/*
 * i386 and 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@kvack.org>
 */
#include <linux/sched.h>
#include <linux/err.h>
#include <linux/init.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 spinlock in the semaphore's waitqueue head.
 *
 * 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);
}

void __sched __down(struct semaphore *sem)
{
        struct task_struct *tsk = current;
        DECLARE_WAITQUEUE(wait, tsk);
        unsigned long flags;

        tsk->state = TASK_UNINTERRUPTIBLE;
        spin_lock_irqsave(&sem->wait.lock, flags);
        add_wait_queue_exclusive_locked(&sem->wait, &wait);

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

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

                schedule();

                spin_lock_irqsave(&sem->wait.lock, flags);
                tsk->state = TASK_UNINTERRUPTIBLE;
        }
        remove_wait_queue_locked(&sem->wait, &wait);
        wake_up_locked(&sem->wait);
        spin_unlock_irqrestore(&sem->wait.lock, flags);
        tsk->state = TASK_RUNNING;
}

int __sched __down_interruptible(struct semaphore *sem)
{
        int retval = 0;
        struct task_struct *tsk = current;
        DECLARE_WAITQUEUE(wait, tsk);
        unsigned long flags;

        tsk->state = TASK_INTERRUPTIBLE;
        spin_lock_irqsave(&sem->wait.lock, flags);
        add_wait_queue_exclusive_locked(&sem->wait, &wait);

        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 in
                 * wait_queue_head. The "-1" is because we're
                 * still hoping to get the semaphore.
                 */
                if (!atomic_add_negative(sleepers - 1, &sem->count)) {
                        sem->sleepers = 0;
                        break;
                }
                sem->sleepers = 1;      /* us - see -1 above */
                spin_unlock_irqrestore(&sem->wait.lock, flags);

                schedule();

                spin_lock_irqsave(&sem->wait.lock, flags);
                tsk->state = TASK_INTERRUPTIBLE;
        }
        remove_wait_queue_locked(&sem->wait, &wait);
        wake_up_locked(&sem->wait);
        spin_unlock_irqrestore(&sem->wait.lock, flags);

        tsk->state = TASK_RUNNING;
        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(&sem->wait.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 in the
         * wait_queue_head.
         */
        if (!atomic_add_negative(sleepers, &sem->count)) {
                wake_up_locked(&sem->wait);
        }

        spin_unlock_irqrestore(&sem->wait.lock, flags);
        return 1;
}