Re: [PATCH 10/11] VFS: take a shared lock for create/remove directory operations.

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

On Fri, Dec 20, 2024 at 01:54:28PM +1100, NeilBrown wrote:

> Once the exclusive "update" lock is obtained on the dentry we must make
> sure it wasn't unlinked or renamed while we slept.  If it was we repeat
> the lookup.

> +		if (
> +			/* Was unlinked while we waited ?*/
> +			d_unhashed(dentry) ||
> +			/* Or was dentry renamed ?? */
> +			dentry->d_parent != base ||
> +			dentry->d_name.hash != last->hash ||
> +			!d_same_name(dentry, base, last)
> +		) {
> +			rcu_read_unlock();
> +			spin_unlock(&dentry->d_lock);
> +			lock_map_release(&dentry->d_update_map);
> +			dput(dentry);
> +			goto retry;
> +		}
> +		rcu_read_unlock();
> +	}
> +	dentry->d_flags |= DCACHE_PAR_UPDATE;
> +	spin_unlock(&dentry->d_lock);

... and now __d_unalias() moves it to new place, making all the checks
you've just done completely useless.