linux-kernel - Re: [PATCH mm-unstable v2 1/5] cgroup: clarify css sibling linkage is protected by cgroup

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Message-ID: <mh6fy7l44s34x3nz5u2rvp7qpc7bpt4ah4ruy5ixeuj3p6ggs3@2bbetdmdgjxt>
Date: Wed, 14 Aug 2024 11:00:15 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Kinsey Ho <kinseyho@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 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>
Subject: Re: [PATCH mm-unstable v2 1/5] cgroup: clarify css sibling linkage
 is protected by cgroup_mutex or RCU

On Tue, Aug 13, 2024 at 08:47:11PM GMT, Kinsey Ho <kinseyho@...gle.com> wrote:
> --- 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;

Thanks, this is good.

>  
> 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

I'd say these comments are unnecessary given the functions have
cgroup_assert_mutex_or_rcu_locked() but if it helps overall
understanding in broader context, why not.

Reviewed-by: Michal Koutný <mkoutny@...e.com>

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