[PATCH 1/2] virtio-scsi: avoid race between unplug and transport event

[Stefan Hajnoczi]

Only report a transport reset event to the guest after the SCSIDevice
has been unrealized by qdev_simple_device_unplug_cb().

qdev_simple_device_unplug_cb() sets the SCSIDevice's qdev.realized field
to false so that scsi_device_find/get() no longer see it.

scsi_target_emulate_report_luns() also needs to be updated to filter out
SCSIDevices that are unrealized.

These changes ensure that the guest driver does not see the SCSIDevice
that's being unplugged if it responds very quickly to the transport
reset event.

I noticed this issue when reading the code and have not reproduced it.

Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
 hw/scsi/scsi-bus.c    |  3 ++-
 hw/scsi/virtio-scsi.c | 18 +++++++++---------
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index c97176110c..f9bd064833 100644
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -487,7 +487,8 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq 
*r)
             DeviceState *qdev = kid->child;
             SCSIDevice *dev = SCSI_DEVICE(qdev);
 
-            if (dev->channel == channel && dev->id == id && dev->lun != 0) {
+            if (dev->channel == channel && dev->id == id && dev->lun != 0 &&
+                qatomic_load_acquire(&dev->qdev.realized)) {
                 store_lun(tmp, dev->lun);
                 g_byte_array_append(buf, tmp, 8);
                 len += 8;
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 612c525d9d..000961446c 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -1063,15 +1063,6 @@ static void virtio_scsi_hotunplug(HotplugHandler 
*hotplug_dev, DeviceState *dev,
     SCSIDevice *sd = SCSI_DEVICE(dev);
     AioContext *ctx = s->ctx ?: qemu_get_aio_context();
 
-    if (virtio_vdev_has_feature(vdev, VIRTIO_SCSI_F_HOTPLUG)) {
-        virtio_scsi_acquire(s);
-        virtio_scsi_push_event(s, sd,
-                               VIRTIO_SCSI_T_TRANSPORT_RESET,
-                               VIRTIO_SCSI_EVT_RESET_REMOVED);
-        scsi_bus_set_ua(&s->bus, SENSE_CODE(REPORTED_LUNS_CHANGED));
-        virtio_scsi_release(s);
-    }
-
     aio_disable_external(ctx);
     qdev_simple_device_unplug_cb(hotplug_dev, dev, errp);
     aio_enable_external(ctx);
@@ -1082,6 +1073,15 @@ static void virtio_scsi_hotunplug(HotplugHandler 
*hotplug_dev, DeviceState *dev,
         blk_set_aio_context(sd->conf.blk, qemu_get_aio_context(), NULL);
         virtio_scsi_release(s);
     }
+
+    if (virtio_vdev_has_feature(vdev, VIRTIO_SCSI_F_HOTPLUG)) {
+        virtio_scsi_acquire(s);
+        virtio_scsi_push_event(s, sd,
+                               VIRTIO_SCSI_T_TRANSPORT_RESET,
+                               VIRTIO_SCSI_EVT_RESET_REMOVED);
+        scsi_bus_set_ua(&s->bus, SENSE_CODE(REPORTED_LUNS_CHANGED));
+        virtio_scsi_release(s);
+    }
 }
 
 static struct SCSIBusInfo virtio_scsi_scsi_info = {
-- 
2.39.2