Logo Search packages:      
Sourcecode: linux version File versions  Download package

semaphore.c

/*
 * IA-64 semaphore implementation (derived from x86 version).
 *
 * Copyright (C) 1999-2000, 2002 Hewlett-Packard Co
 *    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.
 *
 * "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.
 */
#include <linux/sched.h>
#include <linux/init.h>

#include <asm/errno.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);
}

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;
}

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.
 */
int
__down_trylock (struct semaphore *sem)
{
      unsigned long flags;
      int sleepers;

      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;
}

Generated by  Doxygen 1.6.0   Back to index