linux-kernel - Re: [PATCH] rcu: Avoid to modify mask_ofl_ipi in sync_rcu_exp_select_node

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Message-ID: <20191008163028.GA136151@google.com>
Date:   Tue, 8 Oct 2019 12:30:28 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, elver@...gle.com,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, rcu@...r.kernel.org
Subject: Re: [PATCH] rcu: Avoid to modify mask_ofl_ipi in
 sync_rcu_exp_select_node_cpus()

On Tue, Oct 08, 2019 at 01:01:40PM +0800, Boqun Feng wrote:
> "mask_ofl_ipi" is used for iterate CPUs which IPIs are needed to send
> to, however in the IPI sending loop, "mask_ofl_ipi" along with another
> variable "mask_ofl_test" might also get modified to record which CPU's
> quiesent state can be reported by sync_rcu_exp_select_node_cpus(). Two
> variables seems to be redundant for such a propose, so this patch clean
> things a little by solely using "mask_ofl_test" for recording and
> "mask_ofl_ipi" for iteration. This would improve the readibility of the
> IPI sending loop in sync_rcu_exp_select_node_cpus().
> 
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---

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

thanks,

 - Joel

>  kernel/rcu/tree_exp.h | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 69c5aa64fcfd..212470018752 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -387,10 +387,10 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
>  		}
>  		ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
>  		put_cpu();
> -		if (!ret) {
> -			mask_ofl_ipi &= ~mask;
> +		/* The CPU responses the IPI, and will report QS itself */
> +		if (!ret)
>  			continue;
> -		}
> +
>  		/* Failed, raced with CPU hotplug operation. */
>  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>  		if ((rnp->qsmaskinitnext & mask) &&
> @@ -401,13 +401,12 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
>  			schedule_timeout_uninterruptible(1);
>  			goto retry_ipi;
>  		}
> -		/* CPU really is offline, so we can ignore it. */
> -		if (!(rnp->expmask & mask))
> -			mask_ofl_ipi &= ~mask;
> +		/* CPU really is offline, and we need its QS to pass GP. */
> +		if (rnp->expmask & mask)
> +			mask_ofl_test |= mask;
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  	}
>  	/* Report quiescent states for those that went offline. */
> -	mask_ofl_test |= mask_ofl_ipi;
>  	if (mask_ofl_test)
>  		rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
>  }
> -- 
> 2.23.0
>