[Qemu-devel] [RFC PATCH 3/4] virtio-blk: Use aio_set_io_event_notifier in dataplane

[Fam Zheng <famz@redhat.com>]

Currently the host notifier is checked by all aio_poll, which is not
safe. For example, in a qmp transaction that takes snapshots or starts
drive-backup on multiple dataplane disks, the atomicity could be broken:
There could be one or more bdrv_drain_all() calls in each transaction
opeartion, which will unconditinally calls one or more aio_poll on the
AioContext:

    qmp transaction
        backup A prepare
            ...
            bdrv_drain_all()
                aio_poll(A)
                aio_poll(B)
                aio_poll(C)
                ...
            ...
        backup B prepare
            ...
            bdrv_drain_all()
                aio_poll(A)
                aio_poll(B)
->              aio_poll(C)
            ...
        snapshot C prepare
            ...
            bdrv_drain_all()
                aio_poll(A)
                aio_poll(B)
                aio_poll(C)
            ...

If the aio_poll(C) in the middle receives a new virtio-blk request from
ioeventfd, a new request will be submitted to C, then snapshot C is
inconsistent.

To avoid that, use aio_set_io_event_notifier so the behavior is the same
as in main loop.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 3db139b..027c5c5 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -283,7 +283,7 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
 
     /* Get this show started by hooking up our callbacks */
     aio_context_acquire(s->ctx);
-    aio_set_event_notifier(s->ctx, &s->host_notifier, handle_notify);
+    aio_set_io_event_notifier(s->ctx, &s->host_notifier, handle_notify);
     aio_context_release(s->ctx);
     return;
 
@@ -319,7 +319,7 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
     aio_context_acquire(s->ctx);
 
     /* Stop notifications for new requests from guest */
-    aio_set_event_notifier(s->ctx, &s->host_notifier, NULL);
+    aio_set_io_event_notifier(s->ctx, &s->host_notifier, NULL);
 
     /* Drain and switch bs back to the QEMU main loop */
     blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context());
-- 
2.4.1