virtio-scsi: Allocate op blocker reason before blocking

s->blocker is really only used in hw/scsi/virtio-scsi.c; the only places
where it is used in hw/scsi/virtio-scsi-dataplane.c is when it is
allocated and when it is freed. That does not make a whole lot of sense
(and is actually wrong because this leads to s->blocker potentially
being NULL when blk_op_block_all() is called in virtio-scsi.c), so move
the allocation and destruction of s->blocker to the device realization
and unrealization in virtio-scsi.c, respectively.

Case in point:

$ echo -e 'eject drv\nquit' | \
    x86_64-softmmu/qemu-system-x86_64 \
        -monitor stdio -machine accel=qtest -display none \
        -object iothread,id=thr -device virtio-scsi-pci,iothread=thr \
        -drive if=none,file=test.qcow2,format=qcow2,id=drv \
        -device scsi-cd,drive=drv

Without this patch:

(qemu) eject drv
[1]    10102 done
       10103 segmentation fault (core dumped)

With this patch:

(qemu) eject drv
Device 'drv' is busy: block device is in use by data plane
(qemu) quit

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <1425057113-26940-1-git-send-email-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 8c437dd..4db3b23 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -903,6 +903,8 @@ static void virtio_scsi_device_realize(DeviceState *dev, Error **errp)
                     virtio_scsi_save, virtio_scsi_load, s);
     s->migration_state_notifier.notify = virtio_scsi_migration_state_changed;
     add_migration_state_change_notifier(&s->migration_state_notifier);
+
+    error_setg(&s->blocker, "block device is in use by data plane");
 }
 
 static void virtio_scsi_instance_init(Object *obj)
@@ -928,6 +930,8 @@ static void virtio_scsi_device_unrealize(DeviceState *dev, Error **errp)
 {
     VirtIOSCSI *s = VIRTIO_SCSI(dev);
 
+    error_free(s->blocker);
+
     unregister_savevm(dev, "virtio-scsi", s);
     remove_migration_state_change_notifier(&s->migration_state_notifier);