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semaphore.c

/*
 *  Generic semaphore code. Buyer beware. Do your own
 * specific changes in <asm/semaphore-helper.h>
 */

#include <linux/sched.h>
#include <linux/init.h>
#include <asm/semaphore-helper.h>

#ifndef CONFIG_RMW_INSNS
spinlock_t semaphore_wake_lock;
#endif

/*
 * Semaphores are implemented using a two-way counter:
 * The "count" variable is decremented for each process
 * that tries to sleep, while the "waking" variable is
 * incremented when the "up()" code goes to wake up waiting
 * processes.
 *
 * Notably, the inline "up()" and "down()" functions can
 * efficiently test if they need to do any extra work (up
 * needs to do something only if count was negative before
 * the increment operation.
 *
 * waking_non_zero() (from asm/semaphore.h) must execute
 * atomically.
 *
 * When __up() is called, the count was negative before
 * incrementing it, and we need to wake up somebody.
 *
 * This routine adds one to the count of processes that need to
 * wake up and exit.  ALL waiting processes actually wake up but
 * only the one that gets to the "waking" field first will gate
 * through and acquire the semaphore.  The others will go back
 * to sleep.
 *
 * Note that these functions are only called when there is
 * contention on the lock, and as such all this is the
 * "non-critical" part of the whole semaphore business. The
 * critical part is the inline stuff in <asm/semaphore.h>
 * where we want to avoid any extra jumps and calls.
 */
void __up(struct semaphore *sem)
{
      wake_one_more(sem);
      wake_up(&sem->wait);
}

/*
 * Perform the "down" function.  Return zero for semaphore acquired,
 * return negative for signalled out of the function.
 *
 * If called from __down, the return is ignored and the wait loop is
 * not interruptible.  This means that a task waiting on a semaphore
 * using "down()" cannot be killed until someone does an "up()" on
 * the semaphore.
 *
 * If called from __down_interruptible, the return value gets checked
 * upon return.  If the return value is negative then the task continues
 * with the negative value in the return register (it can be tested by
 * the caller).
 *
 * Either form may be used in conjunction with "up()".
 *
 */


#define DOWN_HEAD(task_state)                               \
                                                      \
                                                      \
      current->state = (task_state);                              \
      add_wait_queue(&sem->wait, &wait);                    \
                                                      \
      /*                                              \
       * Ok, we're set up.  sem->count is known to be less than zero    \
       * so we must wait.                                   \
       *                                              \
       * We can let go the lock for purposes of waiting.          \
       * We re-acquire it after awaking so as to protect          \
       * all semaphore operations.                          \
       *                                              \
       * If "up()" is called before we call waking_non_zero() then      \
       * we will catch it right away.  If it is called later then \
       * we will have to go through a wakeup cycle to catch it.   \
       *                                              \
       * Multiple waiters contend for the semaphore lock to see   \
       * who gets to gate through and who has to wait some more.  \
       */                                             \
      for (;;) {

#define DOWN_TAIL(task_state)             \
            current->state = (task_state);      \
      }                             \
      current->state = TASK_RUNNING;            \
      remove_wait_queue(&sem->wait, &wait);

void __sched __down(struct semaphore * sem)
{
      DECLARE_WAITQUEUE(wait, current);

      DOWN_HEAD(TASK_UNINTERRUPTIBLE)
      if (waking_non_zero(sem))
            break;
      schedule();
      DOWN_TAIL(TASK_UNINTERRUPTIBLE)
}

int __sched __down_interruptible(struct semaphore * sem)
{
      DECLARE_WAITQUEUE(wait, current);
      int ret = 0;

      DOWN_HEAD(TASK_INTERRUPTIBLE)

      ret = waking_non_zero_interruptible(sem, current);
      if (ret)
      {
            if (ret == 1)
                  /* ret != 0 only if we get interrupted -arca */
                  ret = 0;
            break;
      }
      schedule();
      DOWN_TAIL(TASK_INTERRUPTIBLE)
      return ret;
}

int __down_trylock(struct semaphore * sem)
{
      return waking_non_zero_trylock(sem);
}

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