1 files changed, 94 insertions, 0 deletions
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 90c33ebdd6b..8f64b459fbd 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -534,6 +534,100 @@ static inline struct cgroup* task_cgroup(struct task_struct *task,
 	return task_subsys_state(task, subsys_id)->cgroup;
 }
 
+/**
+ * cgroup_for_each_child - iterate through children of a cgroup
+ * @pos: the cgroup * to use as the loop cursor
+ * @cgroup: cgroup whose children to walk
+ *
+ * Walk @cgroup's children.  Must be called under rcu_read_lock().  A child
+ * cgroup which hasn't finished ->post_create() or already has finished
+ * ->pre_destroy() may show up during traversal and it's each subsystem's
+ * responsibility to verify that each @pos is alive.
+ *
+ * If a subsystem synchronizes against the parent in its ->post_create()
+ * and before starting iterating, a cgroup which finished ->post_create()
+ * is guaranteed to be visible in the future iterations.
+ */
+#define cgroup_for_each_child(pos, cgroup)				\
+	list_for_each_entry_rcu(pos, &(cgroup)->children, sibling)
+
+struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
+					  struct cgroup *cgroup);
+
+/**
+ * cgroup_for_each_descendant_pre - pre-order walk of a cgroup's descendants
+ * @pos: the cgroup * to use as the loop cursor
+ * @cgroup: cgroup whose descendants to walk
+ *
+ * Walk @cgroup's descendants.  Must be called under rcu_read_lock().  A
+ * descendant cgroup which hasn't finished ->post_create() or already has
+ * finished ->pre_destroy() may show up during traversal and it's each
+ * subsystem's responsibility to verify that each @pos is alive.
+ *
+ * If a subsystem synchronizes against the parent in its ->post_create()
+ * and before starting iterating, and synchronizes against @pos on each
+ * iteration, any descendant cgroup which finished ->post_create() is
+ * guaranteed to be visible in the future iterations.
+ *
+ * In other words, the following guarantees that a descendant can't escape
+ * state updates of its ancestors.
+ *
+ * my_post_create(@cgrp)
+ * {
+ *	Lock @cgrp->parent and @cgrp;
+ *	Inherit state from @cgrp->parent;
+ *	Unlock both.
+ * }
+ *
+ * my_update_state(@cgrp)
+ * {
+ *	Lock @cgrp;
+ *	Update @cgrp's state;
+ *	Unlock @cgrp;
+ *
+ *	cgroup_for_each_descendant_pre(@pos, @cgrp) {
+ *		Lock @pos;
+ *		Verify @pos is alive and inherit state from @pos->parent;
+ *		Unlock @pos;
+ *	}
+ * }
+ *
+ * As long as the inheriting step, including checking the parent state, is
+ * enclosed inside @pos locking, double-locking the parent isn't necessary
+ * while inheriting.  The state update to the parent is guaranteed to be
+ * visible by walking order and, as long as inheriting operations to the
+ * same @pos are atomic to each other, multiple updates racing each other
+ * still result in the correct state.  It's guaranateed that at least one
+ * inheritance happens for any cgroup after the latest update to its
+ * parent.
+ *
+ * If checking parent's state requires locking the parent, each inheriting
+ * iteration should lock and unlock both @pos->parent and @pos.
+ *
+ * Alternatively, a subsystem may choose to use a single global lock to
+ * synchronize ->post_create() and ->pre_destroy() against tree-walking
+ * operations.
+ */
+#define cgroup_for_each_descendant_pre(pos, cgroup)			\
+	for (pos = cgroup_next_descendant_pre(NULL, (cgroup)); (pos);	\
+	     pos = cgroup_next_descendant_pre((pos), (cgroup)))
+
+struct cgroup *cgroup_next_descendant_post(struct cgroup *pos,
+					   struct cgroup *cgroup);
+
+/**
+ * cgroup_for_each_descendant_post - post-order walk of a cgroup's descendants
+ * @pos: the cgroup * to use as the loop cursor
+ * @cgroup: cgroup whose descendants to walk
+ *
+ * Similar to cgroup_for_each_descendant_pre() but performs post-order
+ * traversal instead.  Note that the walk visibility guarantee described in
+ * pre-order walk doesn't apply the same to post-order walks.
+ */
+#define cgroup_for_each_descendant_post(pos, cgroup)			\
+	for (pos = cgroup_next_descendant_post(NULL, (cgroup)); (pos);	\
+	     pos = cgroup_next_descendant_post((pos), (cgroup)))
+
 /* A cgroup_iter should be treated as an opaque object */
 struct cgroup_iter {
 	struct list_head *cg_link;