arch/avr32/kernel/semaphore.c

   1 /*
   2  * AVR32 sempahore implementation.
   3  *
   4  * Copyright (C) 2004-2006 Atmel Corporation
   5  *
   6  * Based on linux/arch/i386/kernel/semaphore.c
   7  *  Copyright (C) 1999 Linus Torvalds
   8  *
   9  * This program is free software; you can redistribute it and/or modify
  10  * it under the terms of the GNU General Public License version 2 as
  11  * published by the Free Software Foundation.
  12  */
  13
  14 #include <linux/sched.h>
  15 #include <linux/errno.h>
  16 #include <linux/module.h>
  17
  18 #include <asm/semaphore.h>
  19 #include <asm/atomic.h>
  20
  21 /*
  22  * Semaphores are implemented using a two-way counter:
  23  * The "count" variable is decremented for each process
  24  * that tries to acquire the semaphore, while the "sleeping"
  25  * variable is a count of such acquires.
  26  *
  27  * Notably, the inline "up()" and "down()" functions can
  28  * efficiently test if they need to do any extra work (up
  29  * needs to do something only if count was negative before
  30  * the increment operation.
  31  *
  32  * "sleeping" and the contention routine ordering is protected
  33  * by the spinlock in the semaphore's waitqueue head.
  34  *
  35  * Note that these functions are only called when there is
  36  * contention on the lock, and as such all this is the
  37  * "non-critical" part of the whole semaphore business. The
  38  * critical part is the inline stuff in <asm/semaphore.h>
  39  * where we want to avoid any extra jumps and calls.
  40  */
  41
  42 /*
  43  * Logic:
  44  *  - only on a boundary condition do we need to care. When we go
  45  *    from a negative count to a non-negative, we wake people up.
  46  *  - when we go from a non-negative count to a negative do we
  47  *    (a) synchronize with the "sleeper" count and (b) make sure
  48  *    that we're on the wakeup list before we synchronize so that
  49  *    we cannot lose wakeup events.
  50  */
  51
  52 void __up(struct semaphore *sem)
  53 {
  54         wake_up(&sem->wait);
  55 }
  56 EXPORT_SYMBOL(__up);
  57
  58 void __sched __down(struct semaphore *sem)
  59 {
  60         struct task_struct *tsk = current;
  61         DECLARE_WAITQUEUE(wait, tsk);
  62         unsigned long flags;
  63
  64         tsk->state = TASK_UNINTERRUPTIBLE;
  65         spin_lock_irqsave(&sem->wait.lock, flags);
  66         add_wait_queue_exclusive_locked(&sem->wait, &wait);
  67
  68         sem->sleepers++;
  69         for (;;) {
  70                 int sleepers = sem->sleepers;
  71
  72                 /*
  73                  * Add "everybody else" into it. They aren't
  74                  * playing, because we own the spinlock in
  75                  * the wait_queue_head.
  76                  */
  77                 if (atomic_add_return(sleepers - 1, &sem->count) >= 0) {
  78                         sem->sleepers = 0;
  79                         break;
  80                 }
  81                 sem->sleepers = 1;      /* us - see -1 above */
  82                 spin_unlock_irqrestore(&sem->wait.lock, flags);
  83
  84                 schedule();
  85
  86                 spin_lock_irqsave(&sem->wait.lock, flags);
  87                 tsk->state = TASK_UNINTERRUPTIBLE;
  88         }
  89         remove_wait_queue_locked(&sem->wait, &wait);
  90         wake_up_locked(&sem->wait);
  91         spin_unlock_irqrestore(&sem->wait.lock, flags);
  92         tsk->state = TASK_RUNNING;
  93 }
  94 EXPORT_SYMBOL(__down);
  95
  96 int __sched __down_interruptible(struct semaphore *sem)
  97 {
  98         int retval = 0;
  99         struct task_struct *tsk = current;
 100         DECLARE_WAITQUEUE(wait, tsk);
 101         unsigned long flags;
 102
 103         tsk->state = TASK_INTERRUPTIBLE;
 104         spin_lock_irqsave(&sem->wait.lock, flags);
 105         add_wait_queue_exclusive_locked(&sem->wait, &wait);
 106
 107         sem->sleepers++;
 108         for (;;) {
 109                 int sleepers = sem->sleepers;
 110
 111                 /*
 112                  * With signals pending, this turns into the trylock
 113                  * failure case - we won't be sleeping, and we can't
 114                  * get the lock as it has contention. Just correct the
 115                  * count and exit.
 116                  */
 117                 if (signal_pending(current)) {
 118                         retval = -EINTR;
 119                         sem->sleepers = 0;
 120                         atomic_add(sleepers, &sem->count);
 121                         break;
 122                 }
 123
 124                 /*
 125                  * Add "everybody else" into it. They aren't
 126                  * playing, because we own the spinlock in
 127                  * the wait_queue_head.
 128                  */
 129                 if (atomic_add_return(sleepers - 1, &sem->count) >= 0) {
 130                         sem->sleepers = 0;
 131                         break;
 132                 }
 133                 sem->sleepers = 1;      /* us - see -1 above */
 134                 spin_unlock_irqrestore(&sem->wait.lock, flags);
 135
 136                 schedule();
 137
 138                 spin_lock_irqsave(&sem->wait.lock, flags);
 139                 tsk->state = TASK_INTERRUPTIBLE;
 140         }
 141         remove_wait_queue_locked(&sem->wait, &wait);
 142         wake_up_locked(&sem->wait);
 143         spin_unlock_irqrestore(&sem->wait.lock, flags);
 144
 145         tsk->state = TASK_RUNNING;
 146         return retval;
 147 }
 148 EXPORT_SYMBOL(__down_interruptible);