From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754404AbcIBRtJ (ORCPT ); Fri, 2 Sep 2016 13:49:09 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:37037 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754364AbcIBRtE (ORCPT ); Fri, 2 Sep 2016 13:49:04 -0400 X-IBM-Helo: d03dlp03.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com Date: Fri, 2 Sep 2016 10:00:56 -0700 From: "Paul E. McKenney" To: Jens Axboe Cc: Tejun Heo , kernel test robot , lkp@01.org, Jens Axboe , LKML Subject: Re: [writeback] 8bc4ad9498: INFO: suspicious RCU usage. ] Reply-To: paulmck@linux.vnet.ibm.com References: <57c8012a.zlFYe5dba5wTaKD+%xiaolong.ye@intel.com> <25429877-af98-30b2-1980-6686a7b7b78f@fb.com> <20160902145622.GH12660@htj.duckdns.org> <20160902153601.GM16261@linux.vnet.ibm.com> <1a774b3f-9fdc-b620-9a35-d3e7ff18177a@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1a774b3f-9fdc-b620-9a35-d3e7ff18177a@fb.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16090217-0028-0000-0000-00000581327B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00005698; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000184; SDB=6.00753010; UDB=6.00356090; IPR=6.00525837; BA=6.00004687; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00012560; XFM=3.00000011; UTC=2016-09-02 17:49:01 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16090217-0029-0000-0000-00002EE68429 Message-Id: <20160902170056.GN16261@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-09-02_05:,, 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=8.0.1-1604210000 definitions=main-1609020237 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 02, 2016 at 09:49:25AM -0600, Jens Axboe wrote: > On 09/02/2016 09:36 AM, Paul E. McKenney wrote: > >On Fri, Sep 02, 2016 at 10:56:22AM -0400, Tejun Heo wrote: > >>(cc'ing Paul, hi!) > >> > >>Hello, > >> > >>On Thu, Sep 01, 2016 at 02:13:34PM -0600, Jens Axboe wrote: > >>>On 09/01/2016 04:21 AM, kernel test robot wrote: > >>>>[ 7.323356] cdrom: Uniform CD-ROM driver Revision: 3.20 > >>>>[ 7.334239] > >>>>[ 7.337256] =============================== > >>>>[ 7.340532] [ INFO: suspicious RCU usage. ] > >>>>[ 7.342419] 4.8.0-rc4-00008-g8bc4ad9 #1 Not tainted > >>>>[ 7.347065] ------------------------------- > >>>>[ 7.350132] include/linux/cgroup.h:435 suspicious rcu_dereference_check() usage! > > > >Lockdep does not believe that any locks are held, correct? > > Which is correct, the queue lock has been dropped at this point. Note that lockdep believes that the rcu_read_lock() has also been dropped, otherwise it would have listed it. > >>>>[ 7.410074] Call Trace: > >>>>[ 7.411328] [] dump_stack+0x82/0xb8 > >>>>[ 7.413982] [] lockdep_rcu_suspicious+0xf7/0x100 > >>>>[ 7.415828] [] bio_blkcg+0x89/0x93 > >>>>[ 7.417336] [] check_blkcg_changed+0x58/0x1b8 > >>>>[ 7.428722] [] cfq_set_request+0xd1/0x2a3 > >>>>[ 7.439690] [] elv_set_request+0x1f/0x24 > >>>>[ 7.442157] [] get_request+0x38f/0x77f > >>>>[ 7.447449] [] blk_get_request+0x65/0xa8 > >>>>[ 7.449868] [] ide_cd_queue_pc+0x76/0x19d > >>>>[ 7.453757] [] cdrom_check_status+0x51/0x53 > >>>>[ 7.455372] [] ide_cdrom_check_events_real+0x20/0x3f > >>>>[ 7.457294] [] cdrom_update_events+0x18/0x21 > >>>>[ 7.458987] [] cdrom_check_events+0x12/0x1f > >>>>[ 7.460713] [] idecd_check_events+0x1c/0x1e > >>>>[ 7.462393] [] disk_check_events+0x47/0x103 > >>>>[ 7.464129] [] disk_events_workfn+0x1c/0x1e > >>>>[ 7.465844] [] process_one_work+0x272/0x4ee > >>>>[ 7.467462] [] worker_thread+0x1eb/0x2c9 > >> > >>The warning is from > >> > >>#define task_css_set_check(task, __c) \ > >> rcu_dereference_check((task)->cgroups, \ > >> lockdep_is_held(&cgroup_mutex) || \ > >> lockdep_is_held(&css_set_lock) || \ > >> ((task)->flags & PF_EXITING) || (__c)) > >> > >>which is used by bio_blkcg() which is called by the following code in > >>check_blkcg_changed(). > >> > >> rcu_read_lock(); > >> serial_nr = bio_blkcg(bio)->css.serial_nr; > >> rcu_read_unlock(); > >> > >>So, I have no idea. It looks like rcu_dereference_check() is being > >>called with rcu read locked but still triggering suspicious RCU usage > >>warning. > > > >Perhaps there is an rcu_read_unlock() somewhere on the code path? > > > >>The code hasn't changed for quite a while now, so it's also really > >>weird that it's triggering now. Paul, does anything ring a bell? > > > >I have not see something like this recently. > > The question is if it's really new, or just re-triggering because the > writeback branch changes that function? It's further down though, so > can't impact the RCU section. And the writeback changes don't have any > RCU code in them... At this point, I have to suggest "git bisect"... Thanx, Paul