/* *  linux/arch/S390/kernel/semaphore.c * *  S390 version *    Copyright (C) 1998-2000 IBM Corporation *    Author(s): Martin Schwidefsky * *  Derived from "linux/arch/i386/kernel/semaphore.c *    Copyright (C) 1999, Linus Torvalds * */#include <linux/sched.h>#include <asm/semaphore.h>/* * Semaphores are implemented using a two-way counter: * The "count" variable is decremented for each process * that tries to acquire the semaphore, while the "sleeping" * 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. * * "sleeping" 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. *//* * 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 "sleeper" 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;}