From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751336AbdISCse (ORCPT ); Mon, 18 Sep 2017 22:48:34 -0400 Received: from LGEAMRELO11.lge.com ([156.147.23.51]:40594 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750783AbdISCsc (ORCPT ); Mon, 18 Sep 2017 22:48:32 -0400 X-Original-SENDERIP: 156.147.1.151 X-Original-MAILFROM: byungchul.park@lge.com X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Tue, 19 Sep 2017 11:48:22 +0900 From: Byungchul Park To: "Paul E. McKenney" Cc: Steven Rostedt , Neeraj Upadhyay , josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, linux-kernel@vger.kernel.org, sramana@codeaurora.org, prsood@codeaurora.org, pkondeti@codeaurora.org, markivx@codeaurora.org, peterz@infradead.org, kernel-team@lge.com Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu Message-ID: <20170919024822.GG5994@X58A-UD3R> References: <20170918111105.15f687da@gandalf.local.home> <20170918160125.GL3521@linux.vnet.ibm.com> <20170918121213.312c82b0@gandalf.local.home> <20170918162412.GM3521@linux.vnet.ibm.com> <20170918122931.0e3341f3@gandalf.local.home> <20170918165527.GN3521@linux.vnet.ibm.com> <20170918235311.GA20177@linux.vnet.ibm.com> <20170919015027.GD5994@X58A-UD3R> <20170919020610.GF5994@X58A-UD3R> <20170919023329.GA3521@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170919023329.GA3521@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 18, 2017 at 07:33:29PM -0700, Paul E. McKenney wrote: > > > Hello Paul and Steven, > > > > > > This is saying: > > > > > > Thread A > > > -------- > > > takedown_cpu() > > > irq_lock_sparse() > > > wait_for_completion(&st->done) // Wait for completion of B > > > irq_unlock_sparse() > > > > > > Thread B > > > -------- > > > cpuhp_invoke_callback() > > > irq_lock_sparse() // Wait for A to irq_unlock_sparse() > > > (on the way going to complete(&st->done)) > > > > > > So, lockdep consider this as a deadlock. > > > Is it possible to happen? > > > > In addition, if it's impossible, then we should fix lock class > > assignments so that the locks actually have different classes. > > Interesting, and thank you for the analysis! > > The strange thing is that the way you describe it, this would be a > deterministic deadlock. Yet CPU hotplug operations complete just fine > in my tests. What am I missing here? Hi, :) Lockdep basically reports either (1) an actual deadlock happened at the time or (2) a deadlock possibility, even w/o LOCKDEP_CROSSRELEASE. Both are useful. But LOCKDEP_CROSSRELEASE can only do the latter. IOW, the deadlock would actually happen _only_ when the two threads(A and B) run simultaniously. In your case, those two threads might run at different timings. So it's not an actual deadlock, but still has a possibility for the problem to happen later. > Thanx, Paul > > > > Thanks, > > > Byungchul > > > > > > > [ 35.313943] > > > > [ 35.313943] 3 locks held by torture_onoff/766: > > > > [ 35.313943] #0: (cpu_add_remove_lock){+.+.}, at: [] do_cpu_down+0x22/0x50 > > > > [ 35.313943] #1: (cpu_hotplug_lock.rw_sem){++++}, at: [] percpu_down_write+0x21/0xf0 > > > > [ 35.313943] #2: (sparse_irq_lock){+.+.}, at: [] irq_lock_sparse+0x12/0x20 > > > > [ 35.313943] > > > > [ 35.313943] stack backtrace: > > > > [ 35.313943] CPU: 7 PID: 766 Comm: torture_onoff Not tainted 4.13.0-rc4+ #1 > > > > [ 35.313943] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014 > > > > [ 35.313943] Call Trace: > > > > [ 35.313943] dump_stack+0x67/0x97 > > > > [ 35.313943] print_circular_bug+0x21d/0x330 > > > > [ 35.313943] ? add_lock_to_list.isra.31+0xc0/0xc0 > > > > [ 35.313943] check_prev_add+0x401/0x800 > > > > [ 35.313943] ? wake_up_q+0x70/0x70 > > > > [ 35.313943] __lock_acquire+0x1100/0x11a0 > > > > [ 35.313943] ? __lock_acquire+0x1100/0x11a0 > > > > [ 35.313943] ? add_lock_to_list.isra.31+0xc0/0xc0 > > > > [ 35.313943] lock_acquire+0x9e/0x1e0 > > > > [ 35.313943] ? takedown_cpu+0x86/0xf0 > > > > [ 35.313943] wait_for_completion+0x36/0x130 > > > > [ 35.313943] ? takedown_cpu+0x86/0xf0 > > > > [ 35.313943] ? stop_machine_cpuslocked+0xb9/0xd0 > > > > [ 35.313943] ? cpuhp_invoke_callback+0x8b0/0x8b0 > > > > [ 35.313943] ? cpuhp_complete_idle_dead+0x10/0x10 > > > > [ 35.313943] takedown_cpu+0x86/0xf0 > > > > [ 35.313943] cpuhp_invoke_callback+0xa7/0x8b0 > > > > [ 35.313943] cpuhp_down_callbacks+0x3d/0x80 > > > > [ 35.313943] _cpu_down+0xbb/0xf0 > > > > [ 35.313943] do_cpu_down+0x39/0x50 > > > > [ 35.313943] cpu_down+0xb/0x10 > > > > [ 35.313943] torture_offline+0x75/0x140 > > > > [ 35.313943] torture_onoff+0x102/0x1e0 > > > > [ 35.313943] kthread+0x142/0x180 > > > > [ 35.313943] ? torture_kthread_stopping+0x70/0x70 > > > > [ 35.313943] ? kthread_create_on_node+0x40/0x40 > > > > [ 35.313943] ret_from_fork+0x27/0x40 > >