[Qemu-devel] [RFC v3 3/3] virtio-scsi: fix iothread deadlock on 'cont'

[Stefan Hajnoczi <stefanha@redhat.com>]

When the 'cont' command resumes guest execution the vm change state
handlers are invoked.  Unfortunately there is no explicit ordering
between vm change state handlers.  When two layers of code both use vm
change state handlers, we don't control which handler runs first.

virtio-scsi with iothreads hits a deadlock when a failed SCSI command is
restarted and completes before the iothread is re-initialized.

This patch makes sure that DMA restart happens after the iothread has
been started again.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/scsi/virtio-scsi.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 839f120256..236a0ee873 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -846,12 +846,28 @@ static void virtio_scsi_hotunplug(HotplugHandler *hotplug_dev, DeviceState *dev,
     qdev_simple_device_unplug_cb(hotplug_dev, dev, errp);
 }
 
+static void virtio_scsi_vmstate_change(VirtIODevice *vdev, bool running)
+{
+    VirtIOSCSI *s = VIRTIO_SCSI(vdev);
+
+    if (running) {
+        scsi_bus_dma_restart(&s->bus);
+    }
+}
+
 static struct SCSIBusInfo virtio_scsi_scsi_info = {
     .tcq = true,
     .max_channel = VIRTIO_SCSI_MAX_CHANNEL,
     .max_target = VIRTIO_SCSI_MAX_TARGET,
     .max_lun = VIRTIO_SCSI_MAX_LUN,
 
+    /* We call scsi_bus_dma_restart() ourselves to control the ordering between
+     * ->start_ioeventfd() and DMA restart.  Do it in
+     * virtio_scsi_vmstate_change(), which is called by the core virtio code
+     * after ->start_ioeventfd().
+     */
+    .custom_dma_restart = true,
+
     .complete = virtio_scsi_command_complete,
     .cancel = virtio_scsi_request_cancelled,
     .change = virtio_scsi_change,
@@ -986,6 +1002,7 @@ static void virtio_scsi_class_init(ObjectClass *klass, void *data)
     vdc->reset = virtio_scsi_reset;
     vdc->start_ioeventfd = virtio_scsi_dataplane_start;
     vdc->stop_ioeventfd = virtio_scsi_dataplane_stop;
+    vdc->vmstate_change = virtio_scsi_vmstate_change;
     hc->plug = virtio_scsi_hotplug;
     hc->unplug = virtio_scsi_hotunplug;
 }
-- 
2.21.0