Re: [PATCH 1/2] llist: avoid memory tearing for llist_node

["Paul E. McKenney" <paulmck@kernel.org>]

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@linux.dev>

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