[Qemu-devel] [PATCH v2] virtio-scsi: Allocate op blocker reason before blocking

[Max Reitz <mreitz@redhat.com>]

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>
---
v2:
- Put the reproducer into the commit message [Markus] and modified its
  wording to be more fitting of a commit message ("Case in point"
  instead of the imperative "Try").
- As noted by Fam on my bdrv_close_all() series, there can be multiple
  block devices per virtio-scsi bus; therefore, it's wrong to delete the
  blocker if one of these is unplugged. Instead, just allocate the
  blocker with the virtio-scsi device itself and free it when the device
  is unrealized.
---
 hw/scsi/virtio-scsi-dataplane.c | 4 ----
 hw/scsi/virtio-scsi.c           | 4 ++++
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
index 03a1e8c..9b775d4 100644
--- a/hw/scsi/virtio-scsi-dataplane.c
+++ b/hw/scsi/virtio-scsi-dataplane.c
@@ -211,8 +211,6 @@ void virtio_scsi_dataplane_start(VirtIOSCSI *s)
 
     s->dataplane_starting = true;
 
-    assert(!s->blocker);
-    error_setg(&s->blocker, "block device is in use by data plane");
     /* Set up guest notifier (irq) */
     rc = k->set_guest_notifiers(qbus->parent, vs->conf.num_queues + 2, true);
     if (rc != 0) {
@@ -279,8 +277,6 @@ void virtio_scsi_dataplane_stop(VirtIOSCSI *s)
     if (!s->dataplane_started || s->dataplane_stopping) {
         return;
     }
-    error_free(s->blocker);
-    s->blocker = NULL;
     s->dataplane_stopping = true;
     assert(s->ctx == iothread_get_aio_context(vs->conf.iothread));
 
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 9e2c718..9c2a272 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -904,6 +904,8 @@ 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)
@@ -929,6 +931,8 @@ 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);
 
-- 
2.1.0