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[davej-history.git] / arch / i386 / kernel / semaphore.c

/*
 * 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(
".align 4\n"
".globl __down_failed\n"
"__down_failed:\n\t"
        "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"
        "ret"
);

asm(
".align 4\n"
".globl __down_failed_interruptible\n"
"__down_failed_interruptible:\n\t"
        "pushl %edx\n\t"
        "pushl %ecx\n\t"
        "call __down_interruptible\n\t"
        "popl %ecx\n\t"
        "popl %edx\n\t"
        "ret"
);

asm(
".align 4\n"
".globl __down_failed_trylock\n"
"__down_failed_trylock:\n\t"
        "pushl %edx\n\t"
        "pushl %ecx\n\t"
        "call __down_trylock\n\t"
        "popl %ecx\n\t"
        "popl %edx\n\t"
        "ret"
);

asm(
".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"
);

asm(
"
.align 4
.globl __down_read_failed
__down_read_failed:
        pushl   %edx
        pushl   %ecx
        jnc     2f

3:      call    down_read_failed_biased

1:      popl    %ecx
        popl    %edx
        ret

2:      call    down_read_failed
        " LOCK "subl    $1,(%eax)
        jns     1b
        jnc     2b
        jmp     3b
"
);

asm(
"
.align 4
.globl __down_write_failed
__down_write_failed:
        pushl   %edx
        pushl   %ecx
        jnc     2f

3:      call    down_write_failed_biased

1:      popl    %ecx
        popl    %edx
        ret

2:      call    down_write_failed
        " LOCK "subl    $" RW_LOCK_BIAS_STR ",(%eax)
        jz      1b
        jnc     2b
        jmp     3b
"
);

struct rw_semaphore *FASTCALL(rwsem_wake_readers(struct rw_semaphore *sem));
struct rw_semaphore *FASTCALL(rwsem_wake_writer(struct rw_semaphore *sem));

struct rw_semaphore *FASTCALL(down_read_failed_biased(struct rw_semaphore *sem));
struct rw_semaphore *FASTCALL(down_write_failed_biased(struct rw_semaphore *sem));
struct rw_semaphore *FASTCALL(down_read_failed(struct rw_semaphore *sem));
struct rw_semaphore *FASTCALL(down_write_failed(struct rw_semaphore *sem));

struct rw_semaphore *down_read_failed_biased(struct rw_semaphore *sem)
{
        struct task_struct *tsk = current;
        DECLARE_WAITQUEUE(wait, tsk);

        add_wait_queue(&sem->wait, &wait);      /* put ourselves at the head of the list */

        for (;;) {
                if (sem->read_bias_granted && xchg(&sem->read_bias_granted, 0))
                        break;
                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                if (!sem->read_bias_granted)
                        schedule();
        }

        remove_wait_queue(&sem->wait, &wait);
        tsk->state = TASK_RUNNING;

        return sem;
}

struct rw_semaphore *down_write_failed_biased(struct rw_semaphore *sem)
{
        struct task_struct *tsk = current;
        DECLARE_WAITQUEUE(wait, tsk);

        add_wait_queue_exclusive(&sem->write_bias_wait, &wait); /* put ourselves at the end of the list */

        for (;;) {
                if (sem->write_bias_granted && xchg(&sem->write_bias_granted, 0))
                        break;
                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                if (!sem->write_bias_granted)
                        schedule();
        }

        remove_wait_queue(&sem->write_bias_wait, &wait);
        tsk->state = TASK_RUNNING;

        /* if the lock is currently unbiased, awaken the sleepers
         * FIXME: this wakes up the readers early in a bit of a
         * stampede -> bad!
         */
        if (atomic_read(&sem->count) >= 0)
                wake_up(&sem->wait);

        return sem;
}

/* Wait for the lock to become unbiased.  Readers
 * are non-exclusive. =)
 */
struct rw_semaphore *down_read_failed(struct rw_semaphore *sem)
{
        struct task_struct *tsk = current;
        DECLARE_WAITQUEUE(wait, tsk);

        __up_read(sem); /* this takes care of granting the lock */

        add_wait_queue(&sem->wait, &wait);

        while (atomic_read(&sem->count) < 0) {
                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                if (atomic_read(&sem->count) >= 0)
                        break;
                schedule();
        }

        remove_wait_queue(&sem->wait, &wait);
        tsk->state = TASK_RUNNING;

        return sem;
}

/* Wait for the lock to become unbiased. Since we're
 * a writer, we'll make ourselves exclusive.
 */
struct rw_semaphore *down_write_failed(struct rw_semaphore *sem)
{
        struct task_struct *tsk = current;
        DECLARE_WAITQUEUE(wait, tsk);

        __up_write(sem);        /* this takes care of granting the lock */

        add_wait_queue_exclusive(&sem->wait, &wait);

        while (atomic_read(&sem->count) < 0) {
                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                if (atomic_read(&sem->count) >= 0)
                        break;  /* we must attempt to acquire or bias the lock */
                schedule();
        }

        remove_wait_queue(&sem->wait, &wait);
        tsk->state = TASK_RUNNING;

        return sem;
}

asm(
"
.align 4
.globl __rwsem_wake
__rwsem_wake:
        pushl   %edx
        pushl   %ecx

        jz      1f
        call    rwsem_wake_readers
        jmp     2f

1:      call    rwsem_wake_writer

2:      popl    %ecx
        popl    %edx
        ret
"
);

/* Called when someone has done an up that transitioned from
 * negative to non-negative, meaning that the lock has been
 * granted to whomever owned the bias.
 */
struct rw_semaphore *rwsem_wake_readers(struct rw_semaphore *sem)
{
        if (xchg(&sem->read_bias_granted, 1))
                BUG();
        wake_up(&sem->wait);
        return sem;
}

struct rw_semaphore *rwsem_wake_writer(struct rw_semaphore *sem)
{
        if (xchg(&sem->write_bias_granted, 1))
                BUG();
        wake_up(&sem->write_bias_wait);
        return sem;
}

#if defined(CONFIG_SMP)
asm(
"
.align  4
.globl  __write_lock_failed
__write_lock_failed:
        " LOCK "addl    $" RW_LOCK_BIAS_STR ",(%eax)
1:      cmpl    $" RW_LOCK_BIAS_STR ",(%eax)
        jne     1b

        " LOCK "subl    $" RW_LOCK_BIAS_STR ",(%eax)
        jnz     __write_lock_failed
        ret


.align  4
.globl  __read_lock_failed
__read_lock_failed:
        lock ; incl     (%eax)
1:      cmpl    $1,(%eax)
        js      1b

        lock ; decl     (%eax)
        js      __read_lock_failed
        ret
"
);
#endif