4 files changed, 32 insertions, 13 deletions

diff --git a/block.c b/block.c
index 29e931e217..8440712ca0 100644
--- a/block.c
+++ b/block.c
@@ -2251,24 +2251,19 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
         if (child->role->detach) {
             child->role->detach(child);
         }
-        if (old_bs->quiesce_counter && child->role->drained_end) {
-            int num = old_bs->quiesce_counter;
-            if (child->role->parent_is_bds) {
-                num -= bdrv_drain_all_count;
-            }
-            assert(num >= 0);
-            for (i = 0; i < num; i++) {
-                child->role->drained_end(child);
-            }
+        while (child->parent_quiesce_counter) {
+            bdrv_parent_drained_end_single(child);
         }
         QLIST_REMOVE(child, next_parent);
+    } else {
+        assert(child->parent_quiesce_counter == 0);
     }
 
     child->bs = new_bs;
 
     if (new_bs) {
         QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
-        if (new_bs->quiesce_counter && child->role->drained_begin) {
+        if (new_bs->quiesce_counter) {
             int num = new_bs->quiesce_counter;
             if (child->role->parent_is_bds) {
                 num -= bdrv_drain_all_count;
diff --git a/block/io.c b/block/io.c
index 24a18759fd..1e618f9a37 100644
--- a/block/io.c
+++ b/block/io.c
@@ -55,6 +55,15 @@ void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
     }
 }
 
+void bdrv_parent_drained_end_single(BdrvChild *c)
+{
+    assert(c->parent_quiesce_counter > 0);
+    c->parent_quiesce_counter--;
+    if (c->role->drained_end) {
+        c->role->drained_end(c);
+    }
+}
+
 void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
                              bool ignore_bds_parents)
 {
@@ -64,9 +73,7 @@ void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
         if (c == ignore || (ignore_bds_parents && c->role->parent_is_bds)) {
             continue;
         }
-        if (c->role->drained_end) {
-            c->role->drained_end(c);
-        }
+        bdrv_parent_drained_end_single(c);
     }
 }
 
@@ -96,6 +103,7 @@ static bool bdrv_parent_drained_poll(BlockDriverState *bs, BdrvChild *ignore,
 
 void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll)
 {
+    c->parent_quiesce_counter++;
     if (c->role->drained_begin) {
         c->role->drained_begin(c);
     }
diff --git a/include/block/block.h b/include/block/block.h
index 734c9d2f76..bff3317696 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -618,6 +618,13 @@ void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
 void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll);
 
 /**
+ * bdrv_parent_drained_end_single:
+ *
+ * End a quiesced section for the parent of @c.
+ */
+void bdrv_parent_drained_end_single(BdrvChild *c);
+
+/**
  * bdrv_parent_drained_end:
  *
  * End a quiesced section of all users of @bs. This is part of
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 50902531b7..f5b044fcdb 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -729,6 +729,15 @@ struct BdrvChild {
      */
     bool frozen;
 
+    /*
+     * How many times the parent of this child has been drained
+     * (through role->drained_*).
+     * Usually, this is equal to bs->quiesce_counter (potentially
+     * reduced by bdrv_drain_all_count).  It may differ while the
+     * child is entering or leaving a drained section.
+     */
+    int parent_quiesce_counter;
+
     QLIST_ENTRY(BdrvChild) next;
     QLIST_ENTRY(BdrvChild) next_parent;
 };