本文主要分析accept()的阻塞等待和唤醒。
内核版本：3.6
Author：zhangskd @ csdn blog
等待队列
（1）socket的等待队列
/*
* @sk_wq: sock wait queue head and async head
*/
struct sock {
...
struct socket_wq __rcu *sk_wq; /* 套接字的等待队列 */
...
};struct socket_wq {
/* Note: wait MUST be first field of socket_wq */
wait_queue_head_t wait; /* 等待队列 */
struct fasync_struct *fasync_list; /* 异步文件操作 */
struct rcu_head rcu; /* 更新时的回调函数 */
} __cacheline_aligned_in_smp;
struct __wait_queue_head {
spinlock_t lock;
struct list_head task_list;
};
typedef struct __wait_queue_head wait_queue_head_t; /* 等待队列头 */
（2）进程的等待任务
struct __wait_queue {
unsigned int flags;
#define WQ_FLAG_EXCLUSIVE 0x01
void *private; /* 指向当前的进程控制块 */
wait_queue_func_t func; /* 唤醒函数 */
struct list_head task_list; /* 用于链接入等待队列 */
};
typedef struct __wait_queue wait_queue_t;
typedef int (*wait_queue_func_t) (wait_queue_t *wait, unsigned mode, int flags, void *key);
int default_wake_function(wait_queue_t *wait, unsigned mode, int flags, void *key);
初始化等待任务。
#define DEFINE_WAIT(name) DEFINE_WAIT_FUNC(name, autoremove_wake_function)
#define DEFINE_WAIT_FUNC(name, function)
wait_queue_t name = {
.private = current,
.func = function,
.task_list = LIST_HEAD_INIT((name).task_list),
}
int autoremove_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
int ret = default_wake_function(wait, mode, sync, key); /* 默认的唤醒函数 */
if (ret)
list_del_init(&wait->task_list); /* 从等待队列中删除，初始化此等待任务 */
return ret;
}
获取sock的等待队列。
static inline wait_queue_head_t *sk_sleep(struct sock *sk)
{
BUILD_BUG_ON(offsetof(struct socket_wq, wait) != 0);
return &rcu_dereference_raw(sk->sk_wq)->wait;
}
把等待任务加入到等待队列中，设置当前进程的状态。
void prepare_to_wait_exclusive(wait_queue_head_t *q, wait_queue_t *wait, int state)
{
unsigned long flags;
/* 这个标志表示一次只唤醒一个等待任务，避免惊群现象 */
wait->flags |= WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&q->lock, flags);
if (list_empty(&wait->task_list))
__add_wait_queue_tail(q, wait); /* 把此等待任务加入到等待队列中 */
set_current_state(state); /* 设置当前进程的状态 */
spin_unlock_irqrestore(&q->lock, flags);
}
static inline void __add_wait_queue_tail(wait_queue_head_t *head, wait_queue_t *new)
{
list_add_tail(&new->task_list, &head->task_list);
}
#define set_current_state(state_value)
set_mb(current->state, (state_value))
（3）accept()的阻塞等待
accept()超时时间为sk->sk_rcvtimeo，在sock_init_data()中初始化为MAX_SCHEDULE_TIMEOUT，表示无限等待。
/* Wait for an incoming connection, avoid race conditions.
* This must be called with the socket locked.
*/
static int inet_csk_wait_for_connect(struct sock *sk, long timeo)
{
struct inet_connection_sock *icsk = inet_csk(sk);
DEFINE_WAIT(wait); /* 初始化等待任务 */
int err;
for (; ;) {
/* 把等待任务加入到socket的等待队列中，把进程状态设置为TASK_INTERRUPTIBLE */
prepare_to_wait_exclusive(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
release_sock(sk); /* 等下可能要睡觉了，先释放 */
if (reqsk_queue_empty(&icsk->icsk_accept_queue)) /* 如果全连接队列为空 */
timeo = schedule_timeout(timeo); /* 进入睡眠，直到超时或收到信号 */
lock_sock(sk); /* 醒来后重新上锁 */
err = 0;
if (! reqsk_queue_empty(&icsk->icsk_accept_queue)) /* 全连接队列不为空时，退出 */
break;
err = -EINVAL;
if (sk->sk_state != TCP_LISTEN) /* 如果sock不处于监听状态了，退出 */
break;
err = sock_intr_errno(timeo);
/* 如果进程有待处理的信号，退出。
* 因为timeo默认为MAX_SCHEDULE_TIMEOUT，所以err默认为-ERESTARTSYS。
* 接下来会重新调用此函数，所以accept()依然阻塞。
*/
if (signal_pending(current))
break;
err = -EAGAIN;
if (! timeo) /* 如果等待超时，即超过用户设置的sk->sk_rcvtimeo，退出 */
break;
}
finish_wait(sk_sleep(sk), &wait);
return err;
}/**
* schedule_timeout - sleep until timeout
* @timeout: timeout value in jiffies
*
* Make the current task sleep until @timeout jiffies have elapsed. The routine
* will return immediately unless the current task state has been set (see set_current_state()).
*
* You can set the task state as follows -
* %TASK_UNINTERRUPTIBLE - at least @timeout jiffies are guaranteed to pass before
* the routine returns. The routine will return 0.
*
* %TASK_INTERRUPTIBLE - the routine may return early if a signal is delivered to the
* current task. In this case the remaining time in jiffies will be returned, or 0 if the timer
* expired in time.
*
* The current task state is guaranteed to be TASK_RUNNING when this routine returns.
*
* Specifying a @timeout value of %MAX_SCHEDULE_TIMEOUT will schedule the CPU
* away without a bound on the timeout. In this case the return value will be
* %MAX_SCHEDULE_TIMEOUT.
*
* In all cases the return value is guaranteed to be non-negative.
*/
signed long __sched schedule_timeout(signed long timeout) {}
因为sk->sk_rcvtimeo默认值为MAX_SCHEDULE_TIMEOUT，所以返回-ERESTARTSYS，即告诉系统
重新执行accept()的系统调用。
static inline int sock_intr_errno(long timeo)
{
return timeo == MAX_SCHEDULE_TIMEOUT? -ERESTARTSYS : -EINTR; /* Interrupted system call */
}
从等待队列中删除等待任务，把当前进程的状态置为可运行。
/**
* finish_wait - clean up after waiting in a queue
* @q: waitqueue waited on，等待队列头
* @wait: wait descriptor，等待任务
*
* Sets current thread back to running state and removes the wait
* descriptor from the given waitqueue if still queued.
*/
void finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
__set_current_state(TASK_RUNNING);
if (! list_empty_careful(&wait->task_list)) {
spin_lock_irqsave(&q->lock, flags);
list_del_init(&wait->task_list); /* 从等待队列中删除，初始化此等待任务 */
spin_unlock_irqrestore(&q->lock, flags);
}
}
（4）accept()的唤醒
当收到客户端的ACK后，经过如下调用：
tcp_v4_rcv
tcp_v4_do_rcv
tcp_child_process
sock_def_readable
wake_up_interruptible_sync_poll
__wake_up_sync_key
__wake_up_common
最终调用我们给等待任务注册的唤醒函数。
我们来看下accept()是如何避免惊群现象的。
static void __wake_up_common(wait_queue_head_t *q, unsigned int mode, int nr_exclusive,
int wake_flags, void *key)
{
wait_queue_t *curr, *next;
list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
unsigned flags = curr->flags;
if (curr->func(curr, mode, wake_flags, key) && (flags & WQ_FLAG_EXCLUSIVE)
!--nr_exclusive)
break;
}
}
初始化等待任务时，flags |= WQ_FLAG_EXCLUSIVE。传入的nr_exclusive为1，表示只允许唤醒一个等待任务。
所以这里只会唤醒一个等待的进程，不会导致惊群现象。