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