Re: [PATCH v2 2/2] RDMA/rxe: Fix up RCU usage for rxe_ns_pernet_sk6().

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Apr 25, 2026 at 06:04:14AM +0000, Kuniyuki Iwashima wrote:
> @@ -133,16 +133,21 @@ static struct dst_entry *rxe_find_route6(struct rxe_qp *qp,
>  					 struct in6_addr *saddr,
>  					 struct in6_addr *daddr)
>  {
> -	struct dst_entry *ndst;
> +	struct dst_entry *ndst = NULL;
>  	struct flowi6 fl6 = {};
> +	struct sock *sk;
>  
>  	fl6.flowi6_oif = ndev->ifindex;
>  	memcpy(&fl6.saddr, saddr, sizeof(*saddr));
>  	memcpy(&fl6.daddr, daddr, sizeof(*daddr));
>  	fl6.flowi6_proto = IPPROTO_UDP;
>  
> -	ndst = ip6_dst_lookup_flow(net, rxe_ns_pernet_sk6(net), &fl6, NULL);
> -	if (IS_ERR(ndst)) {
> +	rcu_read_lock();
> +	sk = rxe_ns_pernet_sk6(net);
> +	if (sk)
> +		ndst = ip6_dst_lookup_flow(net, sk, &fl6, NULL);
> +	rcu_read_unlock();
> +	if (IS_ERR_OR_NULL(ndst)) {
>  		rxe_dbg_qp(qp, "no route to %pI6\n", daddr);
>  		return NULL;
>  	}

This idea seems correct, but ..

>  struct sock *rxe_ns_pernet_sk6(struct net *net)
>  {
>  	struct rxe_ns_sock *ns_sk = net_generic(net, rxe_pernet_id);
> -	struct sock *sk;
> -
> -	rcu_read_lock();
> -	sk = rcu_dereference(ns_sk->rxe_sk6);
> -	rcu_read_unlock();
>
> -	return sk;
> +	return rcu_dereference(ns_sk->rxe_sk6);
>  }

This is called from several places that don't hold the rcu so it can't
quite work like this.

Jason