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

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Date:	Wed, 23 Jul 2014 06:50:24 -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 08:59:06AM -0400, Pranith Kumar wrote:
> On Wed, Jul 23, 2014 at 8:21 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > 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
> 
> I guess you must be thrown off by the complementary checks, the first
> check is for cpu_online() and second is for cpu_is_offline(). :)
> 
> Previously, if a cpu is offline, the first condition is false and the
> second condition is true, so we return from the second if() condition.
> The same semantics are being preserved.

Fair enough!

Nevertheless, I am not seeing this as a simplification.

							Thanx, Paul

> --
> Pranith.
> 
> 
> 
> >
> >> ---
> >>  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
> >>
> >
> 
> 
> 
> -- 
> Pranith
> 

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