149 lines
4.6 KiB
C
149 lines
4.6 KiB
C
/*
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* AVR32 sempahore implementation.
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*
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* Copyright (C) 2004-2006 Atmel Corporation
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*
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* Based on linux/arch/i386/kernel/semaphore.c
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* Copyright (C) 1999 Linus Torvalds
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*/
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#include <linux/sched.h>
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#include <linux/errno.h>
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#include <linux/module.h>
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#include <asm/semaphore.h>
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#include <asm/atomic.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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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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EXPORT_SYMBOL(__up);
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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_return(sleepers - 1, &sem->count) >= 0) {
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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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EXPORT_SYMBOL(__down);
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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 the trylock
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* failure case - we won't be sleeping, and we can't
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* get the lock as it has contention. Just correct the
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* count 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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* the wait_queue_head.
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*/
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if (atomic_add_return(sleepers - 1, &sem->count) >= 0) {
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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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EXPORT_SYMBOL(__down_interruptible);
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