linux-kernel - [PATCH mm-unstable v4 1/5] cgroup: clarify css sibling linkage is protected by cgroup

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Message-ID: <20240905003058.1859929-2-kinseyho@google.com>
Date: Thu,  5 Sep 2024 00:30:50 +0000
From: Kinsey Ho <kinseyho@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	Yosry Ahmed <yosryahmed@...gle.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>, mkoutny@...e.com, 
	"T . J . Mercier" <tjmercier@...gle.com>, Hugh Dickins <hughd@...gle.com>, Kinsey Ho <kinseyho@...gle.com>
Subject: [PATCH mm-unstable v4 1/5] cgroup: clarify css sibling linkage is
 protected by cgroup_mutex or RCU

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@...gle.com>
Reviewed-by: Michal Koutný <mkoutny@...e.com>
Signed-off-by: Kinsey Ho <kinseyho@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Muchun Song <muchun.song@...ux.dev>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Tejun Heo <tj@...nel.org>
Cc: Zefan Li <lizefan.x@...edance.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: T.J. Mercier <tjmercier@...gle.com>
---
 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