2 files changed, 33 insertions, 10 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ee041a01a67e..d0ad3794b947 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -688,9 +688,9 @@ struct cgroup *cgroup_next_sibling(struct cgroup *pos);
 /**
  * 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
+ * @cgrp: cgroup whose children to walk
  *
- * Walk @cgroup's children.  Must be called under rcu_read_lock().  A child
+ * Walk @cgrp's children.  Must be called under rcu_read_lock().  A child
  * cgroup which hasn't finished ->css_online() or already has finished
  * ->css_offline() may show up during traversal and it's each subsystem's
  * responsibility to verify that each @pos is alive.
@@ -698,9 +698,15 @@ struct cgroup *cgroup_next_sibling(struct cgroup *pos);
  * If a subsystem synchronizes against the parent in its ->css_online() and
  * before starting iterating, a cgroup which finished ->css_online() is
  * guaranteed to be visible in the future iterations.
+ *
+ * It is allowed to temporarily drop RCU read lock during iteration.  The
+ * caller is responsible for ensuring that @pos remains accessible until
+ * the start of the next iteration by, for example, bumping the css refcnt.
  */
-#define cgroup_for_each_child(pos, cgroup)                              \
-        list_for_each_entry_rcu(pos, &(cgroup)->children, sibling)
+#define cgroup_for_each_child(pos, cgrp)                                \
+        for ((pos) = list_first_or_null_rcu(&(cgrp)->children,          \
+                                            struct cgroup, sibling);    \
+             (pos); (pos) = cgroup_next_sibling((pos)))
 
 struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
                                           struct cgroup *cgroup);
@@ -759,6 +765,10 @@ struct cgroup *cgroup_rightmost_descendant(struct cgroup *pos);
  * Alternatively, a subsystem may choose to use a single global lock to
  * synchronize ->css_online() and ->css_offline() against tree-walking
  * operations.
+ *
+ * It is allowed to temporarily drop RCU read lock during iteration.  The
+ * caller is responsible for ensuring that @pos remains accessible until
+ * the start of the next iteration by, for example, bumping the css refcnt.
  */
 #define cgroup_for_each_descendant_pre(pos, cgroup)                     \
         for (pos = cgroup_next_descendant_pre(NULL, (cgroup)); (pos);   \
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b87c7a5a5497..fefc41c1a147 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3031,6 +3031,11 @@ EXPORT_SYMBOL_GPL(cgroup_next_sibling);
  *
  * To be used by cgroup_for_each_descendant_pre().  Find the next
  * descendant to visit for pre-order traversal of @cgroup's descendants.
+ *
+ * While this function requires RCU read locking, it doesn't require the
+ * whole traversal to be contained in a single RCU critical section.  This
+ * function will return the correct next descendant as long as both @pos
+ * and @cgroup are accessible and @pos is a descendant of @cgroup.
  */
 struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
                                           struct cgroup *cgroup)
@@ -3050,11 +3055,9 @@ struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
 
         /* no child, visit my or the closest ancestor's next sibling */
         while (pos != cgroup) {
-                next = list_entry_rcu(pos->sibling.next, struct cgroup,
-                                      sibling);
-                if (&next->sibling != &pos->parent->children)
+                next = cgroup_next_sibling(pos);
+                if (next)
                         return next;
-
                 pos = pos->parent;
         }
 
@@ -3069,6 +3072,11 @@ EXPORT_SYMBOL_GPL(cgroup_next_descendant_pre);
  * Return the rightmost descendant of @pos.  If there's no descendant,
  * @pos is returned.  This can be used during pre-order traversal to skip
  * subtree of @pos.
+ *
+ * While this function requires RCU read locking, it doesn't require the
+ * whole traversal to be contained in a single RCU critical section.  This
+ * function will return the correct rightmost descendant as long as @pos is
+ * accessible.
  */
 struct cgroup *cgroup_rightmost_descendant(struct cgroup *pos)
 {
@@ -3108,6 +3116,11 @@ static struct cgroup *cgroup_leftmost_descendant(struct cgroup *pos)
  *
  * To be used by cgroup_for_each_descendant_post().  Find the next
  * descendant to visit for post-order traversal of @cgroup's descendants.
+ *
+ * While this function requires RCU read locking, it doesn't require the
+ * whole traversal to be contained in a single RCU critical section.  This
+ * function will return the correct next descendant as long as both @pos
+ * and @cgroup are accessible and @pos is a descendant of @cgroup.
  */
 struct cgroup *cgroup_next_descendant_post(struct cgroup *pos,
                                            struct cgroup *cgroup)
@@ -3123,8 +3136,8 @@ struct cgroup *cgroup_next_descendant_post(struct cgroup *pos,
         }
 
         /* if there's an unvisited sibling, visit its leftmost descendant */
-        next = list_entry_rcu(pos->sibling.next, struct cgroup, sibling);
-        if (&next->sibling != &pos->parent->children)
+        next = cgroup_next_sibling(pos);
+        if (next)
                 return cgroup_leftmost_descendant(next);
 
         /* no sibling left, visit parent */