LXR linux/arch/sparc/kernel/semaphore.c

   1/* $Id: semaphore.c,v 1.7 2001/04/18 21:06:05 davem Exp $ */
   2
   3/* sparc32 semaphore implementation, based on i386 version */
   4
   5#include <linux/sched.h>
   6#include <linux/errno.h>
   7#include <linux/init.h>
   8
   9#include <asm/semaphore.h>
  10
  11/*
  12 * Semaphores are implemented using a two-way counter:
  13 * The "count" variable is decremented for each process
  14 * that tries to acquire the semaphore, while the "sleeping"
  15 * variable is a count of such acquires.
  16 *
  17 * Notably, the inline "up()" and "down()" functions can
  18 * efficiently test if they need to do any extra work (up
  19 * needs to do something only if count was negative before
  20 * the increment operation.
  21 *
  22 * "sleeping" and the contention routine ordering is
  23 * protected by the semaphore spinlock.
  24 *
  25 * Note that these functions are only called when there is
  26 * contention on the lock, and as such all this is the
  27 * "non-critical" part of the whole semaphore business. The
  28 * critical part is the inline stuff in <asm/semaphore.h>
  29 * where we want to avoid any extra jumps and calls.
  30 */
  31
  32/*
  33 * Logic:
  34 *  - only on a boundary condition do we need to care. When we go
  35 *    from a negative count to a non-negative, we wake people up.
  36 *  - when we go from a non-negative count to a negative do we
  37 *    (a) synchronize with the "sleeper" count and (b) make sure
  38 *    that we're on the wakeup list before we synchronize so that
  39 *    we cannot lose wakeup events.
  40 */
  41
  42void __up(struct semaphore *sem)
  43{
  44        wake_up(&sem->wait);
  45}
  46
  47static DEFINE_SPINLOCK(semaphore_lock);
  48
  49void __sched __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 (!atomic24_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 __sched __down_interruptible(struct semaphore * sem)
  83{
  84        int retval = 0;
  85        struct task_struct *tsk = current;
  86        DECLARE_WAITQUEUE(wait, tsk);
  87        tsk->state = TASK_INTERRUPTIBLE;
  88        add_wait_queue_exclusive(&sem->wait, &wait);
  89
  90        spin_lock_irq(&semaphore_lock);
  91        sem->sleepers ++;
  92        for (;;) {
  93                int sleepers = sem->sleepers;
  94
  95                /*
  96                 * With signals pending, this turns into
  97                 * the trylock failure case - we won't be
  98                 * sleeping, and we* can't get the lock as
  99                 * it has contention. Just correct the count
 100                 * and exit.
 101                 */
 102                if (signal_pending(current)) {
 103                        retval = -EINTR;
 104                        sem->sleepers = 0;
 105                        atomic24_add(sleepers, &sem->count);
 106                        break;
 107                }
 108
 109                /*
 110                 * Add "everybody else" into it. They aren't
 111                 * playing, because we own the spinlock. The
 112                 * "-1" is because we're still hoping to get
 113                 * the lock.
 114                 */
 115                if (!atomic24_add_negative(sleepers - 1, &sem->count)) {
 116                        sem->sleepers = 0;
 117                        break;
 118                }
 119                sem->sleepers = 1;      /* us - see -1 above */
 120                spin_unlock_irq(&semaphore_lock);
 121
 122                schedule();
 123                tsk->state = TASK_INTERRUPTIBLE;
 124                spin_lock_irq(&semaphore_lock);
 125        }
 126        spin_unlock_irq(&semaphore_lock);
 127        tsk->state = TASK_RUNNING;
 128        remove_wait_queue(&sem->wait, &wait);
 129        wake_up(&sem->wait);
 130        return retval;
 131}
 132
 133/*
 134 * Trylock failed - make sure we correct for
 135 * having decremented the count.
 136 */
 137int __down_trylock(struct semaphore * sem)
 138{
 139        int sleepers;
 140        unsigned long flags;
 141
 142        spin_lock_irqsave(&semaphore_lock, flags);
 143        sleepers = sem->sleepers + 1;
 144        sem->sleepers = 0;
 145
 146        /*
 147         * Add "everybody else" and us into it. They aren't
 148         * playing, because we own the spinlock.
 149         */
 150        if (!atomic24_add_negative(sleepers, &sem->count))
 151                wake_up(&sem->wait);
 152
 153        spin_unlock_irqrestore(&semaphore_lock, flags);
 154        return 1;
 155}
 156