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

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Message-ID: <20140723134116.GP11241@linux.vnet.ibm.com>
Date:	Wed, 23 Jul 2014 06:41:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU

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@...ux.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@...il.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

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