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/*
* IA-64 semaphore implementation (derived from x86 version).
*
* Copyright (C) 1999-2000 Hewlett-Packard Co
* Copyright (C) 1999-2000 David Mosberger-Tang <davidm@hpl.hp.com>
*/
/*
* Semaphores are implemented using a two-way counter: The "count"
* variable is decremented for each process that tries to acquire the
* semaphore, while the "sleepers" 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.
*
* "sleepers" 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.
*/
#include <linux/sched.h>
#include <asm/semaphore.h>
/*
* 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 "sleepers" 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.
*/
int
__down_trylock (struct semaphore *sem)
{
unsigned long flags;
int sleepers;
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;
}
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