virtio: console: add locking around c_ovq operations

commit 9ba5c80b1aea8648a3efe5f22dc1f7cacdfbeeb8 upstream.

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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 4b56551..d5ae736 100644 (file)
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -132,6 +132,7 @@ 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;
@@ -457,11 +458,14 @@ 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) >= 0) {
                virtqueue_kick(vq);
                while (!virtqueue_get_buf(vq, &len))
                        cpu_relax();
        }
+       spin_unlock(&portdev->c_ovq_lock);
        return 0;
 }
 
@@ -1722,6 +1726,7 @@ static int __devinit 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,