[PATCH v2] rcu/segcblist: Use WRITE_ONCE() for rclp->len decrement

[PATCH v2] rcu/segcblist: Use WRITE_ONCE() for rclp->len decrement

[Kaushlendra Kumar]

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@intel.com>
---
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

Re: [PATCH v2] rcu/segcblist: Use WRITE_ONCE() for rclp->len decrement

[Davidlohr Bueso]

On Thu, 11 Sep 2025, 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@intel.com>

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>

Re: [PATCH v2] rcu/segcblist: Use WRITE_ONCE() for rclp->len decrement

[Paul E. McKenney]

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@intel.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
>