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  * Also see Documentation/mutex-design.txt.
  14  */
  15 #include <linux/mutex.h>
  16 #include <linux/sched.h>
  17 #include <linux/module.h>
  18 #include <linux/spinlock.h>
  19 #include <linux/interrupt.h>
  20 #include <linux/debug_locks.h>
  21
  22 /*
  23  * In the DEBUG case we are using the "NULL fastpath" for mutexes,
  24  * which forces all calls into the slowpath:
  25  */
  26 #ifdef CONFIG_DEBUG_MUTEXES
  27 # include "mutex-debug.h"
  28 # include <asm-generic/mutex-null.h>
  29 #else
  30 # include "mutex.h"
  31 # include <asm/mutex.h>
  32 #endif
  33
  34 /***
  35  * mutex_init - initialize the mutex
  36  * @lock: the mutex to be initialized
  37  *
  38  * Initialize the mutex to unlocked state.
  39  *
  40  * It is not allowed to initialize an already locked mutex.
  41  */
  42 void
  43 __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
  44 {
  45         atomic_set(&lock->count, 1);
  46         spin_lock_init(&lock->wait_lock);
  47         INIT_LIST_HEAD(&lock->wait_list);
  48
  49         debug_mutex_init(lock, name, key);
  50 }
  51
  52 EXPORT_SYMBOL(__mutex_init);
  53
  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  */
  60 static void fastcall noinline __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  */
  84 void inline fastcall __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 }
  93
  94 EXPORT_SYMBOL(mutex_lock);
  95
  96 static void fastcall noinline __sched
  97 __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 fastcall __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         __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
 117 }
 118
 119 EXPORT_SYMBOL(mutex_unlock);
 120
 121 /*
 122  * Lock a mutex (possibly interruptible), slowpath:
 123  */
 124 static inline int __sched
 125 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass)
 126 {
 127         struct task_struct *task = current;
 128         struct mutex_waiter waiter;
 129         unsigned int old_val;
 130         unsigned long flags;
 131
 132         spin_lock_mutex(&lock->wait_lock, flags);
 133
 134         debug_mutex_lock_common(lock, &waiter);
 135         mutex_acquire(&lock->dep_map, subclass, 0, _RET_IP_);
 136         debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
 137
 138         /* add waiting tasks to the end of the waitqueue (FIFO): */
 139         list_add_tail(&waiter.list, &lock->wait_list);
 140         waiter.task = task;
 141
 142         for (;;) {
 143                 /*
 144                  * Lets try to take the lock again - this is needed even if
 145                  * we get here for the first time (shortly after failing to
 146                  * acquire the lock), to make sure that we get a wakeup once
 147                  * it's unlocked. Later on, if we sleep, this is the
 148                  * operation that gives us the lock. We xchg it to -1, so
 149                  * that when we release the lock, we properly wake up the
 150                  * other waiters:
 151                  */
 152                 old_val = atomic_xchg(&lock->count, -1);
 153                 if (old_val == 1)
 154                         break;
 155
 156                 /*
 157                  * got a signal? (This code gets eliminated in the
 158                  * TASK_UNINTERRUPTIBLE case.)
 159                  */
 160                 if (unlikely(state == TASK_INTERRUPTIBLE &&
 161                                                 signal_pending(task))) {
 162                         mutex_remove_waiter(lock, &waiter, task_thread_info(task));
 163                         mutex_release(&lock->dep_map, 1, _RET_IP_);
 164                         spin_unlock_mutex(&lock->wait_lock, flags);
 165
 166                         debug_mutex_free_waiter(&waiter);
 167                         return -EINTR;
 168                 }
 169                 __set_task_state(task, state);
 170
 171                 /* didnt get the lock, go to sleep: */
 172                 spin_unlock_mutex(&lock->wait_lock, flags);
 173                 schedule();
 174                 spin_lock_mutex(&lock->wait_lock, flags);
 175         }
 176
 177         /* got the lock - rejoice! */
 178         mutex_remove_waiter(lock, &waiter, task_thread_info(task));
 179         debug_mutex_set_owner(lock, task_thread_info(task));
 180
 181         /* set it to 0 if there are no waiters left: */
 182         if (likely(list_empty(&lock->wait_list)))
 183                 atomic_set(&lock->count, 0);
 184
 185         spin_unlock_mutex(&lock->wait_lock, flags);
 186
 187         debug_mutex_free_waiter(&waiter);
 188
 189         return 0;
 190 }
 191
 192 static void fastcall noinline __sched
 193 __mutex_lock_slowpath(atomic_t *lock_count)
 194 {
 195         struct mutex *lock = container_of(lock_count, struct mutex, count);
 196
 197         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0);
 198 }
 199
 200 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 201 void __sched
 202 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
 203 {
 204         might_sleep();
 205         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass);
 206 }
 207
 208 EXPORT_SYMBOL_GPL(mutex_lock_nested);
 209
 210 int __sched
 211 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
 212 {
 213         might_sleep();
 214         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, subclass);
 215 }
 216
 217 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
 218 #endif
 219
 220 /*
 221  * Release the lock, slowpath:
 222  */
 223 static fastcall inline void
 224 __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
 225 {
 226         struct mutex *lock = container_of(lock_count, struct mutex, count);
 227         unsigned long flags;
 228
 229         spin_lock_mutex(&lock->wait_lock, flags);
 230         mutex_release(&lock->dep_map, nested, _RET_IP_);
 231         debug_mutex_unlock(lock);
 232
 233         /*
 234          * some architectures leave the lock unlocked in the fastpath failure
 235          * case, others need to leave it locked. In the later case we have to
 236          * unlock it here
 237          */
 238         if (__mutex_slowpath_needs_to_unlock())
 239                 atomic_set(&lock->count, 1);
 240
 241         if (!list_empty(&lock->wait_list)) {
 242                 /* get the first entry from the wait-list: */
 243                 struct mutex_waiter *waiter =
 244                                 list_entry(lock->wait_list.next,
 245                                            struct mutex_waiter, list);
 246
 247                 debug_mutex_wake_waiter(lock, waiter);
 248
 249                 wake_up_process(waiter->task);
 250         }
 251
 252         debug_mutex_clear_owner(lock);
 253
 254         spin_unlock_mutex(&lock->wait_lock, flags);
 255 }
 256
 257 /*
 258  * Release the lock, slowpath:
 259  */
 260 static fastcall noinline void
 261 __mutex_unlock_slowpath(atomic_t *lock_count)
 262 {
 263         __mutex_unlock_common_slowpath(lock_count, 1);
 264 }
 265
 266 /*
 267  * Here come the less common (and hence less performance-critical) APIs:
 268  * mutex_lock_interruptible() and mutex_trylock().
 269  */
 270 static int fastcall noinline __sched
 271 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
 272
 273 /***
 274  * mutex_lock_interruptible - acquire the mutex, interruptable
 275  * @lock: the mutex to be acquired
 276  *
 277  * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 278  * been acquired or sleep until the mutex becomes available. If a
 279  * signal arrives while waiting for the lock then this function
 280  * returns -EINTR.
 281  *
 282  * This function is similar to (but not equivalent to) down_interruptible().
 283  */
 284 int fastcall __sched mutex_lock_interruptible(struct mutex *lock)
 285 {
 286         might_sleep();
 287         return __mutex_fastpath_lock_retval
 288                         (&lock->count, __mutex_lock_interruptible_slowpath);
 289 }
 290
 291 EXPORT_SYMBOL(mutex_lock_interruptible);
 292
 293 static int fastcall noinline __sched
 294 __mutex_lock_interruptible_slowpath(atomic_t *lock_count)
 295 {
 296         struct mutex *lock = container_of(lock_count, struct mutex, count);
 297
 298         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0);
 299 }
 300
 301 /*
 302  * Spinlock based trylock, we take the spinlock and check whether we
 303  * can get the lock:
 304  */
 305 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 306 {
 307         struct mutex *lock = container_of(lock_count, struct mutex, count);
 308         unsigned long flags;
 309         int prev;
 310
 311         spin_lock_mutex(&lock->wait_lock, flags);
 312
 313         prev = atomic_xchg(&lock->count, -1);
 314         if (likely(prev == 1)) {
 315                 debug_mutex_set_owner(lock, current_thread_info());
 316                 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
 317         }
 318         /* Set it back to 0 if there are no waiters: */
 319         if (likely(list_empty(&lock->wait_list)))
 320                 atomic_set(&lock->count, 0);
 321
 322         spin_unlock_mutex(&lock->wait_lock, flags);
 323
 324         return prev == 1;
 325 }
 326
 327 /***
 328  * mutex_trylock - try acquire the mutex, without waiting
 329  * @lock: the mutex to be acquired
 330  *
 331  * Try to acquire the mutex atomically. Returns 1 if the mutex
 332  * has been acquired successfully, and 0 on contention.
 333  *
 334  * NOTE: this function follows the spin_trylock() convention, so
 335  * it is negated to the down_trylock() return values! Be careful
 336  * about this when converting semaphore users to mutexes.
 337  *
 338  * This function must not be used in interrupt context. The
 339  * mutex must be released by the same task that acquired it.
 340  */
 341 int fastcall __sched mutex_trylock(struct mutex *lock)
 342 {
 343         return __mutex_fastpath_trylock(&lock->count,
 344                                         __mutex_trylock_slowpath);
 345 }
 346
 347 EXPORT_SYMBOL(mutex_trylock);