Re: [PATCH 7/8] VFS: use new dentry locking for create/remove/rename

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

On Mon, Jun 09, 2025 at 05:34:12PM +1000, NeilBrown wrote:
> After taking the directory lock (or locks) we now lock the target
> dentries.  This is pointless at present but will allow us to remove the
> taking of the directory lock in a future patch.
> 
> MORE WORDS

Such as "why doesn't it deadlock?", presumably, seeing that you have

> @@ -2003,7 +2003,14 @@ struct dentry *lookup_and_lock_hashed(struct qstr *last,
>  
>  	inode_lock_nested(base->d_inode, I_MUTEX_PARENT);
>  
> +retry:
>  	dentry = lookup_one_qstr(last, base, lookup_flags);
> +	if (!IS_ERR(dentry) &&
> +	    !dentry_lock(dentry, base, last, TASK_UNINTERRUPTIBLE)) {

... take dentry lock inside ->i_rwsem on parent and

>  bool lock_and_check_dentry(struct dentry *child, struct dentry *parent)
>  {
> -	inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);
> -	if (child->d_parent == parent) {
> -		/* get the child to balance with dentry_unlock which puts it. */
> -		dget(child);
> -		return true;
> +	if (!dentry_lock(child, NULL, NULL, TASK_UNINTERRUPTIBLE))
> +		return false;
> +	if (child->d_parent != parent) {
> +		__dentry_unlock(child);
> +		return false;
>  	}
> -	inode_unlock(d_inode(parent));
> -	return false;
> +	/* get the child to balance with dentry_unlock() which puts it. */
> +	dget(child);
> +	inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);

... do the same in opposite order?

How could that possibly work?