block: Honour graph read lock even in the main thread

[PATCH 4/8] qemu-img: Take graph lock more selectively

Posted by Kevin Wolf 2 years, 9 months ago

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>
---
 qemu-img.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 9f9f0a7629..27f48051b0 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2938,8 +2938,6 @@ 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
@@ -2947,7 +2945,10 @@ static BlockGraphInfoList *collect_image_info_list(bool image_opts,
          * 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);
-- 
2.40.1

Re: [PATCH 4/8] qemu-img: Take graph lock more selectively

Posted by Eric Blake 2 years, 9 months ago

On Wed, May 10, 2023 at 10:35:57PM +0200, Kevin Wolf wrote:
> 
> 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>
> ---
>  qemu-img.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org