block: assert that graph read and writes are performed correctly

Remove the old assert_bdrv_graph_writable, and replace it with
the new version using graph-lock API.

See the function documentation for more information.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20221207131838.239125-14-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -1406,7 +1406,7 @@ static void bdrv_child_cb_attach(BdrvChild *child)
 {
     BlockDriverState *bs = child->opaque;
 
-    assert_bdrv_graph_writable(bs);
+    assert_bdrv_graph_writable();
     QLIST_INSERT_HEAD(&bs->children, child, next);
     if (bs->drv->is_filter || (child->role & BDRV_CHILD_FILTERED)) {
         /*
@@ -1452,7 +1452,7 @@ static void bdrv_child_cb_detach(BdrvChild *child)
         bdrv_backing_detach(child);
     }
 
-    assert_bdrv_graph_writable(bs);
+    assert_bdrv_graph_writable();
     QLIST_REMOVE(child, next);
     if (child == bs->backing) {
         assert(child != bs->file);

block/graph-lock.c

@@ -259,3 +259,14 @@ void bdrv_graph_rdunlock_main_loop(void)
     GLOBAL_STATE_CODE();
     assert(!qemu_in_coroutine());
 }
+
+void assert_bdrv_graph_readable(void)
+{
+    assert(qemu_in_main_thread() || reader_count());
+}
+
+void assert_bdrv_graph_writable(void)
+{
+    assert(qemu_in_main_thread());
+    assert(qatomic_read(&has_writer));
+}

include/block/block_int-global-state.h

@@ -310,21 +310,4 @@ 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_H */

include/block/graph-lock.h

@@ -135,6 +135,21 @@ void coroutine_fn bdrv_graph_co_rdunlock(void);
 void bdrv_graph_rdlock_main_loop(void);
 void bdrv_graph_rdunlock_main_loop(void);
 
+/*
+ * assert_bdrv_graph_readable:
+ * Make sure that the reader is either the main loop,
+ * or there is at least a reader helding the rdlock.
+ * In this way an incoming writer is aware of the read and waits.
+ */
+void assert_bdrv_graph_readable(void);
+
+/*
+ * assert_bdrv_graph_writable:
+ * Make sure that the writer is the main loop and has set @has_writer,
+ * so that incoming readers will pause.
+ */
+void assert_bdrv_graph_writable(void);
+
 typedef struct GraphLockable { } GraphLockable;
 
 /*