From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 03B83C2BA83 for ; Fri, 14 Feb 2020 18:23:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C6EE02086A for ; Fri, 14 Feb 2020 18:23:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581704592; bh=2zzEizZPgz3jQw2F25+EGzqeylL8hud7mBY/PTOc2w4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=L13xctH8gjLkGllo0z+PudMtjefPnYm2dbCk6s1gB6WUdiSvGeOhF+lZ3YrSe9oA2 ks+8cpdlfWzsFmQaZfwglOh3sKdcreZSa9IaO+eHOSD/gqMXdTMSdl1GwvOTcCYGac +xo8S83orKAm+0107p2cKVm3T0NCzJjseIw2iNqw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729923AbgBNPt0 (ORCPT ); Fri, 14 Feb 2020 10:49:26 -0500 Received: from mail.kernel.org ([198.145.29.99]:52266 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729881AbgBNPtY (ORCPT ); Fri, 14 Feb 2020 10:49:24 -0500 Received: from sasha-vm.mshome.net (c-73-47-72-35.hsd1.nh.comcast.net [73.47.72.35]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 09AAB2467E; Fri, 14 Feb 2020 15:49:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581695363; bh=2zzEizZPgz3jQw2F25+EGzqeylL8hud7mBY/PTOc2w4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=vRm0TM8Oz+otXwoxMj0biWcCNrZ78wI//grU706soTyfWJrFfIeIXSBI4jBKxGEts xRKQpMGn4jc1FSfwcBzgP/SQqH3OdhbDgKv2pKjcJT6TQOyjHLt9gn5k1kdRevSwsd 5oeLYM1bKkyiS2qnvPIywrPK7DNk18kyjcQECcck= From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Peter Zijlstra , "Paul E. McKenney" , Tejun Heo , Sasha Levin Subject: [PATCH AUTOSEL 5.5 022/542] cpu/hotplug, stop_machine: Fix stop_machine vs hotplug order Date: Fri, 14 Feb 2020 10:40:14 -0500 Message-Id: <20200214154854.6746-22-sashal@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200214154854.6746-1-sashal@kernel.org> References: <20200214154854.6746-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Peter Zijlstra [ Upstream commit 45178ac0cea853fe0e405bf11e101bdebea57b15 ] Paul reported a very sporadic, rcutorture induced, workqueue failure. When the planets align, the workqueue rescuer's self-migrate fails and then triggers a WARN for running a work on the wrong CPU. Tejun then figured that set_cpus_allowed_ptr()'s stop_one_cpu() call could be ignored! When stopper->enabled is false, stop_machine will insta complete the work, without actually doing the work. Worse, it will not WARN about this (we really should fix this). It turns out there is a small window where a freshly online'ed CPU is marked 'online' but doesn't yet have the stopper task running: BP AP bringup_cpu() __cpu_up(cpu, idle) --> start_secondary() ... cpu_startup_entry() bringup_wait_for_ap() wait_for_ap_thread() <-- cpuhp_online_idle() while (1) do_idle() ... available to run kthreads ... stop_machine_unpark() stopper->enable = true; Close this by moving the stop_machine_unpark() into cpuhp_online_idle(), such that the stopper thread is ready before we start the idle loop and schedule. Reported-by: "Paul E. McKenney" Debugged-by: Tejun Heo Signed-off-by: Peter Zijlstra (Intel) Tested-by: "Paul E. McKenney" Signed-off-by: Sasha Levin --- kernel/cpu.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/kernel/cpu.c b/kernel/cpu.c index 4dc279ed3b2d7..9c706af713fbc 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -525,8 +525,7 @@ static int bringup_wait_for_ap(unsigned int cpu) if (WARN_ON_ONCE((!cpu_online(cpu)))) return -ECANCELED; - /* Unpark the stopper thread and the hotplug thread of the target cpu */ - stop_machine_unpark(cpu); + /* Unpark the hotplug thread of the target cpu */ kthread_unpark(st->thread); /* @@ -1089,8 +1088,8 @@ void notify_cpu_starting(unsigned int cpu) /* * Called from the idle task. Wake up the controlling task which brings the - * stopper and the hotplug thread of the upcoming CPU up and then delegates - * the rest of the online bringup to the hotplug thread. + * hotplug thread of the upcoming CPU up and then delegates the rest of the + * online bringup to the hotplug thread. */ void cpuhp_online_idle(enum cpuhp_state state) { @@ -1100,6 +1099,12 @@ void cpuhp_online_idle(enum cpuhp_state state) if (state != CPUHP_AP_ONLINE_IDLE) return; + /* + * Unpart the stopper thread before we start the idle loop (and start + * scheduling); this ensures the stopper task is always available. + */ + stop_machine_unpark(smp_processor_id()); + st->state = CPUHP_AP_ONLINE_IDLE; complete_ap_thread(st, true); } -- 2.20.1