This adds a macro to get the css of a tset (using task_get_css()) by
just grabbing a ref to the first item in the tset (since there is a
guarantee that all tasks in a tset share a css).

This is in preparation for implementing the pids cgroup subsystem.

Signed-off-by: Aleksa Sarai <cyphar-gVpy/LI/lHzQT0dZR+AlfA@public.gmane.org>
---
 include/linux/cgroup.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 886a883..773846d 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h

@@ -639,6 +639,18 @@ struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset);
 	for ((task) = cgroup_taskset_first((tset)); (task);		\
 	     (task) = cgroup_taskset_next((tset)))
 
+/**
+ * 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)