Subject: Re: [PATCH] macvtap: fix two races

From: Vlad Yasevich <[email-masked]>
Date: Fri, 09 Aug 2013 13:16:10 -0400

On 08/08/2013 11:06 AM, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
>
> Since commit ac4e4af1e59e1 ("macvtap: Consistently use rcu functions"),
> Thomas gets two different warnings :
>
> BUG: using smp_processor_id() in preemptible [00000000] code: vhost-45891/45892
> caller is macvtap_do_read+0x45c/0x600 [macvtap]
> CPU: 1 PID: 45892 Comm: vhost-45891 Not tainted 3.11.0-bisecttest #13
> Call Trace:
> ([<00000000001126ee>] show_trace+0x126/0x144)
>   [<00000000001127d2>] show_stack+0xc6/0xd4
>   [<000000000068bcec>] dump_stack+0x74/0xd8
>   [<0000000000481066>] debug_smp_processor_id+0xf6/0x114
>   [<000003ff802e9a18>] macvtap_do_read+0x45c/0x600 [macvtap]
>   [<000003ff802e9c1c>] macvtap_recvmsg+0x60/0x88 [macvtap]
>   [<000003ff80318c5e>] handle_rx+0x5b2/0x800 [vhost_net]
>   [<000003ff8028f77c>] vhost_worker+0x15c/0x1c4 [vhost]
>   [<000000000015f3ac>] kthread+0xd8/0xe4
>   [<00000000006934a6>] kernel_thread_starter+0x6/0xc
>   [<00000000006934a0>] kernel_thread_starter+0x0/0xc
>
> And
>
> 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
> 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]
>
>
> In the first case, macvtap_put_user() calls macvlan_count_rx()
> in a preempt-able context, and this is not allowed.
>
> In the second case, macvtap_get_user() calls
> macvlan_start_xmit() with BH enabled, and this is not allowed.
>
> Reported-by: Thomas Huth <[email-masked]>
> Bisected-by: Thomas Huth <[email-masked]>
> Signed-off-by: Eric Dumazet <[email-masked]>
> Tested-by: Thomas Huth <[email-masked]>
> Cc: Vlad Yasevich <[email-masked]>
> ---
>   drivers/net/macvtap.c |   12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
>
> 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();
> +	}

I was looking at this a bit more and I think this call to 
macvlan_count_rx() is double counting the packets.

In macvlan_handle_frame(), we call macvlan_count_rx() after we call
vlan->receive().  For macvtap, receive() function is essentially 
macvtap_forward() which just tacks the data onto the queue.

Then, the above code counts the data again as we pull it off the queue
socket queue to give to the user.

-vlad

>   	rcu_read_unlock();
>
>   	return ret ? ret : copied;
>
>