arch/m68k/kernel/semaphore.c

   1 /*
   2  *  Generic semaphore code. Buyer beware. Do your own
   3  * specific changes in <asm/semaphore-helper.h>
   4  */
   5
   6 #include <linux/config.h>
   7 #include <linux/sched.h>
   8 #include <linux/init.h>
   9 #include <asm/semaphore-helper.h>
  10
  11 #ifndef CONFIG_RMW_INSNS
  12 spinlock_t semaphore_wake_lock;
  13 #endif
  14
  15 /*
  16  * Semaphores are implemented using a two-way counter:
  17  * The "count" variable is decremented for each process
  18  * that tries to sleep, while the "waking" variable is
  19  * incremented when the "up()" code goes to wake up waiting
  20  * processes.
  21  *
  22  * Notably, the inline "up()" and "down()" functions can
  23  * efficiently test if they need to do any extra work (up
  24  * needs to do something only if count was negative before
  25  * the increment operation.
  26  *
  27  * waking_non_zero() (from asm/semaphore.h) must execute
  28  * atomically.
  29  *
  30  * When __up() is called, the count was negative before
  31  * incrementing it, and we need to wake up somebody.
  32  *
  33  * This routine adds one to the count of processes that need to
  34  * wake up and exit.  ALL waiting processes actually wake up but
  35  * only the one that gets to the "waking" field first will gate
  36  * through and acquire the semaphore.  The others will go back
  37  * to sleep.
  38  *
  39  * Note that these functions are only called when there is
  40  * contention on the lock, and as such all this is the
  41  * "non-critical" part of the whole semaphore business. The
  42  * critical part is the inline stuff in <asm/semaphore.h>
  43  * where we want to avoid any extra jumps and calls.
  44  */
  45 void __up(struct semaphore *sem)
  46 {
  47         wake_one_more(sem);
  48         wake_up(&sem->wait);
  49 }
  50
  51 /*
  52  * Perform the "down" function.  Return zero for semaphore acquired,
  53  * return negative for signalled out of the function.
  54  *
  55  * If called from __down, the return is ignored and the wait loop is
  56  * not interruptible.  This means that a task waiting on a semaphore
  57  * using "down()" cannot be killed until someone does an "up()" on
  58  * the semaphore.
  59  *
  60  * If called from __down_interruptible, the return value gets checked
  61  * upon return.  If the return value is negative then the task continues
  62  * with the negative value in the return register (it can be tested by
  63  * the caller).
  64  *
  65  * Either form may be used in conjunction with "up()".
  66  *
  67  */
  68
  69
  70 #define DOWN_HEAD(task_state)                                           \
  71                                                                         \
  72                                                                         \
  73         current->state = (task_state);                                  \
  74         add_wait_queue(&sem->wait, &wait);                              \
  75                                                                         \
  76         /*                                                              \
  77          * Ok, we're set up.  sem->count is known to be less than zero  \
  78          * so we must wait.                                             \
  79          *                                                              \
  80          * We can let go the lock for purposes of waiting.              \
  81          * We re-acquire it after awaking so as to protect              \
  82          * all semaphore operations.                                    \
  83          *                                                              \
  84          * If "up()" is called before we call waking_non_zero() then    \
  85          * we will catch it right away.  If it is called later then     \
  86          * we will have to go through a wakeup cycle to catch it.       \
  87          *                                                              \
  88          * Multiple waiters contend for the semaphore lock to see       \
  89          * who gets to gate through and who has to wait some more.      \
  90          */                                                             \
  91         for (;;) {
  92
  93 #define DOWN_TAIL(task_state)                   \
  94                 current->state = (task_state);  \
  95         }                                       \
  96         current->state = TASK_RUNNING;          \
  97         remove_wait_queue(&sem->wait, &wait);
  98
  99 void __sched __down(struct semaphore * sem)
 100 {
 101         DECLARE_WAITQUEUE(wait, current);
 102
 103         DOWN_HEAD(TASK_UNINTERRUPTIBLE)
 104         if (waking_non_zero(sem))
 105                 break;
 106         schedule();
 107         DOWN_TAIL(TASK_UNINTERRUPTIBLE)
 108 }
 109
 110 int __sched __down_interruptible(struct semaphore * sem)
 111 {
 112         DECLARE_WAITQUEUE(wait, current);
 113         int ret = 0;
 114
 115         DOWN_HEAD(TASK_INTERRUPTIBLE)
 116
 117         ret = waking_non_zero_interruptible(sem, current);
 118         if (ret)
 119         {
 120                 if (ret == 1)
 121                         /* ret != 0 only if we get interrupted -arca */
 122                         ret = 0;
 123                 break;
 124         }
 125         schedule();
 126         DOWN_TAIL(TASK_INTERRUPTIBLE)
 127         return ret;
 128 }
 129
 130 int __down_trylock(struct semaphore * sem)
 131 {
 132         return waking_non_zero_trylock(sem);
 133 }