From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752632AbbDCKjN (ORCPT ); Fri, 3 Apr 2015 06:39:13 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:55453 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752530AbbDCKjI (ORCPT ); Fri, 3 Apr 2015 06:39:08 -0400 Message-ID: <551E6DBD.6070401@linux.vnet.ibm.com> Date: Fri, 03 Apr 2015 16:08:53 +0530 From: Preeti U Murthy User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: nico@linaro.org, tglx@linutronix.de, hpa@zytor.com, linux-kernel@vger.kernel.org, mingo@kernel.org, linux-tip-commits@vger.kernel.org Subject: Re: [tip:timers/core] clockevents: Fix cpu_down() race for hrtimer based broadcasting References: <20150330092410.24979.59887.stgit@preeti.in.ibm.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15040310-0009-0000-0000-000009DCEBCC Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/02/2015 08:00 PM, tip-bot for Preeti U Murthy wrote: > Commit-ID: 345527b1edce8df719e0884500c76832a18211c3 > Gitweb: http://git.kernel.org/tip/345527b1edce8df719e0884500c76832a18211c3 > Author: Preeti U Murthy > AuthorDate: Mon, 30 Mar 2015 14:59:19 +0530 > Committer: Ingo Molnar > CommitDate: Thu, 2 Apr 2015 14:25:39 +0200 > > clockevents: Fix cpu_down() race for hrtimer based broadcasting > > It was found when doing a hotplug stress test on POWER, that the > machine either hit softlockups or rcu_sched stall warnings. The > issue was traced to commit: > > 7cba160ad789 ("powernv/cpuidle: Redesign idle states management") > > which exposed the cpu_down() race with hrtimer based broadcast mode: > > 5d1638acb9f6 ("tick: Introduce hrtimer based broadcast") > > The race is the following: > > Assume CPU1 is the CPU which holds the hrtimer broadcasting duty > before it is taken down. > > CPU0 CPU1 > > cpu_down() take_cpu_down() > disable_interrupts() > > cpu_die() > > while (CPU1 != CPU_DEAD) { > msleep(100); > switch_to_idle(); > stop_cpu_timer(); > schedule_broadcast(); > } > > tick_cleanup_cpu_dead() > take_over_broadcast() > > So after CPU1 disabled interrupts it cannot handle the broadcast > hrtimer anymore, so CPU0 will be stuck forever. > > Fix this by explicitly taking over broadcast duty before cpu_die(). > > This is a temporary workaround. What we really want is a callback > in the clockevent device which allows us to do that from the dying > CPU by pushing the hrtimer onto a different cpu. That might involve > an IPI and is definitely more complex than this immediate fix. > > Changelog was picked up from: > > https://lkml.org/lkml/2015/2/16/213 > > Suggested-by: Thomas Gleixner > Tested-by: Nicolas Pitre > Signed-off-by: Preeti U. Murthy > Cc: linuxppc-dev@lists.ozlabs.org > Cc: mpe@ellerman.id.au > Cc: nicolas.pitre@linaro.org > Cc: peterz@infradead.org > Cc: rjw@rjwysocki.net > Fixes: http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html > Link: http://lkml.kernel.org/r/20150330092410.24979.59887.stgit@preeti.in.ibm.com > [ Merged it to the latest timer tree, renamed the callback, tidied up the changelog. ] > Signed-off-by: Ingo Molnar Can this be marked for stable too please? Regards Preeti U Murthy