2 files changed, 30 insertions, 1 deletions

diff --git a/include/block/block.h b/include/block/block.h
index a81645e..60f0047 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -612,6 +612,9 @@ void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll);
  * bdrv_parent_drained_end_single:
  *
  * End a quiesced section for the parent of @c.
+ *
+ * This polls @bs's AioContext until all scheduled sub-drained_ends
+ * have settled, which may result in graph changes.
  */
 void bdrv_parent_drained_end_single(BdrvChild *c);
 
@@ -661,10 +664,32 @@ void bdrv_subtree_drained_begin(BlockDriverState *bs);
  * bdrv_drained_end:
  *
  * End a quiescent section started by bdrv_drained_begin().
+ *
+ * This polls @bs's AioContext until all scheduled sub-drained_ends
+ * have settled.  On one hand, that may result in graph changes.  On
+ * the other, this requires that all involved nodes (@bs and all of
+ * its parents) are in the same AioContext, and that the caller has
+ * acquired it.
+ * If there are any nodes that are in different contexts from @bs,
+ * these contexts must not be acquired.
  */
 void bdrv_drained_end(BlockDriverState *bs);
 
 /**
+ * bdrv_drained_end_no_poll:
+ *
+ * Same as bdrv_drained_end(), but do not poll for the subgraph to
+ * actually become unquiesced.  Therefore, no graph changes will occur
+ * with this function.
+ *
+ * *drained_end_counter is incremented for every background operation
+ * that is scheduled, and will be decremented for every operation once
+ * it settles.  The caller must poll until it reaches 0.  The counter
+ * should be accessed using atomic operations only.
+ */
+void bdrv_drained_end_no_poll(BlockDriverState *bs, int *drained_end_counter);
+
+/**
  * End a quiescent section started by bdrv_subtree_drained_begin().
  */
 void bdrv_subtree_drained_end(BlockDriverState *bs);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index f5b044f..3aa1e83 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -664,11 +664,15 @@ struct BdrvChildRole {
      * These functions must not change the graph (and therefore also must not
      * call aio_poll(), which could change the graph indirectly).
      *
+     * If drained_end() schedules background operations, it must atomically
+     * increment *drained_end_counter for each such operation and atomically
+     * decrement it once the operation has settled.
+     *
      * Note that this can be nested. If drained_begin() was called twice, new
      * I/O is allowed only after drained_end() was called twice, too.
      */
     void (*drained_begin)(BdrvChild *child);
-    void (*drained_end)(BdrvChild *child);
+    void (*drained_end)(BdrvChild *child, int *drained_end_counter);
 
     /*
      * Returns whether the parent has pending requests for the child. This