Re: [PATCH v7 2/3] fs: introduce lock_rename_child() helper

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Sep 21, 2022 at 07:43:37AM +0900, Namjae Jeon wrote:


FWIW, it probably needs a few comments:

// c1 and p2 should be on the same fs
> +struct dentry *lock_rename_child(struct dentry *c1, struct dentry *p2)
> +{
> +	if (READ_ONCE(c1->d_parent) == p2) {
		// hopefully won't need to touch ->s_vfs_rename_mutex at all.
> +		inode_lock_nested(p2->d_inode, I_MUTEX_PARENT);
		// now that p2 is locked, nobody can move in or out of it,
		// so the test below is safe
> +		if (likely(c1->d_parent == p2))
> +			return NULL;
> +
		// c1 got moved out of p2 while we'd been taking locks;
		// unlock and fall back to slow case
> +		inode_unlock(p2->d_inode);
> +	}
> +
> +	mutex_lock(&c1->d_sb->s_vfs_rename_mutex);
	// nobody can move out of any directories on this fs
> +	if (likely(c1->d_parent != p2))
> +		return lock_two_directories(c1->d_parent, p2);
> +
	// c1 got moved into p2 while we were taking locks;
	// we need p2 locked and ->s_vfs_rename_mutex unlocked,
	// for consistency with lock_rename().
> +	inode_lock_nested(p2->d_inode, I_MUTEX_PARENT);
> +	mutex_unlock(&c1->d_sb->s_vfs_rename_mutex);
> +	return NULL;
> +}
> +EXPORT_SYMBOL(lock_rename_child);