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 __always_inline void fastcall __mutex_init(struct mutex *lock, const char *name)
  43 {
  44         atomic_set(&lock->count, 1);
  45         spin_lock_init(&lock->wait_lock);
  46         INIT_LIST_HEAD(&lock->wait_list);
  47
  48         debug_mutex_init(lock, name);
  49 }
  50
  51 EXPORT_SYMBOL(__mutex_init);
  52
  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 void fastcall noinline __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 inline fastcall __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 }
  92
  93 EXPORT_SYMBOL(mutex_lock);
  94
  95 static void fastcall noinline __sched
  96 __mutex_unlock_slowpath(atomic_t *lock_count);
  97
  98 /***
  99  * mutex_unlock - release the mutex
 100  * @lock: the mutex to be released
 101  *
 102  * Unlock a mutex that has been locked by this task previously.
 103  *
 104  * This function must not be used in interrupt context. Unlocking
 105  * of a not locked mutex is not allowed.
 106  *
 107  * This function is similar to (but not equivalent to) up().
 108  */
 109 void fastcall __sched mutex_unlock(struct mutex *lock)
 110 {
 111         /*
 112          * The unlocking fastpath is the 0->1 transition from 'locked'
 113          * into 'unlocked' state:
 114          */
 115         __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
 116 }
 117
 118 EXPORT_SYMBOL(mutex_unlock);
 119
 120 /*
 121  * Lock a mutex (possibly interruptible), slowpath:
 122  */
 123 static inline int __sched
 124 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass)
 125 {
 126         struct task_struct *task = current;
 127         struct mutex_waiter waiter;
 128         unsigned int old_val;
 129         unsigned long flags;
 130
 131         spin_lock_mutex(&lock->wait_lock, flags);
 132
 133         debug_mutex_lock_common(lock, &waiter);
 134         debug_mutex_add_waiter(lock, &waiter, task->thread_info);
 135
 136         /* add waiting tasks to the end of the waitqueue (FIFO): */
 137         list_add_tail(&waiter.list, &lock->wait_list);
 138         waiter.task = task;
 139
 140         for (;;) {
 141                 /*
 142                  * Lets try to take the lock again - this is needed even if
 143                  * we get here for the first time (shortly after failing to
 144                  * acquire the lock), to make sure that we get a wakeup once
 145                  * it's unlocked. Later on, if we sleep, this is the
 146                  * operation that gives us the lock. We xchg it to -1, so
 147                  * that when we release the lock, we properly wake up the
 148                  * other waiters:
 149                  */
 150                 old_val = atomic_xchg(&lock->count, -1);
 151                 if (old_val == 1)
 152                         break;
 153
 154                 /*
 155                  * got a signal? (This code gets eliminated in the
 156                  * TASK_UNINTERRUPTIBLE case.)
 157                  */
 158                 if (unlikely(state == TASK_INTERRUPTIBLE &&
 159                                                 signal_pending(task))) {
 160                         mutex_remove_waiter(lock, &waiter, task->thread_info);
 161                         spin_unlock_mutex(&lock->wait_lock, flags);
 162
 163                         debug_mutex_free_waiter(&waiter);
 164                         return -EINTR;
 165                 }
 166                 __set_task_state(task, state);
 167
 168                 /* didnt get the lock, go to sleep: */
 169                 spin_unlock_mutex(&lock->wait_lock, flags);
 170                 schedule();
 171                 spin_lock_mutex(&lock->wait_lock, flags);
 172         }
 173
 174         /* got the lock - rejoice! */
 175         mutex_remove_waiter(lock, &waiter, task->thread_info);
 176         debug_mutex_set_owner(lock, task->thread_info);
 177
 178         /* set it to 0 if there are no waiters left: */
 179         if (likely(list_empty(&lock->wait_list)))
 180                 atomic_set(&lock->count, 0);
 181
 182         spin_unlock_mutex(&lock->wait_lock, flags);
 183
 184         debug_mutex_free_waiter(&waiter);
 185
 186         return 0;
 187 }
 188
 189 static void fastcall noinline __sched
 190 __mutex_lock_slowpath(atomic_t *lock_count)
 191 {
 192         struct mutex *lock = container_of(lock_count, struct mutex, count);
 193
 194         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0);
 195 }
 196
 197 /*
 198  * Release the lock, slowpath:
 199  */
 200 static fastcall inline void
 201 __mutex_unlock_common_slowpath(atomic_t *lock_count)
 202 {
 203         struct mutex *lock = container_of(lock_count, struct mutex, count);
 204         unsigned long flags;
 205
 206         spin_lock_mutex(&lock->wait_lock, flags);
 207         debug_mutex_unlock(lock);
 208
 209         /*
 210          * some architectures leave the lock unlocked in the fastpath failure
 211          * case, others need to leave it locked. In the later case we have to
 212          * unlock it here
 213          */
 214         if (__mutex_slowpath_needs_to_unlock())
 215                 atomic_set(&lock->count, 1);
 216
 217         if (!list_empty(&lock->wait_list)) {
 218                 /* get the first entry from the wait-list: */
 219                 struct mutex_waiter *waiter =
 220                                 list_entry(lock->wait_list.next,
 221                                            struct mutex_waiter, list);
 222
 223                 debug_mutex_wake_waiter(lock, waiter);
 224
 225                 wake_up_process(waiter->task);
 226         }
 227
 228         debug_mutex_clear_owner(lock);
 229
 230         spin_unlock_mutex(&lock->wait_lock, flags);
 231 }
 232
 233 /*
 234  * Release the lock, slowpath:
 235  */
 236 static fastcall noinline void
 237 __mutex_unlock_slowpath(atomic_t *lock_count)
 238 {
 239         __mutex_unlock_common_slowpath(lock_count);
 240 }
 241
 242 /*
 243  * Here come the less common (and hence less performance-critical) APIs:
 244  * mutex_lock_interruptible() and mutex_trylock().
 245  */
 246 static int fastcall noinline __sched
 247 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
 248
 249 /***
 250  * mutex_lock_interruptible - acquire the mutex, interruptable
 251  * @lock: the mutex to be acquired
 252  *
 253  * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 254  * been acquired or sleep until the mutex becomes available. If a
 255  * signal arrives while waiting for the lock then this function
 256  * returns -EINTR.
 257  *
 258  * This function is similar to (but not equivalent to) down_interruptible().
 259  */
 260 int fastcall __sched mutex_lock_interruptible(struct mutex *lock)
 261 {
 262         might_sleep();
 263         return __mutex_fastpath_lock_retval
 264                         (&lock->count, __mutex_lock_interruptible_slowpath);
 265 }
 266
 267 EXPORT_SYMBOL(mutex_lock_interruptible);
 268
 269 static int fastcall noinline __sched
 270 __mutex_lock_interruptible_slowpath(atomic_t *lock_count)
 271 {
 272         struct mutex *lock = container_of(lock_count, struct mutex, count);
 273
 274         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0);
 275 }
 276
 277 /*
 278  * Spinlock based trylock, we take the spinlock and check whether we
 279  * can get the lock:
 280  */
 281 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 282 {
 283         struct mutex *lock = container_of(lock_count, struct mutex, count);
 284         unsigned long flags;
 285         int prev;
 286
 287         spin_lock_mutex(&lock->wait_lock, flags);
 288
 289         prev = atomic_xchg(&lock->count, -1);
 290         if (likely(prev == 1))
 291                 debug_mutex_set_owner(lock, current_thread_info());
 292
 293         /* Set it back to 0 if there are no waiters: */
 294         if (likely(list_empty(&lock->wait_list)))
 295                 atomic_set(&lock->count, 0);
 296
 297         spin_unlock_mutex(&lock->wait_lock, flags);
 298
 299         return prev == 1;
 300 }
 301
 302 /***
 303  * mutex_trylock - try acquire the mutex, without waiting
 304  * @lock: the mutex to be acquired
 305  *
 306  * Try to acquire the mutex atomically. Returns 1 if the mutex
 307  * has been acquired successfully, and 0 on contention.
 308  *
 309  * NOTE: this function follows the spin_trylock() convention, so
 310  * it is negated to the down_trylock() return values! Be careful
 311  * about this when converting semaphore users to mutexes.
 312  *
 313  * This function must not be used in interrupt context. The
 314  * mutex must be released by the same task that acquired it.
 315  */
 316 int fastcall mutex_trylock(struct mutex *lock)
 317 {
 318         return __mutex_fastpath_trylock(&lock->count,
 319                                         __mutex_trylock_slowpath);
 320 }
 321
 322 EXPORT_SYMBOL(mutex_trylock);