[PATCH 1/8] virtio_queue_aio_attach_host_notifier: remove AioContext lock

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

virtio_queue_aio_attach_host_notifier() and
virtio_queue_aio_attach_host_notifier_nopoll() run always in the
main loop, so there is no need to protect them with AioContext
lock.

On the other side, virtio_queue_aio_detach_host_notifier() runs
in a bh in the iothread context, but it is always scheduled
(thus serialized) by the main loop. Therefore removing the
AioContext lock is safe, but unfortunately we can't do it
right now since bdrv_set_aio_context() and
aio_wait_bh_oneshot() still need to have it.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 14 ++++++++++++--
 hw/block/virtio-blk.c           |  2 ++
 hw/scsi/virtio-scsi-dataplane.c | 12 ++++++++++--
 3 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 49276e46f2..f9224f23d2 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -167,6 +167,8 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
     Error *local_err = NULL;
     int r;
 
+    GLOBAL_STATE_CODE();
+
     if (vblk->dataplane_started || s->starting) {
         return 0;
     }
@@ -243,13 +245,11 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
     }
 
     /* Get this show started by hooking up our callbacks */
-    aio_context_acquire(s->ctx);
     for (i = 0; i < nvqs; i++) {
         VirtQueue *vq = virtio_get_queue(s->vdev, i);
 
         virtio_queue_aio_attach_host_notifier(vq, s->ctx);
     }
-    aio_context_release(s->ctx);
     return 0;
 
   fail_aio_context:
@@ -304,6 +304,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
     unsigned i;
     unsigned nvqs = s->conf->num_queues;
 
+    GLOBAL_STATE_CODE();
+
     if (!vblk->dataplane_started || s->stopping) {
         return;
     }
@@ -318,6 +320,14 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
     trace_virtio_blk_data_plane_stop(s);
 
     aio_context_acquire(s->ctx);
+    /*
+     * TODO: virtio_blk_data_plane_stop_bh() does not need the AioContext lock,
+     * because even though virtio_queue_aio_detach_host_notifier() runs in
+     * Iothread context, such calls are serialized by the BQL held (this
+     * function runs in the main loop).
+     * On the other side, virtio_queue_aio_attach_host_notifier* always runs
+     * in the main loop, therefore it doesn't need the AioContext lock.
+     */
     aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
 
     /* Drain and try to switch bs back to the QEMU main loop. If other users
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index e9ba752f6b..8d0590cc76 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -121,6 +121,8 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
     VirtIOBlock *s = next->dev;
     VirtIODevice *vdev = VIRTIO_DEVICE(s);
 
+    IO_CODE();
+
     aio_context_acquire(blk_get_aio_context(s->conf.conf.blk));
     while (next) {
         VirtIOBlockReq *req = next;
diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
index 8bb6e6acfc..7080e9caa9 100644
--- a/hw/scsi/virtio-scsi-dataplane.c
+++ b/hw/scsi/virtio-scsi-dataplane.c
@@ -91,6 +91,8 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
     VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
     VirtIOSCSI *s = VIRTIO_SCSI(vdev);
 
+    GLOBAL_STATE_CODE();
+
     if (s->dataplane_started ||
         s->dataplane_starting ||
         s->dataplane_fenced) {
@@ -136,7 +138,6 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
 
     memory_region_transaction_commit();
 
-    aio_context_acquire(s->ctx);
     virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);
     virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx);
 
@@ -146,7 +147,6 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
 
     s->dataplane_starting = false;
     s->dataplane_started = true;
-    aio_context_release(s->ctx);
     return 0;
 
 fail_host_notifiers:
@@ -193,6 +193,14 @@ void virtio_scsi_dataplane_stop(VirtIODevice *vdev)
     s->dataplane_stopping = true;
 
     aio_context_acquire(s->ctx);
+    /*
+     * TODO: virtio_scsi_dataplane_stop_bh() does not need the AioContext lock,
+     * because even though virtio_queue_aio_detach_host_notifier() runs in
+     * Iothread context, such calls are serialized by the BQL held (this
+     * function runs in the main loop).
+     * On the other side, virtio_queue_aio_attach_host_notifier* always runs
+     * in the main loop, therefore it doesn't need the AioContext lock.
+     */
     aio_wait_bh_oneshot(s->ctx, virtio_scsi_dataplane_stop_bh, s);
     aio_context_release(s->ctx);
 
-- 
2.31.1