arch/sparc64/kernel/semaphore.c

   1 /* $Id: semaphore.c,v 1.1 1999/08/30 10:00:50 davem Exp $
   2  *  Generic semaphore code. Buyer beware. Do your own
   3  * specific changes in <asm/semaphore-helper.h>
   4  */
   5
   6 #include <linux/sched.h>
   7 #include <asm/semaphore-helper.h>
   8
   9 /*
  10  * Semaphores are implemented using a two-way counter:
  11  * The "count" variable is decremented for each process
  12  * that tries to sleep, while the "waking" variable is
  13  * incremented when the "up()" code goes to wake up waiting
  14  * processes.
  15  *
  16  * Notably, the inline "up()" and "down()" functions can
  17  * efficiently test if they need to do any extra work (up
  18  * needs to do something only if count was negative before
  19  * the increment operation.
  20  *
  21  * waking_non_zero() (from asm/semaphore.h) must execute
  22  * atomically.
  23  *
  24  * When __up() is called, the count was negative before
  25  * incrementing it, and we need to wake up somebody.
  26  *
  27  * This routine adds one to the count of processes that need to
  28  * wake up and exit.  ALL waiting processes actually wake up but
  29  * only the one that gets to the "waking" field first will gate
  30  * through and acquire the semaphore.  The others will go back
  31  * to sleep.
  32  *
  33  * Note that these functions are only called when there is
  34  * contention on the lock, and as such all this is the
  35  * "non-critical" part of the whole semaphore business. The
  36  * critical part is the inline stuff in <asm/semaphore.h>
  37  * where we want to avoid any extra jumps and calls.
  38  */
  39 void __up(struct semaphore *sem)
  40 {
  41         wake_one_more(sem);
  42         wake_up(&sem->wait);
  43 }
  44
  45 /*
  46  * Perform the "down" function.  Return zero for semaphore acquired,
  47  * return negative for signalled out of the function.
  48  *
  49  * If called from __down, the return is ignored and the wait loop is
  50  * not interruptible.  This means that a task waiting on a semaphore
  51  * using "down()" cannot be killed until someone does an "up()" on
  52  * the semaphore.
  53  *
  54  * If called from __down_interruptible, the return value gets checked
  55  * upon return.  If the return value is negative then the task continues
  56  * with the negative value in the return register (it can be tested by
  57  * the caller).
  58  *
  59  * Either form may be used in conjunction with "up()".
  60  *
  61  */
  62
  63 #define DOWN_VAR                                \
  64         struct task_struct *tsk = current;      \
  65         wait_queue_t wait;                      \
  66         init_waitqueue_entry(&wait, tsk);
  67
  68 #define DOWN_HEAD(task_state)                                           \
  69                                                                         \
  70                                                                         \
  71         tsk->state = (task_state);                                      \
  72         add_wait_queue(&sem->wait, &wait);                              \
  73                                                                         \
  74         /*                                                              \
  75          * Ok, we're set up.  sem->count is known to be less than zero  \
  76          * so we must wait.                                             \
  77          *                                                              \
  78          * We can let go the lock for purposes of waiting.              \
  79          * We re-acquire it after awaking so as to protect              \
  80          * all semaphore operations.                                    \
  81          *                                                              \
  82          * If "up()" is called before we call waking_non_zero() then    \
  83          * we will catch it right away.  If it is called later then     \
  84          * we will have to go through a wakeup cycle to catch it.       \
  85          *                                                              \
  86          * Multiple waiters contend for the semaphore lock to see       \
  87          * who gets to gate through and who has to wait some more.      \
  88          */                                                             \
  89         for (;;) {
  90
  91 #define DOWN_TAIL(task_state)                   \
  92                 tsk->state = (task_state);      \
  93         }                                       \
  94         tsk->state = TASK_RUNNING;              \
  95         remove_wait_queue(&sem->wait, &wait);
  96
  97 void __down(struct semaphore * sem)
  98 {
  99         DOWN_VAR
 100         DOWN_HEAD(TASK_UNINTERRUPTIBLE)
 101         if (waking_non_zero(sem))
 102                 break;
 103         schedule();
 104         DOWN_TAIL(TASK_UNINTERRUPTIBLE)
 105 }
 106
 107 int __down_interruptible(struct semaphore * sem)
 108 {
 109         int ret = 0;
 110         DOWN_VAR
 111         DOWN_HEAD(TASK_INTERRUPTIBLE)
 112
 113         ret = waking_non_zero_interruptible(sem, tsk);
 114         if (ret)
 115         {
 116                 if (ret == 1)
 117                         /* ret != 0 only if we get interrupted -arca */
 118                         ret = 0;
 119                 break;
 120         }
 121         schedule();
 122         DOWN_TAIL(TASK_INTERRUPTIBLE)
 123         return ret;
 124 }
 125
 126 int __down_trylock(struct semaphore * sem)
 127 {
 128         return waking_non_zero_trylock(sem);
 129 }