Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754519Ab3IXUf2 (ORCPT ); Tue, 24 Sep 2013 16:35:28 -0400 Received: from merlin.infradead.org ([205.233.59.134]:58451 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753618Ab3IXUf1 (ORCPT ); Tue, 24 Sep 2013 16:35:27 -0400 Date: Tue, 24 Sep 2013 22:35:12 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: "Paul E. McKenney" , Steven Rostedt , 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: <20130924203512.GS9326@twins.programming.kicks-ass.net> References: <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> <20130924180005.GA7148@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130924180005.GA7148@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1935 Lines: 64 On Tue, Sep 24, 2013 at 08:00:05PM +0200, Oleg Nesterov wrote: > 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... So the reason I put it in was because of the inline; it could possibly make it do: test 0, current->cpuhp_ref je label1: inc current->cpuhp_ref label2: do_something(); label1: inc %gs:__preempt_count test 0, __cpuhp_writer jne label3 inc %gs:__cpuhp_refcount label5 dec %gs:__preempt_count je label4 jmp label2 label3: call __get_online_cpus(); jmp label5 label4: call ____preempt_schedule(); jmp label2 In which case the recursive fast path doesn't have a barrier() between taking the ref and starting do_something(). I wanted to make absolutely sure nothing of do_something leaked before the label2 thing. The other labels all have barrier() from the preempt_count ops. -- 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/