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);
}
}