Re: [PATCH net v2] wireguard: wait for per-peer crypto during removal

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Sun, Sep 13, 2026 at 07:19:08AM -0500, Chris J Arges wrote:
> Calling peer_remove_after_dead() currently flushes device-wide packet
> crypto and handshake workqueues while holding RTNL. This is problematic as
> unrelated peers can continue adding work to those queues, blocking other
> tasks that want to take the RTNL lock.
> 
> Instead, this patch tracks pending crypto handoffs for each peer using a
> counter. After marking the peer dead, synchronize_net() prevents new
> submissions. Next, wait for pending crypto workers to schedule TX work and
> for RX NAPI to drain the peer's RX queue. Then flush only the peer's TX
> packet and handshake work.
> 
> This scopes teardown synchronization to the removed peer and prevents
> unrelated peers from extending the RTNL hold time.
>  struct wg_device;
> @@ -161,6 +162,7 @@ static inline int wg_queue_enqueue_per_device_and_peer(
>  	 */
>  	if (unlikely(!wg_prev_queue_enqueue(peer_queue, skb)))
>  		return -ENOSPC;
> +	atomic_inc(&PACKET_PEER(skb)->packet_crypt_pending);
>  
>  	/* Then we queue it up in the device queue, which consumes the
>  	 * packet as soon as it can.
> @@ -182,6 +184,8 @@ static inline void wg_queue_enqueue_per_peer_tx(struct sk_buff *skb, enum packet
>  	atomic_set_release(&PACKET_CB(skb)->state, state);
>  	queue_work_on(wg_cpumask_choose_online(&peer->serial_work_cpu, peer->internal_id),
>  		      peer->device->packet_crypt_wq, &peer->transmit_packet_work);
> +	if (atomic_dec_and_test(&peer->packet_crypt_pending))
> +		wake_up_var(&peer->packet_crypt_pending);
>  	wg_peer_put(peer);
>  }
>  
> diff --git a/drivers/net/wireguard/receive.c b/drivers/net/wireguard/receive.c
> index 824bbefce61c..bb35e3205491 100644
> --- a/drivers/net/wireguard/receive.c
> +++ b/drivers/net/wireguard/receive.c
> @@ -476,9 +476,11 @@ int wg_packet_rx_poll(struct napi_struct *napi, int budget)
>  
>  next:
>  		wg_noise_keypair_put(keypair, false);
> -		wg_peer_put(peer);
>  		if (unlikely(free))
>  			dev_kfree_skb(skb);
> +		if (atomic_dec_and_test(&peer->packet_crypt_pending))
> +			wake_up_var(&peer->packet_crypt_pending);
> +		wg_peer_put(peer);

This adds two atomic updates to a per-peer counter for every RX packet
and TX batch. That could introduce contention across crypto workers. I
suppose it'd be good to see some measurements in if this changes
anything. Certainly it should change _something_. Question is by how
much.

Jason