nbd: use BlockDriverState refcnt

Previously, nbd calls drive_get_ref() on the drive of bs. A BDS doesn't
always have associated dinfo, which nbd doesn't care either. We already
have BDS ref count, so use it to make it safe for a BDS w/o blockdev.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

blockdev-nbd.c

@@ -69,12 +69,6 @@ static void nbd_close_notifier(Notifier *n, void *data)
     g_free(cn);
 }
 
-static void nbd_server_put_ref(NBDExport *exp)
-{
-    BlockDriverState *bs = nbd_export_get_blockdev(exp);
-    drive_put_ref(drive_get_by_blockdev(bs));
-}
-
 void qmp_nbd_server_add(const char *device, bool has_writable, bool writable,
                         Error **errp)
 {
@@ -105,11 +99,9 @@ void qmp_nbd_server_add(const char *device, bool has_writable, bool writable,
         writable = false;
     }
 
-    exp = nbd_export_new(bs, 0, -1, writable ? 0 : NBD_FLAG_READ_ONLY,
-                         nbd_server_put_ref);
+    exp = nbd_export_new(bs, 0, -1, writable ? 0 : NBD_FLAG_READ_ONLY, NULL);
 
     nbd_export_set_name(exp, device);
-    drive_get_ref(drive_get_by_blockdev(bs));
 
     n = g_malloc0(sizeof(NBDCloseNotifier));
     n->n.notify = nbd_close_notifier;

nbd.c

@@ -882,6 +882,7 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset,
     exp->nbdflags = nbdflags;
     exp->size = size == -1 ? bdrv_getlength(bs) : size;
     exp->close = close;
+    bdrv_ref(bs);
     return exp;
 }
 
@@ -928,6 +929,10 @@ void nbd_export_close(NBDExport *exp)
     }
     nbd_export_set_name(exp, NULL);
     nbd_export_put(exp);
+    if (exp->bs) {
+        bdrv_unref(exp->bs);
+        exp->bs = NULL;
+    }
 }
 
 void nbd_export_get(NBDExport *exp)