stream: Replace subtree drain with a single node drain

The subtree drain was introduced in commit b1e1af394d as a way to avoid
graph changes between finding the base node and changing the block graph
as necessary on completion of the image streaming job.

The block graph could change between these two points because
bdrv_set_backing_hd() first drains the parent node, which involved
polling and can do anything.

Subtree draining was an imperfect way to make this less likely (because
with it, fewer callbacks are called during this window). Everyone agreed
that it's not really the right solution, and it was only committed as a
stopgap solution.

This replaces the subtree drain with a solution that simply drains the
parent node before we try to find the base node, and then call a version
of bdrv_set_backing_hd() that doesn't drain, but just asserts that the
parent node is already drained.

This way, any graph changes caused by draining happen before we start
looking at the graph and things stay consistent between finding the base
node and changing the graph.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20221118174110.55183-10-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -3426,14 +3426,15 @@ static int bdrv_set_backing_noperm(BlockDriverState *bs,
     return bdrv_set_file_or_backing_noperm(bs, backing_hd, true, tran, 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 ret;
     Transaction *tran = tran_new();
 
     GLOBAL_STATE_CODE();
-    bdrv_drained_begin(bs);
+    assert(bs->quiesce_counter > 0);
 
     ret = bdrv_set_backing_noperm(bs, backing_hd, tran, errp);
     if (ret < 0) {
@@ -3443,7 +3444,17 @@ int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
     ret = bdrv_refresh_perms(bs, tran, errp);
 out:
     tran_finalize(tran, ret);
+    return ret;
+}
 
+int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
+                        Error **errp)
+{
+    int ret;
+    GLOBAL_STATE_CODE();
+
+    bdrv_drained_begin(bs);
+    ret = bdrv_set_backing_hd_drained(bs, backing_hd, errp);
     bdrv_drained_end(bs);
 
     return ret;

block/stream.c

@@ -64,13 +64,16 @@ static int stream_prepare(Job *job)
     bdrv_cor_filter_drop(s->cor_filter_bs);
     s->cor_filter_bs = NULL;
 
-    bdrv_subtree_drained_begin(s->above_base);
+    /*
+     * bdrv_set_backing_hd() requires that unfiltered_bs is drained. Drain
+     * already here and use bdrv_set_backing_hd_drained() instead because
+     * the polling during drained_begin() might change the graph, and if we do
+     * this only later, we may end up working with the wrong base node (or it
+     * might even have gone away by the time we want to use it).
+     */
+    bdrv_drained_begin(unfiltered_bs);
 
     base = bdrv_filter_or_cow_bs(s->above_base);
-    if (base) {
-        bdrv_ref(base);
-    }
-
     unfiltered_base = bdrv_skip_filters(base);
 
     if (bdrv_cow_child(unfiltered_bs)) {
@@ -82,7 +85,13 @@ static int stream_prepare(Job *job)
             }
         }
 
-        bdrv_set_backing_hd(unfiltered_bs, base, &local_err);
+        bdrv_set_backing_hd_drained(unfiltered_bs, base, &local_err);
+
+        /*
+         * This call will do I/O, so the graph can change again from here on.
+         * We have already completed the graph change, so we are not in danger
+         * of operating on the wrong node any more if this happens.
+         */
         ret = bdrv_change_backing_file(unfiltered_bs, base_id, base_fmt, false);
         if (local_err) {
             error_report_err(local_err);
@@ -92,10 +101,7 @@ static int stream_prepare(Job *job)
     }
 
 out:
-    if (base) {
-        bdrv_unref(base);
-    }
-    bdrv_subtree_drained_end(s->above_base);
+    bdrv_drained_end(unfiltered_bs);
     return ret;
 }
 

include/block/block-global-state.h

@@ -82,6 +82,9 @@ int bdrv_open_file_child(const char *filename,
 BlockDriverState *bdrv_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 bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
                            const char *bdref_key, Error **errp);
 BlockDriverState *bdrv_open(const char *filename, const char *reference,