linux-kernel - vhost-net: is there a race for sock in handle

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Date:	Thu, 3 May 2012 16:33:55 +0800
From:	Liu ping fan <kernelfans@...il.com>
To:	netdev@...r.kernel.org
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: vhost-net: is there a race for sock in handle_tx/rx?

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
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