/*
 * x86_64 semaphore implementation.
 *
 * (C) Copyright 1999 Linus Torvalds
 *
 * Portions Copyright 1999 Red Hat, Inc.
 *
 *	This program is free software; you can redistribute it and/or
 *	modify it under the terms of the GNU General Public License
 *	as published by the Free Software Foundation; either version
 *	2 of the License, or (at your option) any later version.
 *
 * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@redhat.com>
 */
#include <linux/config.h>
#include <linux/sched.h>
#include <linux/worktodo.h>

#include <asm/semaphore.h>

/*
 * Semaphores are implemented using a two-way counter:
 * The "count" variable is decremented for each process
 * that tries to acquire the semaphore, while the "sleeping"
 * variable is a count of such acquires.
 *
 * Notably, the inline "up()" and "down()" functions can
 * efficiently test if they need to do any extra work (up
 * needs to do something only if count was negative before
 * the increment operation.
 *
 * "sleeping" and the contention routine ordering is
 * protected by the semaphore spinlock.
 *
 * Note that these functions are only called when there is
 * contention on the lock, and as such all this is the
 * "non-critical" part of the whole semaphore business. The
 * critical part is the inline stuff in <asm/semaphore.h>
 * where we want to avoid any extra jumps and calls.
 */

/*
 * Logic:
 *  - only on a boundary condition do we need to care. When we go
 *    from a negative count to a non-negative, we wake people up.
 *  - when we go from a non-negative count to a negative do we
 *    (a) synchronize with the "sleeper" count and (b) make sure
 *    that we're on the wakeup list before we synchronize so that
 *    we cannot lose wakeup events.
 */

void __up(struct semaphore *sem)
{
	wake_up(&sem->wait);
}

static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;

void __down(struct semaphore * sem)
{
	struct task_struct *tsk = current;
	DECLARE_WAITQUEUE(wait, tsk);
	tsk->state = TASK_UNINTERRUPTIBLE;
	add_wait_queue_exclusive(&sem->wait, &wait);

	spin_lock_irq(&semaphore_lock);
	sem->sleepers++;
	for (;;) {
		int sleepers = sem->sleepers;

		/*
		 * Add "everybody else" into it. They aren't
		 * playing, because we own the spinlock.
		 */
		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
			sem->sleepers = 0;
			break;
		}
		sem->sleepers = 1;	/* us - see -1 above */
		spin_unlock_irq(&semaphore_lock);

		schedule();
		tsk->state = TASK_UNINTERRUPTIBLE;
		spin_lock_irq(&semaphore_lock);
	}
	spin_unlock_irq(&semaphore_lock);
	remove_wait_queue(&sem->wait, &wait);
	tsk->state = TASK_RUNNING;
	wake_up(&sem->wait);
}

int __down_interruptible(struct semaphore * sem)
{
	int retval = 0;
	struct task_struct *tsk = current;
	DECLARE_WAITQUEUE(wait, tsk);
	tsk->state = TASK_INTERRUPTIBLE;
	add_wait_queue_exclusive(&sem->wait, &wait);

	spin_lock_irq(&semaphore_lock);
	sem->sleepers ++;
	for (;;) {
		int sleepers = sem->sleepers;

		/*
		 * With signals pending, this turns into
		 * the trylock failure case - we won't be
		 * sleeping, and we* can't get the lock as
		 * it has contention. Just correct the count
		 * and exit.
		 */
		if (signal_pending(current)) {
			retval = -EINTR;
			sem->sleepers = 0;
			atomic_add(sleepers, &sem->count);
			break;
		}

		/*
		 * Add "everybody else" into it. They aren't
		 * playing, because we own the spinlock. The
		 * "-1" is because we're still hoping to get
		 * the lock.
		 */
		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
			sem->sleepers = 0;
			break;
		}
		sem->sleepers = 1;	/* us - see -1 above */
		spin_unlock_irq(&semaphore_lock);

		schedule();
		tsk->state = TASK_INTERRUPTIBLE;
		spin_lock_irq(&semaphore_lock);
	}
	spin_unlock_irq(&semaphore_lock);
	tsk->state = TASK_RUNNING;
	remove_wait_queue(&sem->wait, &wait);
	wake_up(&sem->wait);
	return retval;
}

/*
 * Trylock failed - make sure we correct for
 * having decremented the count.
 *
 * We could have done the trylock with a
 * single "cmpxchg" without failure cases,
 * but then it wouldn't work on a 386.
 */
int __down_trylock(struct semaphore * sem)
{
	int sleepers;
	unsigned long flags;

	spin_lock_irqsave(&semaphore_lock, flags);
	sleepers = sem->sleepers + 1;
	sem->sleepers = 0;

	/*
	 * Add "everybody else" and us into it. They aren't
	 * playing, because we own the spinlock.
	 */
	if (!atomic_add_negative(sleepers, &sem->count))
		wake_up(&sem->wait);

	spin_unlock_irqrestore(&semaphore_lock, flags);
	return 1;
}

void __wtd_down(struct semaphore * sem, struct worktodo *wtd);
void __wtd_down_from_wakeup(struct semaphore * sem, struct worktodo *wtd);

void __wtd_down_action(void *data)
{
	struct worktodo *wtd = data;
	struct semaphore *sem;

	wtd_pop(wtd);
	sem = wtd->data;

	__wtd_down_from_wakeup(sem, wtd);
}

void __wtd_down_waiter(wait_queue_t *wait)
{
	struct worktodo *wtd = (struct worktodo *)wait;
	struct semaphore *sem = wtd->data;

	__remove_wait_queue(&sem->wait, &wtd->wait);
	wtd_push(wtd, __wtd_down_action, wtd);
	wtd_queue(wtd);
}

void wtd_down_common(struct semaphore * sem, struct worktodo *wtd,
                       int do_incr)
{
	int gotit;
	int sleepers;

	init_waitqueue_func_entry(&wtd->wait, __wtd_down_waiter);
	wtd->data = sem;

	spin_lock_irq(&semaphore_lock);
	sem->sleepers += do_incr;
	sleepers = sem->sleepers;
	gotit = add_wait_queue_exclusive_cond(&sem->wait, &wtd->wait,
			atomic_add_negative(sleepers - 1, &sem->count));
	if (gotit)
		sem->sleepers = 0;
	else
		sem->sleepers = 1;
	spin_unlock_irq(&semaphore_lock);

	if (gotit) {
		wake_up(&sem->wait);
		wtd_queue(wtd);
	}
}
 
void __wtd_down(struct semaphore * sem, struct worktodo *wtd)
{
	wtd_down_common(sem, wtd, 1);
}

/*
 * Same as __wtd_down, but sem->sleepers is not incremented when
 * coming from a wakeup.
 */
void __wtd_down_from_wakeup(struct semaphore * sem, struct worktodo *wtd)
{
	wtd_down_common(sem, wtd, 0);
}

