Linux 等待队列

1. Linux内核中等待队列简介

　　Linux 内核等待队列可以用于许多用途，如中断、进程同步、以及定时。等待队列实现了在事件上的条件等待：希望等待特定事件的进程把自己放进合适的队列，

并且放弃控制权限。因此等待队列表示一组睡眠的进程，当某一事件为真时，有内核唤醒。

2. Linux 内核等待队列中重要数据结构

2.1 等待队列头

struct __wait_queue_head {
    spinlock_t lock;
    struct list_head task_list;
};
typedef struct __wait_queue_head wait_queue_head_t;

2.2 等待队列睡眠过程

　　使用等待等待队列通常会定义一个等待队列头，static wait_queue_head_t wq，然后调用wait_event_*函数等待某条件，condition 的当前进程插入到等待队列wq中

并且睡眠，一直等到condition的条件满足后，内核在将对等队列上某一进程或者所有进程唤醒。

#define __wait_event_interruptible(wq, condition, ret)            \
do {                                    \
    DEFINE_WAIT(__wait);                        \
                                    \
    for (;;) {                            \
        prepare_to_wait(&wq, &__wait, TASK_INTERRUPTIBLE);    \
        if (condition)                        \
            break;                        \
        if (!signal_pending(current)) {                \
            schedule();                    \
            continue;                    \
        }                            \
        ret = -ERESTARTSYS;                    \
        break;                            \
    }                                \
    finish_wait(&wq, &__wait);                    \
} while (0)

/**
 * wait_event_interruptible - sleep until a condition gets true
 * @wq: the waitqueue to wait on
 * @condition: a C expression for the event to wait for
 *
 * The process is put to sleep (TASK_INTERRUPTIBLE) until the
 * @condition evaluates to true or a signal is received.
 * The @condition is checked each time the waitqueue @wq is woken up.
 *
 * wake_up() has to be called after changing any variable that could
 * change the result of the wait condition.
 *
 * The function will return -ERESTARTSYS if it was interrupted by a
 * signal and 0 if @condition evaluated to true.
 */
#define wait_event_interruptible(wq, condition)                \
({                                    \
    int __ret = 0;                            \
    if (!(condition))                        \
        __wait_event_interruptible(wq, condition, __ret);    \
    __ret;                                \
})

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