linux-kernel - Re: [PATCH v2] rcu/segcblist: Use WRITE

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Message-ID: <d0f20036-70d7-4dcb-9745-99341fd1c501@paulmck-laptop>
Date: Thu, 11 Sep 2025 12:41:43 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Kaushlendra Kumar <kaushlendra.kumar@...el.com>
Cc: dave@...olabs.net, josh@...htriplett.org, frederic@...nel.org,
	neeraj.upadhyay@...nel.org, rostedt@...dmis.org,
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH v2] rcu/segcblist: Use WRITE_ONCE() for rclp->len
 decrement

On Thu, Sep 11, 2025 at 09:56:41PM +0530, Kaushlendra Kumar wrote:
> Use WRITE_ONCE() for rclp->len decrement in rcu_cblist_dequeue() to
> maintain consistency with rcu_cblist_enqueue(), which already uses
> WRITE_ONCE() for rclp->len increment. It maintains consistent access
> patterns to the rclp->len field across both enqueue and dequeue operations.
> 
> Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@...el.com>

Again, why is this patch needed?

Please keep in mind that unmarked normal C-language loads and stores have
value in that KCSAN can detect data races involving them.  If we decorate
all loads and stores with READ_ONCE() and WRITE_ONCE(), respectively,
KCSAN is unable to detect buggy concurrent accesses.

							Thanx, Paul

> ---
> Changes in v2:
> - Revised justification to focus on code consistency rather than
>   concurrent access claims.
> - Revised commit message to use imperative mood throughout per kernel
>   documentation guidelines (suggested by Markus Elfring).
> 
>  kernel/rcu/rcu_segcblist.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 1693ea22ef1b..e10b36e9de54 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -71,7 +71,7 @@ struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp)
>  	rhp = rclp->head;
>  	if (!rhp)
>  		return NULL;
> -	rclp->len--;
> +	WRITE_ONCE(rclp->len, rclp->len - 1);
>  	rclp->head = rhp->next;
>  	if (!rclp->head)
>  		rclp->tail = &rclp->head;
> -- 
> 2.34.1
>