From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757351AbaGWOBs (ORCPT ); Wed, 23 Jul 2014 10:01:48 -0400 Received: from mail-la0-f46.google.com ([209.85.215.46]:36612 "EHLO mail-la0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756030AbaGWOBr (ORCPT ); Wed, 23 Jul 2014 10:01:47 -0400 MIME-Version: 1.0 In-Reply-To: <20140723134116.GP11241@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> From: Pranith Kumar Date: Wed, 23 Jul 2014 10:01:14 -0400 Message-ID: Subject: Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU To: Paul McKenney Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? -- Pranith