Re: [PATCH] fs: Fix bug in lock_rename_child that can cause deadlock

[Christian Brauner <brauner@xxxxxxxxxx>]

On Mon, Jun 19, 2023 at 09:37:14PM +0800, Li Dong wrote:
> Function xx causes a deadlock，because s_vfs_rename_mutex was not released when return
> 
> Signed-off-by: Li Dong <lidong@xxxxxxxx>
> ---

This is a cross-directory rename which requires s_vfs_rename_mutex to be
held. You're suggesting to drop it, violating basic locking assumptions
with dire consequences:

lock_rename_child()
{
        c1->d_parent != p2
        -> acquire s_vfs_rename_mutex
}

pairs with

old_parent = c1->d_parent;
unlock_rename(old_parent, p2)
{
        if (old_parent != p2)
        -> release s_vfs_rename_mutex
}

See the usage in ksmbd:

trap = lock_rename_child(old_child, new_path.dentry);
old_parent = dget(old_child->d_parent);
unlock_rename(old_parent, new_path.dentry);