103 lines
2.4 KiB
C
103 lines
2.4 KiB
C
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/*
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* Semaphore implementation Copyright (c) 2001 Matthew Wilcox, Hewlett-Packard
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*/
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#include <linux/sched.h>
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#include <linux/spinlock.h>
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#include <linux/errno.h>
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#include <linux/init.h>
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/*
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* Semaphores are complex as we wish to avoid using two variables.
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* `count' has multiple roles, depending on its value. If it is positive
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* or zero, there are no waiters. The functions here will never be
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* called; see <asm/semaphore.h>
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*
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* When count is -1 it indicates there is at least one task waiting
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* for the semaphore.
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*
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* When count is less than that, there are '- count - 1' wakeups
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* pending. ie if it has value -3, there are 2 wakeups pending.
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*
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* Note that these functions are only called when there is contention
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* on the lock, and as such all this is the "non-critical" part of the
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* whole semaphore business. The critical part is the inline stuff in
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* <asm/semaphore.h> where we want to avoid any extra jumps and calls.
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*/
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void __up(struct semaphore *sem)
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{
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sem->count--;
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wake_up(&sem->wait);
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}
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#define wakers(count) (-1 - count)
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#define DOWN_HEAD \
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int ret = 0; \
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DECLARE_WAITQUEUE(wait, current); \
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\
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/* Note that someone is waiting */ \
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if (sem->count == 0) \
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sem->count = -1; \
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\
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/* protected by the sentry still -- use unlocked version */ \
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wait.flags = WQ_FLAG_EXCLUSIVE; \
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__add_wait_queue_tail(&sem->wait, &wait); \
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lost_race: \
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spin_unlock_irq(&sem->sentry); \
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#define DOWN_TAIL \
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spin_lock_irq(&sem->sentry); \
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if (wakers(sem->count) == 0 && ret == 0) \
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goto lost_race; /* Someone stole our wakeup */ \
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__remove_wait_queue(&sem->wait, &wait); \
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current->state = TASK_RUNNING; \
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if (!waitqueue_active(&sem->wait) && (sem->count < 0)) \
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sem->count = wakers(sem->count);
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#define UPDATE_COUNT \
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sem->count += (sem->count < 0) ? 1 : - 1;
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void __sched __down(struct semaphore * sem)
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{
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DOWN_HEAD
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for(;;) {
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set_task_state(current, TASK_UNINTERRUPTIBLE);
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/* we can _read_ this without the sentry */
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if (sem->count != -1)
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break;
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schedule();
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}
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DOWN_TAIL
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UPDATE_COUNT
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}
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int __sched __down_interruptible(struct semaphore * sem)
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{
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DOWN_HEAD
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for(;;) {
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set_task_state(current, TASK_INTERRUPTIBLE);
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/* we can _read_ this without the sentry */
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if (sem->count != -1)
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break;
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if (signal_pending(current)) {
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ret = -EINTR;
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break;
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}
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schedule();
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}
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DOWN_TAIL
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if (!ret) {
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UPDATE_COUNT
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}
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return ret;
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}
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