kernel/mutex.c

   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
  39 void
  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
  50 EXPORT_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  */
  59 static __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  */
  83 void __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
  94 EXPORT_SYMBOL(mutex_lock);
  95 #endif
  96
  97 static __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  */
 110 void __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
 127 EXPORT_SYMBOL(mutex_unlock);
 128
 129 /*
 130  * Lock a mutex (possibly interruptible), slowpath:
 131  */
 132 static inline int __sched
 133 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 134                 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(&lock->dep_map, subclass, 0, 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
 249 done:
 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
 268 void __sched
 269 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
 270 {
 271         might_sleep();
 272         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, _RET_IP_);
 273 }
 274
 275 EXPORT_SYMBOL_GPL(mutex_lock_nested);
 276
 277 int __sched
 278 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
 279 {
 280         might_sleep();
 281         return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);
 282 }
 283 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
 284
 285 int __sched
 286 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
 287 {
 288         might_sleep();
 289         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
 290                                    subclass, _RET_IP_);
 291 }
 292
 293 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
 294 #endif
 295
 296 /*
 297  * Release the lock, slowpath:
 298  */
 299 static inline void
 300 __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
 301 {
 302         struct mutex *lock = container_of(lock_count, struct mutex, count);
 303         unsigned long flags;
 304
 305         spin_lock_mutex(&lock->wait_lock, flags);
 306         mutex_release(&lock->dep_map, nested, _RET_IP_);
 307         debug_mutex_unlock(lock);
 308
 309         /*
 310          * some architectures leave the lock unlocked in the fastpath failure
 311          * case, others need to leave it locked. In the later case we have to
 312          * unlock it here
 313          */
 314         if (__mutex_slowpath_needs_to_unlock())
 315                 atomic_set(&lock->count, 1);
 316
 317         if (!list_empty(&lock->wait_list)) {
 318                 /* get the first entry from the wait-list: */
 319                 struct mutex_waiter *waiter =
 320                                 list_entry(lock->wait_list.next,
 321                                            struct mutex_waiter, list);
 322
 323                 debug_mutex_wake_waiter(lock, waiter);
 324
 325                 wake_up_process(waiter->task);
 326         }
 327
 328         spin_unlock_mutex(&lock->wait_lock, flags);
 329 }
 330
 331 /*
 332  * Release the lock, slowpath:
 333  */
 334 static __used noinline void
 335 __mutex_unlock_slowpath(atomic_t *lock_count)
 336 {
 337         __mutex_unlock_common_slowpath(lock_count, 1);
 338 }
 339
 340 #ifndef CONFIG_DEBUG_LOCK_ALLOC
 341 /*
 342  * Here come the less common (and hence less performance-critical) APIs:
 343  * mutex_lock_interruptible() and mutex_trylock().
 344  */
 345 static noinline int __sched
 346 __mutex_lock_killable_slowpath(atomic_t *lock_count);
 347
 348 static noinline int __sched
 349 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
 350
 351 /**
 352  * mutex_lock_interruptible - acquire the mutex, interruptible
 353  * @lock: the mutex to be acquired
 354  *
 355  * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 356  * been acquired or sleep until the mutex becomes available. If a
 357  * signal arrives while waiting for the lock then this function
 358  * returns -EINTR.
 359  *
 360  * This function is similar to (but not equivalent to) down_interruptible().
 361  */
 362 int __sched mutex_lock_interruptible(struct mutex *lock)
 363 {
 364         int ret;
 365
 366         might_sleep();
 367         ret =  __mutex_fastpath_lock_retval
 368                         (&lock->count, __mutex_lock_interruptible_slowpath);
 369         if (!ret)
 370                 mutex_set_owner(lock);
 371
 372         return ret;
 373 }
 374
 375 EXPORT_SYMBOL(mutex_lock_interruptible);
 376
 377 int __sched mutex_lock_killable(struct mutex *lock)
 378 {
 379         int ret;
 380
 381         might_sleep();
 382         ret = __mutex_fastpath_lock_retval
 383                         (&lock->count, __mutex_lock_killable_slowpath);
 384         if (!ret)
 385                 mutex_set_owner(lock);
 386
 387         return ret;
 388 }
 389 EXPORT_SYMBOL(mutex_lock_killable);
 390
 391 static __used noinline void __sched
 392 __mutex_lock_slowpath(atomic_t *lock_count)
 393 {
 394         struct mutex *lock = container_of(lock_count, struct mutex, count);
 395
 396         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, _RET_IP_);
 397 }
 398
 399 static noinline int __sched
 400 __mutex_lock_killable_slowpath(atomic_t *lock_count)
 401 {
 402         struct mutex *lock = container_of(lock_count, struct mutex, count);
 403
 404         return __mutex_lock_common(lock, TASK_KILLABLE, 0, _RET_IP_);
 405 }
 406
 407 static noinline int __sched
 408 __mutex_lock_interruptible_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_INTERRUPTIBLE, 0, _RET_IP_);
 413 }
 414 #endif
 415
 416 /*
 417  * Spinlock based trylock, we take the spinlock and check whether we
 418  * can get the lock:
 419  */
 420 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 421 {
 422         struct mutex *lock = container_of(lock_count, struct mutex, count);
 423         unsigned long flags;
 424         int prev;
 425
 426         spin_lock_mutex(&lock->wait_lock, flags);
 427
 428         prev = atomic_xchg(&lock->count, -1);
 429         if (likely(prev == 1)) {
 430                 mutex_set_owner(lock);
 431                 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
 432         }
 433
 434         /* Set it back to 0 if there are no waiters: */
 435         if (likely(list_empty(&lock->wait_list)))
 436                 atomic_set(&lock->count, 0);
 437
 438         spin_unlock_mutex(&lock->wait_lock, flags);
 439
 440         return prev == 1;
 441 }
 442
 443 /**
 444  * mutex_trylock - try to acquire the mutex, without waiting
 445  * @lock: the mutex to be acquired
 446  *
 447  * Try to acquire the mutex atomically. Returns 1 if the mutex
 448  * has been acquired successfully, and 0 on contention.
 449  *
 450  * NOTE: this function follows the spin_trylock() convention, so
 451  * it is negated from the down_trylock() return values! Be careful
 452  * about this when converting semaphore users to mutexes.
 453  *
 454  * This function must not be used in interrupt context. The
 455  * mutex must be released by the same task that acquired it.
 456  */
 457 int __sched mutex_trylock(struct mutex *lock)
 458 {
 459         int ret;
 460
 461         ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
 462         if (ret)
 463                 mutex_set_owner(lock);
 464
 465         return ret;
 466 }
 467 EXPORT_SYMBOL(mutex_trylock);
 468
 469 /**
 470  * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
 471  * @cnt: the atomic which we are to dec
 472  * @lock: the mutex to return holding if we dec to 0
 473  *
 474  * return true and hold lock if we dec to 0, return false otherwise
 475  */
 476 int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
 477 {
 478         /* dec if we can't possibly hit 0 */
 479         if (atomic_add_unless(cnt, -1, 1))
 480                 return 0;
 481         /* we might hit 0, so take the lock */
 482         mutex_lock(lock);
 483         if (!atomic_dec_and_test(cnt)) {
 484                 /* when we actually did the dec, we didn't hit 0 */
 485                 mutex_unlock(lock);
 486                 return 0;
 487         }
 488         /* we hit 0, and we hold the lock */
 489         return 1;
 490 }
 491 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);