/*
 * i386 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;
}


/*
 * The semaphore operations have a special calling sequence that
 * allow us to do a simpler in-line version of them. These routines
 * need to convert that sequence back into the C sequence when
 * there is contention on the semaphore.
 *
 * %ecx contains the semaphore pointer on entry. Save the C-clobbered
 * registers (%eax, %edx and %ecx) except %eax when used as a return
 * value..
 */
asm(
".text\n"
".align 4\n"
".globl __down_failed\n"
"__down_failed:\n\t"
#if defined(CONFIG_FRAME_POINTER)
        "pushl %ebp\n\t"
        "movl  %esp,%ebp\n\t"
#endif
        "pushl %eax\n\t"
        "pushl %edx\n\t"
        "pushl %ecx\n\t"
        "call __down\n\t"
        "popl %ecx\n\t"
        "popl %edx\n\t"
        "popl %eax\n\t"
#if defined(CONFIG_FRAME_POINTER)
        "movl %ebp,%esp\n\t"
        "popl %ebp\n\t"
#endif
        "ret"
);

asm(
".text\n"
".align 4\n"
".globl __down_failed_interruptible\n"
"__down_failed_interruptible:\n\t"
#if defined(CONFIG_FRAME_POINTER)
        "pushl %ebp\n\t"
        "movl  %esp,%ebp\n\t"
#endif
        "pushl %edx\n\t"
        "pushl %ecx\n\t"
        "call __down_interruptible\n\t"
        "popl %ecx\n\t"
        "popl %edx\n\t"
#if defined(CONFIG_FRAME_POINTER)
        "movl %ebp,%esp\n\t"
        "popl %ebp\n\t"
#endif
        "ret"
);

asm(
".text\n"
".align 4\n"
".globl __down_failed_trylock\n"
"__down_failed_trylock:\n\t"
#if defined(CONFIG_FRAME_POINTER)
        "pushl %ebp\n\t"
        "movl  %esp,%ebp\n\t"
#endif
        "pushl %edx\n\t"
        "pushl %ecx\n\t"
        "call __down_trylock\n\t"
        "popl %ecx\n\t"
        "popl %edx\n\t"
#if defined(CONFIG_FRAME_POINTER)
        "movl %ebp,%esp\n\t"
        "popl %ebp\n\t"
#endif
        "ret"
);

asm(
".text\n"
".align 4\n"
".globl __up_wakeup\n"
"__up_wakeup:\n\t"
        "pushl %eax\n\t"
        "pushl %edx\n\t"
        "pushl %ecx\n\t"
        "call __up\n\t"
        "popl %ecx\n\t"
        "popl %edx\n\t"
        "popl %eax\n\t"
        "ret"
);

/*
 * rw spinlock fallbacks
 */
#if defined(CONFIG_SMP)
asm(
".text\n"
".align 4\n"
".globl __write_lock_failed\n"
"__write_lock_failed:\n\t"
        LOCK "addl      $" RW_LOCK_BIAS_STR ",(%eax)\n"
"1:     rep; nop\n\t"
        "cmpl   $" RW_LOCK_BIAS_STR ",(%eax)\n\t"
        "jne    1b\n\t"
        LOCK "subl      $" RW_LOCK_BIAS_STR ",(%eax)\n\t"
        "jnz    __write_lock_failed\n\t"
        "ret"
);

asm(
".text\n"
".align 4\n"
".globl __read_lock_failed\n"
"__read_lock_failed:\n\t"
        LOCK "incl      (%eax)\n"
"1:     rep; nop\n\t"
        "cmpl   $1,(%eax)\n\t"
        "js     1b\n\t"
        LOCK "decl      (%eax)\n\t"
        "js     __read_lock_failed\n\t"
        "ret"
);
#endif