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
  21 /*
  22  * In the DEBUG case we are using the "NULL fastpath" for mutexes,
  23  * which forces all calls into the slowpath:
  24  */
  25 #ifdef CONFIG_DEBUG_MUTEXES
  26 # include "mutex-debug.h"
  27 # include <asm-generic/mutex-null.h>
  28 #else
  29 # include "mutex.h"
  30 # include <asm/mutex.h>
  31 #endif
  32
  33 /***
  34  * mutex_init - initialize the mutex
  35  * @lock: the mutex to be initialized
  36  *
  37  * Initialize the mutex to unlocked state.
  38  *
  39  * It is not allowed to initialize an already locked mutex.
  40  */
  41 void fastcall __mutex_init(struct mutex *lock, const char *name)
  42 {
  43         atomic_set(&lock->count, 1);
  44         spin_lock_init(&lock->wait_lock);
  45         INIT_LIST_HEAD(&lock->wait_list);
  46
  47         debug_mutex_init(lock, name);
  48 }
  49
  50 EXPORT_SYMBOL(__mutex_init);
  51
  52 /*
  53  * We split the mutex lock/unlock logic into separate fastpath and
  54  * slowpath functions, to reduce the register pressure on the fastpath.
  55  * We also put the fastpath first in the kernel image, to make sure the
  56  * branch is predicted by the CPU as default-untaken.
  57  */
  58 static void fastcall noinline __sched
  59 __mutex_lock_slowpath(atomic_t *lock_count __IP_DECL__);
  60
  61 /***
  62  * mutex_lock - acquire the mutex
  63  * @lock: the mutex to be acquired
  64  *
  65  * Lock the mutex exclusively for this task. If the mutex is not
  66  * available right now, it will sleep until it can get it.
  67  *
  68  * The mutex must later on be released by the same task that
  69  * acquired it. Recursive locking is not allowed. The task
  70  * may not exit without first unlocking the mutex. Also, kernel
  71  * memory where the mutex resides mutex must not be freed with
  72  * the mutex still locked. The mutex must first be initialized
  73  * (or statically defined) before it can be locked. memset()-ing
  74  * the mutex to 0 is not allowed.
  75  *
  76  * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
  77  *   checks that will enforce the restrictions and will also do
  78  *   deadlock debugging. )
  79  *
  80  * This function is similar to (but not equivalent to) down().
  81  */
  82 void fastcall __sched mutex_lock(struct mutex *lock)
  83 {
  84         might_sleep();
  85         /*
  86          * The locking fastpath is the 1->0 transition from
  87          * 'unlocked' into 'locked' state.
  88          */
  89         __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
  90 }
  91
  92 EXPORT_SYMBOL(mutex_lock);
  93
  94 static void fastcall noinline __sched
  95 __mutex_unlock_slowpath(atomic_t *lock_count __IP_DECL__);
  96
  97 /***
  98  * mutex_unlock - release the mutex
  99  * @lock: the mutex to be released
 100  *
 101  * Unlock a mutex that has been locked by this task previously.
 102  *
 103  * This function must not be used in interrupt context. Unlocking
 104  * of a not locked mutex is not allowed.
 105  *
 106  * This function is similar to (but not equivalent to) up().
 107  */
 108 void fastcall __sched mutex_unlock(struct mutex *lock)
 109 {
 110         /*
 111          * The unlocking fastpath is the 0->1 transition from 'locked'
 112          * into 'unlocked' state:
 113          */
 114         __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
 115 }
 116
 117 EXPORT_SYMBOL(mutex_unlock);
 118
 119 /*
 120  * Lock a mutex (possibly interruptible), slowpath:
 121  */
 122 static inline int __sched
 123 __mutex_lock_common(struct mutex *lock, long state __IP_DECL__)
 124 {
 125         struct task_struct *task = current;
 126         struct mutex_waiter waiter;
 127         unsigned int old_val;
 128
 129         debug_mutex_init_waiter(&waiter);
 130
 131         spin_lock_mutex(&lock->wait_lock);
 132
 133         debug_mutex_add_waiter(lock, &waiter, task->thread_info, ip);
 134
 135         /* add waiting tasks to the end of the waitqueue (FIFO): */
 136         list_add_tail(&waiter.list, &lock->wait_list);
 137         waiter.task = task;
 138
 139         for (;;) {
 140                 /*
 141                  * Lets try to take the lock again - this is needed even if
 142                  * we get here for the first time (shortly after failing to
 143                  * acquire the lock), to make sure that we get a wakeup once
 144                  * it's unlocked. Later on, if we sleep, this is the
 145                  * operation that gives us the lock. We xchg it to -1, so
 146                  * that when we release the lock, we properly wake up the
 147                  * other waiters:
 148                  */
 149                 old_val = atomic_xchg(&lock->count, -1);
 150                 if (old_val == 1)
 151                         break;
 152
 153                 /*
 154                  * got a signal? (This code gets eliminated in the
 155                  * TASK_UNINTERRUPTIBLE case.)
 156                  */
 157                 if (unlikely(state == TASK_INTERRUPTIBLE &&
 158                                                 signal_pending(task))) {
 159                         mutex_remove_waiter(lock, &waiter, task->thread_info);
 160                         spin_unlock_mutex(&lock->wait_lock);
 161
 162                         debug_mutex_free_waiter(&waiter);
 163                         return -EINTR;
 164                 }
 165                 __set_task_state(task, state);
 166
 167                 /* didnt get the lock, go to sleep: */
 168                 spin_unlock_mutex(&lock->wait_lock);
 169                 schedule();
 170                 spin_lock_mutex(&lock->wait_lock);
 171         }
 172
 173         /* got the lock - rejoice! */
 174         mutex_remove_waiter(lock, &waiter, task->thread_info);
 175         debug_mutex_set_owner(lock, task->thread_info __IP__);
 176
 177         /* set it to 0 if there are no waiters left: */
 178         if (likely(list_empty(&lock->wait_list)))
 179                 atomic_set(&lock->count, 0);
 180
 181         spin_unlock_mutex(&lock->wait_lock);
 182
 183         debug_mutex_free_waiter(&waiter);
 184
 185         DEBUG_WARN_ON(list_empty(&lock->held_list));
 186         DEBUG_WARN_ON(lock->owner != task->thread_info);
 187
 188         return 0;
 189 }
 190
 191 static void fastcall noinline __sched
 192 __mutex_lock_slowpath(atomic_t *lock_count __IP_DECL__)
 193 {
 194         struct mutex *lock = container_of(lock_count, struct mutex, count);
 195
 196         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE __IP__);
 197 }
 198
 199 /*
 200  * Release the lock, slowpath:
 201  */
 202 static fastcall noinline void
 203 __mutex_unlock_slowpath(atomic_t *lock_count __IP_DECL__)
 204 {
 205         struct mutex *lock = container_of(lock_count, struct mutex, count);
 206
 207         DEBUG_WARN_ON(lock->owner != current_thread_info());
 208
 209         spin_lock_mutex(&lock->wait_lock);
 210
 211         /*
 212          * some architectures leave the lock unlocked in the fastpath failure
 213          * case, others need to leave it locked. In the later case we have to
 214          * unlock it here
 215          */
 216         if (__mutex_slowpath_needs_to_unlock())
 217                 atomic_set(&lock->count, 1);
 218
 219         debug_mutex_unlock(lock);
 220
 221         if (!list_empty(&lock->wait_list)) {
 222                 /* get the first entry from the wait-list: */
 223                 struct mutex_waiter *waiter =
 224                                 list_entry(lock->wait_list.next,
 225                                            struct mutex_waiter, list);
 226
 227                 debug_mutex_wake_waiter(lock, waiter);
 228
 229                 wake_up_process(waiter->task);
 230         }
 231
 232         debug_mutex_clear_owner(lock);
 233
 234         spin_unlock_mutex(&lock->wait_lock);
 235 }
 236
 237 /*
 238  * Here come the less common (and hence less performance-critical) APIs:
 239  * mutex_lock_interruptible() and mutex_trylock().
 240  */
 241 static int fastcall noinline __sched
 242 __mutex_lock_interruptible_slowpath(atomic_t *lock_count __IP_DECL__);
 243
 244 /***
 245  * mutex_lock_interruptible - acquire the mutex, interruptable
 246  * @lock: the mutex to be acquired
 247  *
 248  * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 249  * been acquired or sleep until the mutex becomes available. If a
 250  * signal arrives while waiting for the lock then this function
 251  * returns -EINTR.
 252  *
 253  * This function is similar to (but not equivalent to) down_interruptible().
 254  */
 255 int fastcall __sched mutex_lock_interruptible(struct mutex *lock)
 256 {
 257         might_sleep();
 258         return __mutex_fastpath_lock_retval
 259                         (&lock->count, __mutex_lock_interruptible_slowpath);
 260 }
 261
 262 EXPORT_SYMBOL(mutex_lock_interruptible);
 263
 264 static int fastcall noinline __sched
 265 __mutex_lock_interruptible_slowpath(atomic_t *lock_count __IP_DECL__)
 266 {
 267         struct mutex *lock = container_of(lock_count, struct mutex, count);
 268
 269         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE __IP__);
 270 }
 271
 272 /*
 273  * Spinlock based trylock, we take the spinlock and check whether we
 274  * can get the lock:
 275  */
 276 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 277 {
 278         struct mutex *lock = container_of(lock_count, struct mutex, count);
 279         int prev;
 280
 281         spin_lock_mutex(&lock->wait_lock);
 282
 283         prev = atomic_xchg(&lock->count, -1);
 284         if (likely(prev == 1))
 285                 debug_mutex_set_owner(lock, current_thread_info() __RET_IP__);
 286         /* Set it back to 0 if there are no waiters: */
 287         if (likely(list_empty(&lock->wait_list)))
 288                 atomic_set(&lock->count, 0);
 289
 290         spin_unlock_mutex(&lock->wait_lock);
 291
 292         return prev == 1;
 293 }
 294
 295 /***
 296  * mutex_trylock - try acquire the mutex, without waiting
 297  * @lock: the mutex to be acquired
 298  *
 299  * Try to acquire the mutex atomically. Returns 1 if the mutex
 300  * has been acquired successfully, and 0 on contention.
 301  *
 302  * NOTE: this function follows the spin_trylock() convention, so
 303  * it is negated to the down_trylock() return values! Be careful
 304  * about this when converting semaphore users to mutexes.
 305  *
 306  * This function must not be used in interrupt context. The
 307  * mutex must be released by the same task that acquired it.
 308  */
 309 int fastcall mutex_trylock(struct mutex *lock)
 310 {
 311         return __mutex_fastpath_trylock(&lock->count,
 312                                         __mutex_trylock_slowpath);
 313 }
 314
 315 EXPORT_SYMBOL(mutex_trylock);