Re: [PATCH] NFS: Fix directory delegation verifier checks

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

[cc to fsdevel added]

On Wed, Dec 31, 2025 at 09:52:35PM +0000, Trond Myklebust wrote:

> +static void nfs_clear_verifier_directory(struct inode *dir)
> +{
> +	struct dentry *this_parent;
> +	struct dentry *dentry;
> +	struct inode *inode;
> +
> +	if (hlist_empty(&dir->i_dentry))
> +		return;
> +	this_parent =
> +		hlist_entry(dir->i_dentry.first, struct dentry, d_u.d_alias);
> +
> +	spin_lock(&this_parent->d_lock);
> +	nfs_unset_verifier_delegated(&this_parent->d_time);
> +	dentry = d_first_child(this_parent);
> +	hlist_for_each_entry_from(dentry, d_sib) {
> +		if (unlikely(dentry->d_flags & DCACHE_DENTRY_CURSOR))
> +			continue;
> +		inode = d_inode_rcu(dentry);
> +		if (inode &&
> +		    NFS_PROTO(inode)->have_delegation(inode, FMODE_READ, 0))
> +			continue;

What's to stop the inode from being freed right under you?  You are *not*
guaranteed to be holding rcu_read_lock(), unless I'm missing something
in the call chain, so... what's going on here?