block: move drain outside of bdrv_try_change_aio_context()

This is part of resolving the deadlock mentioned in commit "block:
move draining out of bdrv_change_aio_context() and mark GRAPH_RDLOCK".

Convert the function to a _locked() version that has to be called with
the graph lock held and add a convenience wrapper that has to be
called with the graph unlocked, which drains and takes the lock
itself. Since bdrv_try_change_aio_context() is global state code, the
wrapper is too.

Callers are adapted to use the appropriate variant, depending on
whether the caller already holds the lock. In the
test_set_aio_context() unit test, prior drains can be removed, because
draining already happens inside the new wrapper.

Note that bdrv_attach_child_common_abort(), bdrv_attach_child_common()
and bdrv_root_unref_child() hold the graph lock and are not actually
allowed to drain either. This will be addressed in the following
commits.

Functions like qmp_blockdev_mirror() query the nodes to act on before
draining and locking. In theory, draining could invalidate those nodes.
This kind of issue is not addressed by these commits.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-10-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/blockdev.c b/blockdev.c
index 3982f9776b..750beba41f 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3601,12 +3601,13 @@ void qmp_x_blockdev_set_iothread(const char *node_name, StrOrNull *iothread,
     AioContext *new_context;
     BlockDriverState *bs;
 
-    GRAPH_RDLOCK_GUARD_MAINLOOP();
+    bdrv_drain_all_begin();
+    bdrv_graph_rdlock_main_loop();
 
     bs = bdrv_find_node(node_name);
     if (!bs) {
         error_setg(errp, "Failed to find node with node-name='%s'", node_name);
-        return;
+        goto out;
     }
 
     /* Protects against accidents. */
@@ -3614,14 +3615,14 @@ void qmp_x_blockdev_set_iothread(const char *node_name, StrOrNull *iothread,
         error_setg(errp, "Node %s is associated with a BlockBackend and could "
                          "be in use (use force=true to override this check)",
                          node_name);
-        return;
+        goto out;
     }
 
     if (iothread->type == QTYPE_QSTRING) {
         IOThread *obj = iothread_by_id(iothread->u.s);
         if (!obj) {
             error_setg(errp, "Cannot find iothread %s", iothread->u.s);
-            return;
+            goto out;
         }
 
         new_context = iothread_get_aio_context(obj);
@@ -3629,7 +3630,11 @@ void qmp_x_blockdev_set_iothread(const char *node_name, StrOrNull *iothread,
         new_context = qemu_get_aio_context();
     }
 
-    bdrv_try_change_aio_context(bs, new_context, NULL, errp);
+    bdrv_try_change_aio_context_locked(bs, new_context, NULL, errp);
+
+out:
+    bdrv_graph_rdunlock_main_loop();
+    bdrv_drain_all_end();
 }
 
 QemuOptsList qemu_common_drive_opts = {