Explicitly document that css sibling/descendant linkage is protected by
cgroup_mutex or RCU. Also, document in css_next_descendant_pre() and
similar functions that it isn't necessary to hold a ref on @pos.
The following changes in this patchset rely on this clarification
for simplification in memcg iteration code.
Suggested-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Kinsey Ho <kinseyho@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: T.J. Mercier <tjmercier@xxxxxxxxxx>
---
include/linux/cgroup-defs.h | 6 +++++-
kernel/cgroup/cgroup.c | 16 +++++++++-------
2 files changed, 14 insertions(+), 8 deletions(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 7fc2d0195f56..ca7e912b8355 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -172,7 +172,11 @@ struct cgroup_subsys_state {
/* reference count - access via css_[try]get() and css_put() */
struct percpu_ref refcnt;
- /* siblings list anchored at the parent's ->children */
+ /*
+ * siblings list anchored at the parent's ->children
+ *
+ * linkage is protected by cgroup_mutex or RCU
+ */
struct list_head sibling;
struct list_head children;
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 0a97cb2ef124..ece2316e2bca 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4602,8 +4602,9 @@ struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
*
* While this function requires cgroup_mutex or RCU read locking, it
* doesn't require the whole traversal to be contained in a single critical
- * section. This function will return the correct next descendant as long
- * as both @pos and @root are accessible and @pos is a descendant of @root.
+ * section. Additionally, it isn't necessary to hold onto a reference to @pos.
+ * This function will return the correct next descendant as long as both @pos
+ * and @root are accessible and @pos is a descendant of @root.
*
* If a subsystem synchronizes ->css_online() and the start of iteration, a
* css which finished ->css_online() is guaranteed to be visible in the
@@ -4651,8 +4652,9 @@ EXPORT_SYMBOL_GPL(css_next_descendant_pre);
*
* While this function requires cgroup_mutex or RCU read locking, it
* doesn't require the whole traversal to be contained in a single critical
- * section. This function will return the correct rightmost descendant as
- * long as @pos is accessible.
+ * section. Additionally, it isn't necessary to hold onto a reference to @pos.
+ * This function will return the correct rightmost descendant as long as @pos
+ * is accessible.
*/
struct cgroup_subsys_state *
css_rightmost_descendant(struct cgroup_subsys_state *pos)
@@ -4696,9 +4698,9 @@ css_leftmost_descendant(struct cgroup_subsys_state *pos)
*
* While this function requires cgroup_mutex or RCU read locking, it
* doesn't require the whole traversal to be contained in a single 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.
+ * section. Additionally, it isn't necessary to hold onto a reference to @pos.
+ * This function will return the correct next descendant as long as both @pos
+ * and @cgroup are accessible and @pos is a descendant of @cgroup.
*
* If a subsystem synchronizes ->css_online() and the start of iteration, a
* css which finished ->css_online() is guaranteed to be visible in the
--
2.46.0.469.g59c65b2a67-goog
