qemu-img: Take graph lock more selectively

If we take a reader lock, we can't call any functions that take a writer
lock internally without causing deadlocks once the reader lock is
actually enforced in the main thread, too. Take the reader lock only
where it is actually needed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230510203601.418015-5-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -2938,16 +2938,17 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
         }
         bs = blk_bs(blk);
 
-        GRAPH_RDLOCK_GUARD_MAINLOOP();
-
         /*
          * Note that the returned BlockGraphInfo object will not have
          * information about this image's backing node, because we have opened
          * it with BDRV_O_NO_BACKING.  Printing this object will therefore not
          * duplicate the backing chain information that we obtain by walking
          * the chain manually here.
          */
+        bdrv_graph_rdlock_main_loop();
         bdrv_query_block_graph_info(bs, &info, &err);
+        bdrv_graph_rdunlock_main_loop();
+
         if (err) {
             error_report_err(err);
             blk_unref(blk);