arch/s390/kernel/semaphore.c

   1 /*
   2  *  linux/arch/S390/kernel/semaphore.c
   3  *
   4  *  S390 version
   5  *    Copyright (C) 1998-2000 IBM Corporation
   6  *    Author(s): Martin Schwidefsky
   7  *
   8  *  Derived from "linux/arch/i386/kernel/semaphore.c
   9  *    Copyright (C) 1999, Linus Torvalds
  10  *
  11  */
  12 #include <linux/sched.h>
  13
  14 #include <asm/semaphore.h>
  15
  16 /*
  17  * Semaphores are implemented using a two-way counter:
  18  * The "count" variable is decremented for each process
  19  * that tries to acquire the semaphore, while the "sleeping"
  20  * variable is a count of such acquires.
  21  *
  22  * Notably, the inline "up()" and "down()" functions can
  23  * efficiently test if they need to do any extra work (up
  24  * needs to do something only if count was negative before
  25  * the increment operation.
  26  *
  27  * "sleeping" and the contention routine ordering is
  28  * protected by the semaphore spinlock.
  29  *
  30  * Note that these functions are only called when there is
  31  * contention on the lock, and as such all this is the
  32  * "non-critical" part of the whole semaphore business. The
  33  * critical part is the inline stuff in <asm/semaphore.h>
  34  * where we want to avoid any extra jumps and calls.
  35  */
  36
  37 /*
  38  * Logic:
  39  *  - only on a boundary condition do we need to care. When we go
  40  *    from a negative count to a non-negative, we wake people up.
  41  *  - when we go from a non-negative count to a negative do we
  42  *    (a) synchronize with the "sleeper" count and (b) make sure
  43  *    that we're on the wakeup list before we synchronize so that
  44  *    we cannot lose wakeup events.
  45  */
  46
  47 void __up(struct semaphore *sem)
  48 {
  49         wake_up(&sem->wait);
  50 }
  51
  52 static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
  53
  54 void __down(struct semaphore * sem)
  55 {
  56         struct task_struct *tsk = current;
  57         DECLARE_WAITQUEUE(wait, tsk);
  58         tsk->state = TASK_UNINTERRUPTIBLE;
  59         add_wait_queue_exclusive(&sem->wait, &wait);
  60
  61         spin_lock_irq(&semaphore_lock);
  62         sem->sleepers++;
  63         for (;;) {
  64                 int sleepers = sem->sleepers;
  65
  66                 /*
  67                  * Add "everybody else" into it. They aren't
  68                  * playing, because we own the spinlock.
  69                  */
  70                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  71                         sem->sleepers = 0;
  72                         break;
  73                 }
  74                 sem->sleepers = 1;      /* us - see -1 above */
  75                 spin_unlock_irq(&semaphore_lock);
  76
  77                 schedule();
  78                 tsk->state = TASK_UNINTERRUPTIBLE;
  79                 spin_lock_irq(&semaphore_lock);
  80         }
  81         spin_unlock_irq(&semaphore_lock);
  82         remove_wait_queue(&sem->wait, &wait);
  83         tsk->state = TASK_RUNNING;
  84         wake_up(&sem->wait);
  85 }
  86
  87 int __down_interruptible(struct semaphore * sem)
  88 {
  89         int retval = 0;
  90         struct task_struct *tsk = current;
  91         DECLARE_WAITQUEUE(wait, tsk);
  92         tsk->state = TASK_INTERRUPTIBLE;
  93         add_wait_queue_exclusive(&sem->wait, &wait);
  94
  95         spin_lock_irq(&semaphore_lock);
  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. The
 117                  * "-1" is because we're still hoping to get
 118                  * the lock.
 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_irq(&semaphore_lock);
 126
 127                 schedule();
 128                 tsk->state = TASK_INTERRUPTIBLE;
 129                 spin_lock_irq(&semaphore_lock);
 130         }
 131         spin_unlock_irq(&semaphore_lock);
 132         tsk->state = TASK_RUNNING;
 133         remove_wait_queue(&sem->wait, &wait);
 134         wake_up(&sem->wait);
 135         return retval;
 136 }
 137
 138 /*
 139  * Trylock failed - make sure we correct for
 140  * having decremented the count.
 141  */
 142 int __down_trylock(struct semaphore * sem)
 143 {
 144         unsigned long flags;
 145         int sleepers;
 146
 147         spin_lock_irqsave(&semaphore_lock, flags);
 148         sleepers = sem->sleepers + 1;
 149         sem->sleepers = 0;
 150
 151         /*
 152          * Add "everybody else" and us into it. They aren't
 153          * playing, because we own the spinlock.
 154          */
 155         if (!atomic_add_negative(sleepers, &sem->count))
 156                 wake_up(&sem->wait);
 157
 158         spin_unlock_irqrestore(&semaphore_lock, flags);
 159         return 1;
 160 }