+ cgroup-clarify-css-sibling-linkage-is-protected-by-cgroup_mutex-or-rcu.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: cgroup: clarify css sibling linkage is protected by cgroup_mutex or RCU
has been added to the -mm mm-unstable branch.  Its filename is
     cgroup-clarify-css-sibling-linkage-is-protected-by-cgroup_mutex-or-rcu.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/cgroup-clarify-css-sibling-linkage-is-protected-by-cgroup_mutex-or-rcu.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Kinsey Ho <kinseyho@google.com>
Subject: cgroup: clarify css sibling linkage is protected by cgroup_mutex or RCU
Date: Tue, 27 Aug 2024 23:07:38 +0000

Pach series "Improve mem_cgroup_iter()", v3.

Incremental cgroup iteration is being used again [1].  This patchset
improves the reliability of mem_cgroup_iter().  It also improves
simplicity and code readability.

[1] https://lore.kernel.org/20240514202641.2821494-1-hannes@cmpxchg.org/


This patch (of 5):

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.

Link: https://lkml.kernel.org/r/20240827230753.2073580-1-kinseyho@google.com
Link: https://lkml.kernel.org/r/20240827230753.2073580-2-kinseyho@google.com
Suggested-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Kinsey Ho <kinseyho@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/cgroup-defs.h |    6 +++++-
 kernel/cgroup/cgroup.c      |   16 +++++++++-------
 2 files changed, 14 insertions(+), 8 deletions(-)

--- a/include/linux/cgroup-defs.h~cgroup-clarify-css-sibling-linkage-is-protected-by-cgroup_mutex-or-rcu
+++ a/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;
 
--- a/kernel/cgroup/cgroup.c~cgroup-clarify-css-sibling-linkage-is-protected-by-cgroup_mutex-or-rcu
+++ a/kernel/cgroup/cgroup.c
@@ -4602,8 +4602,9 @@ struct cgroup_subsys_state *css_next_chi
  *
  * 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_pr
  *
  * 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_su
  *
  * 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
_

Patches currently in -mm which might be from kinseyho@google.com are

cgroup-clarify-css-sibling-linkage-is-protected-by-cgroup_mutex-or-rcu.patch
mm-dont-hold-css-refcnt-during-traversal.patch
mm-increment-gen-before-restarting-traversal.patch
mm-restart-if-multiple-traversals-raced.patch
mm-clean-up-mem_cgroup_iter.patch