Linux虚拟文件系统之文件系统卸载（sys_umount())

 Linux中卸载文件系统由umount系统调用实现，入口函数为sys_umount()。较于文件系统的安装较为简单，下面是具体的实现。

 /*sys_umont系统调用*/
 SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
 {
     struct path path;
     int retval;
     /*找到装载点的vfsmount实例和dentry实例，二者包装
     在一个nameidata结构中*/
     retval = user_path(name, &path);
     if (retval)
         goto out;
     retval = -EINVAL;
     /*如果查找的最终目录不是文件系统的挂载点*/
     if (path.dentry != path.mnt->mnt_root)
         goto dput_and_out;
     /*如果要卸载的文件系统还没有安装在命名空间中*/
     if (!check_mnt(path.mnt))
         goto dput_and_out;

     retval = -EPERM;
     /*如果用户不具有卸载文件系统的特权*/
     if (!capable(CAP_SYS_ADMIN))
         goto dput_and_out;
     /*实际umount工作*/
     retval = do_umount(path.mnt, flags);
 dput_and_out:
     /* we mustn't call path_put() as that would clear mnt_expiry_mark */
     dput(path.dentry);
     mntput_no_expire(path.mnt);
 out:
     return retval;
 }

卸载实际工作

 static int do_umount(struct vfsmount *mnt, int flags)
 {
     /*从vfsmount对象的mnt_sb字段检索超级块对象sb的地址*/
     struct super_block *sb = mnt->mnt_sb;
     int retval;
     /*初始化umount_list，该链表在后面的释放中会做临时链表
     用*/
     LIST_HEAD(umount_list);

     retval = security_sb_umount(mnt, flags);
     if (retval)
         return retval;

     /*
      * Allow userspace to request a mountpoint be expired rather than
      * unmounting unconditionally. Unmount only happens if:
      *  (1) the mark is already set (the mark is cleared by mntput())
      *  (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
      */
      /*如果设置了MNT_EXPIRE标志，即要标记挂载点“到期”*/
     if (flags & MNT_EXPIRE) {
         /*若要卸载的文件系统是根文件系统或者同时设置了
         MNT_FORCE或MNT_DETACH，则返回-EINVAL*/
         if (mnt == current->fs->root.mnt ||
             flags & (MNT_FORCE | MNT_DETACH))
             return -EINVAL;
         /*检查vfsmount的引用计数，若不为2，则返回-EBUSY，
         要卸载的文件系统在卸载的时候不能有引用者，
         这个2代表vfsmount的父vfsmount和sys_umount()对本对象的引用*/
         if (atomic_read(&mnt->mnt_count) != 2)
             return -EBUSY;
         /*设置vfsmount对象的mnt_expiry_mark字段为1。*/
         if (!xchg(&mnt->mnt_expiry_mark, 1))
             return -EAGAIN;
     }

     /*
      * If we may have to abort operations to get out of this
      * mount, and they will themselves hold resources we must
      * allow the fs to do things. In the Unix tradition of
      * 'Gee thats tricky lets do it in userspace' the umount_begin
      * might fail to complete on the first run through as other tasks
      * must return, and the like. Thats for the mount program to worry
      * about for the moment.
      */
      /*如果用户要求强制卸载操作，则调用umount_begin
      超级块操作中断任何正在进行的安装操作*/
     /*当然如果特定的文件系统定义了下面函数则调用它*/
     if (flags & MNT_FORCE && sb->s_op->umount_begin) {
         sb->s_op->umount_begin(sb);
     }

     /*
      * No sense to grab the lock for this test, but test itself looks
      * somewhat bogus. Suggestions for better replacement?
      * Ho-hum... In principle, we might treat that as umount + switch
      * to rootfs. GC would eventually take care of the old vfsmount.
      * Actually it makes sense, especially if rootfs would contain a
      * /reboot - static binary that would close all descriptors and
      * call reboot(9). Then init(8) could umount root and exec /reboot.
      */
      /*如果要卸载的文件系统是根文件系统，且用户
      并不要求真正地把它卸载下来（即设置了MNT_DETACH标志，
      这个标志仅仅标记挂载点为不能再访问，知道挂载不busy
      时才卸载），则调用do_remount_sb()重新安装根文件系统为只
      读并终止，并返回do_remount_sb()的返回值。*/
     if (mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
         /*
          * Special case for "unmounting" root ...
          * we just try to remount it readonly.
          */
         down_write(&sb->s_umount);
         if (!(sb->s_flags & MS_RDONLY))
             retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
         up_write(&sb->s_umount);
         return retval;
     }

     down_write(&namespace_sem);
     /*为进行写操作而获取当前进程的namespace_sem读/写信号量和vfsmount_lock自旋锁*/
     spin_unlock(&vfsmount_lock);
     spin_lock(&vfsmount_lock);
     event++;

     if (!(flags & MNT_DETACH))
         shrink_submounts(mnt, &umount_list);

     retval = -EBUSY;
     /*如果已安装文件系统不包含任何子安装文件系统的安装点，或者用户要求强制
     卸载文件系统，则调用umount_tree()卸载文件系统（及其所有子文件系统）。*/
     if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
         if (!list_empty(&mnt->mnt_list))
             /*完成实际的底层的卸载文件系统的任务。首先他将mnt的所有孩子移动至kill链表中，
             也就是传递进去的umount_list，然后将kill链表中的所有的vfsmount对象的一些字段设为无效状态。
             */
             umount_tree(mnt, 1, &umount_list);
         retval = 0;
     }

     if (retval)
         security_sb_umount_busy(mnt);
     /*释放vfsmount_lock自旋锁和当前进程的namespace_sem读/写信号量*/
     up_write(&namespace_sem);
     /*减小相应文件系统根目录的目录项对象和已经安装文件系统
     描述符的引用计数器值，这些计数器值由path_lookup()增加*/
     release_mounts(&umount_list);
     return retval;
 }

从内核链表中脱离

 /*完成实际的底层的卸载文件系统的任务。首先他将mnt的所有子移动至kill链表中，
 也就是传递进去的umount_list，然后将kill链表中的所有的vfsmount对象的一些字段设为无效状态。
 */
 void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
 {
     struct vfsmount *p;

     for (p = mnt; p; p = next_mnt(p, mnt))
         list_move(&p->mnt_hash, kill);

     if (propagate)
         propagate_umount(kill);

     list_for_each_entry(p, kill, mnt_hash) {
         list_del_init(&p->mnt_expire);
         list_del_init(&p->mnt_list);
         __touch_mnt_namespace(p->mnt_ns);
         p->mnt_ns = NULL;
         list_del_init(&p->mnt_child);
         if (p->mnt_parent != p) {
             p->mnt_parent->mnt_ghosts++;
             p->mnt_mountpoint->d_mounted--;
         }
         change_mnt_propagation(p, MS_PRIVATE);
     }
 }

释放引用计数

 void release_mounts(struct list_head *head)
 {
     struct vfsmount *mnt;
     while (!list_empty(head)) {
         mnt = list_first_entry(head, struct vfsmount, mnt_hash);
         list_del_init(&mnt->mnt_hash);
         if (mnt->mnt_parent != mnt) {
             struct dentry *dentry;
             struct vfsmount *m;
             spin_lock(&vfsmount_lock);
             dentry = mnt->mnt_mountpoint;
             m = mnt->mnt_parent;
             mnt->mnt_mountpoint = mnt->mnt_root;
             mnt->mnt_parent = mnt;
             m->mnt_ghosts--;
             spin_unlock(&vfsmount_lock);
             /*下面两个函数为减小引用计数，减到0时释放*/
             dput(dentry);
             mntput(m);
         }
         /*vfsmount对象所占的内存空间最终在mntput()函数中释放*/
         mntput(mnt);
     }
 }