This function has two callers; one wants it to poll, the other does not.

To implement this parameter, we also need a new BdrvChildRole method,
namely .drained_end_unquiesce().  This function differs from
.drained_end() in that it should poll and is thus allowed to modify the
block graph.

Note that currently every BDS child's .drained_end() implementation is
simply broken in exactly that regard; the current implementation is
actually fit for .drained_end_unquiesce().  However, at this point we
cannot implement .drained_end() correctly, so a later patch in this
series will address that issue.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 include/block/block.h     |  5 ++++-
 include/block/block_int.h | 14 ++++++++++++++
 block.c                   |  2 +-
 block/io.c                | 10 +++++++---
 4 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index 3c084e8222..d98237f0f1 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -620,8 +620,11 @@ void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll);
  * bdrv_parent_drained_end_single:
  *
  * End a quiesced section for the parent of @c.
+ *
+ * If @poll is true, poll until the parent is unquiesced.  This may
+ * lead to changes in the block graph.
  */
-void bdrv_parent_drained_end_single(BdrvChild *c);
+void bdrv_parent_drained_end_single(BdrvChild *c, bool poll);
 
 /**
  * bdrv_parent_drained_end:
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 7f62907932..fce9a9e7ee 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -670,6 +670,20 @@ struct BdrvChildRole {
     void (*drained_begin)(BdrvChild *child);
     void (*drained_end)(BdrvChild *child);
 
+    /*
+     * Parents that require polling to actually become unquiesced
+     * should implement this function in addition to .drained_end().
+     * In it, the parent should end the drain and aio_poll() until it
+     * is no longer quiesced.
+     *
+     * Thus, in contrast to .drained_end(), this function is allowed
+     * to change the graph.
+     *
+     * (This pointer may be NULL, in which case .drained_end() is
+     * called instead.)
+     */
+    void (*drained_end_unquiesce)(BdrvChild *child);
+
     /*
      * Returns whether the parent has pending requests for the child. This
      * callback is polled after .drained_begin() has been called until all
diff --git a/block.c b/block.c
index f7d7d8ccef..31b85df0f0 100644
--- a/block.c
+++ b/block.c
@@ -2252,7 +2252,7 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
             child->role->detach(child);
         }
         while (child->parent_quiesce_counter) {
-            bdrv_parent_drained_end_single(child);
+            bdrv_parent_drained_end_single(child, true);
         }
         QLIST_REMOVE(child, next_parent);
     } else {
diff --git a/block/io.c b/block/io.c
index 426ad5b4a1..5f30baa8e5 100644
--- a/block/io.c
+++ b/block/io.c
@@ -55,12 +55,16 @@ void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
     }
 }
 
-void bdrv_parent_drained_end_single(BdrvChild *c)
+void bdrv_parent_drained_end_single(BdrvChild *c, bool poll)
 {
     assert(c->parent_quiesce_counter > 0);
     c->parent_quiesce_counter--;
     if (c->role->drained_end) {
-        c->role->drained_end(c);
+        if (poll && c->role->drained_end_unquiesce) {
+            c->role->drained_end_unquiesce(c);
+        } else {
+            c->role->drained_end(c);
+        }
     }
 }
 
@@ -73,7 +77,7 @@ void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
         if (c == ignore || (ignore_bds_parents && c->role->parent_is_bds)) {
             continue;
         }
-        bdrv_parent_drained_end_single(c);
+        bdrv_parent_drained_end_single(c, false);
     }
 }
 
-- 
2.21.0