Subject: Re: [PATCH net-next v3 2/4] netdevsim: forward skbs from one connected port to another

From: Jiri Pirko <[email-masked]>
Date: Sun, 17 Dec 2023 12:36:46 +0100

Sun, Dec 17, 2023 at 03:59:53AM CET, [email-masked] wrote:
>On 2023-12-16 01:22, Jiri Pirko wrote:
>> Fri, Dec 15, 2023 at 07:31:42PM CET, [email-masked] wrote:
>>> On 2023-12-15 02:45, Jiri Pirko wrote:
>>>> Thu, Dec 14, 2023 at 10:24:41PM CET, [email-masked] wrote:
>>>>> Forward skbs sent from one netdevsim port to its connected netdevsim
>>>>> port using dev_forward_skb, in a spirit similar to veth.
>>>>
>>>> Perhaps better to write "dev_forward_skb()" to make obvious you talk
>>>> about function.
>>>
>>> Sorry, it's a bad habit at this point :)
>>>
>>>>
>>>>
>>>>>
>>>>> Signed-off-by: David Wei <[email-masked]>
>>>>> ---
>>>>> drivers/net/netdevsim/netdev.c | 23 ++++++++++++++++++-----
>>>>> 1 file changed, 18 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
>>>>> index e290c54b0e70..c5f53b1dbdcc 100644
>>>>> --- a/drivers/net/netdevsim/netdev.c
>>>>> +++ b/drivers/net/netdevsim/netdev.c
>>>>> @@ -29,19 +29,33 @@
>>>>> static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>>>> {
>>>>> 	struct netdevsim *ns = netdev_priv(dev);
>>>>> +	struct netdevsim *peer_ns;
>>>>> +	int ret = NETDEV_TX_OK;
>>>>>
>>>>> +	rcu_read_lock();
>>>>
>>>> Why do you need to be in rcu read locked section here?
>>>
>>> So the RCU protected pointer `peer` does not change during the critical
>>> section. Veth does something similar in its xmit() for its peer.
>> 
>> RCU does not work like this. Please check out the documentation.
>
>When destroying a netdevsim in nsim_destroy(), rtnl_lock is held which prevents
>concurrent destruction of netdevsims. Then, unregister_netdevice() will
>eventually call synchronize_rcu_expedited().
>
>Let's say we have two netdevsims, A linked with B, where A->peer is B and
>B->peer is A.
>
>If we're destroying B in nsim_destroy(), then we first do
>rcu_assign_pointer(A->peer, NULL). Of course, any read-side critical sections
>that dereferenced a non-NULL A->peer won't be affected by this update.
>
>Then B's nsim_destroy() calls unregister_netdevice(), followed eventually by
>synchronize_rcu_expedited(). As I understand RCU, this will wait for one RCU
>grace period, or any nsim_start_xmit() that started _before_ B's
>rcu_assign_pointer(A->peer, NULL) to complete.
>
>RCU docs say that the caller of synchronize_rcu() upon return may be again
>concurrent w/ another nsim_start_xmit() reader. But they should see NULL for
>A->peer ptr due to the rcu_assign_pointer(A->peer, NULL) update during B's
>nsim_destroy(). So after synchronize_rcu() no skb from A should be forwarded to
>B anymore.
>
>In fact, it looks like since v5.0 being in a softirq handler serves as an RCU
>read-side critical section. So the rcu_read_lock() here in nsim_start_xmit() is
>actually redundant.
>
>I believe this is veth's intention too. There is a comment in veth_dellink()
>that says the pair of peer devices are guaranteed to be not freed before one
>RCU grace period.
>
>As long as adding/removing links is also under rtnl_lock, I think with RCU
>guarantees discussed above we will be SMP safe. Does this seem right to you?
>
>> 
>> 
>>>
>>>>
>>>>
>>>>> 	if (!nsim_ipsec_tx(ns, skb))
>>>>> -		goto out;
>>>>> +		goto err;
>>>>
>>>> Not sure why you need to rename the label. Why "out" is not okay?
>>>>
>>>>>
>>>>> 	u64_stats_update_begin(&ns->syncp);
>>>>> 	ns->tx_packets++;
>>>>> 	ns->tx_bytes += skb->len;
>>>>> 	u64_stats_update_end(&ns->syncp);
>>>>>
>>>>> -out:
>>>>> -	dev_kfree_skb(skb);

My point is, why don't take rcu_read_lock() here.

>>>>> +	peer_ns = rcu_dereference(ns->peer);
>>>>> +	if (!peer_ns)
>>>>> +		goto err;
>>>>
>>>> This is definitelly not an error path, "err" label name is misleading.
>>>
>>> That's fair, I can change it back. Lots has changed since my original
>>> intentions.
>>>
>>>>
>>>>
>>>>> +
>>>>> +	skb_tx_timestamp(skb);
>>>>> +	if (unlikely(dev_forward_skb(peer_ns->netdev, skb) == NET_RX_DROP))
>>>>> +		ret = NET_XMIT_DROP;
>>>>
>>>> Hmm, can't you track dropped packets in ns->tx_dropped and expose in
>>>> nsim_get_stats64() ?
>>>
>>> I can add this.
>>>
>>>>
>>>>
>>>>>
>>>>> -	return NETDEV_TX_OK;
>>>>> +	rcu_read_unlock();
>>>>> +	return ret;
>>>>> +
>>>>> +err:
>>>>> +	rcu_read_unlock();
>>>>> +	dev_kfree_skb(skb);
>>>>> +	return ret;
>>>>> }
>>>>>
>>>>> static void nsim_set_rx_mode(struct net_device *dev)
>>>>> @@ -302,7 +316,6 @@ static void nsim_setup(struct net_device *dev)
>>>>> 	eth_hw_addr_random(dev);
>>>>>
>>>>> 	dev->tx_queue_len = 0;
>>>>> -	dev->flags |= IFF_NOARP;
>>>>> 	dev->flags &= ~IFF_MULTICAST;
>>>>> 	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE |
>>>>> 			   IFF_NO_QUEUE;
>>>>> -- 
>>>>> 2.39.3
>>>>>