From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964832AbcGZS5l (ORCPT ); Tue, 26 Jul 2016 14:57:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50210 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932345AbcGZS5j (ORCPT ); Tue, 26 Jul 2016 14:57:39 -0400 Date: Tue, 26 Jul 2016 20:57:36 +0200 From: Oleg Nesterov To: Andrew Morton , Ingo Molnar Cc: Dave Anderson , Peter Zijlstra , "Paul E. McKenney" , Wang Shu , linux-kernel@vger.kernel.org, Tejun Heo , Thomas Gleixner Subject: [PATCH 1/1] stop_machine: touch_nmi_watchdog() after MULTI_STOP_PREPARE Message-ID: <20160726185736.GB4088@redhat.com> References: <20160725162332.GA23935@redhat.com> <20160726185712.GA4088@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160726185712.GA4088@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Tue, 26 Jul 2016 18:57:38 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Suppose that stop_machine(fn) hangs because fn() hangs. In this case NMI hard-lockup can be triggered on another CPU which does nothing wrong and the trace from nmi_panic() won't help to investigate the problem. And this change "fixes" the problem we (seem to) hit in practice. - stop_two_cpus(0, 1) races with show_state_filter() running on CPU_0. - CPU_1 already spins in MULTI_STOP_PREPARE state, it detects the soft lockup and tries to report the problem. - show_state_filter() enables preemption, CPU_0 calls multi_cpu_stop() which goes to MULTI_STOP_DISABLE_IRQ state and disables interrupts. - CPU_1 spends more than 10 seconds trying to flush the log buffer to the slow serial console. - NMI interrupt on CPU_0 (which now waits for CPU_1) calls nmi_panic(). Reported-by: Wang Shu Signed-off-by: Oleg Nesterov --- kernel/stop_machine.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index a467e6c..4a1ca5f 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -21,6 +21,7 @@ #include #include #include +#include /* * Structure to determine completion condition and record errors. May @@ -209,6 +210,13 @@ static int multi_cpu_stop(void *data) break; } ack_state(msdata); + } else if (curstate > MULTI_STOP_PREPARE) { + /* + * At this stage all other CPUs we depend on must spin + * in the same loop. Any reason for hard-lockup should + * be detected and reported on their side. + */ + touch_nmi_watchdog(); } } while (curstate != MULTI_STOP_EXIT); -- 2.5.0