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

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Message-ID: <CAJhHMCAZ__1xkEou44iux=Z6v6NAKPrTvWob6tqorm23nNygOA@mail.gmail.com>
Date:	Wed, 23 Jul 2014 10:12:54 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.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 9:50 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> 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.
>

I am not sure what you mean here, do you mean that both the checks are
actually required?


>> --
>> 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
>>
>



-- 
Pranith
--
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