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