block: Drain recursively with a single BDRV_POLL_WHILE()

Anything can happen inside BDRV_POLL_WHILE(), including graph
changes that may interfere with its callers (e.g. child list iteration
in recursive callers of bdrv_do_drained_begin).

Switch to a single BDRV_POLL_WHILE() call for the whole subtree at the
end of bdrv_do_drained_begin() to avoid such effects. The recursion
happens now inside the loop condition. As the graph can only change
between bdrv_drain_poll() calls, but not inside of it, doing the
recursion here is safe.

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

1 files changed, 6 insertions, 3 deletions

diff --git a/include/block/block.h b/include/block/block.h
index cebbb39c6c..254ed2e4c9 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -570,10 +570,13 @@ void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore);
 /**
  * bdrv_drain_poll:
  *
- * Poll for pending requests in @bs and its parents (except for
- * @ignore_parent). This is part of bdrv_drained_begin.
+ * Poll for pending requests in @bs, its parents (except for @ignore_parent),
+ * and if @recursive is true its children as well.
+ *
+ * This is part of bdrv_drained_begin.
  */
-bool bdrv_drain_poll(BlockDriverState *bs, BdrvChild *ignore_parent);
+bool bdrv_drain_poll(BlockDriverState *bs, bool recursive,
+                     BdrvChild *ignore_parent);
 
 /**
  * bdrv_drained_begin: