arch/m68knommu/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/err.h>
   9 #include <linux/init.h>
  10 #include <asm/semaphore-helper.h>
  11
  12 #ifndef CONFIG_RMW_INSNS
  13 spinlock_t semaphore_wake_lock;
  14 #endif
  15
  16 /*
  17  * Semaphores are implemented using a two-way counter:
  18  * The "count" variable is decremented for each process
  19  * that tries to sleep, while the "waking" variable is
  20  * incremented when the "up()" code goes to wake up waiting
  21  * processes.
  22  *
  23  * Notably, the inline "up()" and "down()" functions can
  24  * efficiently test if they need to do any extra work (up
  25  * needs to do something only if count was negative before
  26  * the increment operation.
  27  *
  28  * waking_non_zero() (from asm/semaphore.h) must execute
  29  * atomically.
  30  *
  31  * When __up() is called, the count was negative before
  32  * incrementing it, and we need to wake up somebody.
  33  *
  34  * This routine adds one to the count of processes that need to
  35  * wake up and exit.  ALL waiting processes actually wake up but
  36  * only the one that gets to the "waking" field first will gate
  37  * through and acquire the semaphore.  The others will go back
  38  * to sleep.
  39  *
  40  * Note that these functions are only called when there is
  41  * contention on the lock, and as such all this is the
  42  * "non-critical" part of the whole semaphore business. The
  43  * critical part is the inline stuff in <asm/semaphore.h>
  44  * where we want to avoid any extra jumps and calls.
  45  */
  46 void __up(struct semaphore *sem)
  47 {
  48         wake_one_more(sem);
  49         wake_up(&sem->wait);
  50 }
  51
  52 /*
  53  * Perform the "down" function.  Return zero for semaphore acquired,
  54  * return negative for signalled out of the function.
  55  *
  56  * If called from __down, the return is ignored and the wait loop is
  57  * not interruptible.  This means that a task waiting on a semaphore
  58  * using "down()" cannot be killed until someone does an "up()" on
  59  * the semaphore.
  60  *
  61  * If called from __down_interruptible, the return value gets checked
  62  * upon return.  If the return value is negative then the task continues
  63  * with the negative value in the return register (it can be tested by
  64  * the caller).
  65  *
  66  * Either form may be used in conjunction with "up()".
  67  *
  68  */
  69
  70
  71 #define DOWN_HEAD(task_state)                                           \
  72                                                                         \
  73                                                                         \
  74         current->state = (task_state);                                  \
  75         add_wait_queue(&sem->wait, &wait);                              \
  76                                                                         \
  77         /*                                                              \
  78          * Ok, we're set up.  sem->count is known to be less than zero  \
  79          * so we must wait.                                             \
  80          *                                                              \
  81          * We can let go the lock for purposes of waiting.              \
  82          * We re-acquire it after awaking so as to protect              \
  83          * all semaphore operations.                                    \
  84          *                                                              \
  85          * If "up()" is called before we call waking_non_zero() then    \
  86          * we will catch it right away.  If it is called later then     \
  87          * we will have to go through a wakeup cycle to catch it.       \
  88          *                                                              \
  89          * Multiple waiters contend for the semaphore lock to see       \
  90          * who gets to gate through and who has to wait some more.      \
  91          */                                                             \
  92         for (;;) {
  93
  94 #define DOWN_TAIL(task_state)                   \
  95                 current->state = (task_state);  \
  96         }                                       \
  97         current->state = TASK_RUNNING;          \
  98         remove_wait_queue(&sem->wait, &wait);
  99
 100 void __sched __down(struct semaphore * sem)
 101 {
 102         DECLARE_WAITQUEUE(wait, current);
 103
 104         DOWN_HEAD(TASK_UNINTERRUPTIBLE)
 105         if (waking_non_zero(sem))
 106                 break;
 107         schedule();
 108         DOWN_TAIL(TASK_UNINTERRUPTIBLE)
 109 }
 110
 111 int __sched __down_interruptible(struct semaphore * sem)
 112 {
 113         DECLARE_WAITQUEUE(wait, current);
 114         int ret = 0;
 115
 116         DOWN_HEAD(TASK_INTERRUPTIBLE)
 117
 118         ret = waking_non_zero_interruptible(sem, current);
 119         if (ret)
 120         {
 121                 if (ret == 1)
 122                         /* ret != 0 only if we get interrupted -arca */
 123                         ret = 0;
 124                 break;
 125         }
 126         schedule();
 127         DOWN_TAIL(TASK_INTERRUPTIBLE)
 128         return ret;
 129 }
 130
 131 int __down_trylock(struct semaphore * sem)
 132 {
 133         return waking_non_zero_trylock(sem);
 134 }