From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757573AbaGWOOa (ORCPT ); Wed, 23 Jul 2014 10:14:30 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:33124 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751936AbaGWOO3 (ORCPT ); Wed, 23 Jul 2014 10:14:29 -0400 Date: Wed, 23 Jul 2014 07:14:24 -0700 From: "Paul E. McKenney" To: Pranith Kumar Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Subject: Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU Message-ID: <20140723141424.GT11241@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1406092194-13004-1-git-send-email-bobby.prani@gmail.com> <1406092194-13004-5-git-send-email-bobby.prani@gmail.com> <20140723120907.GG11241@linux.vnet.ibm.com> <20140723134116.GP11241@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14072314-0928-0000-0000-0000039315FA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 23, 2014 at 10:01:14AM -0400, Pranith Kumar wrote: > On Wed, Jul 23, 2014 at 9:41 AM, Paul E. McKenney > wrote: > > 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 > >> 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 > >> > > >> > 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? > > > > The only scenario which is unsafe is when this thread is preempted and > scheduled on a dying CPU. In that case we should turn off interrupts. > > Actually, checking the code, I see that interrupts are turned off > already before we call _call_rcu_core(). So I am not sure such a case > will happen. On the other hand invoke_rcu_core() will be a one line > function which might as well be in-lined to avoid the double cpu > online check. > > What am I missing? I am not sure. Perhaps the fact that __call_rcu_core() doesn't call invoke_rcu_core() unless the condition holds (which means that you cannnot remove the check from __call_rcu_core()) or maybe the fact that invoke_rcu_core() is called from many other places, which means that you might not be able to remove the check from invoke_rcu_core(). Thanx, Paul