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[/] [test_project/] [trunk/] [linux_sd_driver/] [lib/] [semaphore-sleepers.c] - Blame information for rev 62

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1 62 marcus.erl
/*
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 * i386 and x86-64 semaphore implementation.
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 *
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 * (C) Copyright 1999 Linus Torvalds
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 *
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 * Portions Copyright 1999 Red Hat, Inc.
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 *
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 *      This program is free software; you can redistribute it and/or
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 *      modify it under the terms of the GNU General Public License
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 *      as published by the Free Software Foundation; either version
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 *      2 of the License, or (at your option) any later version.
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 *
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 * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
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 */
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#include <linux/sched.h>
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#include <linux/err.h>
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#include <linux/init.h>
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#include <asm/semaphore.h>
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/*
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 * Semaphores are implemented using a two-way counter:
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 * The "count" variable is decremented for each process
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 * that tries to acquire the semaphore, while the "sleeping"
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 * variable is a count of such acquires.
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 *
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 * Notably, the inline "up()" and "down()" functions can
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 * efficiently test if they need to do any extra work (up
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 * needs to do something only if count was negative before
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 * the increment operation.
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 *
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 * "sleeping" and the contention routine ordering is protected
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 * by the spinlock in the semaphore's waitqueue head.
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 *
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 * Note that these functions are only called when there is
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 * contention on the lock, and as such all this is the
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 * "non-critical" part of the whole semaphore business. The
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 * critical part is the inline stuff in <asm/semaphore.h>
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 * where we want to avoid any extra jumps and calls.
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 */
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/*
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 * Logic:
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 *  - only on a boundary condition do we need to care. When we go
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 *    from a negative count to a non-negative, we wake people up.
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 *  - when we go from a non-negative count to a negative do we
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 *    (a) synchronize with the "sleeper" count and (b) make sure
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 *    that we're on the wakeup list before we synchronize so that
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 *    we cannot lose wakeup events.
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 */
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fastcall void __up(struct semaphore *sem)
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{
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        wake_up(&sem->wait);
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}
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fastcall void __sched __down(struct semaphore * sem)
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{
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        struct task_struct *tsk = current;
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        DECLARE_WAITQUEUE(wait, tsk);
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        unsigned long flags;
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        tsk->state = TASK_UNINTERRUPTIBLE;
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        spin_lock_irqsave(&sem->wait.lock, flags);
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        add_wait_queue_exclusive_locked(&sem->wait, &wait);
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        sem->sleepers++;
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        for (;;) {
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                int sleepers = sem->sleepers;
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                /*
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                 * Add "everybody else" into it. They aren't
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                 * playing, because we own the spinlock in
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                 * the wait_queue_head.
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                 */
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                if (!atomic_add_negative(sleepers - 1, &sem->count)) {
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                        sem->sleepers = 0;
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                        break;
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                }
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                sem->sleepers = 1;      /* us - see -1 above */
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                spin_unlock_irqrestore(&sem->wait.lock, flags);
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                schedule();
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                spin_lock_irqsave(&sem->wait.lock, flags);
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                tsk->state = TASK_UNINTERRUPTIBLE;
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        }
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        remove_wait_queue_locked(&sem->wait, &wait);
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        wake_up_locked(&sem->wait);
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        spin_unlock_irqrestore(&sem->wait.lock, flags);
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        tsk->state = TASK_RUNNING;
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}
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fastcall int __sched __down_interruptible(struct semaphore * sem)
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{
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        int retval = 0;
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        struct task_struct *tsk = current;
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        DECLARE_WAITQUEUE(wait, tsk);
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        unsigned long flags;
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        tsk->state = TASK_INTERRUPTIBLE;
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        spin_lock_irqsave(&sem->wait.lock, flags);
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        add_wait_queue_exclusive_locked(&sem->wait, &wait);
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        sem->sleepers++;
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        for (;;) {
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                int sleepers = sem->sleepers;
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                /*
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                 * With signals pending, this turns into
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                 * the trylock failure case - we won't be
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                 * sleeping, and we* can't get the lock as
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                 * it has contention. Just correct the count
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                 * and exit.
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                 */
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                if (signal_pending(current)) {
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                        retval = -EINTR;
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                        sem->sleepers = 0;
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                        atomic_add(sleepers, &sem->count);
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                        break;
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                }
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                /*
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                 * Add "everybody else" into it. They aren't
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                 * playing, because we own the spinlock in
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                 * wait_queue_head. The "-1" is because we're
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                 * still hoping to get the semaphore.
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                 */
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                if (!atomic_add_negative(sleepers - 1, &sem->count)) {
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                        sem->sleepers = 0;
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                        break;
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                }
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                sem->sleepers = 1;      /* us - see -1 above */
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                spin_unlock_irqrestore(&sem->wait.lock, flags);
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                schedule();
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                spin_lock_irqsave(&sem->wait.lock, flags);
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                tsk->state = TASK_INTERRUPTIBLE;
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        }
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        remove_wait_queue_locked(&sem->wait, &wait);
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        wake_up_locked(&sem->wait);
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        spin_unlock_irqrestore(&sem->wait.lock, flags);
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        tsk->state = TASK_RUNNING;
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        return retval;
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}
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/*
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 * Trylock failed - make sure we correct for
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 * having decremented the count.
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 *
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 * We could have done the trylock with a
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 * single "cmpxchg" without failure cases,
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 * but then it wouldn't work on a 386.
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 */
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fastcall int __down_trylock(struct semaphore * sem)
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{
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        int sleepers;
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        unsigned long flags;
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        spin_lock_irqsave(&sem->wait.lock, flags);
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        sleepers = sem->sleepers + 1;
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        sem->sleepers = 0;
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        /*
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         * Add "everybody else" and us into it. They aren't
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         * playing, because we own the spinlock in the
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         * wait_queue_head.
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         */
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        if (!atomic_add_negative(sleepers, &sem->count)) {
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                wake_up_locked(&sem->wait);
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        }
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        spin_unlock_irqrestore(&sem->wait.lock, flags);
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        return 1;
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}

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