netdev - Re: vhost-net: is there a race for sock in handle

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Date:	Thu, 3 May 2012 11:41:15 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Liu ping fan <kernelfans@...il.com>
Cc:	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: vhost-net: is there a race for sock in handle_tx/rx?

On Thu, May 03, 2012 at 04:33:55PM +0800, Liu ping fan wrote:
> Hi,
> 
> During reading the vhost-net code, I find the following,
> 
> static void handle_tx(struct vhost_net *net)
> {
> 	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> 	unsigned out, in, s;
> 	int head;
> 	struct msghdr msg = {
> 		.msg_name = NULL,
> 		.msg_namelen = 0,
> 		.msg_control = NULL,
> 		.msg_controllen = 0,
> 		.msg_iov = vq->iov,
> 		.msg_flags = MSG_DONTWAIT,
> 	};
> 	size_t len, total_len = 0;
> 	int err, wmem;
> 	size_t hdr_size;
> 	struct socket *sock;
> 	struct vhost_ubuf_ref *uninitialized_var(ubufs);
> 	bool zcopy;
> 
> 	/* TODO: check that we are running from vhost_worker? */
> 	sock = rcu_dereference_check(vq->private_data, 1);
> 	if (!sock)
> 		return;
> 
>            --------------------------------> Qemu calls
> vhost_net_set_backend() to set a new backend fd, and close
> @oldsock->file. And  sock->file refcnt==0.
> 
>                                               Can vhost_worker prevent
> itself from such situation? And how?
> 
> 	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
>        .........................................................................
> 
> Is it a race?
> 
> Thanks and regards,
> pingfan

See comment before void __rcu *private_data in vhost.h


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