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