kernel/semaphore.c

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

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

/*
 * Some notes on the implementation:
 *
 * down_trylock() and up() can be called from interrupt context.
 * So we have to disable interrupts when taking the lock.
 *
 * The ->count variable defines how many more tasks can acquire the
 * semaphore.  If it's zero, there may be tasks waiting on the list.
 */

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);

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);

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);

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 and 1 if it is contended.
 *
 * 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);

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);

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);
}