linux-kernel - Re: [PATCH 1/2] llist: avoid memory tearing for llist

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Message-ID: <e33d07a0-8470-44a8-88c1-10dfcb1171ea@paulmck-laptop>
Date: Thu, 3 Jul 2025 15:24:21 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Tejun Heo <tj@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
	JP Kobryn <inwardvessel@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Huang <huang.ying.caritas@...il.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Alexei Starovoitov <ast@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Michal Koutný <mkoutny@...e.com>,
	bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 1/2] llist: avoid memory tearing for llist_node

On Thu, Jul 03, 2025 at 01:00:11PM -0700, Shakeel Butt wrote:
> Before the commit 36df6e3dbd7e ("cgroup: make css_rstat_updated nmi
> safe"), the struct llist_node is expected to be private to the one
> inserting the node to the lockless list or the one removing the node
> from the lockless list. After the mentioned commit, the llist_node in
> the rstat code is per-cpu shared between the stacked contexts i.e.
> process, softirq, hardirq & nmi. It is possible the compiler may tear
> the loads or stores of llist_node. Let's avoid that.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  include/linux/llist.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/llist.h b/include/linux/llist.h
> index 27b17f64bcee..607b2360c938 100644
> --- a/include/linux/llist.h
> +++ b/include/linux/llist.h
> @@ -83,7 +83,7 @@ static inline void init_llist_head(struct llist_head *list)
>   */
>  static inline void init_llist_node(struct llist_node *node)
>  {
> -	node->next = node;
> +	WRITE_ONCE(node->next, node);
>  }
>  
>  /**
> @@ -97,7 +97,7 @@ static inline void init_llist_node(struct llist_node *node)
>   */
>  static inline bool llist_on_list(const struct llist_node *node)
>  {
> -	return node->next != node;
> +	return READ_ONCE(node->next) != node;
>  }
>  
>  /**
> @@ -220,7 +220,7 @@ static inline bool llist_empty(const struct llist_head *head)
>  
>  static inline struct llist_node *llist_next(struct llist_node *node)
>  {
> -	return node->next;
> +	return READ_ONCE(node->next);
>  }
>  
>  /**
> -- 
> 2.47.1
>