virtio-scsi: fixed virtio_scsi_ctx_check failed when detaching scsi disk

commit a6f230c move blockbackend back to main AioContext on unplug. It set the AioContext of
SCSIDevice to the main AioContex, but s->ctx is still the iothread AioContex（if the scsi controller
is configure with iothread）. So if there are having in-flight requests during unplug, a failing assertion
happend. The bt is below:
(gdb) bt
#0  0x0000ffff86aacbd0 in raise () from /lib64/libc.so.6
#1  0x0000ffff86aadf7c in abort () from /lib64/libc.so.6
#2  0x0000ffff86aa6124 in __assert_fail_base () from /lib64/libc.so.6
#3  0x0000ffff86aa61a4 in __assert_fail () from /lib64/libc.so.6
#4  0x0000000000529118 in virtio_scsi_ctx_check (d=<optimized out>, s=<optimized out>, s=<optimized out>) at /home/qemu-4.0.0/hw/scsi/virtio-scsi.c:246
#5  0x0000000000529ec4 in virtio_scsi_handle_cmd_req_prepare (s=0x2779ec00, req=0xffff740397d0) at /home/qemu-4.0.0/hw/scsi/virtio-scsi.c:559
#6  0x000000000052a228 in virtio_scsi_handle_cmd_vq (s=0x2779ec00, vq=0xffff7c6d7110) at /home/qemu-4.0.0/hw/scsi/virtio-scsi.c:603
#7  0x000000000052afa8 in virtio_scsi_data_plane_handle_cmd (vdev=<optimized out>, vq=0xffff7c6d7110) at /home/qemu-4.0.0/hw/scsi/virtio-scsi-dataplane.c:59
#8  0x000000000054d94c in virtio_queue_host_notifier_aio_poll (opaque=<optimized out>) at /home/qemu-4.0.0/hw/virtio/virtio.c:2452

assert(blk_get_aio_context(d->conf.blk) == s->ctx) failed.

To avoid assertion failed,  moving the "if" after qdev_simple_device_unplug_cb.

In addition, to avoid another qemu crash below, add aio_disable_external before
qdev_simple_device_unplug_cb, which disable the further processing of external clients
when doing qdev_simple_device_unplug_cb.
(gdb) bt
#0  scsi_req_unref (req=0xffff6802c6f0) at hw/scsi/scsi-bus.c:1283
#1  0x00000000005294a4 in virtio_scsi_handle_cmd_req_submit (req=<optimized out>,
    s=<optimized out>) at /home/qemu-4.0.0/hw/scsi/virtio-scsi.c:589
#2  0x000000000052a2a8 in virtio_scsi_handle_cmd_vq (s=s@entry=0x9c90e90,
    vq=vq@entry=0xffff7c05f110) at /home/qemu-4.0.0/hw/scsi/virtio-scsi.c:625
#3  0x000000000052afd8 in virtio_scsi_data_plane_handle_cmd (vdev=<optimized out>,
    vq=0xffff7c05f110) at /home/qemu-4.0.0/hw/scsi/virtio-scsi-dataplane.c:60
#4  0x000000000054d97c in virtio_queue_host_notifier_aio_poll (opaque=<optimized out>)
    at /home/qemu-4.0.0/hw/virtio/virtio.c:2447
#5  0x00000000009b204c in run_poll_handlers_once (ctx=ctx@entry=0x6efea40,
    timeout=timeout@entry=0xffff7d7f7308) at util/aio-posix.c:521
#6  0x00000000009b2b64 in run_poll_handlers (ctx=ctx@entry=0x6efea40,
    max_ns=max_ns@entry=4000, timeout=timeout@entry=0xffff7d7f7308) at util/aio-posix.c:559
#7  0x00000000009b2ca0 in try_poll_mode (ctx=ctx@entry=0x6efea40, timeout=0xffff7d7f7308,
    timeout@entry=0xffff7d7f7348) at util/aio-posix.c:594
#8  0x00000000009b31b8 in aio_poll (ctx=0x6efea40, blocking=blocking@entry=true)
    at util/aio-posix.c:636
#9  0x00000000006973cc in iothread_run (opaque=0x6ebd800) at iothread.c:75
#10 0x00000000009b592c in qemu_thread_start (args=0x6efef60) at util/qemu-thread-posix.c:502
#11 0x0000ffff8057f8bc in start_thread () from /lib64/libpthread.so.0
#12 0x0000ffff804e5f8c in thread_start () from /lib64/libc.so.6
(gdb) p bus
$1 = (SCSIBus *) 0x0

Signed-off-by: Zhengui li <lizhengui@huawei.com>
Message-Id: <1563696502-7972-1-git-send-email-lizhengui@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1563829520-17525-1-git-send-email-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/virtio-scsi.c

@@ -832,6 +832,7 @@ static void virtio_scsi_hotunplug(HotplugHandler *hotplug_dev, DeviceState *dev,
     VirtIODevice *vdev = VIRTIO_DEVICE(hotplug_dev);
     VirtIOSCSI *s = VIRTIO_SCSI(vdev);
     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);
@@ -841,14 +842,16 @@ static void virtio_scsi_hotunplug(HotplugHandler *hotplug_dev, DeviceState *dev,
         virtio_scsi_release(s);
     }
 
+    aio_disable_external(ctx);
+    qdev_simple_device_unplug_cb(hotplug_dev, dev, errp);
+    aio_enable_external(ctx);
+
     if (s->ctx) {
         virtio_scsi_acquire(s);
         /* If other users keep the BlockBackend in the iothread, that's ok */
         blk_set_aio_context(sd->conf.blk, qemu_get_aio_context(), NULL);
         virtio_scsi_release(s);
     }
-
-    qdev_simple_device_unplug_cb(hotplug_dev, dev, errp);
 }
 
 static struct SCSIBusInfo virtio_scsi_scsi_info = {