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                 "1:     ll      %0, %2                                  \n"
  46                 "       sra     %1, %0, 31                              \n"
  47                 "       not     %1                                      \n"
  48                 "       and     %1, %0, %1                              \n"
  49                 "       add     %1, %1, %3                              \n"
  50                 "       sc      %1, %2                                  \n"
  51                 "       beqzl   %1, 1b                                  \n"
  52                 : "=&r" (old_count), "=&r" (tmp), "=m" (sem->count)
  53                 : "r" (incr), "m" (sem->count));
  54         } else if (cpu_has_llsc) {
  55                 __asm__ __volatile__(
  56                 "1:     ll      %0, %2                                  \n"
  57                 "       sra     %1, %0, 31                              \n"
  58                 "       not     %1                                      \n"
  59                 "       and     %1, %0, %1                              \n"
  60                 "       add     %1, %1, %3                              \n"
  61                 "       sc      %1, %2                                  \n"
  62                 "       beqz    %1, 1b                                  \n"
  63                 : "=&r" (old_count), "=&r" (tmp), "=m" (sem->count)
  64                 : "r" (incr), "m" (sem->count));
  65         } else {
  66                 static DEFINE_SPINLOCK(semaphore_lock);
  67                 unsigned long flags;
  68
  69                 spin_lock_irqsave(&semaphore_lock, flags);
  70                 old_count = atomic_read(&sem->count);
  71                 tmp = max_t(int, old_count, 0) + incr;
  72                 atomic_set(&sem->count, tmp);
  73                 spin_unlock_irqrestore(&semaphore_lock, flags);
  74         }
  75
  76         return old_count;
  77 }
  78
  79 void __up(struct semaphore *sem)
  80 {
  81         /*
  82          * Note that we incremented count in up() before we came here,
  83          * but that was ineffective since the result was <= 0, and
  84          * any negative value of count is equivalent to 0.
  85          * This ends up setting count to 1, unless count is now > 0
  86          * (i.e. because some other cpu has called up() in the meantime),
  87          * in which case we just increment count.
  88          */
  89         __sem_update_count(sem, 1);
  90         wake_up(&sem->wait);
  91 }
  92
  93 EXPORT_SYMBOL(__up);
  94
  95 /*
  96  * Note that when we come in to __down or __down_interruptible,
  97  * we have already decremented count, but that decrement was
  98  * ineffective since the result was < 0, and any negative value
  99  * of count is equivalent to 0.
 100  * Thus it is only when we decrement count from some value > 0
 101  * that we have actually got the semaphore.
 102  */
 103 void __sched __down(struct semaphore *sem)
 104 {
 105         struct task_struct *tsk = current;
 106         DECLARE_WAITQUEUE(wait, tsk);
 107
 108         __set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 109         add_wait_queue_exclusive(&sem->wait, &wait);
 110
 111         /*
 112          * Try to get the semaphore.  If the count is > 0, then we've
 113          * got the semaphore; we decrement count and exit the loop.
 114          * If the count is 0 or negative, we set it to -1, indicating
 115          * that we are asleep, and then sleep.
 116          */
 117         while (__sem_update_count(sem, -1) <= 0) {
 118                 schedule();
 119                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 120         }
 121         remove_wait_queue(&sem->wait, &wait);
 122         __set_task_state(tsk, TASK_RUNNING);
 123
 124         /*
 125          * If there are any more sleepers, wake one of them up so
 126          * that it can either get the semaphore, or set count to -1
 127          * indicating that there are still processes sleeping.
 128          */
 129         wake_up(&sem->wait);
 130 }
 131
 132 EXPORT_SYMBOL(__down);
 133
 134 int __sched __down_interruptible(struct semaphore * sem)
 135 {
 136         int retval = 0;
 137         struct task_struct *tsk = current;
 138         DECLARE_WAITQUEUE(wait, tsk);
 139
 140         __set_task_state(tsk, TASK_INTERRUPTIBLE);
 141         add_wait_queue_exclusive(&sem->wait, &wait);
 142
 143         while (__sem_update_count(sem, -1) <= 0) {
 144                 if (signal_pending(current)) {
 145                         /*
 146                          * A signal is pending - give up trying.
 147                          * Set sem->count to 0 if it is negative,
 148                          * since we are no longer sleeping.
 149                          */
 150                         __sem_update_count(sem, 0);
 151                         retval = -EINTR;
 152                         break;
 153                 }
 154                 schedule();
 155                 set_task_state(tsk, TASK_INTERRUPTIBLE);
 156         }
 157         remove_wait_queue(&sem->wait, &wait);
 158         __set_task_state(tsk, TASK_RUNNING);
 159
 160         wake_up(&sem->wait);
 161         return retval;
 162 }
 163
 164 EXPORT_SYMBOL(__down_interruptible);