From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f44.google.com (mail-pb0-f44.google.com [209.85.160.44]) by kanga.kvack.org (Postfix) with ESMTP id B44126B0031 for ; Tue, 24 Sep 2013 14:07:02 -0400 (EDT) Received: by mail-pb0-f44.google.com with SMTP id xa7so4898209pbc.31 for ; Tue, 24 Sep 2013 11:07:02 -0700 (PDT) Date: Tue, 24 Sep 2013 20:00:05 +0200 From: Oleg Nesterov 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> Sender: owner-linux-mm@kvack.org List-ID: 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 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org