arch/sh/kernel/semaphore.c

   1 /*
   2  * Just taken from alpha implementation.
   3  * This can't work well, perhaps.
   4  */
   5 /*
   6  *  Generic semaphore code. Buyer beware. Do your own
   7  * specific changes in <asm/semaphore-helper.h>
   8  */
   9
  10 #include <linux/errno.h>
  11 #include <linux/sched.h>
  12 #include <linux/wait.h>
  13 #include <linux/init.h>
  14 #include <asm/semaphore.h>
  15 #include <asm/semaphore-helper.h>
  16
  17 DEFINE_SPINLOCK(semaphore_wake_lock);
  18
  19 /*
  20  * Semaphores are implemented using a two-way counter:
  21  * The "count" variable is decremented for each process
  22  * that tries to sleep, while the "waking" variable is
  23  * incremented when the "up()" code goes to wake up waiting
  24  * processes.
  25  *
  26  * Notably, the inline "up()" and "down()" functions can
  27  * efficiently test if they need to do any extra work (up
  28  * needs to do something only if count was negative before
  29  * the increment operation.
  30  *
  31  * waking_non_zero() (from asm/semaphore.h) must execute
  32  * atomically.
  33  *
  34  * When __up() is called, the count was negative before
  35  * incrementing it, and we need to wake up somebody.
  36  *
  37  * This routine adds one to the count of processes that need to
  38  * wake up and exit.  ALL waiting processes actually wake up but
  39  * only the one that gets to the "waking" field first will gate
  40  * through and acquire the semaphore.  The others will go back
  41  * to sleep.
  42  *
  43  * Note that these functions are only called when there is
  44  * contention on the lock, and as such all this is the
  45  * "non-critical" part of the whole semaphore business. The
  46  * critical part is the inline stuff in <asm/semaphore.h>
  47  * where we want to avoid any extra jumps and calls.
  48  */
  49 void __up(struct semaphore *sem)
  50 {
  51         wake_one_more(sem);
  52         wake_up(&sem->wait);
  53 }
  54
  55 /*
  56  * Perform the "down" function.  Return zero for semaphore acquired,
  57  * return negative for signalled out of the function.
  58  *
  59  * If called from __down, the return is ignored and the wait loop is
  60  * not interruptible.  This means that a task waiting on a semaphore
  61  * using "down()" cannot be killed until someone does an "up()" on
  62  * the semaphore.
  63  *
  64  * If called from __down_interruptible, the return value gets checked
  65  * upon return.  If the return value is negative then the task continues
  66  * with the negative value in the return register (it can be tested by
  67  * the caller).
  68  *
  69  * Either form may be used in conjunction with "up()".
  70  *
  71  */
  72
  73 #define DOWN_VAR                                \
  74         struct task_struct *tsk = current;      \
  75         wait_queue_t wait;                      \
  76         init_waitqueue_entry(&wait, tsk);
  77
  78 #define DOWN_HEAD(task_state)                                           \
  79                                                                         \
  80                                                                         \
  81         tsk->state = (task_state);                                      \
  82         add_wait_queue(&sem->wait, &wait);                              \
  83                                                                         \
  84         /*                                                              \
  85          * Ok, we're set up.  sem->count is known to be less than zero  \
  86          * so we must wait.                                             \
  87          *                                                              \
  88          * We can let go the lock for purposes of waiting.              \
  89          * We re-acquire it after awaking so as to protect              \
  90          * all semaphore operations.                                    \
  91          *                                                              \
  92          * If "up()" is called before we call waking_non_zero() then    \
  93          * we will catch it right away.  If it is called later then     \
  94          * we will have to go through a wakeup cycle to catch it.       \
  95          *                                                              \
  96          * Multiple waiters contend for the semaphore lock to see       \
  97          * who gets to gate through and who has to wait some more.      \
  98          */                                                             \
  99         for (;;) {
 100
 101 #define DOWN_TAIL(task_state)                   \
 102                 tsk->state = (task_state);      \
 103         }                                       \
 104         tsk->state = TASK_RUNNING;              \
 105         remove_wait_queue(&sem->wait, &wait);
 106
 107 void __sched __down(struct semaphore * sem)
 108 {
 109         DOWN_VAR
 110         DOWN_HEAD(TASK_UNINTERRUPTIBLE)
 111         if (waking_non_zero(sem))
 112                 break;
 113         schedule();
 114         DOWN_TAIL(TASK_UNINTERRUPTIBLE)
 115 }
 116
 117 int __sched __down_interruptible(struct semaphore * sem)
 118 {
 119         int ret = 0;
 120         DOWN_VAR
 121         DOWN_HEAD(TASK_INTERRUPTIBLE)
 122
 123         ret = waking_non_zero_interruptible(sem, tsk);
 124         if (ret)
 125         {
 126                 if (ret == 1)
 127                         /* ret != 0 only if we get interrupted -arca */
 128                         ret = 0;
 129                 break;
 130         }
 131         schedule();
 132         DOWN_TAIL(TASK_INTERRUPTIBLE)
 133         return ret;
 134 }
 135
 136 int __down_trylock(struct semaphore * sem)
 137 {
 138         return waking_non_zero_trylock(sem);
 139 }