[Qemu-devel] [PATCH v2 2/3] mirror: Keep mirror_top_bs drained after dropping permissions

[Kevin Wolf <kwolf@redhat.com>]

mirror_top_bs is currently implicitly drained through its connection to
the source or the target node. However, the drain section for target_bs
ends early after moving mirror_top_bs from src to target_bs, so that
requests can already be restarted while mirror_top_bs is still present
in the chain, but has dropped all permissions and therefore runs into an
assertion failure like this:

    qemu-system-x86_64: block/io.c:1634: bdrv_co_write_req_prepare:
    Assertion `child->perm & BLK_PERM_WRITE' failed.

Keep mirror_top_bs drained until all graph changes have completed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/mirror.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/mirror.c b/block/mirror.c
index 9f5c59ece1..642d6570cc 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -656,7 +656,10 @@ static int mirror_exit_common(Job *job)
     s->target = NULL;
 
     /* We don't access the source any more. Dropping any WRITE/RESIZE is
-     * required before it could become a backing file of target_bs. */
+     * required before it could become a backing file of target_bs. Not having
+     * these permissions any more means that we can't allow any new requests on
+     * mirror_top_bs from now on, so keep it drained. */
+    bdrv_drained_begin(mirror_top_bs);
     bs_opaque->stop = true;
     bdrv_child_refresh_perms(mirror_top_bs, mirror_top_bs->backing,
                              &error_abort);
@@ -724,6 +727,7 @@ static int mirror_exit_common(Job *job)
     bs_opaque->job = NULL;
 
     bdrv_drained_end(src);
+    bdrv_drained_end(mirror_top_bs);
     s->in_drain = false;
     bdrv_unref(mirror_top_bs);
     bdrv_unref(src);
-- 
2.20.1