SO2 Lecture 03 - Processes

/**
 * wait_event - sleep until a condition gets true
 * @wq_head: the waitqueue to wait on
 * @condition: a C expression for the event to wait for
 *
 * The process is put to sleep (TASK_UNINTERRUPTIBLE) until the
 * @condition evaluates to true. The @condition is checked each time
 * the waitqueue @wq_head is woken up.
 *
 * wake_up() has to be called after changing any variable that could
 * change the result of the wait condition.
 */
#define wait_event(wq_head, condition)            \
do {                                              \
  might_sleep();                                  \
  if (condition)                                  \
          break;                                  \
  __wait_event(wq_head, condition);               \
} while (0)

#define __wait_event(wq_head, condition)                                  \
    (void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0,   \
                        schedule())

/*
 * The below macro ___wait_event() has an explicit shadow of the __ret
 * variable when used from the wait_event_*() macros.
 *
 * This is so that both can use the ___wait_cond_timeout() construct
 * to wrap the condition.
 *
 * The type inconsistency of the wait_event_*() __ret variable is also
 * on purpose; we use long where we can return timeout values and int
 * otherwise.
 */
#define ___wait_event(wq_head, condition, state, exclusive, ret, cmd)    \
({                                                                       \
    __label__ __out;                                                     \
    struct wait_queue_entry __wq_entry;                                  \
    long __ret = ret;       /* explicit shadow */                        \
                                                                         \
    init_wait_entry(&__wq_entry, exclusive ? WQ_FLAG_EXCLUSIVE : 0);     \
    for (;;) {                                                           \
        long __int = prepare_to_wait_event(&wq_head, &__wq_entry, state);\
                                                                         \
        if (condition)                                                   \
            break;                                                       \
                                                                         \
        if (___wait_is_interruptible(state) && __int) {                  \
            __ret = __int;                                               \
            goto __out;                                                  \
        }                                                                \
                                                                         \
        cmd;                                                             \
    }                                                                    \
    finish_wait(&wq_head, &__wq_entry);                                  \
   __out:  __ret;                                                        \
 })

 void init_wait_entry(struct wait_queue_entry *wq_entry, int flags)
 {
    wq_entry->flags = flags;
    wq_entry->private = current;
    wq_entry->func = autoremove_wake_function;
    INIT_LIST_HEAD(&wq_entry->entry);
 }

 long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
 {
     unsigned long flags;
     long ret = 0;

     spin_lock_irqsave(&wq_head->lock, flags);
     if (signal_pending_state(state, current)) {
         /*
          * Exclusive waiter must not fail if it was selected by wakeup,
          * it should "consume" the condition we were waiting for.
          *
          * The caller will recheck the condition and return success if
          * we were already woken up, we can not miss the event because
          * wakeup locks/unlocks the same wq_head->lock.
          *
          * But we need to ensure that set-condition + wakeup after that
          * can't see us, it should wake up another exclusive waiter if
          * we fail.
          */
         list_del_init(&wq_entry->entry);
         ret = -ERESTARTSYS;
     } else {
         if (list_empty(&wq_entry->entry)) {
             if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
                 __add_wait_queue_entry_tail(wq_head, wq_entry);
             else
                 __add_wait_queue(wq_head, wq_entry);
         }
         set_current_state(state);
     }
     spin_unlock_irqrestore(&wq_head->lock, flags);

     return ret;
 }

 static inline void __add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
     list_add(&wq_entry->entry, &wq_head->head);
 }

 static inline void __add_wait_queue_entry_tail(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
     list_add_tail(&wq_entry->entry, &wq_head->head);
 }

 /**
  * finish_wait - clean up after waiting in a queue
  * @wq_head: waitqueue waited on
  * @wq_entry: wait descriptor
  *
  * Sets current thread back to running state and removes
  * the wait descriptor from the given waitqueue if still
  * queued.
  */
 void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
     unsigned long flags;

     __set_current_state(TASK_RUNNING);
     /*
      * We can check for list emptiness outside the lock
      * IFF:
      *  - we use the "careful" check that verifies both
      *    the next and prev pointers, so that there cannot
      *    be any half-pending updates in progress on other
      *    CPU's that we haven't seen yet (and that might
      *    still change the stack area.
      * and
      *  - all other users take the lock (ie we can only
      *    have _one_ other CPU that looks at or modifies
      *    the list).
      */
     if (!list_empty_careful(&wq_entry->entry)) {
         spin_lock_irqsave(&wq_head->lock, flags);
         list_del_init(&wq_entry->entry);
         spin_unlock_irqrestore(&wq_head->lock, flags);
     }
 }