/*
   * Copyright (c) 2008 Intel Corporation
   * Author: Matthew Wilcox <willy@linux.intel.com>
   *
   * Distributed under the terms of the GNU GPL, version 2

   *
   * This file implements counting semaphores.
   * A counting semaphore may be acquired 'n' times before sleeping.
   * See mutex.c for single-acquisition sleeping locks which enforce
   * rules which allow code to be debugged more easily.
   */
  
  /*
   * Some notes on the implementation:
   *
   * The spinlock controls access to the other members of the semaphore.
   * down_trylock() and up() can be called from interrupt context, so we
   * have to disable interrupts when taking the lock.  It turns out various
   * parts of the kernel expect to be able to use down() on a semaphore in
   * interrupt context when they know it will succeed, so we have to use
   * irqsave variants for down(), down_interruptible() and down_killable()
   * too.
   *
   * The ->count variable represents how many more tasks can acquire this
   * semaphore.  If it's zero, there may be tasks waiting on the wait_list.

   */
  
  #include <linux/compiler.h>
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/sched.h>
  #include <linux/semaphore.h>
  #include <linux/spinlock.h>

  #include <linux/ftrace.h>

  static noinline void __down(struct semaphore *sem);
  static noinline int __down_interruptible(struct semaphore *sem);

  static noinline int __down_killable(struct semaphore *sem);

  static noinline int __down_timeout(struct semaphore *sem, long jiffies);

  static noinline void __up(struct semaphore *sem);

  /**
   * down - acquire the semaphore
   * @sem: the semaphore to be acquired
   *
   * Acquires the semaphore.  If no more tasks are allowed to acquire the
   * semaphore, calling this function will put the task to sleep until the
   * semaphore is released.
   *
   * Use of this function is deprecated, please use down_interruptible() or
   * down_killable() instead.
   */

  void down(struct semaphore *sem)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&sem->lock, flags);

  	if (likely(sem->count > 0))
  		sem->count--;
  	else

  		__down(sem);
  	spin_unlock_irqrestore(&sem->lock, flags);
  }
  EXPORT_SYMBOL(down);

  /**
   * down_interruptible - acquire the semaphore unless interrupted
   * @sem: the semaphore to be acquired
   *
   * Attempts to acquire the semaphore.  If no more tasks are allowed to
   * acquire the semaphore, calling this function will put the task to sleep.
   * If the sleep is interrupted by a signal, this function will return -EINTR.
   * If the semaphore is successfully acquired, this function returns 0.
   */

  int down_interruptible(struct semaphore *sem)
  {
  	unsigned long flags;
  	int result = 0;
  
  	spin_lock_irqsave(&sem->lock, flags);

  	if (likely(sem->count > 0))

  		sem->count--;

  	else
  		result = __down_interruptible(sem);

  	spin_unlock_irqrestore(&sem->lock, flags);
  
  	return result;
  }
  EXPORT_SYMBOL(down_interruptible);

  /**
   * down_killable - acquire the semaphore unless killed
   * @sem: the semaphore to be acquired
   *
   * Attempts to acquire the semaphore.  If no more tasks are allowed to
   * acquire the semaphore, calling this function will put the task to sleep.
   * If the sleep is interrupted by a fatal signal, this function will return
   * -EINTR.  If the semaphore is successfully acquired, this function returns
   * 0.
   */

  int down_killable(struct semaphore *sem)
  {
  	unsigned long flags;
  	int result = 0;
  
  	spin_lock_irqsave(&sem->lock, flags);

  	if (likely(sem->count > 0))

  		sem->count--;

  	else
  		result = __down_killable(sem);

  	spin_unlock_irqrestore(&sem->lock, flags);
  
  	return result;
  }
  EXPORT_SYMBOL(down_killable);

  /**
   * down_trylock - try to acquire the semaphore, without waiting
   * @sem: the semaphore to be acquired
   *
   * Try to acquire the semaphore atomically.  Returns 0 if the mutex has

   * been acquired successfully or 1 if it it cannot be acquired.

   *
   * NOTE: This return value is inverted from both spin_trylock and
   * mutex_trylock!  Be careful about this when converting code.
   *
   * Unlike mutex_trylock, this function can be used from interrupt context,
   * and the semaphore can be released by any task or interrupt.
   */
  int down_trylock(struct semaphore *sem)
  {
  	unsigned long flags;
  	int count;
  
  	spin_lock_irqsave(&sem->lock, flags);
  	count = sem->count - 1;
  	if (likely(count >= 0))
  		sem->count = count;
  	spin_unlock_irqrestore(&sem->lock, flags);
  
  	return (count < 0);
  }
  EXPORT_SYMBOL(down_trylock);

  /**
   * down_timeout - acquire the semaphore within a specified time
   * @sem: the semaphore to be acquired
   * @jiffies: how long to wait before failing
   *
   * Attempts to acquire the semaphore.  If no more tasks are allowed to
   * acquire the semaphore, calling this function will put the task to sleep.
   * If the semaphore is not released within the specified number of jiffies,
   * this function returns -ETIME.  It returns 0 if the semaphore was acquired.
   */

  int down_timeout(struct semaphore *sem, long jiffies)
  {
  	unsigned long flags;
  	int result = 0;
  
  	spin_lock_irqsave(&sem->lock, flags);

  	if (likely(sem->count > 0))

  		sem->count--;

  	else
  		result = __down_timeout(sem, jiffies);

  	spin_unlock_irqrestore(&sem->lock, flags);
  
  	return result;
  }
  EXPORT_SYMBOL(down_timeout);

  /**
   * up - release the semaphore
   * @sem: the semaphore to release
   *
   * Release the semaphore.  Unlike mutexes, up() may be called from any
   * context and even by tasks which have never called down().
   */

  void up(struct semaphore *sem)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&sem->lock, flags);

  	if (likely(list_empty(&sem->wait_list)))
  		sem->count++;
  	else

  		__up(sem);
  	spin_unlock_irqrestore(&sem->lock, flags);
  }
  EXPORT_SYMBOL(up);
  
  /* Functions for the contended case */
  
  struct semaphore_waiter {
  	struct list_head list;
  	struct task_struct *task;

  	int up;

  };
  
  /*

   * Because this function is inlined, the 'state' parameter will be
   * constant, and thus optimised away by the compiler.  Likewise the
   * 'timeout' parameter for the cases without timeouts.

   */

  static inline int __sched __down_common(struct semaphore *sem, long state,
  								long timeout)

  {

  	struct task_struct *task = current;
  	struct semaphore_waiter waiter;

  	list_add_tail(&waiter.list, &sem->wait_list);

  	waiter.task = task;
  	waiter.up = 0;

  
  	for (;;) {

  		if (signal_pending_state(state, task))

  			goto interrupted;
  		if (timeout <= 0)
  			goto timed_out;

  		__set_task_state(task, state);
  		spin_unlock_irq(&sem->lock);

  		timeout = schedule_timeout(timeout);

  		spin_lock_irq(&sem->lock);

  		if (waiter.up)
  			return 0;

  	}

   timed_out:
  	list_del(&waiter.list);
  	return -ETIME;
  
   interrupted:

  	list_del(&waiter.list);

  	return -EINTR;

  }
  
  static noinline void __sched __down(struct semaphore *sem)
  {

  	__down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);

  }
  
  static noinline int __sched __down_interruptible(struct semaphore *sem)
  {

  	return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);

  }

  static noinline int __sched __down_killable(struct semaphore *sem)
  {

  	return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
  }
  
  static noinline int __sched __down_timeout(struct semaphore *sem, long jiffies)
  {
  	return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);

  }

  static noinline void __sched __up(struct semaphore *sem)
  {

  	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
  						struct semaphore_waiter, list);

  	list_del(&waiter->list);
  	waiter->up = 1;

  	wake_up_process(waiter->task);

  }