wait_event

Linux中是怎样实现wait 一个事件的？

1】定义一个wait queue head

static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);

2】等待某个事件，当wakeup waitqueue时判断这个事件，也就是condition: a C expression for the event to wait for

struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)

{

struct tty_ldisc *ld;

/* wait_event is a macro */

wait_event(tty_ldisc_wait, (ld = tty_ldisc_try(tty)) != NULL);

return ld;

}

3】唤醒waitqueue

void tty_ldisc_enable(struct tty_struct *tty)

{

set_bit(TTY_LDISC, &tty->flags);

clear_bit(TTY_LDISC_CHANGING, &tty->flags);

wake_up(&tty_ldisc_wait);

}

wait event 实现的原理

/**

* wait_event - 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_UNINTERRUPTIBLE) until the

* @condition evaluates to true. 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.

*/

#define wait_event(wq, condition) \

do { \

if (condition) \

break; \

__wait_event(wq, condition); \

} while (0)

#define __wait_event(wq, condition) \

do { \

DEFINE_WAIT(__wait); \

\

for (;;) { \

prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE); \

if (condition) \

break; \

schedule(); \

} \

finish_wait(&wq, &__wait); \

} while (0)