[RFC] weird use of dget_parent() in xfs_filestream_get_parent()

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

Function in question:

static struct xfs_inode *
xfs_filestream_get_parent(
        struct xfs_inode        *ip)
{
        struct inode            *inode = VFS_I(ip), *dir = NULL;
        struct dentry           *dentry, *parent;

        dentry = d_find_alias(inode);
        if (!dentry)
                goto out;

        parent = dget_parent(dentry);
        if (!parent)
                goto out_dput;

        dir = igrab(d_inode(parent));
        dput(parent);

out_dput:
        dput(dentry);
out:
        return dir ? XFS_I(dir) : NULL;
}

1) dget_parent(dentry) never returns NULL; if you have IS_ROOT(dentry) you
get an equivalent of dget(dentry).  What do you want returned in that
case?

2) why bother with dget_parent() in the first place?  What's wrong with
	spin_lock(&dentry->d_lock); // stabilizes ->d_parent
        dir = igrab(dentry->d_parent->d_inode);
	spin_unlock(&dentry->d_lock);

or, if you intended NULL for root, 
	spin_lock(&dentry->d_lock); // stabilizes ->d_parent
	if (!IS_ROOT(dentry))
		dir = igrab(dentry->d_parent->d_inode);
	spin_unlock(&dentry->d_lock);


Is there anything subtle I'm missing here?