linux-kernel - Re: [PATCH] rcu: Fix existing exp request check in sync_sched_exp_online

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Message-ID: <20210818155151.GV4126399@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 18 Aug 2021 08:51:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, urezki@...il.com,
        frederic@...nel.org, boqun.feng@...il.com
Subject: Re: [PATCH] rcu: Fix existing exp request check in
 sync_sched_exp_online_cleanup()

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@...eaurora.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
>