try_wait_for_completion

/**
 *  try_wait_for_completion - try to decrement a completion without blocking
 *  @x: completion structure
 *
 *  Return: 0 if a decrement cannot be done without blocking
 *       1 if a decrement succeeded.
 *
 *  If a completion is being used as a counting completion,
 *  attempt to decrement the counter without blocking. This
 *  enables us to avoid waiting if the resource the completion
 *  is protecting is not available.
 */
bool try_wait_for_completion(struct completion *x)
{
    unsigned long flags;
    bool ret = true;
 
    /*
     * Since x->done will need to be locked only
     * in the non-blocking case, we check x->done
     * first without taking the lock so we can
     * return early in the blocking case.
     */
    if (!READ_ONCE(x->done))
        return false;
 
    spin_lock_irqsave(&x->wait.lock, flags);
    if (!x->done)
        ret = false;
    else if (x->done != UINT_MAX)
        x->done--;
    spin_unlock_irqrestore(&x->wait.lock, flags);
    return ret;
}
函数源码
