cpu_relax() call is a compiler barrier which forces
 191                 * everything in this loop to be re-loaded. We don't need
 192                 * memory barriers as we'll eventually observe the right
 193                 * values at the cost of a few extra spins.
 194                 */
 195                arch_mutex_cpu_relax();
 196        }
 197#endif
 198        spin_lock_mutex(&lock->wait_lock, flags);
 199
 200        debug_mutex_lock_common(lock, &waiter);
 201        debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
 202
 203        /* add waiting tasks to the end of the waitqueue (FIFO): */
 204        list_add_tail(&waiter.list, &lock->wait_list);
 205        waiter.task = task;
 206
 207        if (atomic_xchg(&lock->count, -1) == 1)
 208                goto done;
 209
 210        lock_contended(&lock->dep_map, ip);
 211
 212        for (;;) {
 213                /*
 214                 * Lets try to take the lock again - this is needed even if
 215                 * we get here for the first time (shortly after failing to
 216                 * acquire the lock), to make sure that we get a wakeup once
 217                 * it's unlocked. Later on, if we sleep, this is the
 218                 * operation that gives us the lock. We xchg it to -1, so
 219                 * that when we release the lock, we properly wake up the
 220                 * other waiters:
 221                 */
 222                if (atomic_xchg(&lock->count, -1) == 1)
 223                        break;
 224
 225                /*
 226                 * got a signal? (This code gets eliminated in the
 227                 * TASK_UNINTERRUPTIBLE case.)
 228                 */
 229                if (unlikely(signal_pending_state(state, task))) {
 230                        mutex_remove_waiter(lock, &waiter,
 231                                            task_thread_info(task));
 232                        mutex_release(&lock->dep_map, 1, ip);
 233                        spin_unlock_mutex(&lock->wait_lock, flags);
 234
 235                        debug_mutex_free_waiter(&waiter);
 236                        preempt_enable();
 237                        return -EINTR;
 238                }
 239                __set_task_state(task, state);
 240
 241                /* didn't get the lock, go to sleep: */
 242                spin_unlock_mutex(&lock->wait_lock, flags);
 243                preempt_enable_no_resched();
 244                schedule();
 245                preempt_disable();
 246                spin_lock_mutex(&lock->wait_lock, flags);
 247        }
 248
 249done:
 250        lock_acquired(&lock->dep_map, ip);
 251        /* got the lock - rejoice! */
 252        mutex_remove_waiter(lock, &waiter, current_thread_info());
 253        mutex_set_owner(lock);
 254
 255        /* set it to 0 if there are no waiters left: */
 256        if (likely(list_empty(&lock->wait_list)))
 257                atomic_set(&lock->count, 0);
 258
 259        spin_unlock_mutex(&lock->wait_lock, flags);
 260
 261        debug_mutex_free_waiter(&waiter);
 262        preempt_enable();
 263
 264        return 0;
 265}
 266
 267#ifdef CONFIG_DEBUG_LOCK_ALLOC
 268void __sched
 269mutex_lock_nested(struct mutex *lock, unsigned int subclass)
 270{
 271        might_sleep();
 272        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
 273}
 274
 275EXPORT_SYMBOL_GPL(mutex_lock_nested);
 276
 277void __sched
 278_mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
 279{
 280        might_sleep();
 281        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
 282}
 283
 284EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
 285
 286int __sched
 287mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
 288{
 289        might_sleep();
 290        return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
 291}
 292EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
 293
 294int __sched
 295mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
 296{
 297        might_sleep();
 298        return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
 299                                   subclass, NULL, _RET_IP_);
 300}
 301
 302EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
 303#endif
 304
 305/*
 306 * Release the lock, slowpath:
 307 */
 308static inline void
 309__mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
 310{
 311        struct mutex *lock = container_of(lock_count, struct mutex, count);
 312        unsigned long flags;
 313
 314        spin_lock_mutex(&lock->wait_lock, flags);
 315        mutex_release(&lock->dep_map, nested, _RET_IP_);
 316        debug_mutex_unlock(lock);
 317
 318        /*
 319         * some architectures leave the lock unlocked in the fastpath failure
 320         * case, others need to leave it locked. In the later case we have to
 321         * unlock it here
 322         */
 323        if (__mutex_slowpath_needs_to_unlock())
 324                atomic_set(&lock->count, 1);
 325
 326        if (!list_empty(&lock->wait_list)) {
 327                /* get the first entry from the wait-list: */
 328                struct mutex_waiter *waiter =
 329                                list_entry(lock->wait_list.next,
 330                                           struct mutex_waiter, list);
 331
 332                debug_mutex_wake_waiter(lock, waiter);
 333
 334                wake_up_process(waiter->task);
 335        }
 336
 337        spin_unlock_mutex(&lock->wait_lock, flags);
 338}
 339
 340/*
 341 * Release the lock, slowpath:
 342 */
 343static __used noinline void
 344__mutex_unlock_slowpath(atomic_t *lock_count)
 345{
 346        __mutex_unlock_common_slowpath(lock_count, 1);
 347}
 348
 349#ifndef CONFIG_DEBUG_LOCK_ALLOC
 350/*
 351 * Here come the less common (and hence less performance-critical) APIs:
 352 * mutex_lock_interruptible() and mutex_trylock().
 353 */
 354static noinline int __sched
 355__mutex_lock_killable_slowpath(atomic_t *lock_count);
 356
 357static noinline int __sched
 358__mutex_lock_interruptible_slowpath(atomic_t *lock_count);
 359
 360/**
 361 * mutex_lock_interruptible - acquire the mutex, interruptible
 362 * @lock: the mutex to be acquired
 363 *
 364 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 365 * been acquired or sleep until the mutex becomes available. If a
 366 * signal arrives while waiting for the lock then this function
 367 * returns -EINTR.
 368 *
 369 * This function is similar to (but not equivalent to) down_interruptible().
 370 */
 371int __sched mutex_lock_interruptible(struct mutex *lock)
 372{
 373        int ret;
 374
 375        might_sleep();
 376        ret =  __mutex_fastpath_lock_retval
 377                        (&lock->count, __mutex_lock_interruptible_slowpath);
 378        if (!ret)
 379                mutex_set_owner(lock);
 380
 381        return ret;
 382}
 383
 384EXPORT_SYMBOL(mutex_lock_interruptible);
 385
 386int __sched mutex_lock_killable(struct mutex *lock)
 387{
 388        int ret;
 389
 390        might_sleep();
 391        ret = __mutex_fastpath_lock_retval
 392                        (&lock->count, __mutex_lock_killable_slowpath);
 393        if (!ret)
 394                mutex_set_owner(lock);
 395
 396        return ret;
 397}
 398EXPORT_SYMBOL(mutex_lock_killable);
 399
 400static __used noinline void __sched
 401__mutex_lock_slowpath(atomic_t *lock_count)
 402{
 403        struct mutex *lock = container_of(lock_count, struct mutex, count);
 404
 405        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
 406}
 407
 408static noinline int __sched
 409__mutex_lock_killable_slowpath(atomic_t *lock_count)
 410{
 411        struct mutex *lock = container_of(lock_count, struct mutex, count);
 412
 413        return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
 414}
 415
 416static noinline int __sched
 417__mutex_lock_interruptible_slowpath(atomic_t *lock_count)
 418{
 419        struct mutex *lock = container_of(lock_count, struct mutex, count);
 420
 421        return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
 422}
 423#endif
 424
 425/*
 426 * Spinlock based trylock, we take the spinlock and check whether we
 427 * can get the lock:
 428 */
 429static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 430{
 431        struct mutex *lock = container_of(lock_count, struct mutex, count);
 432        unsigned long flags;
 433        int prev;
 434
 435        spin_lock_mutex(&lock->wait_lock, flags);
 436
 437        prev = atomic_xchg(&lock->count, -1);
 438        if (likely(prev == 1)) {
 439                mutex_set_owner(lock);
 440                mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
 441        }
 442
 443        /* Set it back to 0 if there are no waiters: */
 444        if (likely(list_empty(&lock->wait_list)))
 445                atomic_set(&lock->count, 0);
 446
 447        spin_unlock_mutex(&lock->wait_lock, flags);
 448
 449        return prev == 1;
 450}
 451
 452/**
 453 * mutex_trylock - try to acquire the mutex, without waiting
 454 * @lock: the mutex to be acquired
 455 *
 456 * Try to acquire the mutex atomically. Returns 1 if the mutex
 457 * has been acquired successfully, and 0 on contention.
 458 *
 459 * NOTE: this function follows the spin_trylock() convention, so
 460 * it is negated from the down_trylock() return values! Be careful
 461 * about this when converting semaphore users to mutexes.
 462 *
 463 * This function must not be used in interrupt context. The
 464 * mutex must be released by the same task that acquired it.
 465 */
 466int __sched mutex_trylock(struct mutex *lock)
 467{
 468        int ret;
 469
 470        ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
 471        if (ret)
 472                mutex_set_owner(lock);
 473
 474        return ret;
 475}
 476EXPORT_SYMBOL(mutex_trylock);
 477
 478/**
 479 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
 480 * @cnt: the atomic which we are to dec
 481 * @lock: the mutex to return holding if we dec to 0
 482 *
 483 * return true and hold lock if we dec to 0, return false otherwise
 484 */
 485int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
 486{
 487        /* dec if we can't possibly hit 0 */
 488        if (atomic_add_unless(cnt, -1, 1))
 489                return 0;
 490        /* we might hit 0, so take the lock */
 491        mutex_lock(lock);
 492        if (!atomic_dec_and_test(cnt)) {
 493                /* when we actually did the dec, we didn't hit 0 */
 494                mutex_unlock(lock);
 495                return 0;
 496        }
 497        /* we hit 0, and we hold the lock */
 498        return 1;
 499}
 500EXPORT_SYMBOL(atomic_dec_and_mutex_lock);
 501
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