arch/ia64/kernel/semaphore.c

   1 /*
   2  * IA-64 semaphore implementation (derived from x86 version).
   3  *
   4  * Copyright (C) 1999-2000, 2002 Hewlett-Packard Co
   5  *      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  * "sleeping" and the contention routine ordering is protected
  19  * by the spinlock in the semaphore's waitqueue head.
  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 #include <linux/init.h>
  28
  29 #include <asm/errno.h>
  30 #include <asm/semaphore.h>
  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 "sleepers" 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
  43 __up (struct semaphore *sem)
  44 {
  45         wake_up(&sem->wait);
  46 }
  47
  48 void __sched __down (struct semaphore *sem)
  49 {
  50         struct task_struct *tsk = current;
  51         DECLARE_WAITQUEUE(wait, tsk);
  52         unsigned long flags;
  53
  54         tsk->state = TASK_UNINTERRUPTIBLE;
  55         spin_lock_irqsave(&sem->wait.lock, flags);
  56         add_wait_queue_exclusive_locked(&sem->wait, &wait);
  57
  58         sem->sleepers++;
  59         for (;;) {
  60                 int sleepers = sem->sleepers;
  61
  62                 /*
  63                  * Add "everybody else" into it. They aren't
  64                  * playing, because we own the spinlock in
  65                  * the wait_queue_head.
  66                  */
  67                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  68                         sem->sleepers = 0;
  69                         break;
  70                 }
  71                 sem->sleepers = 1;      /* us - see -1 above */
  72                 spin_unlock_irqrestore(&sem->wait.lock, flags);
  73
  74                 schedule();
  75
  76                 spin_lock_irqsave(&sem->wait.lock, flags);
  77                 tsk->state = TASK_UNINTERRUPTIBLE;
  78         }
  79         remove_wait_queue_locked(&sem->wait, &wait);
  80         wake_up_locked(&sem->wait);
  81         spin_unlock_irqrestore(&sem->wait.lock, flags);
  82         tsk->state = TASK_RUNNING;
  83 }
  84
  85 int __sched __down_interruptible (struct semaphore * sem)
  86 {
  87         int retval = 0;
  88         struct task_struct *tsk = current;
  89         DECLARE_WAITQUEUE(wait, tsk);
  90         unsigned long flags;
  91
  92         tsk->state = TASK_INTERRUPTIBLE;
  93         spin_lock_irqsave(&sem->wait.lock, flags);
  94         add_wait_queue_exclusive_locked(&sem->wait, &wait);
  95
  96         sem->sleepers ++;
  97         for (;;) {
  98                 int sleepers = sem->sleepers;
  99
 100                 /*
 101                  * With signals pending, this turns into
 102                  * the trylock failure case - we won't be
 103                  * sleeping, and we* can't get the lock as
 104                  * it has contention. Just correct the count
 105                  * and exit.
 106                  */
 107                 if (signal_pending(current)) {
 108                         retval = -EINTR;
 109                         sem->sleepers = 0;
 110                         atomic_add(sleepers, &sem->count);
 111                         break;
 112                 }
 113
 114                 /*
 115                  * Add "everybody else" into it. They aren't
 116                  * playing, because we own the spinlock in
 117                  * wait_queue_head. The "-1" is because we're
 118                  * still hoping to get the semaphore.
 119                  */
 120                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 121                         sem->sleepers = 0;
 122                         break;
 123                 }
 124                 sem->sleepers = 1;      /* us - see -1 above */
 125                 spin_unlock_irqrestore(&sem->wait.lock, flags);
 126
 127                 schedule();
 128
 129                 spin_lock_irqsave(&sem->wait.lock, flags);
 130                 tsk->state = TASK_INTERRUPTIBLE;
 131         }
 132         remove_wait_queue_locked(&sem->wait, &wait);
 133         wake_up_locked(&sem->wait);
 134         spin_unlock_irqrestore(&sem->wait.lock, flags);
 135
 136         tsk->state = TASK_RUNNING;
 137         return retval;
 138 }
 139
 140 /*
 141  * Trylock failed - make sure we correct for having decremented the
 142  * count.
 143  */
 144 int
 145 __down_trylock (struct semaphore *sem)
 146 {
 147         unsigned long flags;
 148         int sleepers;
 149
 150         spin_lock_irqsave(&sem->wait.lock, flags);
 151         sleepers = sem->sleepers + 1;
 152         sem->sleepers = 0;
 153
 154         /*
 155          * Add "everybody else" and us into it. They aren't
 156          * playing, because we own the spinlock in the
 157          * wait_queue_head.
 158          */
 159         if (!atomic_add_negative(sleepers, &sem->count)) {
 160                 wake_up_locked(&sem->wait);
 161         }
 162
 163         spin_unlock_irqrestore(&sem->wait.lock, flags);
 164         return 1;
 165 }