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);
