The rcu_request_urgent_qs_task() function does a cross-CPU store
to ->rcu_urgent_qs, so this commit therefore marks the load in
__rcu_irq_enter_check_tick() with READ_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fae9b4e29c93..aec07f2ec638 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -632,7 +632,7 @@ void __rcu_irq_enter_check_tick(void)
 	// prevents self-deadlock.  So we can safely recheck under the lock.
 	// Note that the nohz_full state currently cannot change.
 	raw_spin_lock_rcu_node(rdp->mynode);
-	if (rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
+	if (READ_ONCE(rdp->rcu_urgent_qs) && !rdp->rcu_forced_tick) {
 		// A nohz_full CPU is in the kernel and RCU needs a
 		// quiescent state.  Turn on the tick!
 		WRITE_ONCE(rdp->rcu_forced_tick, true);
-- 
2.40.1

On 7/17/23 14:03, Paul E. McKenney wrote:
> The rcu_request_urgent_qs_task() function does a cross-CPU store
> to ->rcu_urgent_qs, so this commit therefore marks the load in
> __rcu_irq_enter_check_tick() with READ_ONCE().
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>   kernel/rcu/tree.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index fae9b4e29c93..aec07f2ec638 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -632,7 +632,7 @@ void __rcu_irq_enter_check_tick(void)
>   	// prevents self-deadlock.  So we can safely recheck under the lock.
>   	// Note that the nohz_full state currently cannot change.
>   	raw_spin_lock_rcu_node(rdp->mynode);
> -	if (rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
> +	if (READ_ONCE(rdp->rcu_urgent_qs) && !rdp->rcu_forced_tick) {
>   		// A nohz_full CPU is in the kernel and RCU needs a
>   		// quiescent state.  Turn on the tick!
>   		WRITE_ONCE(rdp->rcu_forced_tick, 

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>


thanks,

- Joel