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         /*
  85          * The locking fastpath is the 1->0 transition from
  86          * 'unlocked' into 'locked' state.
  87          *
  88          * NOTE: if asm/mutex.h is included, then some architectures
  89          * rely on mutex_lock() having _no other code_ here but this
  90          * fastpath. That allows the assembly fastpath to do
  91          * tail-merging optimizations. (If you want to put testcode
  92          * here, do it under #ifndef CONFIG_MUTEX_DEBUG.)
  93          */
  94         __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
  95 }
  96
  97 EXPORT_SYMBOL(mutex_lock);
  98
  99 static void fastcall noinline __sched
 100 __mutex_unlock_slowpath(atomic_t *lock_count __IP_DECL__);
 101
 102 /***
 103  * mutex_unlock - release the mutex
 104  * @lock: the mutex to be released
 105  *
 106  * Unlock a mutex that has been locked by this task previously.
 107  *
 108  * This function must not be used in interrupt context. Unlocking
 109  * of a not locked mutex is not allowed.
 110  *
 111  * This function is similar to (but not equivalent to) up().
 112  */
 113 void fastcall __sched mutex_unlock(struct mutex *lock)
 114 {
 115         /*
 116          * The unlocking fastpath is the 0->1 transition from 'locked'
 117          * into 'unlocked' state:
 118          *
 119          * NOTE: no other code must be here - see mutex_lock() .
 120          */
 121         __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
 122 }
 123
 124 EXPORT_SYMBOL(mutex_unlock);
 125
 126 /*
 127  * Lock a mutex (possibly interruptible), slowpath:
 128  */
 129 static inline int __sched
 130 __mutex_lock_common(struct mutex *lock, long state __IP_DECL__)
 131 {
 132         struct task_struct *task = current;
 133         struct mutex_waiter waiter;
 134         unsigned int old_val;
 135
 136         debug_mutex_init_waiter(&waiter);
 137
 138         spin_lock_mutex(&lock->wait_lock);
 139
 140         debug_mutex_add_waiter(lock, &waiter, task->thread_info, ip);
 141
 142         /* add waiting tasks to the end of the waitqueue (FIFO): */
 143         list_add_tail(&waiter.list, &lock->wait_list);
 144         waiter.task = task;
 145
 146         for (;;) {
 147                 /*
 148                  * Lets try to take the lock again - this is needed even if
 149                  * we get here for the first time (shortly after failing to
 150                  * acquire the lock), to make sure that we get a wakeup once
 151                  * it's unlocked. Later on, if we sleep, this is the
 152                  * operation that gives us the lock. We xchg it to -1, so
 153                  * that when we release the lock, we properly wake up the
 154                  * other waiters:
 155                  */
 156                 old_val = atomic_xchg(&lock->count, -1);
 157                 if (old_val == 1)
 158                         break;
 159
 160                 /*
 161                  * got a signal? (This code gets eliminated in the
 162                  * TASK_UNINTERRUPTIBLE case.)
 163                  */
 164                 if (unlikely(state == TASK_INTERRUPTIBLE &&
 165                                                 signal_pending(task))) {
 166                         mutex_remove_waiter(lock, &waiter, task->thread_info);
 167                         spin_unlock_mutex(&lock->wait_lock);
 168
 169                         debug_mutex_free_waiter(&waiter);
 170                         return -EINTR;
 171                 }
 172                 __set_task_state(task, state);
 173
 174                 /* didnt get the lock, go to sleep: */
 175                 spin_unlock_mutex(&lock->wait_lock);
 176                 schedule();
 177                 spin_lock_mutex(&lock->wait_lock);
 178         }
 179
 180         /* got the lock - rejoice! */
 181         mutex_remove_waiter(lock, &waiter, task->thread_info);
 182         debug_mutex_set_owner(lock, task->thread_info __IP__);
 183
 184         /* set it to 0 if there are no waiters left: */
 185         if (likely(list_empty(&lock->wait_list)))
 186                 atomic_set(&lock->count, 0);
 187
 188         spin_unlock_mutex(&lock->wait_lock);
 189
 190         debug_mutex_free_waiter(&waiter);
 191
 192         DEBUG_WARN_ON(list_empty(&lock->held_list));
 193         DEBUG_WARN_ON(lock->owner != task->thread_info);
 194
 195         return 0;
 196 }
 197
 198 static void fastcall noinline __sched
 199 __mutex_lock_slowpath(atomic_t *lock_count __IP_DECL__)
 200 {
 201         struct mutex *lock = container_of(lock_count, struct mutex, count);
 202
 203         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE __IP__);
 204 }
 205
 206 /*
 207  * Release the lock, slowpath:
 208  */
 209 static fastcall noinline void
 210 __mutex_unlock_slowpath(atomic_t *lock_count __IP_DECL__)
 211 {
 212         struct mutex *lock = container_of(lock_count, struct mutex, count);
 213
 214         DEBUG_WARN_ON(lock->owner != current_thread_info());
 215
 216         spin_lock_mutex(&lock->wait_lock);
 217
 218         /*
 219          * some architectures leave the lock unlocked in the fastpath failure
 220          * case, others need to leave it locked. In the later case we have to
 221          * unlock it here
 222          */
 223         if (__mutex_slowpath_needs_to_unlock())
 224                 atomic_set(&lock->count, 1);
 225
 226         debug_mutex_unlock(lock);
 227
 228         if (!list_empty(&lock->wait_list)) {
 229                 /* get the first entry from the wait-list: */
 230                 struct mutex_waiter *waiter =
 231                                 list_entry(lock->wait_list.next,
 232                                            struct mutex_waiter, list);
 233
 234                 debug_mutex_wake_waiter(lock, waiter);
 235
 236                 wake_up_process(waiter->task);
 237         }
 238
 239         debug_mutex_clear_owner(lock);
 240
 241         spin_unlock_mutex(&lock->wait_lock);
 242 }
 243
 244 /*
 245  * Here come the less common (and hence less performance-critical) APIs:
 246  * mutex_lock_interruptible() and mutex_trylock().
 247  */
 248 static int fastcall noinline __sched
 249 __mutex_lock_interruptible_slowpath(atomic_t *lock_count __IP_DECL__);
 250
 251 /***
 252  * mutex_lock_interruptible - acquire the mutex, interruptable
 253  * @lock: the mutex to be acquired
 254  *
 255  * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 256  * been acquired or sleep until the mutex becomes available. If a
 257  * signal arrives while waiting for the lock then this function
 258  * returns -EINTR.
 259  *
 260  * This function is similar to (but not equivalent to) down_interruptible().
 261  */
 262 int fastcall __sched mutex_lock_interruptible(struct mutex *lock)
 263 {
 264         /* NOTE: no other code must be here - see mutex_lock() */
 265         return __mutex_fastpath_lock_retval
 266                         (&lock->count, __mutex_lock_interruptible_slowpath);
 267 }
 268
 269 EXPORT_SYMBOL(mutex_lock_interruptible);
 270
 271 static int fastcall noinline __sched
 272 __mutex_lock_interruptible_slowpath(atomic_t *lock_count __IP_DECL__)
 273 {
 274         struct mutex *lock = container_of(lock_count, struct mutex, count);
 275
 276         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE __IP__);
 277 }
 278
 279 /*
 280  * Spinlock based trylock, we take the spinlock and check whether we
 281  * can get the lock:
 282  */
 283 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 284 {
 285         struct mutex *lock = container_of(lock_count, struct mutex, count);
 286         int prev;
 287
 288         spin_lock_mutex(&lock->wait_lock);
 289
 290         prev = atomic_xchg(&lock->count, -1);
 291         if (likely(prev == 1))
 292                 debug_mutex_set_owner(lock, current_thread_info() __RET_IP__);
 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);
 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);
 323
 324
 325