/*
 * 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>

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 (state == TASK_INTERRUPTIBLE && signal_pending(task))
                        goto interrupted;
                if (state == TASK_KILLABLE && fatal_signal_pending(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);
}