block: ignore_bds_parents parameter for drain functions

In the future, bdrv_drained_all_begin/end() will drain all invidiual
nodes separately rather than whole subtrees. This means that we don't
want to propagate the drain to all parents any more: If the parent is a
BDS, it will already be drained separately. Recursing to all parents is
unnecessary work and would make it an O(n²) operation.

Prepare the drain function for the changed drain_all by adding an
ignore_bds_parents parameter to the internal implementation that
prevents the propagation of the drain to BDS parents. We still (have to)
propagate it to non-BDS parents like BlockBackends or Jobs because those
are not drained separately.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 11 insertions, 5 deletions

diff --git a/include/block/block.h b/include/block/block.h
index 067d24cc4a..836746e4e1 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -557,7 +557,8 @@ void bdrv_io_unplug(BlockDriverState *bs);
  * Begin a quiesced section of all users of @bs. This is part of
  * bdrv_drained_begin.
  */
-void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore);
+void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
+                               bool ignore_bds_parents);
 
 /**
  * bdrv_parent_drained_end:
@@ -565,18 +566,23 @@ void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore);
  * End a quiesced section of all users of @bs. This is part of
  * bdrv_drained_end.
  */
-void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore);
+void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
+                             bool ignore_bds_parents);
 
 /**
  * bdrv_drain_poll:
  *
  * Poll for pending requests in @bs, its parents (except for @ignore_parent),
- * and if @recursive is true its children as well.
+ * and if @recursive is true its children as well (used for subtree drain).
+ *
+ * If @ignore_bds_parents is true, parents that are BlockDriverStates must
+ * ignore the drain request because they will be drained separately (used for
+ * drain_all).
  *
  * This is part of bdrv_drained_begin.
  */
 bool bdrv_drain_poll(BlockDriverState *bs, bool recursive,
-                     BdrvChild *ignore_parent);
+                     BdrvChild *ignore_parent, bool ignore_bds_parents);
 
 /**
  * bdrv_drained_begin:
@@ -597,7 +603,7 @@ void bdrv_drained_begin(BlockDriverState *bs);
  * running requests to complete.
  */
 void bdrv_do_drained_begin_quiesce(BlockDriverState *bs,
-                                   BdrvChild *parent);
+                                   BdrvChild *parent, bool ignore_bds_parents);
 
 /**
  * Like bdrv_drained_begin, but recursively begins a quiesced section for