From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751417AbbDFE24 (ORCPT ); Mon, 6 Apr 2015 00:28:56 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:48181 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751017AbbDFE2x (ORCPT ); Mon, 6 Apr 2015 00:28:53 -0400 Message-ID: <55220B7D.8020109@linux.vnet.ibm.com> Date: Mon, 06 Apr 2015 09:58:45 +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: Ingo Molnar CC: nico@linaro.org, tglx@linutronix.de, hpa@zytor.com, linux-kernel@vger.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> <551E6DBD.6070401@linux.vnet.ibm.com> <20150403105048.GA27855@gmail.com> In-Reply-To: <20150403105048.GA27855@gmail.com> 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: 15040604-0017-0000-0000-000009DB4E9B Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/03/2015 04:20 PM, Ingo Molnar wrote: > > * Preeti U Murthy wrote: > >> 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? > > It can be forwarded to -stable once it has gone upstream in the merge > window and has gone through some testing upstream as well. The > breakage itself was introduced in the v3.19 merge window, so it's an > older regression - and the fix is not simple either. Ok I see. Thanks Regards Preeti U Murthy > > Thanks, > > Ingo >