linux-kernel - Re: [PATCH 09/16] rcu: Remove redundant check for online cpu

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Date:	Wed, 23 Jul 2014 05:21:12 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/16] rcu: Remove redundant check for online cpu

On Wed, Jul 23, 2014 at 01:09:46AM -0400, Pranith Kumar wrote:
> There are two checks for an online CPU if two if() conditions. This commit
> simplies this by replacing it with only one check for the online CPU.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

I admit that it is very early in the morning my time, but I don't see
this change as preserving the semantics in all cases.  Please recheck
your changes to the second check.

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 5dcbf36..8d598a2 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2602,15 +2602,18 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
>  {
>  	bool needwake;
> 
> +	if (!cpu_online(smp_processor_id()))
> +		return;
> +
>  	/*
>  	 * If called from an extended quiescent state, invoke the RCU
>  	 * core in order to force a re-evaluation of RCU's idleness.
>  	 */
> -	if (!rcu_is_watching() && cpu_online(smp_processor_id()))
> +	if (!rcu_is_watching())
>  		invoke_rcu_core();
> 
> -	/* If interrupts were disabled or CPU offline, don't invoke RCU core. */
> -	if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
> +	/* If interrupts were disabled, don't invoke RCU core. */
> +	if (irqs_disabled_flags(flags))
>  		return;
> 
>  	/*
> -- 
> 2.0.0.rc2
> 

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