Hi Tejun,

quoted

+/**
+ * tset_get_css - obtain and get css for (tset, subsys_id)
+ * @tset: target taskset
+ * @subsys_id: target subsystem id
+ *
+ * Since all of the tasks in a taskset are guaranteed to have the same css, it's
+ * safe to grab the ref of just the first task's css and treat it as though you
+ * have a ref on the taskset's "collective" css.
+ */
+#define tset_get_css(tset, subsys_id)                                        \
+     task_get_css(cgroup_taskset_first(tset), subsys_id)

They aren't tho.  It depends on the hierarchy configuration.  csses
belonging to hierarchies which aren't the current migration target may
differ.

Okay, I was confused by what you said earlier. In this case, we might
as well just move all of the attach accounting to ->attach() and not
deal with reverting it (unless we're guaranteed that the css of each
task in the tset doesn't change between ->can_attach() and
->cancel_attach()).

--
Aleksa Sarai (cyphar)
www.cyphar.com