arch/powerpc/kernel/semaphore.c

   1 /*
   2  * PowerPC-specific semaphore code.
   3  *
   4  * Copyright (C) 1999 Cort Dougan <cort@cs.nmt.edu>
   5  *
   6  * This program is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU General Public License
   8  * as published by the Free Software Foundation; either version
   9  * 2 of the License, or (at your option) any later version.
  10  *
  11  * April 2001 - Reworked by Paul Mackerras <paulus@samba.org>
  12  * to eliminate the SMP races in the old version between the updates
  13  * of `count' and `waking'.  Now we use negative `count' values to
  14  * indicate that some process(es) are waiting for the semaphore.
  15  */
  16
  17 #include <linux/sched.h>
  18 #include <linux/init.h>
  19 #include <linux/module.h>
  20
  21 #include <asm/atomic.h>
  22 #include <asm/semaphore.h>
  23 #include <asm/errno.h>
  24
  25 /*
  26  * Atomically update sem->count.
  27  * This does the equivalent of the following:
  28  *
  29  *      old_count = sem->count;
  30  *      tmp = MAX(old_count, 0) + incr;
  31  *      sem->count = tmp;
  32  *      return old_count;
  33  */
  34 static inline int __sem_update_count(struct semaphore *sem, int incr)
  35 {
  36         int old_count, tmp;
  37
  38         __asm__ __volatile__("\n"
  39 "1:     lwarx   %0,0,%3\n"
  40 "       srawi   %1,%0,31\n"
  41 "       andc    %1,%0,%1\n"
  42 "       add     %1,%1,%4\n"
  43         PPC405_ERR77(0,%3)
  44 "       stwcx.  %1,0,%3\n"
  45 "       bne     1b"
  46         : "=&r" (old_count), "=&r" (tmp), "=m" (sem->count)
  47         : "r" (&sem->count), "r" (incr), "m" (sem->count)
  48         : "cc");
  49
  50         return old_count;
  51 }
  52
  53 void __up(struct semaphore *sem)
  54 {
  55         /*
  56          * Note that we incremented count in up() before we came here,
  57          * but that was ineffective since the result was <= 0, and
  58          * any negative value of count is equivalent to 0.
  59          * This ends up setting count to 1, unless count is now > 0
  60          * (i.e. because some other cpu has called up() in the meantime),
  61          * in which case we just increment count.
  62          */
  63         __sem_update_count(sem, 1);
  64         wake_up(&sem->wait);
  65 }
  66 EXPORT_SYMBOL(__up);
  67
  68 /*
  69  * Note that when we come in to __down or __down_interruptible,
  70  * we have already decremented count, but that decrement was
  71  * ineffective since the result was < 0, and any negative value
  72  * of count is equivalent to 0.
  73  * Thus it is only when we decrement count from some value > 0
  74  * that we have actually got the semaphore.
  75  */
  76 void __sched __down(struct semaphore *sem)
  77 {
  78         struct task_struct *tsk = current;
  79         DECLARE_WAITQUEUE(wait, tsk);
  80
  81         __set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  82         add_wait_queue_exclusive(&sem->wait, &wait);
  83
  84         /*
  85          * Try to get the semaphore.  If the count is > 0, then we've
  86          * got the semaphore; we decrement count and exit the loop.
  87          * If the count is 0 or negative, we set it to -1, indicating
  88          * that we are asleep, and then sleep.
  89          */
  90         while (__sem_update_count(sem, -1) <= 0) {
  91                 schedule();
  92                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  93         }
  94         remove_wait_queue(&sem->wait, &wait);
  95         __set_task_state(tsk, TASK_RUNNING);
  96
  97         /*
  98          * If there are any more sleepers, wake one of them up so
  99          * that it can either get the semaphore, or set count to -1
 100          * indicating that there are still processes sleeping.
 101          */
 102         wake_up(&sem->wait);
 103 }
 104 EXPORT_SYMBOL(__down);
 105
 106 int __sched __down_interruptible(struct semaphore * sem)
 107 {
 108         int retval = 0;
 109         struct task_struct *tsk = current;
 110         DECLARE_WAITQUEUE(wait, tsk);
 111
 112         __set_task_state(tsk, TASK_INTERRUPTIBLE);
 113         add_wait_queue_exclusive(&sem->wait, &wait);
 114
 115         while (__sem_update_count(sem, -1) <= 0) {
 116                 if (signal_pending(current)) {
 117                         /*
 118                          * A signal is pending - give up trying.
 119                          * Set sem->count to 0 if it is negative,
 120                          * since we are no longer sleeping.
 121                          */
 122                         __sem_update_count(sem, 0);
 123                         retval = -EINTR;
 124                         break;
 125                 }
 126                 schedule();
 127                 set_task_state(tsk, TASK_INTERRUPTIBLE);
 128         }
 129         remove_wait_queue(&sem->wait, &wait);
 130         __set_task_state(tsk, TASK_RUNNING);
 131
 132         wake_up(&sem->wait);
 133         return retval;
 134 }
 135 EXPORT_SYMBOL(__down_interruptible);