[PATCH] rcu: Fix existing exp request check in sync_sched_exp_online_cleanup()

[PATCH] rcu: Fix existing exp request check in sync_sched_exp_online_cleanup()

[Neeraj Upadhyay]

The sync_sched_exp_online_cleanup() checks to see if RCU needs
an expedited quiescent state from the incoming CPU, sending it
an IPI if so. Before sending IPI, it checks whether expedited
qs need has been already requested for the incoming CPU, by
checking rcu_data.cpu_no_qs.b.exp for the current cpu, on which
sync_sched_exp_online_cleanup() is running. This works for the
case where incoming CPU is same as self. However, for the case
where incoming CPU is different from self, expedited request
won't get marked, which can potentially delay reporting of
expedited quiescent state for the incoming CPU.

Fixes: e015a3411220 ("rcu: Avoid self-IPI in sync_sched_exp_online_cleanup()")
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
---
 kernel/rcu/tree_exp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index d9e4f8e..f3947c4 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -759,7 +759,7 @@ static void sync_sched_exp_online_cleanup(int cpu)
 	my_cpu = get_cpu();
 	/* Quiescent state either not needed or already requested, leave. */
 	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
-	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp)) {
+	    rdp->cpu_no_qs.b.exp) {
 		put_cpu();
 		return;
 	}
-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, 
hosted by The Linux Foundation

Re: [PATCH] rcu: Fix existing exp request check in sync_sched_exp_online_cleanup()

[Paul E. McKenney]

On Wed, Aug 18, 2021 at 01:34:00PM +0530, Neeraj Upadhyay wrote:
> The sync_sched_exp_online_cleanup() checks to see if RCU needs
> an expedited quiescent state from the incoming CPU, sending it
> an IPI if so. Before sending IPI, it checks whether expedited
> qs need has been already requested for the incoming CPU, by
> checking rcu_data.cpu_no_qs.b.exp for the current cpu, on which
> sync_sched_exp_online_cleanup() is running. This works for the
> case where incoming CPU is same as self. However, for the case
> where incoming CPU is different from self, expedited request
> won't get marked, which can potentially delay reporting of
> expedited quiescent state for the incoming CPU.
> 
> Fixes: e015a3411220 ("rcu: Avoid self-IPI in sync_sched_exp_online_cleanup()")
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>

Good catch!!!  I have queued this for further review and testing, thank
you!

							Thanx, Paul

> ---
>  kernel/rcu/tree_exp.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index d9e4f8e..f3947c4 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -759,7 +759,7 @@ static void sync_sched_exp_online_cleanup(int cpu)
>  	my_cpu = get_cpu();
>  	/* Quiescent state either not needed or already requested, leave. */
>  	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
> -	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp)) {
> +	    rdp->cpu_no_qs.b.exp) {
>  		put_cpu();
>  		return;
>  	}
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, 
> hosted by The Linux Foundation
>