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