[PATCH v5 10/31] block.c: modify .attach and .detach callbacks of child_of_bds

[Emanuele Giuseppe Esposito]

According to the assertions put in the previous patch, we should
first drain and then modify the ->children list. In this way
we prevent other iothreads to read the list while it is being
updated.

In this case, moving the drain won't cause any harm, because
child is a parameter of the drain function so it will still be
included in the operation, despite not being in the list.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index 522a273140..5516c84ec4 100644
--- a/block.c
+++ b/block.c
@@ -1416,6 +1416,7 @@ static void bdrv_child_cb_attach(BdrvChild *child)
 {
     BlockDriverState *bs = child->opaque;
 
+    bdrv_apply_subtree_drain(child, bs);
     assert_bdrv_graph_writable(bs);
     QLIST_INSERT_HEAD(&bs->children, child, next);
 
@@ -1423,7 +1424,6 @@ static void bdrv_child_cb_attach(BdrvChild *child)
         bdrv_backing_attach(child);
     }
 
-    bdrv_apply_subtree_drain(child, bs);
 }
 
 static void bdrv_child_cb_detach(BdrvChild *child)
@@ -1434,10 +1434,9 @@ static void bdrv_child_cb_detach(BdrvChild *child)
         bdrv_backing_detach(child);
     }
 
-    bdrv_unapply_subtree_drain(child, bs);
-
     assert_bdrv_graph_writable(bs);
     QLIST_REMOVE(child, next);
+    bdrv_unapply_subtree_drain(child, bs);
 }
 
 static int bdrv_child_cb_update_filename(BdrvChild *c, BlockDriverState *base,
-- 
2.27.0