linux-kernel - Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU

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Date:	Wed, 23 Jul 2014 05:09:07 -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 04/16] rcu: Remove redundant check for an online CPU

On Wed, Jul 23, 2014 at 01:09:41AM -0400, Pranith Kumar wrote:
> rcu_prcess_callbacks() is the softirq handler for RCU which is raised from
> invoke_rcu_core() which is called from __call_rcu_core().
> 
> Each of these three functions checks if the cpu is online. We can remove the
> redundant ones. This commit removes one of these redundant check.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

Sorry, but no.  There can be a long delay between raise_softirq() and
this function starting, particularly if ksoftirqd gets involved.  The
CPU could easily go offline in the meantime.

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 72e0b1f..044946e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2563,8 +2563,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
>  {
>  	struct rcu_state *rsp;
> 
> -	if (cpu_is_offline(smp_processor_id()))
> -		return;
>  	trace_rcu_utilization(TPS("Start RCU core"));
>  	for_each_rcu_flavor(rsp)
>  		__rcu_process_callbacks(rsp);
> -- 
> 2.0.0.rc2
> 

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