How can I wake up one process from the wait queue?

[Rock Lee]

Hi, everyoneI am implementing a simple driver to experiment with wait queue.Two or more read processes block until a write process changes a flag and call wake_up_interruptible().I expect that a write process will only wake up one read process.However, once a write process calls  wake_up_interruptible() , all the read processes are awaken. How can I wake up one process from the wait queue? Here is the snippet of the simple dirver(just for experimenting, kernel 2.6.18):
static ssize_t rlwait_read(struct file *filp, char __user *userp, size_t size, loff_t *off){    struct rlwait_t *rock = (struct rlwait_t *)filp->private_data;    DECLARE_WAITQUEUE(wait, current);        add_wait_queue(&rock->r_wait_head, &wait);        while (rock->r_flag == 0) {        set_current_state(TASK_INTERRUPTIBLE);        schedule();    }   
    remove_wait_queue(&rock->r_wait_head, &wait);    set_current_state(TASK_RUNNING);    return 0;}
static ssize_t rlwait_write(struct file *filp, const char __user *userp, size_t size, loff_t *off){    struct rlwait_t *rock = (struct rlwait_t *)filp->private_data;            rock->r_flag = 1;    wake_up_interruptible(&rock->r_wait_head);            return size; }
Thans in advice.
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