block: Mark bdrv_set_backing_hd_drained() GRAPH_WRLOCK

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>
Message-ID: <20231027155333.420094-14-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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);
 

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.

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);