Socket层实现系列 — 信号驱动的异步等待

struct sock {
	...
	struct socket_wq __rcu *sk_wq; /* socket的等待队列和异步通知队列 */
	...
}

struct socket_wq {
	/* Note: wait MUST be first field of socket_wq */
	wait_queue_head_t wait; /* 等待队列头 */
	struct fasync_struct *fasync_list; /* 异步通知队列 */
	struct rcu_head *rcu;
};

struct fasync_struct {
	spinlock_t fa_lock;
	int magic;
	int fa_fd; /* 文件描述符 */
	struct fasync_struct *fa_next; /* 用于链入单向链表 */
	struct file *fa_file; /* fa_file->f_owner记录接收信号的进程 */
	struct rcu_head fa_rcu;
};

signal(SIGIO, my_handler); /* set new SIGIO handler */
fcntl(sockfd, F_SETOWN, getpid()); /* set sockfd's owner process */
oflags = fcntl(sockfd, F_GETFL); /* get old sockfd flags */
fcntl(sockfd, F_SETFL, oflags | O_ASYNC); /* set new sockfd flags */

SYSCALL_DEFINE3(fcntl, unsigned int, fd, unsigned int, cmd, unsigned long, arg)
{
	struct fd f = fdget_raw(fd);
	long err = -EBADF; /* Bad file number */

	if (! f.file)
		goto out;

	/* File is opened with O_PATH, almost nothing can be done with it */
	if (unlikely(f.file->f_mode & FMODE_PATH)) {
		if (! check_fcntl_cmd(cmd))
			goto out1;
	}

	err = security_file_fcntl(f.file, cmd, arg);
	if (! err)
		err = do_fcntl(fd, cmd, arg, f.file); /* 实际的处理函数 */

out1:
	fdput(f);
out:
	return err;
}

static long do_fcntl(int fd, unsigned int cmd, unsigned long arg, struct fil *filp)
{
	long err = -EINVAL;

	switch(cmd) {
	...
	case F_SETFL: /* 在这里设置O_ASYNC标志 */
		err = setfl(fd, filp, arg);
		break;
	...
	case F_SETOWN: /* 在这里设置所有者进程 */
		err = f_setown(filp, arg, 1);
		break;
	....
	}

	return err;
}

static int setfl(int fd, struct file *filp, unsigned long arg)
{
	...
	/* ->fasync() is responsible for setting the FASYNC bit. */
	if (((arg ^ filp->f_flags) & FASYNC) && filp->f_op->fasync) {
		error = filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0);

		if (error < 0)
			goto out;
		if (error > 0)
			error = 0;
	}
	...
}

static const struct file_operations socket_file_ops = {
	...
	.fasync = sock_fasync,
	...
};

/* Update the socket async list. */
static int sock_fasync(int fd, struct file *filp, int on)
{
	struct socket *sock = filp->private_data;
	struct sock *sk = sock->sk;
	struct socket_wq *wq; /* Socket的等待队列和异步通知队列 */

	if (sk == NULL)
		return -EINVAL;

	lock_sock(sk);
	wq = rcu_dereference_protected(sock->wq, sock_owned_by_user(sk));

	fasync_helper(fd, filp, on, &wq->fasync_list); /* 使用此函数来插入或删除 */

	/* 设置或取消SOCK_FASYNC标志 */
	if (! wq->fasync_list)
		sock_reset_flag(sk, SOCK_FASYNC);
	else
		sock_set_flag(sk, SOCK_FASYNC);

	release_sock(sk);

	return 0;
}

/*
 * fasync_helper() is used by almost all character device drivers to set up the fasync
 * queue, and for regular files by the file lease code. It returns negative on error, 0 if
 * it did no changes and positive if it added / deleted the entry.
 */

int fasync_helper(int fd, struct file *filp, int on, struct fasync_struct **fapp)
{
	if (! on)
		return fasync_remove_entry(filp, fapp); /* 插入 */

	return fasync_add_entry(fd, filp, fapp); /* 删除 */
}

enum {
	SOCK_WAKE_IO, /* 直接发送SIGIO信号 */
	SOCK_WAKE_WAITD, /* 检测应用程序是否通过recv()类调用来等待接收数据，如果没有才发送SIGIO信号 */
	SOCK_WAKE_SPACE, /* 检测sock的发送队列是否曾经到达上限，如果有的话发送SIGIO信号 */
	SOCK_WAKE_URG, /* 直接发送SIGURG信号 */
};

#define __SI_POLL 0
#define POLL_IN (__SI_POLL | 1) /* data input available, 有接收数据可读 */
#define POLL_OUT (__SI_POLL | 2) /* output buffers available, 有输出缓存可写 */
#define POLL_MSG (__SI_POLL | 3) /* input message available, 有输入消息可读 */
#define POLL_ERR (__SI_POLL | 4) /* i/0 error, I/O错误 */
#define POLL_PRI (__SI_POLL | 5) /* high priority input available, 有紧急数据可读 */
#define POLL_HUP (__SI_POLL | 6) /* device disconnected, 设备关闭或文件关闭，无法继续读写 */

static inline void sk_wake_async(struct sock *sk, int how, int band)
{
	if (sock_flag(sk, SOCK_FASYNC)) /* sock需要支持异步通知 */
		sock_wake_async(sk->sk_socket, how, band);
}

int sock_wake_async(struct socket *sock, int how, int band)
{
	struct socket_wq *wq;

	if (! sock)
		return -1;

	rcu_read_lock();
	wq = rcu_dereference(sock->wq); /* socket的等待队列和异步通知队列 */

	if (! wq || !wq->fasync_list) { /* 如果有队列没有实例 */
		rcu_read_unlock();
		return -1;
	}

	switch(how) {
	/* 检测应用程序是否通过recv()类调用来等待接收数据，如果没有才发送SIGIO信号 */
	case SOCK_WAKE_WAITD:
		if (test_bit(SOCK_ASYNC_WAITDATA, &sock->flags))
			break;
		goto call_kill;

	/* 检测sock的发送队列是否曾经到达上限，如果有的话发送SIGIO信号 */
	case SOCK_WAKE_SPACE:
		if (! test_and_clear_bit(SOCK_ASYNC_NOSPACE, &sock->flags))
			break;
	/* fall_through */

	case SOCK_WAKE_IO: /* 直接发送SIGIO信号 */
call_kill:
			/* 发送SIGIO信号给异步通知队列上的进程，告知IO消息 */
			kill_fasync(&wq->fasync_list, SIGIO, band);
			break;

	case SOCK_WAKE_URG:
			/* 发送SIGURG信号给异步通知队列上的进程 */
			kill_fasync(&wq->fasync_list, SIGURG, band);
	}

	rcu_read_unlock();
	return 0;
}

void kill_fasync(struct fasync_struct **fp, int sig, int band)
{
	/* First a quick test without locking: usually the list is empty. */
	if (*f) {
		rcu_read_lock();
		kill_fasync_rcu(rcu_dereference(*fp), sig, band);
		rcu_read_unlock();
	}
}

static void kill_fasync_rcu(struct fasync_struct *fa, int sig, int band)
{
	while (fa) {
		struct fown_struct *fown;
		unsigned long flags;

		if (fa->magic != FASYNC_MAGIC) {
			printk(KERN_ERR "kill_fasync: bad magic number in fasync_struct!\n");
			return;
		}

		spin_lock_irqsave(&fa->fa_lock, flags);
		if (fa->fa_file) {
			fown = &fa->file->f_owner; /* 持有文件的进程 */

			/* Don't send SIGURG to processes which have not set a queued signum:
			 * SIGURG has its own default signalling mechanism. */

			if (! (sig == SIGURG && fown->signum == 0))
				send_sigio(fown, fa->fa_fd, band); /* 发送信号给持有文件的进程 */
		}
		spin_unlock_irqrestore(&fa->fa_lock, flags);

		fa = rcu_dereference(fa->fa_next); /* 指向下一个异步通知结构体 */
	}
}