arch/mips/kernel/semaphore.c

   1 /*
   2  * MIPS-specific semaphore code.
   3  *
   4  * Copyright (C) 1999 Cort Dougan <cort@cs.nmt.edu>
   5  * Copyright (C) 2004 Ralf Baechle <ralf@linux-mips.org>
   6  *
   7  * This program is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU General Public License
   9  * as published by the Free Software Foundation; either version
  10  * 2 of the License, or (at your option) any later version.
  11  *
  12  * April 2001 - Reworked by Paul Mackerras <paulus@samba.org>
  13  * to eliminate the SMP races in the old version between the updates
  14  * of `count' and `waking'.  Now we use negative `count' values to
  15  * indicate that some process(es) are waiting for the semaphore.
  16  */
  17
  18 #include <linux/module.h>
  19 #include <linux/sched.h>
  20 #include <linux/init.h>
  21 #include <asm/atomic.h>
  22 #include <asm/cpu-features.h>
  23 #include <asm/errno.h>
  24 #include <asm/semaphore.h>
  25 #include <asm/war.h>
  26 /*
  27  * Atomically update sem->count.
  28  * This does the equivalent of the following:
  29  *
  30  *      old_count = sem->count;
  31  *      tmp = MAX(old_count, 0) + incr;
  32  *      sem->count = tmp;
  33  *      return old_count;
  34  *
  35  * On machines without lld/scd we need a spinlock to make the manipulation of
  36  * sem->count and sem->waking atomic.  Scalability isn't an issue because
  37  * this lock is used on UP only so it's just an empty variable.
  38  */
  39 static inline int __sem_update_count(struct semaphore *sem, int incr)
  40 {
  41         int old_count, tmp;
  42
  43         if (cpu_has_llsc && R10000_LLSC_WAR) {
  44                 __asm__ __volatile__(
  45                 "       .set    mips3                                   \n"
  46                 "1:     ll      %0, %2          # __sem_update_count    \n"
  47                 "       sra     %1, %0, 31                              \n"
  48                 "       not     %1                                      \n"
  49                 "       and     %1, %0, %1                              \n"
  50                 "       addu    %1, %1, %3                              \n"
  51                 "       sc      %1, %2                                  \n"
  52                 "       beqzl   %1, 1b                                  \n"
  53                 "       .set    mips0                                   \n"
  54                 : "=&r" (old_count), "=&r" (tmp), "=m" (sem->count)
  55                 : "r" (incr), "m" (sem->count));
  56         } else if (cpu_has_llsc) {
  57                 __asm__ __volatile__(
  58                 "       .set    mips3                                   \n"
  59                 "1:     ll      %0, %2          # __sem_update_count    \n"
  60                 "       sra     %1, %0, 31                              \n"
  61                 "       not     %1                                      \n"
  62                 "       and     %1, %0, %1                              \n"
  63                 "       addu    %1, %1, %3                              \n"
  64                 "       sc      %1, %2                                  \n"
  65                 "       beqz    %1, 1b                                  \n"
  66                 "       .set    mips0                                   \n"
  67                 : "=&r" (old_count), "=&r" (tmp), "=m" (sem->count)
  68                 : "r" (incr), "m" (sem->count));
  69         } else {
  70                 static DEFINE_SPINLOCK(semaphore_lock);
  71                 unsigned long flags;
  72
  73                 spin_lock_irqsave(&semaphore_lock, flags);
  74                 old_count = atomic_read(&sem->count);
  75                 tmp = max_t(int, old_count, 0) + incr;
  76                 atomic_set(&sem->count, tmp);
  77                 spin_unlock_irqrestore(&semaphore_lock, flags);
  78         }
  79
  80         return old_count;
  81 }
  82
  83 void __up(struct semaphore *sem)
  84 {
  85         /*
  86          * Note that we incremented count in up() before we came here,
  87          * but that was ineffective since the result was <= 0, and
  88          * any negative value of count is equivalent to 0.
  89          * This ends up setting count to 1, unless count is now > 0
  90          * (i.e. because some other cpu has called up() in the meantime),
  91          * in which case we just increment count.
  92          */
  93         __sem_update_count(sem, 1);
  94         wake_up(&sem->wait);
  95 }
  96
  97 EXPORT_SYMBOL(__up);
  98
  99 /*
 100  * Note that when we come in to __down or __down_interruptible,
 101  * we have already decremented count, but that decrement was
 102  * ineffective since the result was < 0, and any negative value
 103  * of count is equivalent to 0.
 104  * Thus it is only when we decrement count from some value > 0
 105  * that we have actually got the semaphore.
 106  */
 107 void __sched __down(struct semaphore *sem)
 108 {
 109         struct task_struct *tsk = current;
 110         DECLARE_WAITQUEUE(wait, tsk);
 111
 112         __set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 113         add_wait_queue_exclusive(&sem->wait, &wait);
 114
 115         /*
 116          * Try to get the semaphore.  If the count is > 0, then we've
 117          * got the semaphore; we decrement count and exit the loop.
 118          * If the count is 0 or negative, we set it to -1, indicating
 119          * that we are asleep, and then sleep.
 120          */
 121         while (__sem_update_count(sem, -1) <= 0) {
 122                 schedule();
 123                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 124         }
 125         remove_wait_queue(&sem->wait, &wait);
 126         __set_task_state(tsk, TASK_RUNNING);
 127
 128         /*
 129          * If there are any more sleepers, wake one of them up so
 130          * that it can either get the semaphore, or set count to -1
 131          * indicating that there are still processes sleeping.
 132          */
 133         wake_up(&sem->wait);
 134 }
 135
 136 EXPORT_SYMBOL(__down);
 137
 138 int __sched __down_interruptible(struct semaphore * sem)
 139 {
 140         int retval = 0;
 141         struct task_struct *tsk = current;
 142         DECLARE_WAITQUEUE(wait, tsk);
 143
 144         __set_task_state(tsk, TASK_INTERRUPTIBLE);
 145         add_wait_queue_exclusive(&sem->wait, &wait);
 146
 147         while (__sem_update_count(sem, -1) <= 0) {
 148                 if (signal_pending(current)) {
 149                         /*
 150                          * A signal is pending - give up trying.
 151                          * Set sem->count to 0 if it is negative,
 152                          * since we are no longer sleeping.
 153                          */
 154                         __sem_update_count(sem, 0);
 155                         retval = -EINTR;
 156                         break;
 157                 }
 158                 schedule();
 159                 set_task_state(tsk, TASK_INTERRUPTIBLE);
 160         }
 161         remove_wait_queue(&sem->wait, &wait);
 162         __set_task_state(tsk, TASK_RUNNING);
 163
 164         wake_up(&sem->wait);
 165         return retval;
 166 }
 167
 168 EXPORT_SYMBOL(__down_interruptible);