/*

   * i386 and x86-64 semaphore implementation.

   *
   * (C) Copyright 1999 Linus Torvalds
   *
   * Portions Copyright 1999 Red Hat, Inc.
   *
   *	This program is free software; you can redistribute it and/or
   *	modify it under the terms of the GNU General Public License
   *	as published by the Free Software Foundation; either version
   *	2 of the License, or (at your option) any later version.
   *
   * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
   */

  #include <linux/sched.h>

  #include <linux/err.h>

  #include <linux/init.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 spinlock in the semaphore's waitqueue head.
   *
   * 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.
   */

  fastcall void __up(struct semaphore *sem)

  {
  	wake_up(&sem->wait);
  }

  fastcall void __sched __down(struct semaphore * sem)

  {
  	struct task_struct *tsk = current;
  	DECLARE_WAITQUEUE(wait, tsk);
  	unsigned long flags;
  
  	tsk->state = TASK_UNINTERRUPTIBLE;
  	spin_lock_irqsave(&sem->wait.lock, flags);
  	add_wait_queue_exclusive_locked(&sem->wait, &wait);
  
  	sem->sleepers++;
  	for (;;) {
  		int sleepers = sem->sleepers;
  
  		/*
  		 * Add "everybody else" into it. They aren't
  		 * playing, because we own the spinlock in
  		 * the wait_queue_head.
  		 */
  		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  			sem->sleepers = 0;
  			break;
  		}
  		sem->sleepers = 1;	/* us - see -1 above */
  		spin_unlock_irqrestore(&sem->wait.lock, flags);
  
  		schedule();
  
  		spin_lock_irqsave(&sem->wait.lock, flags);
  		tsk->state = TASK_UNINTERRUPTIBLE;
  	}
  	remove_wait_queue_locked(&sem->wait, &wait);
  	wake_up_locked(&sem->wait);
  	spin_unlock_irqrestore(&sem->wait.lock, flags);
  	tsk->state = TASK_RUNNING;
  }

  fastcall int __sched __down_interruptible(struct semaphore * sem)

  {
  	int retval = 0;
  	struct task_struct *tsk = current;
  	DECLARE_WAITQUEUE(wait, tsk);
  	unsigned long flags;
  
  	tsk->state = TASK_INTERRUPTIBLE;
  	spin_lock_irqsave(&sem->wait.lock, flags);
  	add_wait_queue_exclusive_locked(&sem->wait, &wait);
  
  	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 in
  		 * wait_queue_head. The "-1" is because we're
  		 * still hoping to get the semaphore.
  		 */
  		if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  			sem->sleepers = 0;
  			break;
  		}
  		sem->sleepers = 1;	/* us - see -1 above */
  		spin_unlock_irqrestore(&sem->wait.lock, flags);
  
  		schedule();
  
  		spin_lock_irqsave(&sem->wait.lock, flags);
  		tsk->state = TASK_INTERRUPTIBLE;
  	}
  	remove_wait_queue_locked(&sem->wait, &wait);
  	wake_up_locked(&sem->wait);
  	spin_unlock_irqrestore(&sem->wait.lock, flags);
  
  	tsk->state = TASK_RUNNING;
  	return retval;
  }
  
  /*
   * Trylock failed - make sure we correct for
   * having decremented the count.
   *
   * We could have done the trylock with a
   * single "cmpxchg" without failure cases,
   * but then it wouldn't work on a 386.
   */

  fastcall int __down_trylock(struct semaphore * sem)

  {
  	int sleepers;
  	unsigned long flags;
  
  	spin_lock_irqsave(&sem->wait.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 in the
  	 * wait_queue_head.
  	 */
  	if (!atomic_add_negative(sleepers, &sem->count)) {
  		wake_up_locked(&sem->wait);
  	}
  
  	spin_unlock_irqrestore(&sem->wait.lock, flags);
  	return 1;
  }