Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU

Paul E. McKenney
2014-07-23T12:09:15Z

On Wed, Jul 23, 2014 at 01:09:41AM -0400, Pranith Kumar wrote:
> rcu_prcess_callbacks() is the softirq handler for RCU which is raised from
> invoke_rcu_core() which is called from __call_rcu_core().
>
> Each of these three functions checks if the cpu is online. We can remove the
> redundant ones. This commit removes one of these redundant check.
>
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>

Sorry, but no. There can be a long delay between raise_softirq() and
this function starting, particularly if ksoftirqd gets involved. The
CPU could easily go offline in the meantime.

Thanx, Paul

> ---
> kernel/rcu/tree.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 72e0b1f..044946e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2563,8 +2563,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
> {
> struct rcu_state *rsp;
>
> - if (cpu_is_offline(smp_processor_id()))
> - return;
> trace_rcu_utilization(TPS("Start RCU core"));
> for_each_rcu_flavor(rsp)
> __rcu_process_callbacks(rsp);
> --
> 2.0.0.rc2
>