Re: [PATCH tip/core/rcu 3/4] rcu: stop spurious warnings from synchronize_sched_expedited

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2012-02-21 at 09:58 -0800, Paul E. McKenney wrote:
> From: Hugh Dickins <hughd@google.com>
> 
> synchronize_sched_expedited() is spamming CONFIG_DEBUG_PREEMPT=y
> users with an unintended warning from the cpu_is_offline() check:
> use raw_smp_processor_id() instead of smp_processor_id() there.

This fails to mention why it makes sense to test a random cpu for
offline-ness..

> Signed-off-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/rcutree.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index eacc10b..1050d6d 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -2014,7 +2014,7 @@ void synchronize_sched_expedited(void)
>  	/* Note that atomic_inc_return() implies full memory barrier. */
>  	firstsnap = snap = atomic_inc_return(&sync_sched_expedited_started);
>  	get_online_cpus();
> -	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
> +	WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));
>  
>  	/*
>  	 * Each pass through the following loop attempts to force a