Instead of taking the writer lock internally, require callers to already
hold it when calling bdrv_set_backing_hd_drained(). Basically everthing
in the function needs the lock and its callers may already want to hold
the graph lock and so wouldn't be able to call functions that take it
internally.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 include/block/block-global-state.h | 7 ++++---
 block.c                            | 4 ++--
 block/stream.c                     | 2 ++
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/include/block/block-global-state.h b/include/block/block-global-state.h
index 9a33bd7ef9..a1fd70ec97 100644
--- a/include/block/block-global-state.h
+++ b/include/block/block-global-state.h
@@ -101,9 +101,10 @@ bdrv_co_open_blockdev_ref(BlockdevRef *ref, Error **errp);
 
 int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
                         Error **errp);
-int bdrv_set_backing_hd_drained(BlockDriverState *bs,
-                                BlockDriverState *backing_hd,
-                                Error **errp);
+int GRAPH_WRLOCK
+bdrv_set_backing_hd_drained(BlockDriverState *bs, BlockDriverState *backing_hd,
+                            Error **errp);
+
 int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
                            const char *bdref_key, Error **errp);
 
diff --git a/block.c b/block.c
index 499b147315..d79a6f41f9 100644
--- a/block.c
+++ b/block.c
@@ -3557,7 +3557,6 @@ int bdrv_set_backing_hd_drained(BlockDriverState *bs,
     if (bs->backing) {
         assert(bs->backing->bs->quiesce_counter > 0);
     }
-    bdrv_graph_wrlock(backing_hd);
 
     ret = bdrv_set_backing_noperm(bs, backing_hd, tran, errp);
     if (ret < 0) {
@@ -3567,7 +3566,6 @@ int bdrv_set_backing_hd_drained(BlockDriverState *bs,
     ret = bdrv_refresh_perms(bs, tran, errp);
 out:
     tran_finalize(tran, ret);
-    bdrv_graph_wrunlock();
     return ret;
 }
 
@@ -3580,7 +3578,9 @@ int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
 
     bdrv_ref(drain_bs);
     bdrv_drained_begin(drain_bs);
+    bdrv_graph_wrlock(backing_hd);
     ret = bdrv_set_backing_hd_drained(bs, backing_hd, errp);
+    bdrv_graph_wrunlock();
     bdrv_drained_end(drain_bs);
     bdrv_unref(drain_bs);
 
diff --git a/block/stream.c b/block/stream.c
index 3f5d773535..0b92410c00 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -99,7 +99,9 @@ static int stream_prepare(Job *job)
             }
         }
 
+        bdrv_graph_wrlock(base);
         bdrv_set_backing_hd_drained(unfiltered_bs, base, &local_err);
+        bdrv_graph_wrunlock();
 
         /*
          * This call will do I/O, so the graph can change again from here on.
-- 
2.41.0

On Fri, Oct 27, 2023 at 05:53:22PM +0200, Kevin Wolf wrote:
> Instead of taking the writer lock internally, require callers to already
> hold it when calling bdrv_set_backing_hd_drained(). Basically everthing
> in the function needs the lock and its callers may already want to hold
> the graph lock and so wouldn't be able to call functions that take it
> internally.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/block/block-global-state.h | 7 ++++---
>  block.c                            | 4 ++--
>  block/stream.c                     | 2 ++
>  3 files changed, 8 insertions(+), 5 deletions(-)
>

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org