From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1422780AbcHEPm1 (ORCPT ); Fri, 5 Aug 2016 11:42:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35798 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1422764AbcHEPmY (ORCPT ); Fri, 5 Aug 2016 11:42:24 -0400 Date: Fri, 5 Aug 2016 11:42:23 -0400 From: Mike Snitzer To: Jens Axboe Cc: Benjamin Block , dm-devel@redhat.com, linux-kernel@vger.kernel.org, hch@lst.de, linux-block@vger.kernel.org Subject: Re: bug: using smp_processor_id() in preemptible code in rr_select_path() Message-ID: <20160805154223.GA2340@redhat.com> References: <20160803153545.GE20370@bblock-ThinkPad-W530> <20160805152705.GA17480@redhat.com> <497f02e5-4f3d-8590-3096-fb5e2422bff4@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <497f02e5-4f3d-8590-3096-fb5e2422bff4@kernel.dk> User-Agent: Mutt/1.5.23 (2014-03-12) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Fri, 05 Aug 2016 15:42:23 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 05 2016 at 11:33P -0400, Jens Axboe wrote: > On 08/05/2016 09:27 AM, Mike Snitzer wrote: > >On Wed, Aug 03 2016 at 11:35am -0400, > >Benjamin Block wrote: > > > >>Hej Mike, > >> > >>when running a debug-kernel today with several multipath-devices using > >>the round-robin path selector I noticed that the kernel throws these > >>warnings here: > >> > >>BUG: using smp_processor_id() in preemptible [00000000] code: kdmwork-252:0/881 > >>caller is rr_select_path+0x36/0x108 [dm_round_robin] > >>CPU: 1 PID: 881 Comm: kdmwork-252:0 Not tainted 4.7.0-debug #4 > >> 00000000617679b8 0000000061767a48 0000000000000002 0000000000000000 > >> 0000000061767ae8 0000000061767a60 0000000061767a60 00000000001145d0 > >> 0000000000000000 0000000000b962ae 0000000000bb291e 000000000000000b > >> 0000000061767aa8 0000000061767a48 0000000000000000 0000000000000000 > >> 0700000000b962ae 00000000001145d0 0000000061767a48 0000000061767aa8 > >>Call Trace: > >>([<00000000001144a2>] show_trace+0x8a/0xe0) > >>([<0000000000114586>] show_stack+0x8e/0xf0) > >>([<00000000006c7fdc>] dump_stack+0x9c/0xe0) > >>([<00000000006fbbc0>] check_preemption_disabled+0x108/0x130) > >>([<000003ff80268646>] rr_select_path+0x36/0x108 [dm_round_robin]) > >>([<000003ff80259a42>] choose_path_in_pg+0x42/0xc8 [dm_multipath]) > >>([<000003ff80259b62>] choose_pgpath+0x9a/0x1a0 [dm_multipath]) > >>([<000003ff8025b51a>] __multipath_map.isra.5+0x72/0x228 [dm_multipath]) > >>([<000003ff8025b75e>] multipath_map+0x3e/0x50 [dm_multipath]) > >>([<000003ff80225eb6>] map_request+0x66/0x458 [dm_mod]) > >>([<000003ff802262ec>] map_tio_request+0x44/0x70 [dm_mod]) > >>([<000000000016835a>] kthread_worker_fn+0xf2/0x1d8) > >>([<00000000001681da>] kthread+0x112/0x120) > >>([<000000000098378a>] kernel_thread_starter+0x6/0xc) > >>([<0000000000983784>] kernel_thread_starter+0x0/0xc) > >>no locks held by kdmwork-252:0/881. > >> > >> > >>There are several more of these warnings, but all have the same > >>stack-trace (this is on s390x, but this looks like its only common code) > >>- sometimes the process-context is multipath. > >> > >>Looking at the changes in this function, it rather looks like this is > >>caused by changes made in commit > >>b0b477c7e0dd93f8916d106018ded1331b81bf61 (dm round robin: use percpu > >>'repeat_count' and 'current_path'). > >> > >>The kernel is a stock v4.7 with some debug options enabled (prominently > >>DEBUG_PREEMPT). Need any more info? > > > >As you can see from commit b0b477c7e0dd9 the round-robin path selector > >is now using percpu data (pointer) and a percpu_counter. > > > >I'm really not sure how else to access this percpu data. > > > >Cc'ing LKML, linux-block, Jens and hch to cast a wider net in the hopes > >of getting an answer to how to fix this. > > From a quick look, looks like you are using this_cpu_ptr() without > having preemption disabled. Right, that is what it looked like to me too. I'm just not sure on what the proper pattern is to fix this. I'll look closer though.