LXR linux-old/arch/ia64/kernel/semaphore.c

   1/*
   2 * IA-64 semaphore implementation (derived from x86 version).
   3 *
   4 * Copyright (C) 1999-2000 Hewlett-Packard Co
   5 * Copyright (C) 1999-2000 David Mosberger-Tang <davidm@hpl.hp.com>
   6 */
   7
   8/*
   9 * Semaphores are implemented using a two-way counter: The "count"
  10 * variable is decremented for each process that tries to acquire the
  11 * semaphore, while the "sleepers" variable is a count of such
  12 * acquires.
  13 *
  14 * Notably, the inline "up()" and "down()" functions can efficiently
  15 * test if they need to do any extra work (up needs to do something
  16 * only if count was negative before the increment operation.
  17 *
  18 * "sleepers" and the contention routine ordering is protected by the
  19 * semaphore spinlock.
  20 *
  21 * Note that these functions are only called when there is contention
  22 * on the lock, and as such all this is the "non-critical" part of the
  23 * whole semaphore business. The critical part is the inline stuff in
  24 * <asm/semaphore.h> where we want to avoid any extra jumps and calls.
  25 */
  26#include <linux/sched.h>
  27
  28#include <asm/semaphore.h>
  29
  30/*
  31 * Logic:
  32 *  - Only on a boundary condition do we need to care. When we go
  33 *    from a negative count to a non-negative, we wake people up.
  34 *  - When we go from a non-negative count to a negative do we
  35 *    (a) synchronize with the "sleepers" count and (b) make sure
  36 *    that we're on the wakeup list before we synchronize so that
  37 *    we cannot lose wakeup events.
  38 */
  39
  40void
  41__up (struct semaphore *sem)
  42{
  43        wake_up(&sem->wait);
  44}
  45
  46static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
  47
  48void
  49__down (struct semaphore *sem)
  50{
  51        struct task_struct *tsk = current;
  52        DECLARE_WAITQUEUE(wait, tsk);
  53        tsk->state = TASK_UNINTERRUPTIBLE;
  54        add_wait_queue_exclusive(&sem->wait, &wait);
  55
  56        spin_lock_irq(&semaphore_lock);
  57        sem->sleepers++;
  58        for (;;) {
  59                int sleepers = sem->sleepers;
  60
  61                /*
  62                 * Add "everybody else" into it. They aren't
  63                 * playing, because we own the spinlock.
  64                 */
  65                if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  66                        sem->sleepers = 0;
  67                        break;
  68                }
  69                sem->sleepers = 1;      /* us - see -1 above */
  70                spin_unlock_irq(&semaphore_lock);
  71
  72                schedule();
  73                tsk->state = TASK_UNINTERRUPTIBLE;
  74                spin_lock_irq(&semaphore_lock);
  75        }
  76        spin_unlock_irq(&semaphore_lock);
  77        remove_wait_queue(&sem->wait, &wait);
  78        tsk->state = TASK_RUNNING;
  79        wake_up(&sem->wait);
  80}
  81
  82int
  83__down_interruptible (struct semaphore * sem)
  84{
  85        int retval = 0;
  86        struct task_struct *tsk = current;
  87        DECLARE_WAITQUEUE(wait, tsk);
  88        tsk->state = TASK_INTERRUPTIBLE;
  89        add_wait_queue_exclusive(&sem->wait, &wait);
  90
  91        spin_lock_irq(&semaphore_lock);
  92        sem->sleepers ++;
  93        for (;;) {
  94                int sleepers = sem->sleepers;
  95
  96                /*
  97                 * With signals pending, this turns into
  98                 * the trylock failure case - we won't be
  99                 * sleeping, and we* can't get the lock as
 100                 * it has contention. Just correct the count
 101                 * and exit.
 102                 */
 103                if (signal_pending(current)) {
 104                        retval = -EINTR;
 105                        sem->sleepers = 0;
 106                        atomic_add(sleepers, &sem->count);
 107                        break;
 108                }
 109
 110                /*
 111                 * Add "everybody else" into it. They aren't
 112                 * playing, because we own the spinlock. The
 113                 * "-1" is because we're still hoping to get
 114                 * the lock.
 115                 */
 116                if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 117                        sem->sleepers = 0;
 118                        break;
 119                }
 120                sem->sleepers = 1;      /* us - see -1 above */
 121                spin_unlock_irq(&semaphore_lock);
 122
 123                schedule();
 124                tsk->state = TASK_INTERRUPTIBLE;
 125                spin_lock_irq(&semaphore_lock);
 126        }
 127        spin_unlock_irq(&semaphore_lock);
 128        tsk->state = TASK_RUNNING;
 129        remove_wait_queue(&sem->wait, &wait);
 130        wake_up(&sem->wait);
 131        return retval;
 132}
 133
 134/*
 135 * Trylock failed - make sure we correct for having decremented the
 136 * count.
 137 */
 138int
 139__down_trylock (struct semaphore *sem)
 140{
 141        unsigned long flags;
 142        int sleepers;
 143
 144        spin_lock_irqsave(&semaphore_lock, flags);
 145        sleepers = sem->sleepers + 1;
 146        sem->sleepers = 0;
 147
 148        /*
 149         * Add "everybody else" and us into it. They aren't
 150         * playing, because we own the spinlock.
 151         */
 152        if (!atomic_add_negative(sleepers, &sem->count))
 153                wake_up(&sem->wait);
 154
 155        spin_unlock_irqrestore(&semaphore_lock, flags);
 156        return 1;
 157}
 158