/*
* 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, if positive, defines how many more tasks can
* acquire the semaphore. If negative, it represents how many tasks are
* waiting on the semaphore (*). If zero, no tasks are waiting, and no more
* tasks can acquire the semaphore.
*
* (*) Except for the window between one task calling up() and the task
* sleeping in a __down_common() waking up. In order to avoid a third task
* coming in and stealing the second task's wakeup, we leave the ->count
* negative. If we have a more complex situation, the ->count may become
* zero or negative (eg a semaphore with count = 2, three tasks attempt to
* acquire it, one sleeps, two finish and call up(), the second task to call
* up() notices that the list is empty and just increments count).
*/
static noinline void __down(struct semaphore *sem);
static noinline int __down_interruptible(struct semaphore *sem);
static noinline void __up(struct semaphore *sem);
void down(struct semaphore *sem)
{
unsigned long flags;
spin_lock_irqsave(&sem->lock, flags);
if (unlikely(sem->count-- <= 0))
__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 (unlikely(sem->count-- <= 0))
result = __down_interruptible(sem);
spin_unlock_irqrestore(&sem->lock, flags);
return result;
}
EXPORT_SYMBOL(down_interruptible);
/**
* 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);
void up(struct semaphore *sem)
{
unsigned long flags;
spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count >= 0))
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;
};
/*
* Wake up a process waiting on a semaphore. We need to call this from both
* __up and __down_common as it's possible to race a task into the semaphore
* if it comes in at just the right time between two tasks calling up() and
* a third task waking up. This function assumes the wait_list is already
* checked for being non-empty.
*/
static noinline void __sched __up_down_common(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);
}
/*
* Because this function is inlined, the 'state' parameter will be constant,
* and thus optimised away by the compiler.
*/
static inline int __sched __down_common(struct semaphore *sem, long state)
{
int result = 0;
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;
__set_task_state(task, state);
spin_unlock_irq(&sem->lock);
schedule();
spin_lock_irq(&sem->lock);
if (waiter.up)
goto woken;
}
interrupted:
list_del(&waiter.list);
result = -EINTR;
woken:
/*
* Account for the process which woke us up. For the case where
* we're interrupted, we need to increment the count on our own
* behalf. I don't believe we can hit the case where the
* sem->count hits zero, *and* there's a second task sleeping,
* but it doesn't hurt, that's not a commonly exercised path and
* it's not a performance path either.
*/
if (unlikely((++sem->count >= 0) && !list_empty(&sem->wait_list)))
__up_down_common(sem);
return result;
}
static noinline void __sched __down(struct semaphore *sem)
{
__down_common(sem, TASK_UNINTERRUPTIBLE);
}
static noinline int __sched __down_interruptible(struct semaphore *sem)
{
return __down_common(sem, TASK_INTERRUPTIBLE);
}
static noinline void __sched __up(struct semaphore *sem)
{
if (unlikely(list_empty(&sem->wait_list)))
sem->count++;
else
__up_down_common(sem);
}
