block: add bdrv_graph_wrlock_drained() convenience wrapper

Many write-locked sections are also drained sections. A new
bdrv_graph_wrunlock_drained() wrapper around bdrv_graph_wrunlock() is
introduced, which will begin a drained section first. A global
variable is used so bdrv_graph_wrunlock() knows if it also needs
to end such a drained section. Both the aio_poll call in
bdrv_graph_wrlock() and the aio_bh_poll() in bdrv_graph_wrunlock()
can re-enter a write-locked section. While for the latter, ending the
drain could be moved to before the call, the former requires that the
variable is a counter and not just a boolean.

Since the wrapper calls bdrv_drain_all_begin(), which must be called
with the graph unlocked, mark the wrapper as GRAPH_UNLOCKED too.

The switch to the new helpers was generated with the following
commands and then manually checked:
find . -name '*.c' -exec sed -i -z 's/bdrv_drain_all_begin();\n\s*bdrv_graph_wrlock();/bdrv_graph_wrlock_drained();/g' {} ';'
find . -name '*.c' -exec sed -i -z 's/bdrv_graph_wrunlock();\n\s*bdrv_drain_all_end();/bdrv_graph_wrunlock();/g' {} ';'

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-25-f.ebner@proxmox.com>
[kwolf: Removed redundant GRAPH_UNLOCKED]
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 1 insertions, 3 deletions

diff --git a/blockdev.c b/blockdev.c
index 2e7fda6780..e625534925 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3561,8 +3561,7 @@ void qmp_x_blockdev_change(const char *parent, const char *child,
     BlockDriverState *parent_bs, *new_bs = NULL;
     BdrvChild *p_child;
 
-    bdrv_drain_all_begin();
-    bdrv_graph_wrlock();
+    bdrv_graph_wrlock_drained();
 
     parent_bs = bdrv_lookup_bs(parent, parent, errp);
     if (!parent_bs) {
@@ -3599,7 +3598,6 @@ void qmp_x_blockdev_change(const char *parent, const char *child,
 
 out:
     bdrv_graph_wrunlock();
-    bdrv_drain_all_end();
 }
 
 BlockJobInfoList *qmp_query_block_jobs(Error **errp)