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