block: introduce assert_bdrv_graph_writable

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>
Message-Id: <20220303151616.325444-15-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -1420,6 +1420,7 @@ static void bdrv_child_cb_attach(BdrvChild *child)
 {
     BlockDriverState *bs = child->opaque;
 
+    assert_bdrv_graph_writable(bs);
     QLIST_INSERT_HEAD(&bs->children, child, next);
 
     if (child->role & BDRV_CHILD_COW) {
@@ -1439,6 +1440,7 @@ static void bdrv_child_cb_detach(BdrvChild *child)
 
     bdrv_unapply_subtree_drain(child, bs);
 
+    assert_bdrv_graph_writable(bs);
     QLIST_REMOVE(child, next);
 }
 
@@ -2829,6 +2831,7 @@ static void bdrv_replace_child_noperm(BdrvChild **childp,
         if (child->klass->detach) {
             child->klass->detach(child);
         }
+        assert_bdrv_graph_writable(old_bs);
         QLIST_REMOVE(child, next_parent);
     }
 
@@ -2838,6 +2841,7 @@ static void bdrv_replace_child_noperm(BdrvChild **childp,
     }
 
     if (new_bs) {
+        assert_bdrv_graph_writable(new_bs);
         QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
 
         /*

include/block/block_int-global-state.h

@@ -309,4 +309,21 @@ void bdrv_remove_aio_context_notifier(BlockDriverState *bs,
  */
 void bdrv_drain_all_end_quiesce(BlockDriverState *bs);
 
+/**
+ * Make sure that the function is running under both drain and BQL.
+ * The latter protects from concurrent writings
+ * from the GS API, while the former prevents concurrent reads
+ * from I/O.
+ */
+static inline 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());
+}
+
 #endif /* BLOCK_INT_GLOBAL_STATE */