[PATCH v4 16/20] virtio: make it possible to detach host notifier from any thread

[Stefan Hajnoczi <stefanha@redhat.com>]

virtio_queue_aio_detach_host_notifier() does two things:
1. It removes the fd handler from the event loop.
2. It processes the virtqueue one last time.

The first step can be peformed by any thread and without taking the
AioContext lock.

The second step may need the AioContext lock (depending on the device
implementation) and runs in the thread where request processing takes
place. virtio-blk and virtio-scsi therefore call
virtio_queue_aio_detach_host_notifier() from a BH that is scheduled in
AioContext

Scheduling a BH is undesirable for .drained_begin() functions. The next
patch will introduce a .drained_begin() function that needs to call
virtio_queue_aio_detach_host_notifier().

Move the virtqueue processing out to the callers of
virtio_queue_aio_detach_host_notifier() so that the function can be
called from any thread. This is in preparation for the next patch.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 2 ++
 hw/scsi/virtio-scsi-dataplane.c | 9 +++++++++
 2 files changed, 11 insertions(+)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index b28d81737e..bd7cc6e76b 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -286,8 +286,10 @@ static void virtio_blk_data_plane_stop_bh(void *opaque)
 
     for (i = 0; i < s->conf->num_queues; i++) {
         VirtQueue *vq = virtio_get_queue(s->vdev, i);
+        EventNotifier *host_notifier = virtio_queue_get_host_notifier(vq);
 
         virtio_queue_aio_detach_host_notifier(vq, s->ctx);
+        virtio_queue_host_notifier_read(host_notifier);
     }
 }
 
diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
index 20bb91766e..81643445ed 100644
--- a/hw/scsi/virtio-scsi-dataplane.c
+++ b/hw/scsi/virtio-scsi-dataplane.c
@@ -71,12 +71,21 @@ static void virtio_scsi_dataplane_stop_bh(void *opaque)
 {
     VirtIOSCSI *s = opaque;
     VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
+    EventNotifier *host_notifier;
     int i;
 
     virtio_queue_aio_detach_host_notifier(vs->ctrl_vq, s->ctx);
+    host_notifier = virtio_queue_get_host_notifier(vs->ctrl_vq);
+    virtio_queue_host_notifier_read(host_notifier);
+
     virtio_queue_aio_detach_host_notifier(vs->event_vq, s->ctx);
+    host_notifier = virtio_queue_get_host_notifier(vs->event_vq);
+    virtio_queue_host_notifier_read(host_notifier);
+
     for (i = 0; i < vs->conf.num_queues; i++) {
         virtio_queue_aio_detach_host_notifier(vs->cmd_vqs[i], s->ctx);
+        host_notifier = virtio_queue_get_host_notifier(vs->cmd_vqs[i]);
+        virtio_queue_host_notifier_read(host_notifier);
     }
 }
 
-- 
2.39.2