From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752631AbaEWPey (ORCPT ); Fri, 23 May 2014 11:34:54 -0400 Received: from e28smtp01.in.ibm.com ([122.248.162.1]:46264 "EHLO e28smtp01.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751056AbaEWPex (ORCPT ); Fri, 23 May 2014 11:34:53 -0400 Message-ID: <537F6A53.1080703@linux.vnet.ibm.com> Date: Fri, 23 May 2014 21:03:39 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Peter Zijlstra CC: Frederic Weisbecker , tglx@linutronix.de, mingo@kernel.org, tj@kernel.org, rusty@rustcorp.com.au, akpm@linux-foundation.org, hch@infradead.org, mgorman@suse.de, riel@redhat.com, bp@suse.de, rostedt@goodmis.org, mgalbraith@suse.de, ego@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, oleg@redhat.com, rjw@rjwysocki.net, linux-kernel@vger.kernel.org Subject: Re: [PATCH v6 2/3] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU" References: <20140523101134.17924.31463.stgit@srivatsabhat.in.ibm.com> <20140523101216.17924.62447.stgit@srivatsabhat.in.ibm.com> <20140523132250.GA1768@localhost.localdomain> <537F5F0F.5050802@linux.vnet.ibm.com> <20140523151252.GE30445@twins.programming.kicks-ass.net> <537F66AF.30303@linux.vnet.ibm.com> <20140523153113.GG30445@twins.programming.kicks-ass.net> In-Reply-To: <20140523153113.GG30445@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14052315-4790-0000-0000-00000196A7BF Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/23/2014 09:01 PM, Peter Zijlstra wrote: > On Fri, May 23, 2014 at 08:48:07PM +0530, Srivatsa S. Bhat wrote: >> On 05/23/2014 08:42 PM, Peter Zijlstra wrote: >>> On Fri, May 23, 2014 at 08:15:35PM +0530, Srivatsa S. Bhat wrote: >>>>>> + * During CPU offline, we don't want the other CPUs to send >>>>>> + * IPIs to the active_cpu (the outgoing CPU) *after* it has >>>>>> + * disabled interrupts (because, then it will notice the IPIs >>>>>> + * only after it has gone offline). We can prevent this by >>>>>> + * making the other CPUs disable their interrupts first - that >>>>>> + * way, they will run the stop-machine code with interrupts >>>>>> + * disabled, and hence won't send IPIs after that point. >>> >>> That's complete nonsense, you can send IPIs all you want with interrupts >>> disabled. >>> >> >> True, but that's not what the comment says. It says "you can't send IPIs >> because you are running the *stop-machine* loop, because the stop-machine loop >> doesn't send IPIs itself! The only possibility of sending IPIs from within >> stop-machine is if that CPU can takes an interrupt and the *interrupt handler* >> sends the IPI (like what the block layer used to do) - and we precisely avoid >> that possibility by disabling interrupts. So no IPIs will be sent beyond >> this point. > > but one of those CPUs is running the stop machine function, which calls > CPU_DYING which runs all kinds of nonsense and therefore can send IPIs > all it wants, right? > Yes, but that CPU certainly won't IPI itself! (We are trying to avoid getting IPIs on precisely that CPU - the one which is about to go offline). Regards, Srivatsa S. Bhat