From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932555AbcIBPto (ORCPT ); Fri, 2 Sep 2016 11:49:44 -0400 Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:33905 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932268AbcIBPtl (ORCPT ); Fri, 2 Sep 2016 11:49:41 -0400 Subject: Re: [writeback] 8bc4ad9498: INFO: suspicious RCU usage. ] To: , Tejun Heo 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> CC: kernel test robot , , Jens Axboe , LKML From: Jens Axboe Message-ID: <1a774b3f-9fdc-b620-9a35-d3e7ff18177a@fb.com> Date: Fri, 2 Sep 2016 09:49:25 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 In-Reply-To: <20160902153601.GM16261@linux.vnet.ibm.com> Content-Type: text/plain; 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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. >>>> [ 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... -- Jens Axboe