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/*

 * 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 <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;

}