block: only call aio_poll on the current thread's AioContext

aio_poll is not thread safe; for example bdrv_drain can hang if
the last in-flight I/O operation is completed in the I/O thread after
the main thread has checked bs->in_flight.

The bug remains latent as long as all of it is called within
aio_context_acquire/aio_context_release, but this will change soon.

To fix this, if bdrv_drain is called from outside the I/O thread,
signal the main AioContext through a dummy bottom half.  The event
loop then only runs in the I/O thread.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1477565348-5458-18-git-send-email-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

1 files changed, 1 insertions, 3 deletions

diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
index b173b94949..9424f0e057 100644
--- a/hw/scsi/virtio-scsi-dataplane.c
+++ b/hw/scsi/virtio-scsi-dataplane.c
@@ -189,13 +189,11 @@ void virtio_scsi_dataplane_stop(VirtIOSCSI *s)
     assert(s->ctx == iothread_get_aio_context(vs->conf.iothread));
 
     aio_context_acquire(s->ctx);
-
     virtio_scsi_clear_aio(s);
+    aio_context_release(s->ctx);
 
     blk_drain_all(); /* ensure there are no in-flight requests */
 
-    aio_context_release(s->ctx);
-
     for (i = 0; i < vs->conf.num_queues + 2; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
     }