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   /*
    * kernel/mutex.c
    *
    * Mutexes: blocking mutual exclusion locks
    *
    * Started by Ingo Molnar:
    *
    *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
    *
   * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
   * David Howells for suggestions and improvements.
   *
   *  - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
   *    from the -rt tree, where it was originally implemented for rtmutexes
   *    by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
   *    and Sven Dietrich.
   *
   * Also see Documentation/mutex-design.txt.
   */
  #include <linux/mutex.h>
  #include <linux/sched.h>
  #include <linux/module.h>
  #include <linux/spinlock.h>
  #include <linux/interrupt.h>
  #include <linux/debug_locks.h>
  
  /*
   * In the DEBUG case we are using the "NULL fastpath" for mutexes,
   * which forces all calls into the slowpath:
   */
  #ifdef CONFIG_DEBUG_MUTEXES
  # include "mutex-debug.h"
  # include <asm-generic/mutex-null.h>
  #else
  # include "mutex.h"
  # include <asm/mutex.h>
  #endif
  
  /***
   * mutex_init - initialize the mutex
   * @lock: the mutex to be initialized
   * @key: the lock_class_key for the class; used by mutex lock debugging
   *
   * Initialize the mutex to unlocked state.
   *
   * It is not allowed to initialize an already locked mutex.
   */
  void
  __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
  {
          atomic_set(&lock->count, 1);
          spin_lock_init(&lock->wait_lock);
          INIT_LIST_HEAD(&lock->wait_list);
          mutex_clear_owner(lock);
  
          debug_mutex_init(lock, name, key);
  }
  
  EXPORT_SYMBOL(__mutex_init);
  
  #ifndef CONFIG_DEBUG_LOCK_ALLOC
  /*
   * We split the mutex lock/unlock logic into separate fastpath and
   * slowpath functions, to reduce the register pressure on the fastpath.
   * We also put the fastpath first in the kernel image, to make sure the
   * branch is predicted by the CPU as default-untaken.
   */
  static __used noinline void __sched
  __mutex_lock_slowpath(atomic_t *lock_count);
  
  /***
   * mutex_lock - acquire the mutex
   * @lock: the mutex to be acquired
   *
   * Lock the mutex exclusively for this task. If the mutex is not
   * available right now, it will sleep until it can get it.
   *
   * The mutex must later on be released by the same task that
   * acquired it. Recursive locking is not allowed. The task
   * may not exit without first unlocking the mutex. Also, kernel
   * memory where the mutex resides mutex must not be freed with
   * the mutex still locked. The mutex must first be initialized
   * (or statically defined) before it can be locked. memset()-ing
   * the mutex to 0 is not allowed.
   *
   * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
   *   checks that will enforce the restrictions and will also do
   *   deadlock debugging. )
   *
   * This function is similar to (but not equivalent to) down().
   */
  void __sched mutex_lock(struct mutex *lock)
  {
          might_sleep();
          /*
           * The locking fastpath is the 1->0 transition from
           * 'unlocked' into 'locked' state.
           */
          __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
         mutex_set_owner(lock);
 }
 
 EXPORT_SYMBOL(mutex_lock);
 #endif
 
 static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
 
 /***
  * mutex_unlock - release the mutex
  * @lock: the mutex to be released
  *
  * Unlock a mutex that has been locked by this task previously.
  *
  * This function must not be used in interrupt context. Unlocking
  * of a not locked mutex is not allowed.
  *
  * This function is similar to (but not equivalent to) up().
  */
 void __sched mutex_unlock(struct mutex *lock)
 {
         /*
          * The unlocking fastpath is the 0->1 transition from 'locked'
          * into 'unlocked' state:
          */
 #ifndef CONFIG_DEBUG_MUTEXES
         /*
          * When debugging is enabled we must not clear the owner before time,
          * the slow path will always be taken, and that clears the owner field
          * after verifying that it was indeed current.
          */
         mutex_clear_owner(lock);
 #endif
         __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
 }
 
 EXPORT_SYMBOL(mutex_unlock);
 
 /*
  * Lock a mutex (possibly interruptible), slowpath:
  */
 static inline int __sched
 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                 unsigned long ip)
 {
         struct task_struct *task = current;
         struct mutex_waiter waiter;
         unsigned long flags;
 
         preempt_disable();
         mutex_acquire(&lock->dep_map, subclass, 0, ip);
 #if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES) && \
     !defined(CONFIG_HAVE_DEFAULT_NO_SPIN_MUTEXES)
         /*
          * Optimistic spinning.
          *
          * We try to spin for acquisition when we find that there are no
          * pending waiters and the lock owner is currently running on a
          * (different) CPU.
          *
          * The rationale is that if the lock owner is running, it is likely to
          * release the lock soon.
          *
          * Since this needs the lock owner, and this mutex implementation
          * doesn't track the owner atomically in the lock field, we need to
          * track it non-atomically.
          *
          * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
          * to serialize everything.
          */
 
         for (;;) {
                 struct thread_info *owner;
 
                 /*
                  * If there's an owner, wait for it to either
                  * release the lock or go to sleep.
                  */
                 owner = ACCESS_ONCE(lock->owner);
                 if (owner && !mutex_spin_on_owner(lock, owner))
                         break;
 
                 if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
                         lock_acquired(&lock->dep_map, ip);
                         mutex_set_owner(lock);
                         preempt_enable();
                         return 0;
                 }
 
                 /*
                  * When there's no owner, we might have preempted between the
                  * owner acquiring the lock and setting the owner field. If
                  * we're an RT task that will live-lock because we won't let
                  * the owner complete.
                  */
                 if (!owner && (need_resched() || rt_task(task)))
                         break;
 
                 /*
                  * The cpu_relax() call is a compiler barrier which forces
                  * everything in this loop to be re-loaded. We don't need
                  * memory barriers as we'll eventually observe the right
                  * values at the cost of a few extra spins.
                  */
                 cpu_relax();
         }
 #endif
         spin_lock_mutex(&lock->wait_lock, flags);
 
         debug_mutex_lock_common(lock, &waiter);
         debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
 
         /* add waiting tasks to the end of the waitqueue (FIFO): */
         list_add_tail(&waiter.list, &lock->wait_list);
         waiter.task = task;
 
         if (atomic_xchg(&lock->count, -1) == 1)
                 goto done;
 
         lock_contended(&lock->dep_map, ip);
 
         for (;;) {
                 /*
                  * Lets try to take the lock again - this is needed even if
                  * we get here for the first time (shortly after failing to
                  * acquire the lock), to make sure that we get a wakeup once
                  * it's unlocked. Later on, if we sleep, this is the
                  * operation that gives us the lock. We xchg it to -1, so
                  * that when we release the lock, we properly wake up the
                  * other waiters:
                  */
                 if (atomic_xchg(&lock->count, -1) == 1)
                         break;
 
                 /*
                  * got a signal? (This code gets eliminated in the
                  * TASK_UNINTERRUPTIBLE case.)
                  */
                 if (unlikely(signal_pending_state(state, task))) {
                         mutex_remove_waiter(lock, &waiter,
                                             task_thread_info(task));
                         mutex_release(&lock->dep_map, 1, ip);
                         spin_unlock_mutex(&lock->wait_lock, flags);
 
                         debug_mutex_free_waiter(&waiter);
                         preempt_enable();
                         return -EINTR;
                 }
                 __set_task_state(task, state);
 
                 /* didnt get the lock, go to sleep: */
                 spin_unlock_mutex(&lock->wait_lock, flags);
                 preempt_enable_no_resched();
                 schedule();
                 preempt_disable();
                 spin_lock_mutex(&lock->wait_lock, flags);
         }
 
 done:
         lock_acquired(&lock->dep_map, ip);
         /* got the lock - rejoice! */
         mutex_remove_waiter(lock, &waiter, current_thread_info());
         mutex_set_owner(lock);
 
         /* set it to 0 if there are no waiters left: */
         if (likely(list_empty(&lock->wait_list)))
                 atomic_set(&lock->count, 0);
 
         spin_unlock_mutex(&lock->wait_lock, flags);
 
         debug_mutex_free_waiter(&waiter);
         preempt_enable();
 
         return 0;
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 void __sched
 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
 {
         might_sleep();
         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, _RET_IP_);
 }
 
 EXPORT_SYMBOL_GPL(mutex_lock_nested);
 
 int __sched
 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
 {
         might_sleep();
         return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);
 }
 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
 
 int __sched
 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
 {
         might_sleep();
         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
                                    subclass, _RET_IP_);
 }
 
 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
 #endif
 
 /*
  * Release the lock, slowpath:
  */
 static inline void
 __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
 {
         struct mutex *lock = container_of(lock_count, struct mutex, count);
         unsigned long flags;
 
         spin_lock_mutex(&lock->wait_lock, flags);
         mutex_release(&lock->dep_map, nested, _RET_IP_);
         debug_mutex_unlock(lock);
 
         /*
          * some architectures leave the lock unlocked in the fastpath failure
          * case, others need to leave it locked. In the later case we have to
          * unlock it here
          */
         if (__mutex_slowpath_needs_to_unlock())
                 atomic_set(&lock->count, 1);
 
         if (!list_empty(&lock->wait_list)) {
                 /* get the first entry from the wait-list: */
                 struct mutex_waiter *waiter =
                                 list_entry(lock->wait_list.next,
                                            struct mutex_waiter, list);
 
                 debug_mutex_wake_waiter(lock, waiter);
 
                 wake_up_process(waiter->task);
         }
 
         spin_unlock_mutex(&lock->wait_lock, flags);
 }
 
 /*
  * Release the lock, slowpath:
  */
 static __used noinline void
 __mutex_unlock_slowpath(atomic_t *lock_count)
 {
         __mutex_unlock_common_slowpath(lock_count, 1);
 }
 
 #ifndef CONFIG_DEBUG_LOCK_ALLOC
 /*
  * Here come the less common (and hence less performance-critical) APIs:
  * mutex_lock_interruptible() and mutex_trylock().
  */
 static noinline int __sched
 __mutex_lock_killable_slowpath(atomic_t *lock_count);
 
 static noinline int __sched
 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
 
 /***
  * mutex_lock_interruptible - acquire the mutex, interruptable
  * @lock: the mutex to be acquired
  *
  * Lock the mutex like mutex_lock(), and return 0 if the mutex has
  * been acquired or sleep until the mutex becomes available. If a
  * signal arrives while waiting for the lock then this function
  * returns -EINTR.
  *
  * This function is similar to (but not equivalent to) down_interruptible().
  */
 int __sched mutex_lock_interruptible(struct mutex *lock)
 {
         int ret;
 
         might_sleep();
         ret =  __mutex_fastpath_lock_retval
                         (&lock->count, __mutex_lock_interruptible_slowpath);
         if (!ret)
                 mutex_set_owner(lock);
 
         return ret;
 }
 
 EXPORT_SYMBOL(mutex_lock_interruptible);
 
 int __sched mutex_lock_killable(struct mutex *lock)
 {
         int ret;
 
         might_sleep();
         ret = __mutex_fastpath_lock_retval
                         (&lock->count, __mutex_lock_killable_slowpath);
         if (!ret)
                 mutex_set_owner(lock);
 
         return ret;
 }
 EXPORT_SYMBOL(mutex_lock_killable);
 
 static __used noinline void __sched
 __mutex_lock_slowpath(atomic_t *lock_count)
 {
         struct mutex *lock = container_of(lock_count, struct mutex, count);
 
         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, _RET_IP_);
 }
 
 static noinline int __sched
 __mutex_lock_killable_slowpath(atomic_t *lock_count)
 {
         struct mutex *lock = container_of(lock_count, struct mutex, count);
 
         return __mutex_lock_common(lock, TASK_KILLABLE, 0, _RET_IP_);
 }
 
 static noinline int __sched
 __mutex_lock_interruptible_slowpath(atomic_t *lock_count)
 {
         struct mutex *lock = container_of(lock_count, struct mutex, count);
 
         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, _RET_IP_);
 }
 #endif
 
 /*
  * Spinlock based trylock, we take the spinlock and check whether we
  * can get the lock:
  */
 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 {
         struct mutex *lock = container_of(lock_count, struct mutex, count);
         unsigned long flags;
         int prev;
 
         spin_lock_mutex(&lock->wait_lock, flags);
 
         prev = atomic_xchg(&lock->count, -1);
         if (likely(prev == 1)) {
                 mutex_set_owner(lock);
                 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
         }
 
         /* Set it back to 0 if there are no waiters: */
         if (likely(list_empty(&lock->wait_list)))
                 atomic_set(&lock->count, 0);
 
         spin_unlock_mutex(&lock->wait_lock, flags);
 
         return prev == 1;
 }
 
 /***
  * mutex_trylock - try acquire the mutex, without waiting
  * @lock: the mutex to be acquired
  *
  * Try to acquire the mutex atomically. Returns 1 if the mutex
  * has been acquired successfully, and 0 on contention.
  *
  * NOTE: this function follows the spin_trylock() convention, so
  * it is negated to the down_trylock() return values! Be careful
  * about this when converting semaphore users to mutexes.
  *
  * This function must not be used in interrupt context. The
  * mutex must be released by the same task that acquired it.
  */
 int __sched mutex_trylock(struct mutex *lock)
 {
         int ret;
 
         ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
         if (ret)
                 mutex_set_owner(lock);
 
         return ret;
 }
 EXPORT_SYMBOL(mutex_trylock);
 
 /**
  * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
  * @cnt: the atomic which we are to dec
  * @lock: the mutex to return holding if we dec to 0
  *
  * return true and hold lock if we dec to 0, return false otherwise
  */
 int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
 {
         /* dec if we can't possibly hit 0 */
         if (atomic_add_unless(cnt, -1, 1))
                 return 0;
         /* we might hit 0, so take the lock */
         mutex_lock(lock);
         if (!atomic_dec_and_test(cnt)) {
                 /* when we actually did the dec, we didn't hit 0 */
                 mutex_unlock(lock);
                 return 0;
         }
         /* we hit 0, and we hold the lock */
         return 1;
 }
 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);
 

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