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

Paul E. McKenney
2014-07-23T13:50:33Z

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@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.

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
>