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
  42 void __up(struct semaphore *sem)
  43 {
  44         wake_up(&sem->wait);
  45 }
  46
  47 static DEFINE_SPINLOCK(semaphore_lock);
  48
  49 void __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
  82 int __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  */
 137 int __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 }