Subject: Re: macvtap bug: using smp_processor_id() in preemptible code

From: Thomas Huth <[email-masked]>
Date: Thu, 8 Aug 2013 15:56:34 +0200

Am Thu, 08 Aug 2013 06:21:12 -0700
schrieb Eric Dumazet <[email-masked]>:

> On Thu, 2013-08-08 at 10:25 +0200, Thomas Huth wrote:
> 
> > Thank you very much for your fast reply and the fix, it indeed fixes
> > the messages about macvtap_do_read!
> > However, I now noticed that there are more messages, which I just did
> > not see before because my dmesg output was already flooded with the
> > messages about macvtap_do_read. The other messages are all about
> > macvlan_start_xmit:
> > 
> > BUG: using smp_processor_id() in preemptible [00000000] code: vhost-45897/45898
> > caller is macvlan_start_xmit+0x10a/0x1b4 [macvlan]
> > CPU: 1 PID: 45898 Comm: vhost-45897 Not tainted 3.11.0-bisecttest #16
> >        00000001189b3960 00000001189b3970 0000000000000002 0000000000000000 
> >        00000001189b3a00 00000001189b3978 00000001189b3978 00000000001127b4 
> >        0000000000000000 0000000000000001 0000000000000000 000000000000000b 
> >        0000000000000060 000003fe00000008 0000000000000000 00000001189b39d0 
> >        00000000006ea2f0 00000000001127b4 00000001189b3960 00000001189b39b0 
> > Call Trace:
> > ([<00000000001126ee>] show_trace+0x126/0x144)
> >  [<00000000001127d2>] show_stack+0xc6/0xd4
> >  [<000000000068bdb8>] dump_stack+0x74/0xd4
> >  [<0000000000481132>] debug_smp_processor_id+0xf6/0x114
> >  [<000003ff802b72ca>] macvlan_start_xmit+0x10a/0x1b4 [macvlan]
> >  [<000003ff802ea69a>] macvtap_get_user+0x982/0xbc4 [macvtap]
> >  [<000003ff802ea92a>] macvtap_sendmsg+0x4e/0x60 [macvtap]
> >  [<000003ff8031947c>] handle_tx+0x494/0x5ec [vhost_net]
> >  [<000003ff8028f77c>] vhost_worker+0x15c/0x1c4 [vhost]
> >  [<000000000015f3ac>] kthread+0xd8/0xe4
> >  [<000000000069356e>] kernel_thread_starter+0x6/0xc
> >  [<0000000000693568>] kernel_thread_starter+0x0/0xc
> > 2 locks held by vhost-45897/45898:
> >  #0:  (&vq->mutex){+.+.+.}, at: [<000003ff8031903c>] handle_tx+0x54/0x5ec [vhost_net]
> >  #1:  (rcu_read_lock){.+.+..}, at: [<000003ff802ea53c>] macvtap_get_user+0x824/0xbc4 [macvtap]
> > 
> > Do you also have got an idea how to silence these messages?
> 
> Sure, please try following cumulative patch, thanks !
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index a98fb0e..b51db2a 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -818,10 +818,13 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
>  		skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
>  		skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
>  	}
> -	if (vlan)
> +	if (vlan) {
> +		local_bh_disable();
>  		macvlan_start_xmit(skb, vlan->dev);
> -	else
> +		local_bh_enable();
> +	} else {
>  		kfree_skb(skb);
> +	}
>  	rcu_read_unlock();
> 
>  	return total_len;
> @@ -912,8 +915,11 @@ static ssize_t macvtap_put_user(struct macvtap_queue *q,
>  done:
>  	rcu_read_lock();
>  	vlan = rcu_dereference(q->vlan);
> -	if (vlan)
> +	if (vlan) {
> +		preempt_disable();
>  		macvlan_count_rx(vlan, copied - vnet_hdr_len, ret == 0, 0);
> +		preempt_enable();
> +	}
>  	rcu_read_unlock();
> 
>  	return ret ? ret : copied;

This patch now silences all error messages! Great, thank you very much!
If you submit this patch, you can add my
 Tested-by: Thomas Huth <[email-masked]>
if you like.

 Thomas