From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751361AbdISEFE (ORCPT ); Tue, 19 Sep 2017 00:05:04 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:60016 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750765AbdISEFD (ORCPT ); Tue, 19 Sep 2017 00:05:03 -0400 Date: Mon, 18 Sep 2017 21:04:56 -0700 From: "Paul E. McKenney" To: Byungchul Park 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 Reply-To: paulmck@linux.vnet.ibm.com References: <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> <20170919024822.GG5994@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170919024822.GG5994@X58A-UD3R> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17091904-0024-0000-0000-000002D46C80 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007759; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000230; SDB=6.00919096; UDB=6.00461749; IPR=6.00699323; BA=6.00005596; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00017203; XFM=3.00000015; UTC=2017-09-19 04:05:00 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17091904-0025-0000-0000-000045785FE9 Message-Id: <20170919040456.GC3521@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-09-19_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1709190059 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 19, 2017 at 11:48:22AM +0900, Byungchul Park wrote: > 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. Fair enough, if the wakeup always happened first, deadlock might well be avoided. If the sleep happened first, I suspect deadlock would be deterministic in this case. 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 > > > >