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Why does d_splice_alias need to check IS_ROOT?

Hello list,

I'm insteresting in how hard link and denry lookup work and their

I know that this interface tries to connect an inode to a dentry, but
why does it need
to check whether the inode alias IS_ROOT if the inode represents a directory?
And the code process in different way according to check result.  What
are they used for?

struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
    if (S_ISDIR(inode->i_mode)) {
        struct dentry *new = __d_find_any_alias(inode);
        if (unlikely(new)) {
            if (unlikely(d_ancestor(new, dentry))) {
                /*the code here is to forbidden ancestor hard link*/
            } else if (!IS_ROOT(new)) {
                __d_unalias(inode, dentry, new);
            } else {
                __d_move(new, dentry, false);



Thanks in advance.