[PATCH v4 08/25] block: introduce assert_bdrv_graph_writable

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

We want to be sure that the functions that write the child and
parent list of a bs are under BQL and drain.

BQL prevents from concurrent writings from the GS API, while
drains protect from I/O.

TODO: drains are missing in some functions using this assert.
Therefore a proper assertion will fail. Because adding drains
requires additional discussions, they will be added in future
series.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block.c                                |  5 +++++
 block/io.c                             | 11 +++++++++++
 include/block/block_int-global-state.h | 10 +++++++++-
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 41c5883c5c..94bff5c757 100644
--- a/block.c
+++ b/block.c
@@ -2734,12 +2734,14 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
         if (child->klass->detach) {
             child->klass->detach(child);
         }
+        assert_bdrv_graph_writable(old_bs);
         QLIST_REMOVE(child, next_parent);
     }
 
     child->bs = new_bs;
 
     if (new_bs) {
+        assert_bdrv_graph_writable(new_bs);
         QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
 
         /*
@@ -2940,6 +2942,7 @@ static int bdrv_attach_child_noperm(BlockDriverState *parent_bs,
         return ret;
     }
 
+    assert_bdrv_graph_writable(parent_bs);
     QLIST_INSERT_HEAD(&parent_bs->children, *child, next);
     /*
      * child is removed in bdrv_attach_child_common_abort(), so don't care to
@@ -3140,6 +3143,7 @@ static void bdrv_unset_inherits_from(BlockDriverState *root, BdrvChild *child,
 void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child)
 {
     assert(qemu_in_main_thread());
+    assert_bdrv_graph_writable(parent);
     if (child == NULL) {
         return;
     }
@@ -4903,6 +4907,7 @@ static void bdrv_remove_filter_or_cow_child_abort(void *opaque)
     BdrvRemoveFilterOrCowChild *s = opaque;
     BlockDriverState *parent_bs = s->child->opaque;
 
+    assert_bdrv_graph_writable(parent_bs);
     QLIST_INSERT_HEAD(&parent_bs->children, s->child, next);
     if (s->is_backing) {
         parent_bs->backing = s->child;
diff --git a/block/io.c b/block/io.c
index f271ab3684..1c71e354d6 100644
--- a/block/io.c
+++ b/block/io.c
@@ -740,6 +740,17 @@ void bdrv_drain_all(void)
     bdrv_drain_all_end();
 }
 
+void assert_bdrv_graph_writable(BlockDriverState *bs)
+{
+    /*
+     * TODO: this function is incomplete. Because the users of this
+     * assert lack the necessary drains, check only for BQL.
+     * Once the necessary drains are added,
+     * assert also for qatomic_read(&bs->quiesce_counter) > 0
+     */
+    assert(qemu_in_main_thread());
+}
+
 /**
  * Remove an active request from the tracked requests list
  *
diff --git a/include/block/block_int-global-state.h b/include/block/block_int-global-state.h
index d08e80222c..6bd7746409 100644
--- a/include/block/block_int-global-state.h
+++ b/include/block/block_int-global-state.h
@@ -316,4 +316,12 @@ void bdrv_remove_aio_context_notifier(BlockDriverState *bs,
  */
 void bdrv_drain_all_end_quiesce(BlockDriverState *bs);
 
-#endif /* BLOCK_INT_GLOBAL_STATE*/
+/**
+ * Make sure that the function is either running under
+ * drain and BQL. The latter protects from concurrent writings
+ * from the GS API, while the former prevents concurrent reads
+ * from I/O.
+ */
+void assert_bdrv_graph_writable(BlockDriverState *bs);
+
+#endif /* BLOCK_INT_GLOBAL_STATE */
-- 
2.27.0