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@redhat.com>
  23  */
  24 #include <linux/config.h>
  25 #include <linux/sched.h>
  26 #include <linux/err.h>
  27 #include <linux/init.h>
  28 #include <asm/semaphore.h>
  29
  30 /*
  31  * Semaphores are implemented using a two-way counter:
  32  * The "count" variable is decremented for each process
  33  * that tries to acquire the semaphore, while the "sleeping"
  34  * variable is a count of such acquires.
  35  *
  36  * Notably, the inline "up()" and "down()" functions can
  37  * efficiently test if they need to do any extra work (up
  38  * needs to do something only if count was negative before
  39  * the increment operation.
  40  *
  41  * "sleeping" and the contention routine ordering is protected
  42  * by the spinlock in the semaphore's waitqueue head.
  43  *
  44  * Note that these functions are only called when there is
  45  * contention on the lock, and as such all this is the
  46  * "non-critical" part of the whole semaphore business. The
  47  * critical part is the inline stuff in <asm/semaphore.h>
  48  * where we want to avoid any extra jumps and calls.
  49  */
  50
  51 /*
  52  * Logic:
  53  *  - only on a boundary condition do we need to care. When we go
  54  *    from a negative count to a non-negative, we wake people up.
  55  *  - when we go from a non-negative count to a negative do we
  56  *    (a) synchronize with the "sleeper" count and (b) make sure
  57  *    that we're on the wakeup list before we synchronize so that
  58  *    we cannot lose wakeup events.
  59  */
  60
  61 asmlinkage void __up(struct semaphore *sem)
  62 {
  63         wake_up(&sem->wait);
  64 }
  65
  66 asmlinkage void __sched __down(struct semaphore * sem)
  67 {
  68         struct task_struct *tsk = current;
  69         DECLARE_WAITQUEUE(wait, tsk);
  70         unsigned long flags;
  71
  72         tsk->state = TASK_UNINTERRUPTIBLE;
  73         spin_lock_irqsave(&sem->wait.lock, flags);
  74         add_wait_queue_exclusive_locked(&sem->wait, &wait);
  75
  76         sem->sleepers++;
  77         for (;;) {
  78                 int sleepers = sem->sleepers;
  79
  80                 /*
  81                  * Add "everybody else" into it. They aren't
  82                  * playing, because we own the spinlock in
  83                  * the wait_queue_head.
  84                  */
  85                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  86                         sem->sleepers = 0;
  87                         break;
  88                 }
  89                 sem->sleepers = 1;      /* us - see -1 above */
  90                 spin_unlock_irqrestore(&sem->wait.lock, flags);
  91
  92                 schedule();
  93
  94                 spin_lock_irqsave(&sem->wait.lock, flags);
  95                 tsk->state = TASK_UNINTERRUPTIBLE;
  96         }
  97         remove_wait_queue_locked(&sem->wait, &wait);
  98         wake_up_locked(&sem->wait);
  99         spin_unlock_irqrestore(&sem->wait.lock, flags);
 100         tsk->state = TASK_RUNNING;
 101 }
 102
 103 asmlinkage int __sched __down_interruptible(struct semaphore * sem)
 104 {
 105         int retval = 0;
 106         struct task_struct *tsk = current;
 107         DECLARE_WAITQUEUE(wait, tsk);
 108         unsigned long flags;
 109
 110         tsk->state = TASK_INTERRUPTIBLE;
 111         spin_lock_irqsave(&sem->wait.lock, flags);
 112         add_wait_queue_exclusive_locked(&sem->wait, &wait);
 113
 114         sem->sleepers++;
 115         for (;;) {
 116                 int sleepers = sem->sleepers;
 117
 118                 /*
 119                  * With signals pending, this turns into
 120                  * the trylock failure case - we won't be
 121                  * sleeping, and we* can't get the lock as
 122                  * it has contention. Just correct the count
 123                  * and exit.
 124                  */
 125                 if (signal_pending(current)) {
 126                         retval = -EINTR;
 127                         sem->sleepers = 0;
 128                         atomic_add(sleepers, &sem->count);
 129                         break;
 130                 }
 131
 132                 /*
 133                  * Add "everybody else" into it. They aren't
 134                  * playing, because we own the spinlock in
 135                  * wait_queue_head. The "-1" is because we're
 136                  * still hoping to get the semaphore.
 137                  */
 138                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 139                         sem->sleepers = 0;
 140                         break;
 141                 }
 142                 sem->sleepers = 1;      /* us - see -1 above */
 143                 spin_unlock_irqrestore(&sem->wait.lock, flags);
 144
 145                 schedule();
 146
 147                 spin_lock_irqsave(&sem->wait.lock, flags);
 148                 tsk->state = TASK_INTERRUPTIBLE;
 149         }
 150         remove_wait_queue_locked(&sem->wait, &wait);
 151         wake_up_locked(&sem->wait);
 152         spin_unlock_irqrestore(&sem->wait.lock, flags);
 153
 154         tsk->state = TASK_RUNNING;
 155         return retval;
 156 }
 157
 158 /*
 159  * Trylock failed - make sure we correct for
 160  * having decremented the count.
 161  *
 162  * We could have done the trylock with a
 163  * single "cmpxchg" without failure cases,
 164  * but then it wouldn't work on a 386.
 165  */
 166 asmlinkage int __down_trylock(struct semaphore * sem)
 167 {
 168         int sleepers;
 169         unsigned long flags;
 170
 171         spin_lock_irqsave(&sem->wait.lock, flags);
 172         sleepers = sem->sleepers + 1;
 173         sem->sleepers = 0;
 174
 175         /*
 176          * Add "everybody else" and us into it. They aren't
 177          * playing, because we own the spinlock in the
 178          * wait_queue_head.
 179          */
 180         if (!atomic_add_negative(sleepers, &sem->count)) {
 181                 wake_up_locked(&sem->wait);
 182         }
 183
 184         spin_unlock_irqrestore(&sem->wait.lock, flags);
 185         return 1;
 186 }