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

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Message-ID: <CAJhHMCAoLQPnJYCeqftXwZb6D4ECuJb4g04b4OO5BLf6iC1ENw@mail.gmail.com>
Date:	Wed, 23 Jul 2014 08:59:06 -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 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.

--
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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