Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754596Ab3IXSHr (ORCPT ); Tue, 24 Sep 2013 14:07:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:12027 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754314Ab3IXSHK (ORCPT ); Tue, 24 Sep 2013 14:07:10 -0400 Date: Tue, 24 Sep 2013 20:00:05 +0200 From: Oleg Nesterov To: "Paul E. McKenney" Cc: Steven Rostedt , Peter Zijlstra , Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Thomas Gleixner Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20130924180005.GA7148@redhat.com> References: <20130917162050.GK22421@suse.de> <20130917164505.GG12926@twins.programming.kicks-ass.net> <20130918154939.GZ26785@twins.programming.kicks-ass.net> <20130919143241.GB26785@twins.programming.kicks-ass.net> <20130923175052.GA20991@redhat.com> <20130924123821.GT12926@twins.programming.kicks-ass.net> <20130924160359.GA2739@redhat.com> <20130924124341.64d57912@gandalf.local.home> <20130924170631.GB5059@redhat.com> <20130924174717.GH9093@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130924174717.GH9093@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1122 Lines: 32 On 09/24, Paul E. McKenney wrote: > > On Tue, Sep 24, 2013 at 07:06:31PM +0200, Oleg Nesterov wrote: > > > > If gcc can actually do something wrong, then I suspect this barrier() > > should be unconditional. > > If you are saying that there should be a barrier() on all return paths > from get_online_cpus(), I agree. Paul, Peter, could you provide any (even completely artificial) example to explain me why do we need this barrier() ? I am puzzled. And preempt_enable() already has barrier... get_online_cpus(); do_something(); Yes, we need to ensure gcc doesn't reorder this code so that do_something() comes before get_online_cpus(). But it can't? At least it should check current->cpuhp_ref != 0 first? And if it is non-zero we do not really care, we are already in the critical section and this ->cpuhp_ref has only meaning in put_online_cpus(). Confused... Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/