arch/m32r/kernel/semaphore.c

   1 /*
   2  *  linux/arch/m32r/semaphore.c
   3  *    orig : i386 2.6.4
   4  *
   5  *  M32R semaphore implementation.
   6  *
   7  *      Copyright (c) 2002 - 2004 Hitoshi Yamamoto
   8  */
   9
  10 /*
  11  * i386 semaphore implementation.
  12  *
  13  * (C) Copyright 1999 Linus Torvalds
  14  *
  15  * Portions Copyright 1999 Red Hat, Inc.
  16  *
  17  *      This program is free software; you can redistribute it and/or
  18  *      modify it under the terms of the GNU General Public License
  19  *      as published by the Free Software Foundation; either version
  20  *      2 of the License, or (at your option) any later version.
  21  *
  22  * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
  23  */
  24 #include <linux/sched.h>
  25 #include <linux/err.h>
  26 #include <linux/init.h>
  27 #include <asm/semaphore.h>
  28
  29 /*
  30  * Semaphores are implemented using a two-way counter:
  31  * The "count" variable is decremented for each process
  32  * that tries to acquire the semaphore, while the "sleeping"
  33  * variable is a count of such acquires.
  34  *
  35  * Notably, the inline "up()" and "down()" functions can
  36  * efficiently test if they need to do any extra work (up
  37  * needs to do something only if count was negative before
  38  * the increment operation.
  39  *
  40  * "sleeping" and the contention routine ordering is protected
  41  * by the spinlock in the semaphore's waitqueue head.
  42  *
  43  * Note that these functions are only called when there is
  44  * contention on the lock, and as such all this is the
  45  * "non-critical" part of the whole semaphore business. The
  46  * critical part is the inline stuff in <asm/semaphore.h>
  47  * where we want to avoid any extra jumps and calls.
  48  */
  49
  50 /*
  51  * Logic:
  52  *  - only on a boundary condition do we need to care. When we go
  53  *    from a negative count to a non-negative, we wake people up.
  54  *  - when we go from a non-negative count to a negative do we
  55  *    (a) synchronize with the "sleeper" count and (b) make sure
  56  *    that we're on the wakeup list before we synchronize so that
  57  *    we cannot lose wakeup events.
  58  */
  59
  60 asmlinkage void __up(struct semaphore *sem)
  61 {
  62         wake_up(&sem->wait);
  63 }
  64
  65 asmlinkage void __sched __down(struct semaphore * sem)
  66 {
  67         struct task_struct *tsk = current;
  68         DECLARE_WAITQUEUE(wait, tsk);
  69         unsigned long flags;
  70
  71         tsk->state = TASK_UNINTERRUPTIBLE;
  72         spin_lock_irqsave(&sem->wait.lock, flags);
  73         add_wait_queue_exclusive_locked(&sem->wait, &wait);
  74
  75         sem->sleepers++;
  76         for (;;) {
  77                 int sleepers = sem->sleepers;
  78
  79                 /*
  80                  * Add "everybody else" into it. They aren't
  81                  * playing, because we own the spinlock in
  82                  * the wait_queue_head.
  83                  */
  84                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  85                         sem->sleepers = 0;
  86                         break;
  87                 }
  88                 sem->sleepers = 1;      /* us - see -1 above */
  89                 spin_unlock_irqrestore(&sem->wait.lock, flags);
  90
  91                 schedule();
  92
  93                 spin_lock_irqsave(&sem->wait.lock, flags);
  94                 tsk->state = TASK_UNINTERRUPTIBLE;
  95         }
  96         remove_wait_queue_locked(&sem->wait, &wait);
  97         wake_up_locked(&sem->wait);
  98         spin_unlock_irqrestore(&sem->wait.lock, flags);
  99         tsk->state = TASK_RUNNING;
 100 }
 101
 102 asmlinkage int __sched __down_interruptible(struct semaphore * sem)
 103 {
 104         int retval = 0;
 105         struct task_struct *tsk = current;
 106         DECLARE_WAITQUEUE(wait, tsk);
 107         unsigned long flags;
 108
 109         tsk->state = TASK_INTERRUPTIBLE;
 110         spin_lock_irqsave(&sem->wait.lock, flags);
 111         add_wait_queue_exclusive_locked(&sem->wait, &wait);
 112
 113         sem->sleepers++;
 114         for (;;) {
 115                 int sleepers = sem->sleepers;
 116
 117                 /*
 118                  * With signals pending, this turns into
 119                  * the trylock failure case - we won't be
 120                  * sleeping, and we* can't get the lock as
 121                  * it has contention. Just correct the count
 122                  * and exit.
 123                  */
 124                 if (signal_pending(current)) {
 125                         retval = -EINTR;
 126                         sem->sleepers = 0;
 127                         atomic_add(sleepers, &sem->count);
 128                         break;
 129                 }
 130
 131                 /*
 132                  * Add "everybody else" into it. They aren't
 133                  * playing, because we own the spinlock in
 134                  * wait_queue_head. The "-1" is because we're
 135                  * still hoping to get the semaphore.
 136                  */
 137                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 138                         sem->sleepers = 0;
 139                         break;
 140                 }
 141                 sem->sleepers = 1;      /* us - see -1 above */
 142                 spin_unlock_irqrestore(&sem->wait.lock, flags);
 143
 144                 schedule();
 145
 146                 spin_lock_irqsave(&sem->wait.lock, flags);
 147                 tsk->state = TASK_INTERRUPTIBLE;
 148         }
 149         remove_wait_queue_locked(&sem->wait, &wait);
 150         wake_up_locked(&sem->wait);
 151         spin_unlock_irqrestore(&sem->wait.lock, flags);
 152
 153         tsk->state = TASK_RUNNING;
 154         return retval;
 155 }
 156
 157 /*
 158  * Trylock failed - make sure we correct for
 159  * having decremented the count.
 160  *
 161  * We could have done the trylock with a
 162  * single "cmpxchg" without failure cases,
 163  * but then it wouldn't work on a 386.
 164  */
 165 asmlinkage int __down_trylock(struct semaphore * sem)
 166 {
 167         int sleepers;
 168         unsigned long flags;
 169
 170         spin_lock_irqsave(&sem->wait.lock, flags);
 171         sleepers = sem->sleepers + 1;
 172         sem->sleepers = 0;
 173
 174         /*
 175          * Add "everybody else" and us into it. They aren't
 176          * playing, because we own the spinlock in the
 177          * wait_queue_head.
 178          */
 179         if (!atomic_add_negative(sleepers, &sem->count)) {
 180                 wake_up_locked(&sem->wait);
 181         }
 182
 183         spin_unlock_irqrestore(&sem->wait.lock, flags);
 184         return 1;
 185 }