close系统调用用于关闭文件描述符，其系统调用实现如下所示；
1 /
2 * Careful here! We test whether the file pointer is NULL before
3 * releasing the fd. This ensures that one clone task can‘t release
4 * an fd while another clone is opening it.
5 */
6 SYSCALL_DEFINE1(close, unsigned int, fd)
7 {
8 int retval = __close_fd(current->files, fd);
9
10 /* can‘t restart close syscall because file table entry was cleared */
11 if (unlikely(retval == -ERESTARTSYS ||
12 retval == -ERESTARTNOINTR ||
13 retval == -ERESTARTNOHAND ||
14 retval == -ERESTART_RESTARTBLOCK))
15 retval = -EINTR;
16
17 return retval;
18 }
19 EXPORT_SYMBOL(sys_close);
本文重点在于分析套接字的的close部分，所以简要列出close系统调用通用流程的函数调用关系，如下；
补充：其中重点注意下fput函数，该函数会先现将文件的引用计数-1，然后判断是否为0，为0的时候才会进行继续的流程，也就是说当socket存在多个引用的时候，只有最后一个close才会触发后面的调度销毁流程，也是close与shutdown不同的一个地方；
1 /** 2 * close系统调用函数调用关系 3 *__close_fd 4 * |-->filp_close 5 * |-->fput 将file加入到delayed_fput_list链表 6 * |-->schedule_delayed_work 调度延迟执行队列处理链表 7 * |-->delayed_fput 8 * |-->__fput 9 * |-->file->f_op->release 调用文件的release操作 10 */
可见，在close系统调用中会调用文件的release操作，所以我们本文重点分析socket的release操作实现；
socket实现的文件操作结构如下所示，其中本文讨论的release函数实现为sock_close；
1 /* socket文件操作函数 */
2 static const struct file_operations socket_file_ops = {
3 .owner = THIS_MODULE,
4 .llseek = no_llseek,
5 .read_iter = sock_read_iter,
6 .write_iter = sock_write_iter,
7 .poll = sock_poll,
8 .unlocked_ioctl = sock_ioctl,
9 #ifdef CONFIG_COMPAT
10 .compat_ioctl = compat_sock_ioctl,
11 #endif
12 .mmap = sock_mmap,
13 .release = sock_close,
14 .fasync = sock_fasync,
15 .sendpage = sock_sendpage,
16 .splice_write = generic_splice_sendpage,
17 .splice_read = sock_splice_read,
18 };
在socket_release函数中，会调用socket操作函数release，ipv4对应inet_release;
1 /* 关闭socket */
2 static int sock_close(struct inode *inode, struct file *filp)
3 {
4 /* socket关闭 */
5 sock_release(SOCKET_I(inode));
6 return 0;
7 }
1 /**
2 * sock_release - close a socket
3 * @sock: socket to close
4 *
5 * The socket is released from the protocol stack if it has a release
6 * callback, and the inode is then released if the socket is bound to
7 * an inode not a file.
8 */
9
10 void sock_release(struct socket *sock)
11 {
12 if (sock->ops) {
13 struct module *owner = sock->ops->owner;
14
15 /* 调用socket操作中的release */
16 sock->ops->release(sock);
17 sock->ops = NULL;
18 module_put(owner);
19 }
20
21 if (rcu_dereference_protected(sock->wq, 1)->fasync_list)
22 pr_err("%s: fasync list not empty!\n", __func__);
23
24 /* 减少cpu的套接口数量 */
25 this_cpu_sub(sockets_in_use, 1);
26
27 /* 容错处理 */
28 if (!sock->file) {
29 iput(SOCK_INODE(sock));
30 return;
31 }
32
33 /* 套接口完成关闭，继续执行close系统调用其他流程 */
34 sock->file = NULL;
35 }
inet_release负责退出组播组，根据是否开启linger标记来设置延迟关闭时间，并且调用传输层的close函数，对于tcp来说，其调用的为tcp_close；
1 /*
2 * The peer socket should always be NULL (or else). When we call this
3 * function we are destroying the object and from then on nobody
4 * should refer to it.
5 */
6 int inet_release(struct socket *sock)
7 {
8 struct sock *sk = sock->sk;
9
10 if (sk) {
11 long timeout;
12
13 /* Applications forget to leave groups before exiting */
14 /* 退出组播组 */
15 ip_mc_drop_socket(sk);
16
17 /* If linger is set, we don‘t return until the close
18 * is complete. Otherwise we return immediately. The
19 * actually closing is done the same either way.
20 *
21 * If the close is due to the process exiting, we never
22 * linger..
23 */
24 timeout = 0;
25
26 /*
27 设置了linger标记，进程未在退出，
28 则设置lingertime延迟关闭时间
29 */
30 if (sock_flag(sk, SOCK_LINGER) &&
31 !(current->flags & PF_EXITING))
32 timeout = sk->sk_lingertime;
33 sock->sk = NULL;
34
35 /* 调用传输层的close函数 */
36 sk->sk_prot->close(sk, timeout);
37 }
38 return 0;
39 }
tcp_close分析请移步另外一篇文章<TCP层close系统调用的实现分析>；
原文地址：https://www.cnblogs.com/wanpengcoder/p/11749264.html
时间： 2024-11-06 03:36:27