virtio: console: add locking around c_ovq operations

When multiple ovq operations are being performed (lots of open/close
operations on virtio_console fds), the __send_control_msg() function can
get confused without locking.

A simple recipe to cause badness is:
* create a QEMU VM with two virtio-serial ports
* in the guest, do
  while true;do echo abc >/dev/vport0p1;done
  while true;do echo edf >/dev/vport0p2;done

In one run, this caused a panic in __send_control_msg().  In another, I
got

   virtio_console virtio0: control-o:id 0 is not a head!

This also results repeated messages similar to these on the host:

  qemu-kvm: virtio-serial-bus: Unexpected port id 478762112 for device virtio-serial-bus.0
  qemu-kvm: virtio-serial-bus: Unexpected port id 478762368 for device virtio-serial-bus.0
Reported-by: FuXiangChun <xfu@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Reviewed-by: Asias He <asias@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org

drivers/char/virtio_console.c

@@ -150,6 +150,7 @@ struct ports_device {
 
 	/* To protect the vq operations for the control channel */
 	spinlock_t c_ivq_lock;
+	spinlock_t c_ovq_lock;
 
 	/* The current config space is stored here */
 	struct virtio_console_config config;
@@ -569,11 +570,14 @@ static ssize_t __send_control_msg(struct ports_device *portdev, u32 port_id,
 	vq = portdev->c_ovq;
 
 	sg_init_one(sg, &cpkt, sizeof(cpkt));
+
+	spin_lock(&portdev->c_ovq_lock);
 	if (virtqueue_add_buf(vq, sg, 1, 0, &cpkt, GFP_ATOMIC) == 0) {
 		virtqueue_kick(vq);
 		while (!virtqueue_get_buf(vq, &len))
 			cpu_relax();
 	}
+	spin_unlock(&portdev->c_ovq_lock);
 	return 0;
 }
 
@@ -1997,6 +2001,7 @@ static int virtcons_probe(struct virtio_device *vdev)
 		unsigned int nr_added_bufs;
 
 		spin_lock_init(&portdev->c_ivq_lock);
+		spin_lock_init(&portdev->c_ovq_lock);
 		INIT_WORK(&portdev->control_work, &control_work_handler);
 
 		nr_added_bufs = fill_queue(portdev->c_ivq,