Re: [PATCH] nvme: fix namespace removal list

[Christoph Hellwig <hch@lst.de>]

On Tue, Jun 11, 2024 at 08:20:55AM -0700, Keith Busch wrote:
>  	mutex_lock(&ctrl->namespaces_lock);
>  	list_for_each_entry_safe(ns, next, &ctrl->namespaces, list) {
> -		if (ns->head->ns_id > nsid)
> -			list_splice_init_rcu(&ns->list, &rm_list,
> -					     synchronize_rcu);
> +		if (ns->head->ns_id > nsid) {
> +			list_del_rcu(&ns->list);
> +			list_add_tail_rcu(&ns->list, &rm_list);
> +		}

Is this actually valid for a (S)RCU protected list?  If the entry gets
added to the new list before the grace period has completed, we could
trick a concurrent traversal into following the new list unless I'm
mistaken (although chances I'm mistaken on RCU corner cases aren't that
low..).