[Qemu-devel] [PATCH 4/4] virtio-blk: Clean up start/stop with mutex and BH

[Fam Zheng]

This is to make the dataplane start logic simpler to understand.

Start/stop take the mutex so we don't need the starting flag. The bottom
half is scheduled in the iothread to actually hook up request handlers
with vq.

Suggested-by: Paolo Bonzini <pbonz...@redhat.com>
Signed-off-by: Fam Zheng <f...@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 58 +++++++++++++++++++++++++++++++----------
 1 file changed, 44 insertions(+), 14 deletions(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 36f3d2b..9e5c543 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -26,7 +26,6 @@
 #include "qom/object_interfaces.h"
 
 struct VirtIOBlockDataPlane {
-    bool starting;
     bool stopping;
     bool disabled;
 
@@ -49,6 +48,8 @@ struct VirtIOBlockDataPlane {
 
     /* Operation blocker on BDS */
     Error *blocker;
+
+    QemuMutex start_stop_lock;
 };
 
 /* Raise an interrupt to signal guest, if necessary */
@@ -150,6 +151,7 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, 
VirtIOBlkConf *conf,
     s = g_new0(VirtIOBlockDataPlane, 1);
     s->vdev = vdev;
     s->conf = conf;
+    qemu_mutex_init(&s->start_stop_lock);
 
     if (conf->iothread) {
         s->iothread = conf->iothread;
@@ -184,19 +186,47 @@ void virtio_blk_data_plane_destroy(VirtIOBlockDataPlane 
*s)
     g_free(s);
 }
 
+typedef struct {
+    VirtIOBlock *vblk;
+    QEMUBH *bh;
+} VirtIOBlockStartData;
+
+static void virtio_blk_data_plane_start_bh_cb(void *opaque)
+{
+    VirtIOBlockStartData *data = opaque;
+    VirtIOBlockDataPlane *s = data->vblk->dataplane;
+
+    qemu_mutex_lock(&s->start_stop_lock);
+    if (!data->vblk->dataplane_started) {
+        goto out;
+    }
+    /* Kick right away to begin processing requests already in vring */
+    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
+
+    /* Get this show started by hooking up our callbacks */
+    virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, true, true);
+
+out:
+    qemu_bh_delete(data->bh);
+    g_free(data);
+    qemu_mutex_unlock(&s->start_stop_lock);
+}
+
 /* Context: QEMU global mutex held */
 void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
 {
     BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(s->vdev)));
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
     VirtIOBlock *vblk = VIRTIO_BLK(s->vdev);
+    VirtIOBlockStartData *data;
     int r;
 
-    if (vblk->dataplane_started || s->starting) {
+    qemu_mutex_lock(&s->start_stop_lock);
+    if (vblk->dataplane_started) {
+        qemu_mutex_unlock(&s->start_stop_lock);
         return;
     }
 
-    s->starting = true;
     s->vq = virtio_get_queue(s->vdev, 0);
 
     /* Set up guest notifier (irq) */
@@ -215,27 +245,24 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
         goto fail_host_notifier;
     }
 
-    s->starting = false;
     vblk->dataplane_started = true;
     trace_virtio_blk_data_plane_start(s);
 
     blk_set_aio_context(s->conf->conf.blk, s->ctx);
 
-    /* Kick right away to begin processing requests already in vring */
-    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
-
-    /* Get this show started by hooking up our callbacks */
-    aio_context_acquire(s->ctx);
-    virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, true, true);
-    aio_context_release(s->ctx);
+    data = g_new(VirtIOBlockStartData, 1);
+    data->vblk = vblk;
+    data->bh = aio_bh_new(s->ctx, virtio_blk_data_plane_start_bh_cb, data);
+    qemu_bh_schedule(data->bh);
+    qemu_mutex_unlock(&s->start_stop_lock);
     return;
 
   fail_host_notifier:
     k->set_guest_notifiers(qbus->parent, 1, false);
   fail_guest_notifiers:
     s->disabled = true;
-    s->starting = false;
     vblk->dataplane_started = true;
+    qemu_mutex_unlock(&s->start_stop_lock);
 }
 
 /* Context: QEMU global mutex held */
@@ -245,15 +272,16 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
     VirtIOBlock *vblk = VIRTIO_BLK(s->vdev);
 
+    qemu_mutex_lock(&s->start_stop_lock);
     if (!vblk->dataplane_started || s->stopping) {
-        return;
+        goto out;
     }
 
     /* Better luck next time. */
     if (s->disabled) {
         s->disabled = false;
         vblk->dataplane_started = false;
-        return;
+        goto out;
     }
     s->stopping = true;
     trace_virtio_blk_data_plane_stop(s);
@@ -275,4 +303,6 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
 
     vblk->dataplane_started = false;
     s->stopping = false;
+out:
+    qemu_mutex_unlock(&s->start_stop_lock);
 }
-- 
2.4.3