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

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Date: Sat, 16 Dec 2023 18:59:53 -0800
From: David Wei <dw@...idwei.uk>
To: Jiri Pirko <jiri@...nulli.us>
Cc: Jakub Kicinski <kuba@...nel.org>, Sabrina Dubroca <sd@...asysnail.net>,
 netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v3 2/4] netdevsim: forward skbs from one
 connected port to another

On 2023-12-16 01:22, Jiri Pirko wrote:
> Fri, Dec 15, 2023 at 07:31:42PM CET, dw@...idwei.uk wrote:
>> On 2023-12-15 02:45, Jiri Pirko wrote:
>>> Thu, Dec 14, 2023 at 10:24:41PM CET, dw@...idwei.uk 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 <dw@...idwei.uk>
>>>> ---
>>>> 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);
>>>> +	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
>>>>