block: Add and use bdrv_replace_in_backing_chain()

This cleans up the mess we left behind in the mirror code after the
previous patch. Instead of using bdrv_swap(), just change pointers.

The interface change of the mirror job that callers must consider is
that after job completion, their local BDS pointers still point to the
same node now. qemu-img must change its code accordingly (which makes it
easier to understand); the other callers stays unchanged because after
completion they don't do anything with the BDS, but just with the job,
and the job is still owned by the source BDS.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 7 insertions, 16 deletions

diff --git a/block/mirror.c b/block/mirror.c
index c277691e4d..7e43511832 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -353,6 +353,11 @@ static void mirror_exit(BlockJob *job, void *opaque)
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common);
     MirrorExitData *data = opaque;
     AioContext *replace_aio_context = NULL;
+    BlockDriverState *src = s->common.bs;
+
+    /* Make sure that the source BDS doesn't go away before we called
+     * block_job_completed(). */
+    bdrv_ref(src);
 
     if (s->to_replace) {
         replace_aio_context = bdrv_get_aio_context(s->to_replace);
@@ -367,22 +372,7 @@ static void mirror_exit(BlockJob *job, void *opaque)
         if (bdrv_get_flags(s->target) != bdrv_get_flags(to_replace)) {
             bdrv_reopen(s->target, bdrv_get_flags(to_replace), NULL);
         }
-        bdrv_swap(s->target, to_replace);
-        if (s->common.driver->job_type == BLOCK_JOB_TYPE_COMMIT) {
-            /* drop the bs loop chain formed by the swap: break the loop then
-             * trigger the unref */
-            /* FIXME This duplicates bdrv_set_backing_hd(), except for the
-             * actual detach/unref so that the loop can be broken. When
-             * bdrv_swap() gets replaced, this will become sane again. */
-            BlockDriverState *backing = s->base->backing->bs;
-            assert(s->base->backing_blocker);
-            bdrv_op_unblock_all(backing, s->base->backing_blocker);
-            error_free(s->base->backing_blocker);
-            s->base->backing_blocker = NULL;
-            bdrv_detach_child(s->base->backing);
-            s->base->backing = NULL;
-            bdrv_unref(backing);
-        }
+        bdrv_replace_in_backing_chain(to_replace, s->target);
     }
     if (s->to_replace) {
         bdrv_op_unblock_all(s->to_replace, s->replace_blocker);
@@ -396,6 +386,7 @@ static void mirror_exit(BlockJob *job, void *opaque)
     bdrv_unref(s->target);
     block_job_completed(&s->common, data->ret);
     g_free(data);
+    bdrv_unref(src);
 }
 
 static void coroutine_fn mirror_run(void *opaque)