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

Paul E. McKenney
2014-07-23T13:41:22Z

On Wed, Jul 23, 2014 at 09:23:47AM -0400, Pranith Kumar wrote:
> On Wed, Jul 23, 2014 at 8:09 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > 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.
> >
>
> That makes sense. I guess one of the other two checks in
> __call_rcu_core() or invoke_rcu_core() can go then?

What would have to be the case for this suggested change to be safe?
Does that condition in fact hold?

Thanx, Paul