[PATCH v2 03/10] block/io.c: fix bdrv_child_cb_drained_begin invocations from a coroutine

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

Using bdrv_do_drained_begin_quiesce() in bdrv_child_cb_drained_begin()
is not a good idea: the callback might be called when running
a drain in a coroutine, and bdrv_drained_begin_poll() does not
handle that case, resulting in assertion failure.

Instead, bdrv_do_drained_begin with no recursion and poll
will accomplish the same thing (invoking bdrv_do_drained_begin_quiesce)
but will firstly check if we are already in a coroutine, and exit
from that via bdrv_co_yield_to_drain().

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block.c                  |  2 +-
 block/io.c               |  8 +++++++-
 include/block/block-io.h | 20 +++++++++++---------
 3 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/block.c b/block.c
index 718e4cae8b..372f16f4a0 100644
--- a/block.c
+++ b/block.c
@@ -1211,7 +1211,7 @@ static char *bdrv_child_get_parent_desc(BdrvChild *c)
 static void bdrv_child_cb_drained_begin(BdrvChild *child)
 {
     BlockDriverState *bs = child->opaque;
-    bdrv_do_drained_begin_quiesce(bs, NULL, false);
+    bdrv_drained_begin_no_poll(bs);
 }
 
 static bool bdrv_child_cb_drained_poll(BdrvChild *child)
diff --git a/block/io.c b/block/io.c
index e446782ae0..e77861c464 100644
--- a/block/io.c
+++ b/block/io.c
@@ -523,7 +523,7 @@ static void coroutine_fn bdrv_co_yield_to_drain(BlockDriverState *bs,
     }
 }
 
-void bdrv_do_drained_begin_quiesce(BlockDriverState *bs,
+static void bdrv_do_drained_begin_quiesce(BlockDriverState *bs,
                                    BdrvChild *parent, bool ignore_bds_parents)
 {
     IO_OR_GS_CODE();
@@ -587,6 +587,12 @@ void bdrv_subtree_drained_begin(BlockDriverState *bs)
     bdrv_do_drained_begin(bs, true, NULL, false, true);
 }
 
+void bdrv_drained_begin_no_poll(BlockDriverState *bs)
+{
+    IO_CODE();
+    bdrv_do_drained_begin(bs, false, NULL, false, false);
+}
+
 /**
  * This function does not poll, nor must any of its recursively called
  * functions.  The *drained_end_counter pointee will be incremented
diff --git a/include/block/block-io.h b/include/block/block-io.h
index 5e3f346806..9135b648bf 100644
--- a/include/block/block-io.h
+++ b/include/block/block-io.h
@@ -226,6 +226,17 @@ int coroutine_fn bdrv_co_copy_range(BdrvChild *src, int64_t src_offset,
                                     int64_t bytes, BdrvRequestFlags read_flags,
                                     BdrvRequestFlags write_flags);
 
+/**
+ * bdrv_drained_begin_no_poll:
+ *
+ * Quiesces a BDS like bdrv_drained_begin(), but does not wait for already
+ * running requests to complete.
+ * Same as bdrv_drained_begin(), but do not poll for the subgraph to
+ * actually become unquiesced. Therefore, no graph changes will occur
+ * with this function.
+ */
+void bdrv_drained_begin_no_poll(BlockDriverState *bs);
+
 /**
  * bdrv_drained_end_no_poll:
  *
@@ -332,15 +343,6 @@ bool bdrv_drain_poll(BlockDriverState *bs, bool recursive,
  */
 void bdrv_drained_begin(BlockDriverState *bs);
 
-/**
- * bdrv_do_drained_begin_quiesce:
- *
- * Quiesces a BDS like bdrv_drained_begin(), but does not wait for already
- * running requests to complete.
- */
-void bdrv_do_drained_begin_quiesce(BlockDriverState *bs,
-                                   BdrvChild *parent, bool ignore_bds_parents);
-
 /**
  * Like bdrv_drained_begin, but recursively begins a quiesced section for
  * exclusive access to all child nodes as well.
-- 
2.31.1