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

Pranith Kumar
2014-07-23T12:59:39Z

On Wed, Jul 23, 2014 at 8:21 AM, Paul E. McKenney
<paulmck@linux.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@gmail.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