From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752592Ab0CYM07 (ORCPT ); Thu, 25 Mar 2010 08:26:59 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:60457 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751051Ab0CYM06 (ORCPT ); Thu, 25 Mar 2010 08:26:58 -0400 Message-ID: <4BAB569E.5060103@cn.fujitsu.com> Date: Thu, 25 Mar 2010 20:27:10 +0800 From: Miao Xie Reply-To: miaox@cn.fujitsu.com User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3 MIME-Version: 1.0 To: Oleg Nesterov CC: Peter Zijlstra , Ingo Molnar , Ben Blum , Jiri Slaby , Lai Jiangshan , Li Zefan , Paul Menage , "Rafael J. Wysocki" , Tejun Heo , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/6] kill the broken and deadlockable cpuset_lock/cpuset_cpus_allowed_locked code References: <20100315091003.GA9123@redhat.com> <4BAAD1EB.4020201@cn.fujitsu.com> <20100325101420.GA30779@redhat.com> In-Reply-To: <20100325101420.GA30779@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org on 2010-3-25 18:14, Oleg Nesterov wrote: > On 03/25, Miao Xie wrote: >> >> on 2010-3-15 17:10, Oleg Nesterov wrote: >>> This patch just states the fact the cpusets/cpuhotplug interaction is >>> broken and removes the deadlockable code which only pretends to work. >>> >>> - cpuset_lock() doesn't really work. It is needed for >>> cpuset_cpus_allowed_locked() but we can't take this lock in >>> try_to_wake_up()->select_fallback_rq() path. >>> >>> - cpuset_lock() is deadlockable. Suppose that a task T bound to CPU takes >>> callback_mutex. If cpu_down(CPU) happens before T drops callback_mutex >>> stop_machine() preempts T, then migration_call(CPU_DEAD) tries to take >>> cpuset_lock() and hangs forever because CPU is already dead and thus >>> T can't be scheduled. >> >> The problem what you said don't exist, because the kernel already move T to >> the active cpu when preparing to turn off a CPU. > > we need cpuset_lock() to move T. please look at _cpu_down(). > > OK. > > A task T holds callback_mutex, and it is bound to CPU 1. > > _cpu_down(cpu => 1) is called by the task X. > > _cpu_down()->stop_machine() spawns rt-threads for each cpu, > a thread running on CPU 1 preempts T and calls take_cpu_down() > which removes CPU 1 from online/active masks. > > X continues, and does raw_notifier_call_chain(CPU_DEAD), this > calls migration_call(CPU_DEAD), and _this_ is what move the > tasks from the dead CPU. > > migration_call(CPU_DEAD) calls cpuset_lock() and deadlocks. > > See? But when the kernel want to offline a cpu, it does raw_notifier_call_chain(CPU_DOWN_PREPARE) at first. this calls cpuset_track_online_cpus() to update cpuset's cpus and task->cpus_allowed, and then moves the task running on the dying cpu to the other online cpu. At that time, rt-threads for each cpu have not been created. And when the kernel does migration_call(CPU_DEAD), the rt-threads already exit. the task that holds callback_mutex can run as normal. Miao > > Oleg. > > > >