Subject: Re: [PATCH] vhost_net: clear msg.control for non-zerocopy case during tx

From: Sergei Shtylyov <[email-masked]>
Date: Wed, 05 Jun 2013 17:44:43 +0400

Hello.

On 05-06-2013 11:40, Jason Wang wrote:

> When we decide not use zero-copy, msg.control should be set to NULL otherwise
> macvtap/tap may set zerocopy callbacks which may decrease the kref of ubufs
> wrongly.

> Bug were introduced by commit cedb9bdce099206290a2bdd02ce47a7b253b6a84
> (vhost-net: skip head management if no outstanding).

> This solves the following warnings:

> WARNING: at include/linux/kref.h:47 handle_tx+0x477/0x4b0 [vhost_net]()
> Modules linked in: vhost_net macvtap macvlan tun nfsd exportfs bridge stp llc openvswitch kvm_amd kvm bnx2 megaraid_sas [last unloaded: tun]
> CPU: 5 PID: 8670 Comm: vhost-8668 Not tainted 3.10.0-rc2+ #1566
> Hardware name: Dell Inc. PowerEdge R715/00XHKG, BIOS 1.5.2 04/19/2011
> ffffffffa0198323 ffff88007c9ebd08 ffffffff81796b73 ffff88007c9ebd48
> ffffffff8103d66b 000000007b773e20 ffff8800779f0000 ffff8800779f43f0
> ffff8800779f8418 000000000000015c 0000000000000062 ffff88007c9ebd58
> Call Trace:
> [<ffffffff81796b73>] dump_stack+0x19/0x1e
> [<ffffffff8103d66b>] warn_slowpath_common+0x6b/0xa0
> [<ffffffff8103d6b5>] warn_slowpath_null+0x15/0x20
> [<ffffffffa0197627>] handle_tx+0x477/0x4b0 [vhost_net]
> [<ffffffffa0197690>] handle_tx_kick+0x10/0x20 [vhost_net]
> [<ffffffffa019541e>] vhost_worker+0xfe/0x1a0 [vhost_net]
> [<ffffffffa0195320>] ? vhost_attach_cgroups_work+0x30/0x30 [vhost_net]
> [<ffffffffa0195320>] ? vhost_attach_cgroups_work+0x30/0x30 [vhost_net]
> [<ffffffff81061f46>] kthread+0xc6/0xd0
> [<ffffffff81061e80>] ? kthread_freezable_should_stop+0x70/0x70
> [<ffffffff817a1aec>] ret_from_fork+0x7c/0xb0
> [<ffffffff81061e80>] ? kthread_freezable_should_stop+0x70/0x70

> Signed-off-by: Jason Wang <[email-masked]>
> ---
>   drivers/vhost/net.c |    3 ++-
>   1 files changed, 2 insertions(+), 1 deletions(-)

> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 2b51e23..b07d96b 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -436,7 +436,8 @@ static void handle_tx(struct vhost_net *net)
>   				kref_get(&ubufs->kref);
>   			}
>   			nvq->upend_idx = (nvq->upend_idx + 1) % UIO_MAXIOV;
> -		}
> +		} else

    You have to use {} on the *else* branch if you have it of the *if* 
branch (and vice versa), according to Documentation/CodingStyle.

> +			msg.msg_control = NULL;

WBR, Sergei