Subject: Re: [RFC PATCH net-next] packet: fix using smp_processor_id() in preemptible code

From: Li Zhong <[email-masked]>
Date: Fri, 13 Dec 2013 07:55:03 +0800

On Wed, 2013-12-11 at 10:55 +0100, Daniel Borkmann wrote:
> On 12/12/2013 07:10 AM, Li Zhong wrote:
> > This patches tries to fix the following warning on next-1211 by
> > replacing smp_processor_id() with raw_smp_processor_id().
> 
> I agree with that part. Please send a fix just replacing smp_processor_id()
> into raw_smp_processor_id() for net-next to netdev directly.

Thank you for the review :), patch sent as you suggested. 

> 
> > Also, it seems that we could move skb_set_queue_mapping() into
> > packet_pick_tx_queue(), so we avoid calling it one more time
> > unnecessarily if we are going into the normal dev_queue_xmit() code
> > path.
> 
> I don't agree with that part, I think this can be also beneficiary for
> packets without direct xmit, as in PF_PACKET we don't have a notion of
> "flow" but just raw packets instead, and can keep the mapping local
> depending on the current CPU as we do queue setting elsewhere in the
> stack just as well.

It seems to me that the newly added xmit in packet_sock is
dev_queue_xmit() by default, and in this default case, dev_queue_xmit()
would call netdev_pick_tx(), which would set the skb queue_mapping again
to override the value based on the current CPU. 

Or did I miss something here? 

Thanks, Zhong

> 
> > [   11.120893] BUG: using smp_processor_id() in preemptible [00000000] code: arping/3510
> > [   11.120913] caller is .packet_sendmsg+0xc14/0xe68
> > [   11.120920] CPU: 13 PID: 3510 Comm: arping Not tainted 3.13.0-rc3-next-20131211-dirty #1
> > [   11.120926] Call Trace:
> > [   11.120932] [c0000001f803f6f0] [c0000000000138dc] .show_stack+0x110/0x25c (unreliable)
> > [   11.120942] [c0000001f803f7e0] [c00000000083dd24] .dump_stack+0xa0/0x37c
> > [   11.120951] [c0000001f803f870] [c000000000493fd4] .debug_smp_processor_id+0xfc/0x12c
> > [   11.120959] [c0000001f803f900] [c0000000007eba78] .packet_sendmsg+0xc14/0xe68
> > [   11.120968] [c0000001f803fa80] [c000000000700968] .sock_sendmsg+0xa0/0xe0
> > [   11.120975] [c0000001f803fbf0] [c0000000007014d8] .SyS_sendto+0x100/0x148
> > [   11.120983] [c0000001f803fd60] [c0000000006fff10] .SyS_socketcall+0x1c4/0x2e8
> > [   11.120990] [c0000001f803fe30] [c00000000000a1e4] syscall_exit+0x0/0x9c
> >
> > Signed-off-by: Li Zhong <[email-masked]>
> > ---
> >   net/packet/af_packet.c |   16 ++++++++--------
> >   1 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index 9d70f13..20f6d56 100644
> > --- a/net/packet/af_packet.c
> > +++ b/net/packet/af_packet.c
> > @@ -237,6 +237,13 @@ struct packet_skb_cb {
> >   static void __fanout_unlink(struct sock *sk, struct packet_sock *po);
> >   static void __fanout_link(struct sock *sk, struct packet_sock *po);
> >
> > +static u16 packet_pick_tx_queue(struct net_device *dev, struct sk_buff *skb)
> > +{
> > +	u16 queue_index = (u16)raw_smp_processor_id() % dev->real_num_tx_queues;
> > +	skb_set_queue_mapping(skb, queue_index);
> > +	return queue_index;
> > +}
> > +
> >   static int packet_direct_xmit(struct sk_buff *skb)
> >   {
> >   	struct net_device *dev = skb->dev;
> > @@ -259,7 +266,7 @@ static int packet_direct_xmit(struct sk_buff *skb)
> >   		return NET_XMIT_DROP;
> >   	}
> >
> > -	queue_map = skb_get_queue_mapping(skb);
> > +	queue_map = packet_pick_tx_queue(dev, skb);
> >   	txq = netdev_get_tx_queue(dev, queue_map);
> >
> >   	__netif_tx_lock_bh(txq);
> > @@ -308,11 +315,6 @@ static bool packet_use_direct_xmit(const struct packet_sock *po)
> >   	return po->xmit == packet_direct_xmit;
> >   }
> >
> > -static u16 packet_pick_tx_queue(struct net_device *dev)
> > -{
> > -	return (u16) smp_processor_id() % dev->real_num_tx_queues;
> > -}
> > -
> >   /* register_prot_hook must be invoked with the po->bind_lock held,
> >    * or from a context in which asynchronous accesses to the packet
> >    * socket is not possible (packet_create()).
> > @@ -2219,7 +2221,6 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
> >   			}
> >   		}
> >
> > -		skb_set_queue_mapping(skb, packet_pick_tx_queue(dev));
> >   		skb->destructor = tpacket_destruct_skb;
> >   		__packet_set_status(po, ph, TP_STATUS_SENDING);
> >   		atomic_inc(&po->tx_ring.pending);
> > @@ -2429,7 +2430,6 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> >   	skb->dev = dev;
> >   	skb->priority = sk->sk_priority;
> >   	skb->mark = sk->sk_mark;
> > -	skb_set_queue_mapping(skb, packet_pick_tx_queue(dev));
> >
> >   	if (po->has_vnet_hdr) {
> >   		if (vnet_hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> >
> >
>